TY - JOUR
AU1 - Divagar,, Darsana
AU2 - Jian,, Fuji
AU3 - Cenkowski,, Stefan
AB - Abstract The effect of 105°C steam or hot air on adult mortality of three species of stored-product insect pests outside wheat kernels of 12.5, 14.5 and 16.5% moisture content was investigated. The species were Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae), and Sitophilus oryzae (L.) (Coleoptera: Curculionidae). In the case of S. oryzae, young adults and immature stages inside wheat kernels were also tested. The mortality of insects inside kernels was higher at lower moisture contents of wheat treated with hot air, whereas moisture content did not significantly affect mortality of insects treated with steam. In the hot air treatment, all adults of the three species outside kernels had 100% mortality when the treatment time was 75 s for wheat with 16.5% moisture content, and 60 s for 12.5 and 14.5% wheat. In the steam treatment, the time to reach 100% mortality of adults outside kernels was 1 s at any moisture content and without significantly affecting germination. The young adults and immature stages of S. oryzae inside kernels required 90 s to reach 100% mortality in hot air, whereas 3 s was needed in steam. The treatment to reach 100% mortality of insects inside kernels caused a 20% drop in germination in steam and 81% drop in hot air. stored wheat, insect pest, hot air, steam, mortality Wheat can be infested by stored-product insect pests during storage and transportation. Infestation of insect pests reduces both quality and quantity of the stored food products, which results in millions of dollars of economic losses every year (Karunakaran et al. 2003, Singh et al. 2009). In Western Canada, Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) are the abundant external feeders (Smith and Loschiavo 1978). Sitophilus oryzae (L.) (Coleoptera: Curculionidae) is an important internal feeder (Madrid et al. 1990, Fields et al. 1993, Arthur and Flinn 2000). After mating, female adults of S. oryzae will drill a hole into a grain kernel, oviposit into the cavity, and cover the egg with waxy secretion (egg plug). All larval development and pupation take place inside the kernel. Hatched adults of S. oryzae chew its way out of the kernel, move in grain bulks (Jian et al. 2012), and may hide inside grain kernels feeding on the endosperm. To maintain the quality and quantity of the stored food products, insects inside and outside grain kernels should be killed during storage and transportation of grain. Heat treatment is one of the alternatives to chemical control that can eliminate or control stored-product insects (Fields 1992, Hansen et al. 2011). Many authors reported that most heat treatments are more expensive than the chemical methods (Fields 1992, Boina and Subramanyam 2004, Hansen et al. 2011, Subramanyam et al. 2011) and have a possible adverse effect on the quality of treated commodity. However, heating methods do not leave chemical residues in products like contact insecticides. Heat treatment methods include: solar heating, water based and atmospheric heating, steam treatment, flame treatment, forced hot air heating, electric field treatment, and high-temperature-controlled atmosphere treatment (Hansen et al. 2011). To control various stored-product insect pests, extensive studies have been conducted on the combination of temperature and treatment duration (Jian et al. 2002, Boina and Subramanyam 2004, Subramanyam et al. 2011, Yu et al. 2011). Most researchers used temperatures from 40 to 60°C (Hansen et al. 1997, Beckett and Morton 2003, Mahroof et al. 2003a, Boina and Subramanyam 2004, Yu et al. 2011, Jian et al. 2013). Factors that affect mortality at high temperature are as follows: species (Jian et al. 2002, Mahroof et al. 2003a, Boina and Subramanyam 2004), treatment duration (Jian et al. 2002, Mahroof et al. 2003b, Yu et al. 2011), and life stage (Mahroof et al. 2003b, Boina and Subramanyam 2004, Yu et al. 2011). Treatment at lower temperatures requires longer exposure times than that at higher temperatures. For example, the LT99 (lethal time for 99% mortality) value of pupae of T. castaneum at 46, 54, and 60°C is 780.4, 33.8, and 18.8 min, respectively (Mahroof et al. 2003a). Therefore, treatment at high temperatures will significantly decrease the treatment time, which would result in a less energy usage. However, there are no studies examining the heat tolerance of insect pests at above 100°C in hot air or steam. Steam can be used to control insects in heat-insensitive materials in a short time period with minimal energy usage (Pelletier et al. 1998, Hansen et al. 2011) due to the higher latent heat of steam than that of dry air at the same temperature. Steam treatment was used in sterilizing soil, cotton, woolen products, lumber, and processed food products (Hansen et al. 2011). These studies were only conducted on heat-insensitive materials as heat can damage heat-sensitive biomaterials such as plants (Pelletier et al. 1998), flowers, and foliage (Hansen et al. 1992). Dry seeds such as wheat kernels usually have a lower heat sensitivity than the fresh ones due to the low moisture content of seeds. It is not known whether steam can be used to treat infested wheat without affecting grain quality such as germination. The objectives of this research were to determine 1) mortality of adults of S. oryzae, T. castaneum, and C. ferrugineus outside wheat kernels of various moisture contents and treated in steam or hot air for different exposure times; 2) mortality of larvae, pupae, and young adults of S. oryzae inside wheat kernels exposed to steam or hot air; and 3) germination of wheat kernels under the above treatment conditions. Materials and Methods Wheat Uninfested and clean wheat (cv. Carberry; referred to as sound wheat) was used in this study. Wheat moisture content was determined according to the standard oven drying method (ASABE 2016). The initial wheat moisture content (m.c.; wet basis) was approximately 12%, and its m.c. was adjusted to 12.5, 14.5, or 16.5% by adding the desired amount of water and mixing it in a tumbler for approximately half an hour. After conditioning it to desired moisture contents, wheat samples were stored in sealed plastic bags at 5 ± 1°C until use. Moisture content of the conditioned wheat was measured again before using it, and the SE of moisture content was determined to be ±0.1 percent points. This measured m.c. was reported as the initial m.c. in this article. Insects All rearing of the insects was carried out in the dark at 30 ± 1°C and 70 ± 5% relative humidity (RH). The culture media for T. castaneum, C. ferrugineus, and S. oryzae were white wheat flour with 5% brewer’s yeast (by weight), whole wheat with 5% cracked and 5% wheat germ (by weight), and whole wheat kernels, respectively. The moisture content of the white wheat flour, whole wheat kernel, and wheat germ was 14.5 ± 0.3%. The mass of each culture media was about 0.5–2.5 kg. The culture media were kept at the room temperature for at least 20 min before insects were introduced. Insect cultures had been in the laboratory for 2–4 yr. To get the same age of adults, approximately 1,000 adults were introduced into a jar with about 2 kg of culture media. The adults were sieved out after 48 h. The culture media with eggs were incubated for 33, 27, and 31 d for T. castaneum, C. ferrugineus, and S. oryzae, respectively, and the emerged adults were sieved out and used for the treatment tests. To prevent the selected weevils from entering wheat kernels, the selected weevils were treated within 20 h. The selected adults of T. castaneum and C. ferrugineus were treated within 48 h. To produce the same age of larvae, pupae, and young adults of S. oryzae inside wheat kernels, the culture media with the eggs were incubated for 12, 23, and 28 d, respectively. Infested wheat kernels were randomly taken out from the culture media, and the infestation status was visually examined by using a soft X-ray imaging system (Model: MX-20, Faxitron Bioptics, LLC; Karunakaran et al. 2003). The important features for differentiating stages of S. oryzae were dark feeding cavity (tunnel) in the kernel, dark spot in the head capsule of larvae, articulated margin of the cavity for pupae, and the snout in the head of adults (Milner et al. 1950, Karunakaran et al. 2003). The infested kernels with confirmed insect stages were stained using red food color (Club House, McCormick, London, ON, Canada). The stained kernels were treated within 4 h. Metal Mesh Containers Used for Both Hot Air and Steam Treatments Metal mesh containers (referred to as mesh container) with 54 and 55 mm inner and outer diameters and 41 mm high were used to hold the wheat kernels and insects during treatment (Fig. 1). The bottom of the mesh container was metal mesh with 250-µm opening, which allowed hot air or steam to pass through the mesh and kernels during heat treatment. The mesh container had a 565-mm-long handle. At the handheld side of the handle, there was a 128 × 128 mm metal plate for the hot air treatment (Fig. 1) and a square metal cap (64.5 × 64.5 mm) for the steam treatment (Fig. 2). After the mesh container was inserted into the treatment chamber, the metal plate or metal cap covered the openings of the chamber to prevent heat loss. During the heat treatment in hot air or steam, 30 adults of each species or 30 infested kernels mixed with 4.5 ± 0.2 g sound kernels were placed in the mesh container. A single layer of such kernels was evenly distributed at the bottom of the container. To prevent insect escaping from the container, a thin layer of Teflon was applied on the inner rim of the container. During the heat treatment, the steam or hot air was introduced from bottom of the mesh container. No insects flying out of the mesh container were observed. Fig. 1. Open in new tabDownload slide An oven system for treating insects and wheat kernels. (A) The programmable oven and the metal mesh container inserted into the oven. (B) The metal mesh container outside the oven. (C) The top view of the wheat kernels inside the mesh container. Fig. 1. Open in new tabDownload slide An oven system for treating insects and wheat kernels. (A) The programmable oven and the metal mesh container inserted into the oven. (B) The metal mesh container outside the oven. (C) The top view of the wheat kernels inside the mesh container. Fig. 2. Open in new tabDownload slide A steam system for treating insects and wheat kernels. (A) The steam system with the metal mesh container inserted into the steam chamber. (B) The cross-section view of the steam chamber. Fig. 2. Open in new tabDownload slide A steam system for treating insects and wheat kernels. (A) The steam system with the metal mesh container inserted into the steam chamber. (B) The cross-section view of the steam chamber. Hot Air Treatment The hot air treatment was conducted inside a programmable oven (model T2C-A-WF4, Tenney, New Columbia, PA) at 105 ± 0.3°C. The air velocity inside the oven was 0.7 m/s and was measured by a handheld meter (Turbo meter, David instruments 271 C, Hayward, CA) through an 89-mm-diameter opening at one side wall of the oven (Fig. 1). After the oven reached 105°C, the mesh container holding the selected insects and wheat kernels was quickly inserted into the center of the oven through the opening (Fig. 1). The mesh container with the sample was then quickly removed from the oven after 1, 3, 5, 10, 15, 30, 45, 60, 75, or 90 s. The inserting and retrieving time took less than 1 s. After the mesh container was retrieved from the oven, the sample (insects and wheat kernels) was immediately transferred onto a tray and cooled at room temperature (22 ± 1°C). Each treatment was repeated five times. Steam Treatment The steam system used was designed and fabricated in the lab (Fig. 2) and described by Pronyk et al. (2004). This system was slightly modified by Cenkowski et al. (2007) and Zielinska et al. (2009). The system was set to produce steam at 105 ± 3°C with 0.70 ± 0.05 m/s velocity. This velocity was verified using the method developed by Irvine and Liley (1984). Irvine and Liley (1984) tabularized the relationship between the amount of condensate exiting the steam system in 3 min and the velocity of the steam entering the processing chamber under specific processing temperatures and pressures. During pretest, condensate in 3 min was collected, and the table developed by Irvine and Liley (1984) was used. Then, the required steam velocity was manually adjusted using the steam flow rate valve and maintained at 0.70 ± 0.05 m/s. The steam treatment was conducted inside a chamber with dimensions of 241 × 241 × 356 mm (L × W × D). The steam was introduced at the middle bottom of the chamber (Fig. 2). To prevent condensation of steam on the walls of the chamber, electric strip heaters were used to warm up the chamber walls to 105°C (Zielinska et al. 2009). A square metal pipe with 57.3 ×57.3 mm cross-section and 223 mm long was mounted through the front door of the steam chamber (Fig. 2). The thickness of the square metal pipe wall was 3.4 mm. This pipe was used to guide the inserted mesh container into the center of the steam chamber (Fig. 2). The length of the pipe and the handle of the mesh container were properly scaled, so that the mesh container with the sample was located right above the inlet of steam released to the chamber (Fig. 2). After steam reached the desired temperature and flow rate, the mesh container housing the selected insects and wheat kernels was inserted into the steam chamber through the square pipe (Fig. 2). The mesh container was retrieved from the steam chamber after 1, 3, or 5 s, and then the insects and wheat kernels were immediately transferred onto a tray and cooled at room temperature (22 ± 1°C). Each treatment was repeated five times. Measurement of Temperature Inside and Between Wheat Kernels Four T-type thermocouples connected to data loggers (Onset HOBO 4-channel thermocouple loggers, UX120-014M, Bourne, MA) were used to measure temperatures inside sound and infested kernels, and at the surface of and between the wheat kernels while exposed to steam or hot air. To measure the temperature inside infested kernels, one thermocouple was inserted into a hole (bored by insects) of an infested kernel. To measure the temperature inside sound kernels, one thermocouple was inserted along the major axis of a kernel through a predrilled 0.5-mm-diameter hole. The depth of the individual hole was about half the length of the kernel along its major axis. After inserting thermocouples into the kernels, glue (Permatex High-Temp Gasket Maker, Solon, OH) was applied to seal the holes and fix the thermocouples. To measure temperature between kernels, one thermocouple was inserted between kernels. To measure the temperature at the surface of kernels, one thermocouple was glued on the surfaces of a sound kernel. All the thermocouples were fixed on the wall of the mesh container by electronic tapes. The treatment procedure of the kernels with the thermocouples was the same as the treatment of insects. The total monitoring time was 5 and 90 s for steam and hot air treatment, respectively. The experiment was repeated 12 times in each medium, and different kernels were used for each replicate. This temperature measurement system was also used to monitor the temperature of hot air or steam before the start of or during the heat treatment. When this system was used to monitor the medium temperature, the thermocouples were located at following locations of the programmable oven or steam chamber: one at the center, one at the headspace of the mesh container, and two near the walls at the opposite direction. Determination of Moisture Content Change and Germination Kernels with three different initial moisture contents (12.5, 14.5, and 16.5%) were used to determine their change in moisture before and after the heat treatment. The mass of the samples that included the infested and sound kernels and insects was measured prior to the treatment. Fifteen minutes after each treatment, mass of the sample was measured again. Kernel moisture content after the treatment was calculated based on its initial moisture content and the gained or lost water. Germination of sound kernels was determined before and after the heat treatment by using the standard procedure of the International Seed Testing Association (ISTA 1999). Twenty-five sound kernels were randomly selected for the germination test. In a 90-mm-diameter Petri dish, Whatman no. 3 filter paper was moistened with 7 ml of distilled water, and the 25 kernels were placed on the filter paper. The Petri dishes were covered with lids and kept inside a polyethylene bag at room temperature (23 ± 3°C). Germination was assessed after 1 wk (Manickavasagan et al. 2007, Lehner et al. 2008). The germination of untreated wheat kernels was used as control. There were three replicates. Evaluation of Insect Mortality After each heat treatment, mortality of insects outside wheat kernels was assessed 15 min after the treatment when the kernels reached room temperature. Adults were considered alive if they showed any movement under a microscope when gently touched with a small brush. After this mortality evaluation, the adults were transferred into a 100-ml jar with about 50-g culture media, and mortality was assessed again after 24 and 72 h. To evaluate the mortality of insects inside infested kernels, the treated kernels that were stained by red food color were manually separated from sound kernels and transferred into a jar with 100-g culture medium. The jar with the kernels infested by larvae, pupae, and adults was kept at the rearing conditions for 18, 7, and 1 d, respectively. After this incubation period, the emerged adults were counted, and the infested kernels were dissected under a microscope to confirm the development and mortality of the insects (Evans 1981, Mahroof et al. 2003a). If the insect was moving after splitting kernels, the insect was counted as alive. Insect mortality in the control treatments was determined by placing the adults or infested kernels mixed with 4.5 ± 0.2 or 4.0 ± 0.2 g sound wheat kernels in the mesh container and then leaving the container in the oven or steam chamber at room temperature for 90 and 5 s, respectively. Data Analysis Mortality of insects inside and outside kernels in the control treatments was ≤0.7%. All larvae and pupae emerged to adults after the incubation period if they survived the heat treatment. Therefore, the determined mortality was directly used to conduct the data analysis. Mortalities of insects treated at the same condition but evaluated at different times (15 min, 24 or 72 h) after heat treatment were the same. Therefore, the mortality determined at 15 min after the heat treatment was used for further data analysis. A three-way factorial analysis of variance was performed on mortality of adults of various species outside kernels, various treatment times, and three initial moisture contents (SAS Institute 2018). Mortality of three life stages of S. oryzae at different treatment times and moisture contents was subjected to another three-way factorial analysis. To test whether the germination or final moisture content of wheat was influenced by initial moisture contents and treatment times, two two-way factorial analyses were conducted, respectively. The mortalities at the same treatment condition and time but different initial moisture contents of wheat were compared by using Tukey’s multiple range test (α = 0.05). Tukey’s multiple range test was also conducted to find whether there was a significant difference of germination between control and a treatment time. Mortalities were pooled among tested initial moisture contents of wheat if initial moisture content did not significantly influence mortality. These pooled mortalities ware used to calculate the LT values. Arcsine transformation was used to transfer the mortality or germination prior to statistical test. Lethal times (LT50, LT95, and LT99) were estimated by using probit analysis (Polo Plus, version 2.0, LeOra Software, Berkeley, CA). Results and Discussion Temperature of the Treated Kernels In the 105°C hot air treatment, temperature at the surface of sound kernels and inside the sound and infested kernels reached 102.6 ± 0.6, 74.4 ± 2.3, and 87.6 ± 3.0°C within 90 s, respectively (Fig. 3). In the 105°C steam treatment, temperature at the surface of sound kernels and inside the sound and infested kernels reached 96.7 ± 0.3, 44.5 ± 5.6, and 74 ± 4.9°C within 3 s, respectively. After the samples were removed from the steam chamber, temperature inside kernels kept increasing for another 1 s (Fig. 3). This might be due to the delayed response time (lag time) of the data logger and the latent energy of steam still transferring into the kernels. Fig. 3. Open in new tabDownload slide Temperatures inside sound and infested kernels, and between wheat kernels treated in hot air for 90 s and in steam for 3 s. The temperature between kernels was the same as that at the surface of kernels. The two arrows show the end time of the treatment, and the kernels were moved to the room temperature after the heat treatment. The moisture content of the wheat kernels was 14.5 ± 0.1%. There were 12 replicates for each time point at each measured locations. Fig. 3. Open in new tabDownload slide Temperatures inside sound and infested kernels, and between wheat kernels treated in hot air for 90 s and in steam for 3 s. The temperature between kernels was the same as that at the surface of kernels. The two arrows show the end time of the treatment, and the kernels were moved to the room temperature after the heat treatment. The moisture content of the wheat kernels was 14.5 ± 0.1%. There were 12 replicates for each time point at each measured locations. In both media, temperature inside kernels was lower than the medium temperature, and temperature inside infested kernels was higher than that inside sound kernels. Therefore, insects outside kernels and inside infested kernels experienced higher temperature than the geometric center of sound kernels. A higher heat transfer rate into infested kernels might occur due to the cavity of the infested kernels. A noticeable variation in temperature inside infested and sound kernels (Fig. 3) was possibly caused by the difference in shape and size of wheat kernels and a specific location of thermocouples inside individual kernels. Moisture Content Changes In hot air, the initial moisture content of wheat kernels had a significant effect on the moisture content change when treatment time was >15 s (Table 1, two-way factorial analysis, F60 = 165.94, P < 0.0001). Moisture content of sound kernels decreased with an increase in treatment time. The moisture content loss in the hot air treatment was ≤0.06 percent points when the treatment time was ≤10 s. Table 1. Moisture content (%) of wheat kernels of different initial moisture contents and treated with hot air at 105°C (n = 3 at each data point) Treatment time (s) Initial moisture content (%) 12.5 14.5 16.5 15 12.4 ± 0.0 14.4 ± 0.0 16.4 ± 0.0 30 12.4 ± 0.0 14.4 ± 0.0 16.3 ± 0.1 45 12.3 ± 0.0 14.3 ± 0.0 16.2 ± 0.0 60 12.3 ± 0.0 14.1 ± 0.0 16.0 ± 0.0 75 12.2 ± 0.1 14.1 ± 0.0 15.8 ± 0.1 90 12.1 ± 0.0 13.8 ± 0.1 15.2 ± 0.1 Treatment time (s) Initial moisture content (%) 12.5 14.5 16.5 15 12.4 ± 0.0 14.4 ± 0.0 16.4 ± 0.0 30 12.4 ± 0.0 14.4 ± 0.0 16.3 ± 0.1 45 12.3 ± 0.0 14.3 ± 0.0 16.2 ± 0.0 60 12.3 ± 0.0 14.1 ± 0.0 16.0 ± 0.0 75 12.2 ± 0.1 14.1 ± 0.0 15.8 ± 0.1 90 12.1 ± 0.0 13.8 ± 0.1 15.2 ± 0.1 Open in new tab Table 1. Moisture content (%) of wheat kernels of different initial moisture contents and treated with hot air at 105°C (n = 3 at each data point) Treatment time (s) Initial moisture content (%) 12.5 14.5 16.5 15 12.4 ± 0.0 14.4 ± 0.0 16.4 ± 0.0 30 12.4 ± 0.0 14.4 ± 0.0 16.3 ± 0.1 45 12.3 ± 0.0 14.3 ± 0.0 16.2 ± 0.0 60 12.3 ± 0.0 14.1 ± 0.0 16.0 ± 0.0 75 12.2 ± 0.1 14.1 ± 0.0 15.8 ± 0.1 90 12.1 ± 0.0 13.8 ± 0.1 15.2 ± 0.1 Treatment time (s) Initial moisture content (%) 12.5 14.5 16.5 15 12.4 ± 0.0 14.4 ± 0.0 16.4 ± 0.0 30 12.4 ± 0.0 14.4 ± 0.0 16.3 ± 0.1 45 12.3 ± 0.0 14.3 ± 0.0 16.2 ± 0.0 60 12.3 ± 0.0 14.1 ± 0.0 16.0 ± 0.0 75 12.2 ± 0.1 14.1 ± 0.0 15.8 ± 0.1 90 12.1 ± 0.0 13.8 ± 0.1 15.2 ± 0.1 Open in new tab In the steam treatment, the initial moisture content of wheat kernels had no effect on their moisture change over the treatment times (two-way factorial analysis, F18 = 0, P = 0.998). Moisture content of kernels increased as the treatment time increased due to water condensation on their surfaces. Moisture content gain of the kernels was 1.0 ± 0.3, 2.1 ± 0.3, and 3.1 ± 0.5 percent points in 1, 3, and 5 s of treatment time, respectively. Moisture content gain in the initial stage of steam drying is a common phenomenon caused by the condensation of steam on the surface of kernels (Bonaui et al. 1996, Ramachandran et al. 2017a). Mortality of Adults Outside Kernels Hot Air Treatment In hot air, there was a three-way interaction between treatment times, initial moisture content of wheat kernels, and insect species (F432 = 3.99, P < 0.0001). Mortality increased with increasing treatment time as reported in other studies (Fields 1992, Mahroof et al. 2003a, Boina and Subramanyam 2004). Mortality of the three species at any wheat moisture content was ≤1.9 ± 1.6% when the treatment time was ≤5 s. Adults of three species in 12.5 and 14.5% m.c. kernels reached 100% mortality when the treatment time was 60 s, whereas 75 s was required for insects outside 16.5% m.c. wheat (Fig. 4). Fields (1992) also reported that insects can be killed within 1 min at temperature above 62°C. Fig. 4. Open in new tabDownload slide Mortality of adults outside wheat kernels of 12.5, 14.5, and 16.5% initial moisture content and treated at 105°C with hot air. At each data point, n = 5. Fig. 4. Open in new tabDownload slide Mortality of adults outside wheat kernels of 12.5, 14.5, and 16.5% initial moisture content and treated at 105°C with hot air. At each data point, n = 5. The effect of initial moisture content on mortality of adults was significant at 10 and 15 s for T. castaneum (Tukey’s test, F14 = 5.4 and 8.2, P = 0.020 and 0.006, respectively), 10 and 45 s for C. ferrugineus (Tukey’s test, F14 = 4.1 and 10.2, P = 0.044 and 0.003, respectively), while there was no significant effect at other treatment times or for S. oryzae (Tukey’s test at each treatment time, all F14 < 1.8 and P > 0.2). Therefore, we concluded that the initial moisture content of wheat had a negligible effect on insect mortalities. This conclusion was used as the statistical reason to pool the mortalities at different initial moisture contents for the calculation of lethal time. Previous studies revealed that mortality increased as moisture content decreased (Kirkpatrick et al. 1972, Tilton et al. 1983, Fields 1992). This could be caused by evaporation cooling of water in kernels with a high moisture content. However, these published results were determined by using microwave (Kirkpatrick et al. 1972) or red infrared (Tilton et al. 1983) heating. The LT values for adults of T. castaneum, C. ferrugineus, and S. oryzae outside kernels with 12.5–16.5% m.c. wheat treated with hot air are shown in Table 2. Cryptolestes ferrugineus adults required longer time to reach 50% mortality than adults of T. castaneum and S. oryzae. The calculated LT50 values also indicated that adults of T. castaneum and S. oryzae had similar heat tolerance to hot air (Table 2). Kirkpatrick et al. (1972) also observed that the adults of T. castaneum and S. oryzae had similar heat tolerance to infrared heating. However, the 95% confidence interval (CI) values of the LT95 or LT99 overlapped among all three insect species (Table 2). Table 2. Lethal time (LT) of adults outside wheat kernels with initial moisture content of 12.5–16.5% and treated with hot air at 105°C Insects LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Tribolium castaneum 26.3 (25.0–27.7) 57.7 (53.4–63.0) 79.8 (72.3–89.6) 4.83 ± 0.12 491.04 Cryptolestes ferrugineus 30.9 (28.6–33.2) 71.1 (63.8–81.3) 100.5 (87.2–120.3) 4.54 ± 0.12 972.61 Sitophilus oryzae 25.4 (23.7–27.1) 69.5 (62.6–78.4) 105.4 (92.1–123.8) 3.76 ± 0.09 575.55 Insects LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Tribolium castaneum 26.3 (25.0–27.7) 57.7 (53.4–63.0) 79.8 (72.3–89.6) 4.83 ± 0.12 491.04 Cryptolestes ferrugineus 30.9 (28.6–33.2) 71.1 (63.8–81.3) 100.5 (87.2–120.3) 4.54 ± 0.12 972.61 Sitophilus oryzae 25.4 (23.7–27.1) 69.5 (62.6–78.4) 105.4 (92.1–123.8) 3.76 ± 0.09 575.55 CI (confidence interval). aSlope and χ2 values of the probit analysis, and df (degree of freedom) = 148 for each insect species. The χ2 goodness-of-fit statistic and df for each insect species indicate that the probit model adequately fits the data at P < 0.05 level. Open in new tab Table 2. Lethal time (LT) of adults outside wheat kernels with initial moisture content of 12.5–16.5% and treated with hot air at 105°C Insects LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Tribolium castaneum 26.3 (25.0–27.7) 57.7 (53.4–63.0) 79.8 (72.3–89.6) 4.83 ± 0.12 491.04 Cryptolestes ferrugineus 30.9 (28.6–33.2) 71.1 (63.8–81.3) 100.5 (87.2–120.3) 4.54 ± 0.12 972.61 Sitophilus oryzae 25.4 (23.7–27.1) 69.5 (62.6–78.4) 105.4 (92.1–123.8) 3.76 ± 0.09 575.55 Insects LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Tribolium castaneum 26.3 (25.0–27.7) 57.7 (53.4–63.0) 79.8 (72.3–89.6) 4.83 ± 0.12 491.04 Cryptolestes ferrugineus 30.9 (28.6–33.2) 71.1 (63.8–81.3) 100.5 (87.2–120.3) 4.54 ± 0.12 972.61 Sitophilus oryzae 25.4 (23.7–27.1) 69.5 (62.6–78.4) 105.4 (92.1–123.8) 3.76 ± 0.09 575.55 CI (confidence interval). aSlope and χ2 values of the probit analysis, and df (degree of freedom) = 148 for each insect species. The χ2 goodness-of-fit statistic and df for each insect species indicate that the probit model adequately fits the data at P < 0.05 level. Open in new tab Steam Treatment Adults of three species outside kernels reached 100% mortality when they were treated in 105°C steam for 1 s, whereas mortality of adults was ≤3.5% when they were exposed to hot air at the same temperature for 5 s. The initial moisture content of wheat had no significant effect on insect mortality at any treatment time (Tukey’s test, all F14 < 0.20 and P > 0.22). Treatment time of steam was estimated theoretically by using the law of energy conservation (Ramachandran et al. 2017b), and the estimated time for a 100% mortality of adults outside kernels was also around 1 s. This is due to the superior heat transfer properties and much higher latent energy of steam than hot air at the same temperature (Shibata and Mujumdar 1994). Therefore, insects experienced higher temperatures in a shorter time in steam resulting in a higher mortality. Mortality of Sitophilus oryzae Inside Kernels Hot Air Treatment Life stage did not significantly influence mortality, whereas the treatment time and initial moisture content of wheat showed a significant effect on mortality (three-way factorial analysis, for life stage: F229 = 2.3, P = 0.103; for treatment time: F229 = 117.3, P < 0.001; for initial moisture content: F229 = 17.5, P < 0.001). Mortality of each life stage significantly increased with the increasing treatment time for all initial moisture contents of wheat (Fig. 5); however, no difference in mortality was observed when treatment times were ≤5 s (Tukey’s test at each treatment time, all F14 < 0.25, P ≥ 0.938). Mortality of young adults and immature stages of S. oryzae was higher at a lower initial moisture content than at a higher initial moisture content of wheat (Fig. 5). For instance, young adults required 30.7 and 40.3 s to reach 50% mortality in wheat with 12.5 and 16.5% m.c., respectively (Table 3). Dermott and Evans (1978) concluded that LT values of insects in 14% m.c. wheat was considerably longer than in the 11.3% wheat at 70–80°C. The reason for the fast kill in lower moisture contents was that the specific heat of water (4,200 kJ/kg) was higher than dried grain (1,100–1,400 kJ/kg; Muir and Viravanichai 1972). Therefore, dried kernels had a higher heating rate than that of wet kernels, which led to a higher mortality. This was consistent with the reports in the literature (Evans 1981, Tilton et al. 1983). Table 3. Lethal times (LT) of adults and immature stages of Sitophilus oryzae inside wheat kernels of 12.5–16.5% moisture content and treated with hot air at 105°C Insect stage Moisture content (%) LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Adult 12.5 30.7 (27.4–34.2) 78.0 (65.8–98.1) 114.7 (92.1—155.9) 4.06 ± 0.21 150.42 14.5 35.5 (32.8–38.4) 83.1 (73.3–97.6) 118.1 (100.2–146.70) 4.46 ± 0.25 88.24 16.5 40.3 (36.1–44.8) 90.1 (76.0–115.7) 125.6 (100.6–176.4) 4.71 ± 0.28 174.02 Larva 12.5 32.6 (29.7–35.3) 77.7 (69.2–90.2) 111.4 (95.3–136.9) 4.36 ± 0.25 457.36 14.5 42.9 (40.0–45.5) 76.0 (69.8—85.5) 96.3 (85.6–114.2) 6.63 ± 0.50 86.92 16.5 47.2 (44.7–49.7) 83.8 (77.5–92.7) 106.4 (95.7–122.3) 6.60 ± 0.42 77.99 Pupa 12.5 37.8 (34.6–41.0) 97.8 (85.8–115.5) 145.2 (122.1–181.6) 3.98 ± 0.20 105.25 14.5 44.2 (40.8–47.6) 104.4 (92.0–122.9) 149.0 (126.1–186.1) 4.41 ± 0.24 104.94 16.5 48.7 (44.6—53.0) 114.5 (98.5–141.1) 163.2 (133.6–216.7) 4.43 ± 0.25 139.19 Insect stage Moisture content (%) LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Adult 12.5 30.7 (27.4–34.2) 78.0 (65.8–98.1) 114.7 (92.1—155.9) 4.06 ± 0.21 150.42 14.5 35.5 (32.8–38.4) 83.1 (73.3–97.6) 118.1 (100.2–146.70) 4.46 ± 0.25 88.24 16.5 40.3 (36.1–44.8) 90.1 (76.0–115.7) 125.6 (100.6–176.4) 4.71 ± 0.28 174.02 Larva 12.5 32.6 (29.7–35.3) 77.7 (69.2–90.2) 111.4 (95.3–136.9) 4.36 ± 0.25 457.36 14.5 42.9 (40.0–45.5) 76.0 (69.8—85.5) 96.3 (85.6–114.2) 6.63 ± 0.50 86.92 16.5 47.2 (44.7–49.7) 83.8 (77.5–92.7) 106.4 (95.7–122.3) 6.60 ± 0.42 77.99 Pupa 12.5 37.8 (34.6–41.0) 97.8 (85.8–115.5) 145.2 (122.1–181.6) 3.98 ± 0.20 105.25 14.5 44.2 (40.8–47.6) 104.4 (92.0–122.9) 149.0 (126.1–186.1) 4.41 ± 0.24 104.94 16.5 48.7 (44.6—53.0) 114.5 (98.5–141.1) 163.2 (133.6–216.7) 4.43 ± 0.25 139.19 CI (confidence interval). aSlope and χ2 values of the probit analysis, and df (degree of freedom) = 48 at each moisture content and each insect species. The χ2 goodness-of-fit statistic and df for each stage of Sitophilus oryzae at each moisture content indicate that the probit model adequately fits the data at P < 0.05 level. Open in new tab Table 3. Lethal times (LT) of adults and immature stages of Sitophilus oryzae inside wheat kernels of 12.5–16.5% moisture content and treated with hot air at 105°C Insect stage Moisture content (%) LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Adult 12.5 30.7 (27.4–34.2) 78.0 (65.8–98.1) 114.7 (92.1—155.9) 4.06 ± 0.21 150.42 14.5 35.5 (32.8–38.4) 83.1 (73.3–97.6) 118.1 (100.2–146.70) 4.46 ± 0.25 88.24 16.5 40.3 (36.1–44.8) 90.1 (76.0–115.7) 125.6 (100.6–176.4) 4.71 ± 0.28 174.02 Larva 12.5 32.6 (29.7–35.3) 77.7 (69.2–90.2) 111.4 (95.3–136.9) 4.36 ± 0.25 457.36 14.5 42.9 (40.0–45.5) 76.0 (69.8—85.5) 96.3 (85.6–114.2) 6.63 ± 0.50 86.92 16.5 47.2 (44.7–49.7) 83.8 (77.5–92.7) 106.4 (95.7–122.3) 6.60 ± 0.42 77.99 Pupa 12.5 37.8 (34.6–41.0) 97.8 (85.8–115.5) 145.2 (122.1–181.6) 3.98 ± 0.20 105.25 14.5 44.2 (40.8–47.6) 104.4 (92.0–122.9) 149.0 (126.1–186.1) 4.41 ± 0.24 104.94 16.5 48.7 (44.6—53.0) 114.5 (98.5–141.1) 163.2 (133.6–216.7) 4.43 ± 0.25 139.19 Insect stage Moisture content (%) LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Adult 12.5 30.7 (27.4–34.2) 78.0 (65.8–98.1) 114.7 (92.1—155.9) 4.06 ± 0.21 150.42 14.5 35.5 (32.8–38.4) 83.1 (73.3–97.6) 118.1 (100.2–146.70) 4.46 ± 0.25 88.24 16.5 40.3 (36.1–44.8) 90.1 (76.0–115.7) 125.6 (100.6–176.4) 4.71 ± 0.28 174.02 Larva 12.5 32.6 (29.7–35.3) 77.7 (69.2–90.2) 111.4 (95.3–136.9) 4.36 ± 0.25 457.36 14.5 42.9 (40.0–45.5) 76.0 (69.8—85.5) 96.3 (85.6–114.2) 6.63 ± 0.50 86.92 16.5 47.2 (44.7–49.7) 83.8 (77.5–92.7) 106.4 (95.7–122.3) 6.60 ± 0.42 77.99 Pupa 12.5 37.8 (34.6–41.0) 97.8 (85.8–115.5) 145.2 (122.1–181.6) 3.98 ± 0.20 105.25 14.5 44.2 (40.8–47.6) 104.4 (92.0–122.9) 149.0 (126.1–186.1) 4.41 ± 0.24 104.94 16.5 48.7 (44.6—53.0) 114.5 (98.5–141.1) 163.2 (133.6–216.7) 4.43 ± 0.25 139.19 CI (confidence interval). aSlope and χ2 values of the probit analysis, and df (degree of freedom) = 48 at each moisture content and each insect species. The χ2 goodness-of-fit statistic and df for each stage of Sitophilus oryzae at each moisture content indicate that the probit model adequately fits the data at P < 0.05 level. Open in new tab Fig. 5. Open in new tabDownload slide Mortality of young adults and immature stages of Sitophilus oryzae inside wheat kernels of 12.5, 14.5, and 16.5% initial moisture content and treated at 105°C with hot air. At each data point, n = 5. Fig. 5. Open in new tabDownload slide Mortality of young adults and immature stages of Sitophilus oryzae inside wheat kernels of 12.5, 14.5, and 16.5% initial moisture content and treated at 105°C with hot air. At each data point, n = 5. Figure 5 shows a lack of a clear-cut difference in mortality among young adults and immature stages of S. oryzae, whereas mortality of pupae in wheat with any moisture content was slightly lower than mortality of adults or larvae at 60 and 75 s (Fig. 5). Different studies reported different thermo-tolerance stages of insects. Oosthuizen (1935) ranked life stages of Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae) based on their heat tolerance at 44°C as: pupae > egg > larvae > adults. The later developing stages of psocids are more susceptible to denaturation or coagulation of protein, inactivation of enzymes, and metabolic reactions (Beckett and Morton 2003, Mahroof et al. 2005, Lurie and Jang 2007). During heat treatment, higher moisture content inside body of younger stage of insects than in the old stage might delay the temperature increase of insect bodies. Mahroof et al. (2003b) reported that young larvae had highest thermo-tolerance than old larvae, pupae, and adults of T. castaneum. Thermo-tolerances of different life stages of S. oryzae were similar under our treatment conditions. This lack of discrepancy might be caused by the fast killing effect at 105°C because the difference in body moisture content, denaturation or coagulation of protein, enzyme inactivation, and metabolic reaction might have negligible effects on the insect’s thermo-tolerance at high temperatures. The mortality of S. oryzae adults outside wheat kernels was about two times higher than the mortality inside kernels when hot air treatment times were 10, 15, and 30 s. In this study, the young adults inside wheat kernels might not be exactly similar to elder adults outside wheat kernels because some of the young adults inside kernels might still have a soft cuticle exoskeleton after hatching and before moving out of kernels. This exoskeleton difference might not influence the mortality because the low mortality was mainly caused by the lower temperature experienced by insects inside wheat kernels in comparison to those outside kernels (Fig. 3). Dermott and Evans (1978) and Fleurat-Lessard (1985) tested eight species of stored-product insects and also reported that insects inside seeds had lower mortality than ones outside kernels. As the moisture content of wheat increased, higher LT values were obtained in all three stages of S. oryzae (Table 3), whereas the 95% CI values of these LT did not show a clear difference. The LT50 of adults was lower than the LT50 of pupae, whereas no difference was observed between adults and larvae, and pupae and larvae (Table 3). The 95% CI values of LT95 overlapped with that of LT99 for any insect stage at most initial moisture contents. Lethal time to achieve 99% mortality of adults and immature stages at any initial moisture content was greater than 100 s. These results also supported the similar thermo-tolerance among different life stages of S. oryzae. Steam Treatment All adults and immature stages of S. oryzae reached 100% mortality at 3 s, whereas young adults, larvae, and pupae reached 88.0 ± 6.9, 90.0 ± 9.9, and 81.3 ± 11.7% mortality, respectively, when the exposure time was 1 s (Fig. 6). Mortality among the young adults and immature stages of S. oryzae inside kernels did not show a significant difference (three-way factorial analysis, F108 = 2.95, P = 0.057). Initial moisture content of wheat also did not influence insects’ mortality (three-way factorial analysis, F108 = 2.53, P = 0.085). Mortality was significantly higher at 3-s treatment time than at 1-s treatment time for any stage of insects (Student’s t-test, t9 > 150 for each test, all P < 0.0001). Fig. 6. Open in new tabDownload slide Mortality of adults and immature stages of Sitophilus oryzae inside wheat kernels of 12.5–16.5% initial moisture content and treated at 105°C with steam. At each data point, n = 15. Fig. 6. Open in new tabDownload slide Mortality of adults and immature stages of Sitophilus oryzae inside wheat kernels of 12.5–16.5% initial moisture content and treated at 105°C with steam. At each data point, n = 15. Mortality of young adults and immature stages of S. oryzae inside wheat kernels had larger SE in both media compared with insects outside kernels (Figs. 4–6). This larger SE might be caused by the differences in insect heterogeneity (Fields 1992), kernel size of wheat, and the location of the insects inside kernels. If an insect was located close to the kernel surface, the insect would be exposed to a higher temperature sooner than that at the core location. This conclusion is consistent with the observed temperatures (Fig. 3). Therefore, these mortality variations should be considered in the practice of heat treatment. Germination Hot Air Treatment The initial germination of the sound kernels was 95.1 ± 4.0% at any initial moisture content of wheat. There was no significant difference in germination between control and hot air–treated kernels with 12.5% and 14.5–16.5% m.c. at ≤5 and ≤15 s treatment time, respectively (Student’s t-test at each treatment time, t4 ≤ 1.21 for each test, all P > 0.29). Wheat germination decreased with increasing treatment time (Fig. 7). Also, wheat of 12.5% m.c. had lower germination than wheat of 14.5 and 16.5% m.c. (Fig. 7). Sound kernels with 12.5 and 14.5–16.5% m.c. lost about 50% of germination in 105°C hot air at 29.0 and 51.7 s, respectively, and lost 99% germination at ≥240 s (Table 4). Therefore, hot air treatment at 105°C is not applicable due to the high loss of germination when 100% insect mortality is achieved. Dermott and Evans (1978) also reported that heat treatment reduced the quality of wheat. Table 4. Lethal time (LT) of germination of wheat kernels treated with hot air or steam at 105°C Medium Moisture content (%) df LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Hot air 12.5 31 29.0 (24.9–33.3) 173.2 (131.0–258.1) 363.0 (245.8–639.2) 2.12 ± 0.21 26.41 Hot air 14.5–16.5 64 51.7 (47.7–55.9) 184.8 (152.3–240.8) 313.3 (240.4–451.8) 2.97 ± 0.24 67.84 Steam 12.5–16.5 97 6.6 (5.5–8.7) 81.7 (44.2–214.1) 231.2 (103.7–817.1) 1.51 ± 0.17 53.26 Medium Moisture content (%) df LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Hot air 12.5 31 29.0 (24.9–33.3) 173.2 (131.0–258.1) 363.0 (245.8–639.2) 2.12 ± 0.21 26.41 Hot air 14.5–16.5 64 51.7 (47.7–55.9) 184.8 (152.3–240.8) 313.3 (240.4–451.8) 2.97 ± 0.24 67.84 Steam 12.5–16.5 97 6.6 (5.5–8.7) 81.7 (44.2–214.1) 231.2 (103.7–817.1) 1.51 ± 0.17 53.26 (CI) confidence interval; (df) degree of freedom. aSlope and χ2 values of the probit analysis. The χ2 goodness-of-fit statistic and df for the wheat germination at the specified moisture content and treatment method indicate that the probit model adequately fits the data at P < 0.05 level. Open in new tab Table 4. Lethal time (LT) of germination of wheat kernels treated with hot air or steam at 105°C Medium Moisture content (%) df LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Hot air 12.5 31 29.0 (24.9–33.3) 173.2 (131.0–258.1) 363.0 (245.8–639.2) 2.12 ± 0.21 26.41 Hot air 14.5–16.5 64 51.7 (47.7–55.9) 184.8 (152.3–240.8) 313.3 (240.4–451.8) 2.97 ± 0.24 67.84 Steam 12.5–16.5 97 6.6 (5.5–8.7) 81.7 (44.2–214.1) 231.2 (103.7–817.1) 1.51 ± 0.17 53.26 Medium Moisture content (%) df LT (s) and 95% CI Slopea χ2a LT50 LT95 LT99 Hot air 12.5 31 29.0 (24.9–33.3) 173.2 (131.0–258.1) 363.0 (245.8–639.2) 2.12 ± 0.21 26.41 Hot air 14.5–16.5 64 51.7 (47.7–55.9) 184.8 (152.3–240.8) 313.3 (240.4–451.8) 2.97 ± 0.24 67.84 Steam 12.5–16.5 97 6.6 (5.5–8.7) 81.7 (44.2–214.1) 231.2 (103.7–817.1) 1.51 ± 0.17 53.26 (CI) confidence interval; (df) degree of freedom. aSlope and χ2 values of the probit analysis. The χ2 goodness-of-fit statistic and df for the wheat germination at the specified moisture content and treatment method indicate that the probit model adequately fits the data at P < 0.05 level. Open in new tab Fig. 7. Open in new tabDownload slide Germination of sound kernels treated at 105°C with hot air. a,bDifferent letters at the same treatment time indicated significant difference of the wheat germination (Tukey’s test, F8 ≥ 8.2 at each treatment time, all P < 0.001 except at 90 s. At 90 s, F8 = 1.4, P = 0.316). Fig. 7. Open in new tabDownload slide Germination of sound kernels treated at 105°C with hot air. a,bDifferent letters at the same treatment time indicated significant difference of the wheat germination (Tukey’s test, F8 ≥ 8.2 at each treatment time, all P < 0.001 except at 90 s. At 90 s, F8 = 1.4, P = 0.316). Steam Treatment Initial moisture content did not significantly influence the germination over the treatment times (two-way factorial analysis, F18 = 0, P = 0.998). Germination of the wheat at 1, 3, and 5 s in steam was 82.0 ± 5.8, 74.9 ± 7.6, and 48.9 ± 5.9%, respectively. Wheat kernels lost germination quicker in steam compared with hot air. Germination decreased as the treatment time increased. According to the LT values, wheat treated within steam lost about 50% of germination in 5.5–8.7 s and lost all germination in 103.7–817.1 s (Table 4). Germination corresponding to 100% mortality of all tested insect stages at 3 s was 74.9 ± 7.6%. Therefore, steam could be used to control stored insect pests without tremendously affecting wheat germination. This study found that the 105°C steam could be used to control both internal and external insects, and the wheat germination loss was about 20%. The result of this study could be used in different applications, such as a thin layer of insect-infested wheat can be treated by transferring wheat through a 105°C steam chamber in 3 s. This treatment method could be used for different products after proper treatment time of the product is determined. Steam treatment is already used in different industries for treatment of herbs, spices, teas, seeds, and medical drugs to kill pathogenic bacteria and fungi (Šilingienė et al. 2011). Therefore, steam treatment of insect-infested products has a potential to be adapted by these industries. Conclusions Initial moisture content did not significantly influence mortality of insects outside and inside kernels treated in steam, whereas the mortality of insects inside kernels treated in hot air was significantly influenced by the initial moisture content of wheat. There was no difference in mortality among the species exposed to steam. There was no difference in mortality among the life stages of S. oryaze inside kernels exposed to steam and hot air at 105°C. In 105°C hot air, S. oryzae adults outside wheat kernels showed about two times the mortality when compared with young adults inside wheat kernels prior to hatching when treatment times were 10, 15, and 30 s. In 105°C hot air, adults of T. castaneum, C. ferrugineus, and S. oryzae outside of kernels of 12.5 and 14.5% moisture content had 100% mortality at 60 s, whereas the adults of three species reached 100% mortality at 1 s in 105°C steam. Young adults and immature stages of S. oryzae inside kernels had 100% mortality at 90 s in 105°C hot air and 3 s in 105°C steam. Wheat germination was reduced by 81% in hot air and by 20% in steam when the mortality of insects inside kernels was 100%. Acknowledgments This study is part of an MSc thesis, and we thank Drs. 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TI - Control of Three Species of Stored-Product Insects in Wheat Treated With Steam and Hot Air
JF - Journal of Economic Entomology
DO - 10.1093/jee/toz080
DA - 2019-08-03
UR - https://www.deepdyve.com/lp/oxford-university-press/control-of-three-species-of-stored-product-insects-in-wheat-treated-AtFBjsBmge
SP - 1964
VL - 112
IS - 4
DP - DeepDyve
ER -