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Wood products are highly exposed to infestation by powder post beetles. Dinoderus minutus (bamboo borer) is a wood boring beetle that seriously damage dried bamboo and finished bamboo products. Management of D. minutus using pesticides showed negative effects on environment despite being very costly. By understanding influence of natural climatic conditions on their reproductive behaviour, could help us to develop a cost effective and environmental friendly strategy to cope up with this problem. In the present study, reproductive parameters and egg development of the bamboo borer were determined at 20%, 40%, 56%, 75% and 85% r.h. levels at constant temperature of 30° ± 2°C with 8 L-16D photoregime. From the results, eclosion to first instar larva was recorded at all relative humidities tested. The lowest shortest percentage of hatchability was recorded at 20% and 85% relative humidity with a mean incubation period of 4.63 ± 0.25 and 10.43 ± 0.32 days, respectively. It was noted that pre-ovipositional period decreased from 14.20 ± 0.49 to 7.20 ± 0.31 days as relative humidity increased from 20% to 75% and slightly increased to 8.00 ± 0.37 days at 85% relative humidity. We conclude that female beetles may have a particular hygropreference in oviposition as total egg production increased with increasing relative humidity. Keywords: Bostrychidae, Bamboo borer, Dinoderus minutus, Egg hatchability, Powderpost beetle, Reproductive capacity Introduction environmental problems (Madhun and Freed 1990; Plants are always under the influence of biotic as well as Pimentel 2005; Devi 2007). Repeated application of pesti- abiotic stressful conditions (Anjum et al. 2010; Hakeem cides leads to loss of biodiversity (Bengtsson et al. 2005). et al. 2012). Plant-insect interactions are quite common in The latest trend is to understand the nature of reproduct- nature and sometimes very useful for both species, how- ive biology of these pests and their dependence on several ever, when insects showed negative interactions, resulting environmental conditions (Garcia and Morrell 2009). to cause direct damages to the plants or their products, Various climatic conditions are responsible for the nor- they are termed as pests. Pests are known to attack the mal reproduction, production of eggs as well as egg hatch- economically important plants and their products at both ability. Relative humidity can affect the physiology and pre-harvest as well as post-harvest stages and responsible thus the development, longevity and oviposition of many for huge economic losses (Agboka et al. 2010).To cope up insects. At low relative humidities, development may be with this problem, normally pesticides are used. However, retarded, for example in many pests of stored products; at besides being very costly, the indiscriminate use of these high relative humidities or in saturated air (100% RH), pesticides has now created serious health as well as insects or their eggs may drown or be infected more readily by pathogens (Gullan and Cranston, 2005). Different expos- * Correspondence: email@example.com; firstname.lastname@example.org ureofrelativehumidityonstoredproduct pest, Tribolium Department of Forest Management, Faculty of Forestry, Universiti Putra castaneum, T. confusum (Coleoptera: Tenebrionidae) and Malaysia (UPM), 43400, Serdang, Malaysia Oryzaephilus surinamensis (Coleoptera: Cucujidae) showed Faculty of Forestry, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia increased insect mortality as relative humidity decreased Full list of author information is available at the end of the article © 2013 Norhisham et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Norhisham et al. SpringerPlus 2013, 2:9 Page 2 of 6 http://www.springerplus.com/content/2/1/9 (Jay et al. 1971). Low relative humidity can prevent embryo (NaCl) and Potassium chloride (KCl). The relative hu- development and egg hatching due to loss of lubrication midity was obtained by filling solutions at the bottom of and cuticular softness in insect (Guarneri et al. 2002). High desiccators and monitored using a digital thermohygrom- relative humidity contributes to population increase in eter. Throughout the study, investigation on reproductive stored product pest, as shown in Callosobruchus maculatus parameters and egg hatchability were carried out in Petri under laboratory conditions (Ouedraogoa et al. 1996). Fe- dishes kept in desiccators corresponding to each rela- male Callosobruchus maculatus showed higher fecundity tive humidity and hermetically closed. The desiccators and longer adult lifespan at high humidity. High humidity were put into an incubator with controlled temperature of support reproductive capacity in insects as percentage of 30 ± 2°C. water is correlated with the amount of fat (including eggs in females), which consists of anhydrous mole- Reproductive capacity cules, and with the amount of cuticle, which has a Newly emerged beetles less than 7 days old were used to lower water content than other tissues. Environmental determine the effect of different relative humidities (20%, temperature and humidity affects the developmental 40%, 56%, 75% and 85% r.h.) on reproductive parameters at phase and transpiration through insect body surface constant temperature of 30° ± 2°C with 8 L-16D photore- (Chapman 1975; Guarneri et al. 2002). Insects must gime. Ten pairs of adult beetles were exposed at each rela- keep body water content within certain limits which is tive humidity tested. Each pair of adults were selected and influenced by the degree of the insect cuticle permeability separately introduced into Petri dishes lined with cassava (Willmer, 1982; Raghu et al. 2004). Insect survival is influ- pellet and filter paper. Observation on egg production was enced by its ability to tolerate fluctuations in body water then started after 24 hours of setup. Cassava pellets were influence by humidity (Romoser and Stoffolano, 1998). removed and broken up daily to assess number of eggs laid. The present study was concentrated to understand the The cassava pellet was replaced each time after inspection effect of relative humidity on egg hatchability and repro- on egg production. Data collection on adult longevity and ductive capacity of bamboo borer (Dinoderus minutus). fecundity were collected until complete mortality. This study would help us to develop cost effective and environmental friendly strategies to control insect pests Egg hatchability without any health hazards. The effect of relative humidity on egg hatchability and in- cubation period was inspected under different relative hu- Materials and methods midity levels (20%, 40%, 56%, 75% and 85% r.h.) at Insect Culture constant temperature of 30° ± 2°C with 8 L-16D photore- Adult D. minutus were collected from infested bamboo gime. Newly laid eggs of D. minutus were obtained from culms in Bamboo Incubation Center and Handicraft in an established D. minutus culture on cassava. A total of 50 Simpang Pertang, Negeri Sembilan, Malaysia (2°57’25.61”N eggs were placed equidistant into Petri dishes lined with 102°18’12.69”E). Most bamboo culms infested by D. minutus black filter paper for each relative humidity tested. Each belongs to the species Gigantochloa scortechinii,which is Petri dish was kept in desiccators corresponding to each an important species of bamboo in Malaysia. Infested relative humidity and hermetically closed. The desiccators bamboo culms were kept in a plastic container with di- were put into an incubator with controlled temperature of mension of 30 cm × 20 cm × 25 cm kept in insect rearing 30 ± 2°C. Petri dishes were kept under these conditions room at 75% r.h. and 25 ± 3°C with 8 L-16D photoregime. and observation on egg eclosion and incubation period Newly emerged beetles were collected and introduced into were observed daily under stereomicroscope and com- a new culture jar with cassava block using standards tech- pared. During observation, eggs were removed as required niques of culture described by Abood et al. (2010). with respect to age in days with a fine brush. Each egg was mounted on a concave microscope slide with a cover slip. Control of Relative humidity This method was carried out to minimize the effect of Different salt solutions and distilled water were used to relative humidity during the non-test conditions. control the relative humidity to which eggs and adult bee- tles were exposed. As indicated by Winston and Bates Statistical analysis (1960), Roca and Lazzari (1994) and Dambach and Goehlen Statistical analysis software (SAS version 9.2) was used for (1999), acid or inorganic (non-volatile) salt solutions were the analysis on egg hatching, reproductive capacity and adequate for this purpose, as their water vapour pressure adult longevity at different relative humidities. One Way remained constant for long periods. The liquids used to Analysis of Variance (ANOVA) and Fisher’s least signifi- establish the relative humidities were distilled water, satu- cant difference (LSD) were used to test the effect of rela- rated solutions of Potassium acetae (CH CO K), Sodium tive humidity on reproductive parameter and incubation 3 2 iodide (NaI), Sodium bromide (NaBr), Sodium chloride period. Norhisham et al. SpringerPlus 2013, 2:9 Page 3 of 6 http://www.springerplus.com/content/2/1/9 Results humidity, thus indicating a preference for higher relative Effect of relative humidity on egg hatchability humidity conditions. Similar increment was recorded on The results presented in Table 1 shows a very high signifi- mean ovipositional period which showed an increase from cant effect (F=242.30, DF= 4, 45; p<0.0001) of relative hu- 3.8 ± 0.64 to 54.80 ± 0.94 days. The pre-ovipositional period midity on egg hatchability in D. minutus. There is no decreased with increasing relative humidity levels from 20% significant difference between incubation period at 40%, to 75%. However, between 75% to 85% relative humidity, 56% and 75% relative humidity (p≥0.05; CV=16.63). The there was a significant increase in pre-ovipositional period. lowest and highest relative humidity caused a significant Adult lifespan for both sexes increase with increasing rela- decrease in egg hatchability when compared with the tive humidity. Female D. minutus had longer lifespan com- intermediate relative humidity levels (p≥0.05; CV=16.63). pared to male beetles under all relative humidities tested. Highest eclosion to first instar larva was recorded at There was no significant difference between adult lifespan 75% relative humidity with 86% eclosion to success and for male (p>0.05; CV=7.26) and female (p>0.05; CV=5.19) a mean incubation period of 5.44 ± 0.14 days. Lowest between 85% and 75% and 75% and 56% relative humidity, relative humidity of 20% showed only 8% eclosion to respectively. Earliest mortality for adult beetle was recorded success with a mean incubation period of 4.63 ± 0.25. at 20% relative humidity for male and for female beetle on Percentage of successful eclosion and incubation period day 16 and 18, respectively. Longest lifespan for male beetle increased with increasing relative humidity to 75%. was recorded at 67 day at 75% relative humidity while long- However for 85% relative humidity, the percentage of est lifespan for female beetle was 74 days at 85% relative successful eclosion decreased to 46%. This shows that humidity. egg development was affected by extremes in humidity Fecundity curves for D. minutus under different rela- (Plate 1). Longest incubation period was recorded at tive humidity are shown in Figure 1. Fecundity curve for 85% relative humidity with a mean of 10.43 ± 0.32 days. 40%, 56% and 75% show normal distribution with few fluctuations throughout the oviposition period. The low- Effect of relative humidity on reproductive capacity est and highest relative humidity resulted in different fe- Boring activity on cassava pellet was recorded within cundity curve pattern compared with the intermediate 24 hours of exposure under each relative humidity. Results relative humidity. Peak fertility period was encountered in Table 2 show highly significant relative humidity effect earlier at 75% relative humidity on day 24 with a mean on both D. minutus reproductive capacity (F=388.91, of 1.04 followed by day 27 at 56% and 85% relative hu- DF=4, 45; p<0.0001) and adult lifespan (F=464.70, DF=4, midity with a mean of 0.85 and 1.40, respectively. Peak 45; p<0.0001). However, there is no significant difference fertility period at lower relative humidity occurred at a in pre-ovipositional period between 56% and 75% relative mean of 0.80 on day 27 at 40% relative humidity while at humidity levels (p≥0.05; CV; 13.14). Female D. minutus 20% relative humidity fecundity curve obtained did not showed shorter pre-ovipositional period with increasing present peak fertility period as reproductive parameters relative humidity until 75%. Ovipositional period (p≥0.05; recorded under this condition was very low. CV=8.67) and mean number of eggs laid (p≥0.05; CV=13.14) showed no significant difference between 75% Discussion and 85% relative humidity levels. Shortest ovipositional In order to establish a sustainable pest management, it is period was recorded at 20% relative humidity with a mean essential to know the physiology as well as reproductive of 3.8 ± 0.64 days while the longest ovipositional period capacity of pests viz-a-viz their relations with various was recorded at 85% relative humidity with a mean of natural climatic conditions. Thus present study was 56.50 ± 1.21. Mean number of eggs laid increased from undertaken to investigate the effect of humidity on egg 3.20 ± 0.31 to 56.50 ± 1.21 from 20% to 85% relative hatchability and reproductive biology of the bamboo borer (Dinoderus minutus Fabricus). In our study, we found that egg mortality was highest Table 1 Influence of relative humidity on egg eclosion at 20% and 85% relative humidity levels in Dinoderus Relative Egg development minutus. Dehydration occurred at low relative humidity humidity Incubation period (day) Hatchability due to loss of moisture in the egg, which leads to con- 20% 4.63±0.25c 8% traction and shrinking of both the chorion and the 40% 5.79±0.13b 66% embryo. For eggs, which survived the low relative hu- 56% 5.49±0.18b 74% midity levels up to the prelarval stage, the eggs did hatch 75% 5.44±0.14b 86% but due to dehydration and loss of lubrication and cu- ticular softness, the prelarva could not be released suc- 85% 10.43±0.32a 48% cessfully from the chorion (Figure 2). This effect on eggs Means followed by the same letter in the same column are not significantly different (p≥0.05, Fisher’s least significant difference) N=50. caused by loss of water has been reported by Woods and Norhisham et al. SpringerPlus 2013, 2:9 Page 4 of 6 http://www.springerplus.com/content/2/1/9 Table 2 Effect of relative humidity on reproductive capacity Relative Reproductive parameter humidity Preovipositional period Ovipositional period Mean no. of eggs laid per Female Lifespan Male Lifespan (day) (day) female (day) (day) 20% 14.20±0.49a 3.80±0.64d 3.20±0.31d 22.30±0.79c 20.00±0.62c 40% 11.80±0.44b 37.20±1.34c 28.90±1.01c 56.90±1.20b 49.70±1.15b 56% 9.60±0.38c 51.80±0.49b 43.00±1.65b 68.80±0.68a 61.50±1.29a 75% 7.20±0.31c 54.40±1.33a 66.40±1.21a 70.10±0.70a 60.50±1.19a 85% 8.00±0.37d 56.50±1.21a 68.90±3.07a 69.40±1.52a 59.80±1.13a Means followed by the same letter in the same column are not significantly different (p≥0.05, Fisher’s least significant difference) N=20. Singer (2001) on Lepidoptera. Excessive moisture has From the observations in this study, most eclosion to shown to be detrimental to insect’s survival (Prakash, first instar larva of D. minutus took place in the morning. 2008). At 85% relative humidity, the incubation period During this period, relative humidity reaches its daily max- prolonged and 52% of the total number of egg exposed imum. This may be a way to prevent desiccation of emer- failed to hatch due to excessive moisture, which caused ging larvae. This phenomenan of insect egg hatching at egg mortality. Previously, such negative effect of extreme dawn has also been reported previously in different hete- relative humidity conditions on egg development and ropteran species (Lazzari, 1991; Guarneri et al. 2002) and hatchability has been reported on the stink bug, Nezara the tiger moth, Atteve sciodoxa Meyrick (Bajwa, unpubl. viridula (Linnaeus) (Hirose et al. 2006) and the haema- data 2009). tophagous bug Triatoma brasiliensis (Guarneri et al. Female D. minutus showed heterogeneous ovipositional 2002). The extension of embryonic development duration pattern with most eggs being laid at higher relative hu- at lower humidities (and constant temperature) is a result midities and the mean number of eggs laid increased from of the depression of egg metabolism due to water loss 3.20 ± 0.31 to 56.50 ± 1.21 at 20% to 85% relative humidities, (Zrubek and Woods 2006); low humidities induce in some respectively. The effect of relative humidity on oviposition insect eggs (collembolans, grasshoppers) a dormancy, indicates that females may have a particular hygroprefer- which may last several months (Wigglesworth 1972). ence in oviposition as total egg produced and oviposi- Similar Study by Alex (1983) on the humidity effect on tional period increased with increasing relative humidity. Atherigona soccata showed that low humidity increases However, observation on egg hatchability showed that water loss on the chorion, and resistance to desiccation high relative humidity of 85% resulted with low egg hatch- depends on the ability of the egg to retain water through ability (Table 1). These results might explain how female physical and physiological processes. D. minutus was capable to reproduce at high relative Figure 1 Age-specific fecundity curves of D. minutus at different relative humidities. Norhisham et al. SpringerPlus 2013, 2:9 Page 5 of 6 http://www.springerplus.com/content/2/1/9 a b Figure 2 Effects of different relative humidities on D. minutus egg (x40): a. Newly laid egg; b. shrinkage of egg at 20% relative humidity; c. cuticular softening due to absorption of moisture at 56% relative humidity and d. swelling of prelarva within chorion at 85% relative humidity. humidity at early infestation in bamboo storage. As relative release. High relative humidity at 85% resulted in egg humidity decreased during the processing of bamboo mortality due to excessive moisture. Reproductive cap- culm, egg hatchability increases causing a pest outbreak. acity and incubation period of D. minutus increased Romoser and Stoffolano (1998) stated that low humidity with increasing relative humidity. Mean number of adversely affect the rate of insect oviposition in general. A eggs laid increased from 3.20 ± 0.31 to 56.50 ± 1.21 study by Saha et al. (2011) showed that total progeny of from 20% to 85% relative humidity. Female beetle pre- Xylocoris flavipes decreased at high relative humidity of ferred to lay eggs in higher relative humidity as the 90%. David and Ananthakrishnan (2004) suggested that number of eggs increased with increasing relative the fecundity of an insect is greatest at 70% relative humid- humidity. Pre-ovipositional decreased with increasing re- ity, as seen in Locusta spp. Andrewartha and Birch (1954) lative humidity. However at 85% relative humidity, pre- reported that newly emerged adult migratory locusts did ovipositional period increased slightly compared to 75% not produce eggs below 40% relative humidity. D. minutus relative humidity. Ovipositional period prolonged as rela- response towards relative humidity was found to be similar tive humidity increased. Adult female showed higher fe- to the rice weevil, Sitophylus oryzae, an important pest in cundity characteristics with increasing relative humidity. stored product. Fecundity of S. oryzae increased with in- Mean ovipositional period increased from 3.8 ± 0.64 to creasing moisture content of wheat from 34% to 70% rela- 54.80 ± 0.94 days with increasing relative humidity. The tive humidity (David and Ananthakrishnan, 2004). present study would help scientist to work out the strat- egy/s about the biological control of pests without causing any damage to health, environment as well as economy. Conclusion The effect of relative humidity on D. minutus egg Competing interests The authors declare that they have no competing interests. hatchability showed high mortality at 20% and 85% relative humidity levels compared to that between 40% to 75%. Lowest egg hatchability was recorded at 20% Authors’ contributions NAR and FA develop the research framework and facilitate laboratory and 85% relative humidity levels. D. minutus egg suf- experiment. MR and KRH helped and advised preparations of the fers from loss of moisture that leads to shrinkage of manuscript. All authors have read and approved the final version of the both the embryo and chorion which prevents larval manuscript for publication. Norhisham et al. SpringerPlus 2013, 2:9 Page 6 of 6 http://www.springerplus.com/content/2/1/9 Acknowledgements Raghu S, Drew RAI, Clarke AR (2004) Influence of host plant structure and The authors wish to acknowledge the Ministry of Higher Education, Malaysia, microclimate on the abundance and behaviour of a tephritid fly. J Ins Behav for the FRGS grant and Universiti Putra Malaysia for financial assistance and 17:179–190 facilities. Roca M, Lazzari CR (1994) Effects of the relative humidity on the haematophagous bug Triatoma infestans, Hygropreference and eclosion Author details success. J of Ins Phys 10:901–907 Department of Forest Management, Faculty of Forestry, Universiti Putra Romoser WS, Stoffolano JG Jr (1998) The science of entomology, 4th edn. Malaysia (UPM), 43400, Serdang, Malaysia. 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J of Stor Prod Res 4:325–329 Lazzari CR (1991) Circadian rhythm of egg hatching in Triotoma infestans (Hemiptera: Reduviidae). J Med Entomol 28:740–741 Submit your manuscript to a Madhun YA, Freed VH (1990) Impact of pesticides on the environment. In: journal and beneﬁ t from: Pesticides in the soil environment: processes, impacts, and modeling. SSSA Inc, Madison 7 Convenient online submission Ouedraogoa PA, Soua S, Sanona A, Mongea JP, Huignarda J, Trana B, Credland PF 7 Rigorous peer review (1996) Influence of temperature and humidity on populations of Callosobruchus maculatus (Coleoptera: Bruchidae) and its parasitoid Dinarmus 7 Immediate publication on acceptance basalis (Pteromalidae) in two climatic zones of Burkina Faso. Bull Entomol Res 7 Open access: articles freely available online 86(6):695–702 7 High visibility within the ﬁ eld Pimentel D (2005) Environmental and economic costs of the application of 7 Retaining the copyright to your article pesticides primarily in the United States. Environ, Develop and Sustain 7 (2):229–252 Prakash M (2008) Encyclopaedia of Entomology. In: Insect Ecology. Discovery Submit your next manuscript at 7 springeropen.com Pub, Delhi
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