ISSN 1067-4136, Russian Journal of Ecology, 2017, Vol. 48, No. 4, pp. 377–383. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.I. Ponomarev, G.I. Klobukov, V.V. Napalkova, I.A. Kshnyasev, 2017, published in Ekologiya, 2017, No. 4, pp. 304–311.
Influence of Experimental Conditions on Manifestation of the Group
Effect in the Gypsy Moth, Lymantria dispar L.
V. I. Ponomarev
*, G. I. Klobukov
, V. V. Napalkova
, and I. A. Kshnyasev
Botanical Garden, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620144 Russia
Institute of Plant and Animal Ecology, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620144 Russia
Received May 27, 2016
Abstract⎯The effect of rearing conditions for gypsy moth larvae on parameters of their development (its
duration and mortality) was studied in experiments with early instar larvae reared singly or in groups. The
manifestation of the group effect was analyzed depending on the choice of experimental unit (the larva or the
rearing container) and the amount of volume per larva. The observed effects were evaluated quantitatively.
Keywords: gypsy moth, group effect, survival rate, duration of development
Intensification of intraspecific contacts may lead to
changes in the behavior and physiological features of
individuals within a population. This phenomenon is
termed “group effect” . Edward O. Wilson 
describes this effect as change in behavior and physi-
ology within the species caused by signals that have no
orientation in time or space. This definition gives no
qualitative estimation of the effect, i.e., whether group
rearing has a positive or negative influence on param-
eters of individual development. At the same time,
other definitions often include an estimation criterion.
In particular, it is stated that this effect contributes to
survival and reproduction, provides for more effective
resource utilization, and stimulates metabolism .
The manifestation of group effect in insects is caused
by signals of different nature, including visual , tac-
tile , and chemical stimuli . A considerable effect
of group rearing on morphophysiological parameters
of insects has been noted by many authors. In eruptive
species of phyllophagous insects, such as the gypsy
moth Lymantria dispar (L.), a positive group effect is
manifested mainly in the first and second instar lar-
vae, providing for lower mortality and higher develop-
mental rate . In some cases, however, these param-
eters do not differ between groups of L. dispar larvae
kept at different densities [7, 8], or even an increase in
mortality is observed among young instars larvae kept
in groups . As follows from the definition of group
effect  and published data on the pattern of its man-
ifestation, this is a multifactor and nonlinear phenom-
Analysis of the influence of different signals on the
manifestation of group effect is possible only under
controlled laboratory conditions, which make it possi-
ble to minimize bias caused by unforeseen abiotic con-
ditions. Insects reared in climatic chambers are kept in
containers (Petri dishes, plastic cups, etc.). In studies
on the group effect, some larvae are reared singly,
other in groups. If containers of the same volume are
used, an additional possible source of variation appears,
namely, resource amount per capita per larva. Another
source of variation is a probable influence of larvae of
a certain instar on larvae of different instars, which is
excluded if groups consist of same-instar larvae.
Therefore, prior to planning an experiment, it is nec-
essary to estimate (1) the level of objectivity in the
choice of experimental unit and (2) the influence of
rearing conditions (container volume) on parameters
of larval development.
As defined by Kozlov , “The experimental unit
is the smallest division of material that can be exposed
to the study factor independently of other experimental
units… The measured (estimated) unit is an element of
the experimental unit that serves as a basis for obtaining
an individual estimate (measurement). Although the
experimental unit plays the role of the smallest inde-
pendent element of experimental action, it may (but not
must) consist of several measured units” (p. 54).
Control larvae in laboratory studies on the group
effect are reared singly in individual containers; i.e.,
the experimental and measured units are identical.
The other larvae are reared in groups, and the experi-
mental unit may differ depending on conditions of a
given experiment: this may be a container nested
within i-level of treatment or a larva within container.
In the former case (container), the study object will be