Sublethal Exposure to Diatomaceous Earth Increases Net Fecundity of Flour Beetles (Tribolium confusum) by Inhibiting Egg Cannibalism
Citation: Shostak AW (2014) Sublethal Exposure to Diatomaceous Earth Increases Net Fecundity of Flour Beetles (Tribolium confusum) by Inhibiting Egg
Cannibalism. PLoS ONE 9(2): e88500. doi:10.1371/journal.pone.0088500
Sublethal Exposure to Diatomaceous Earth Increases Net Fecundity of Flour Beetles (Tribolium confusum ) by Inhibiting Egg Cannibalism
Allen W. Shostak 0
Robert B. Srygley, USDA-Agricultural Research Service, United States of America
0 Department of Biological Sciences, University of Alberta , Edmonton, Alberta , Canada
Population regulation results from an interplay of numerous intrinsic and external factors, and for many insects cannibalism is such a factor. This study confirms a previously-reported observation that sublethal exposure to the fossilized remains of diatoms (i.e. diatomaceous earth) increases net fecundity (eggs produced minus eggs destroyed/day) of flour beetles, Tribolium confusum. The aim was to experimentally test two non-mutually-exclusive ecological mechanisms potentially responsible for the increased net fecundity: higher egg production and lower egg cannibalism. Adult T. confusum were maintained at low or high density in medium containing sublethal (0-4%) diatomaceous earth. Net fecundity increased up to 2.16 control values during diatomaceous earth exposure, and returned to control levels following removal from diatomaceous earth. Cannibalism assays on adults showed that diatomaceous earth reduced the number of eggs produced to 0.76 control values at low density and to 0.86 controls at high density, and also reduced egg cannibalism rates of adults to as little as 0.46 control values, but at high density only. Diatomaceous earth also reduced cannibalism by larvae on eggs to 0.36 control values. So, while the presence of diatomaceous earth reduced egg production, net fecundity increased as a result of strong suppression of the normal egg cannibalism by adults and larvae that occurs at high beetle density. Undisturbed cultures containing sublethal diatomaceous earth concentrations reached higher population densities than diatomaceous earth-free controls. Cohort studies on survival from egg to adult indicated that this population increase was due largely to decreased egg cannibalism by adult females. This is the first report of inhibition of egg cannibalism by diatomaceous earth on larval or adult insects. The ability of diatomaceous earth to alter cannibalism behavior without causing mortality makes it an ideal investigative tool for cannibalism studies.
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Cannibalism, or intraspecific predation, occurs in various
invertebrate and vertebrate taxa [1,2] and is considered to be a
normal response to a variety of environmental situations. The
intensity of cannibalism may vary under stressful conditions, and it
can reduce population size before acute resource limitations occur
[2]. Cannibalism is widespread among insects in general and is
particularly common in beetles [3]. Beetles (Insecta: Coleoptera) of
the genus Tribolium have been used as model systems for ecological
and evolutionary study since the work of Chapman [4], and
cannibalism by adults and larvae on all life stages plays a role in
controlling population growth [57]. Egg cannibalism appears to
be a particularly potent mechanism, and it may remove up to 98%
of eggs that are laid [6]. Among adult beetles, females have a much
higher rate of cannibalism on eggs than do males [6]. Among
larvae, egg cannibalism rates increase as larvae age, declining only
just prior to pupation [8]. Egg cannibalism in beetles does not
necessarily fine-tune the population to available resource levels,
and in fact may reduce the population well below environmental
carrying capacity [9]. The effect of cannibalism on egg numbers
has made it important to distinguish between real fecundity,
which is the rate at which eggs are actually oviposited under a
given set of environmental conditions, and net fecundity, which
is eggs produced minus eggs destroyed/day [5]. The ecological
mechanisms for regulating population density may, in turn, be
modified by intrinsic factors such as beetle genetics [10], sex
[11,12], and age [6], and by extrinsic factors such as beetle density
[6,11], environmental conditions [13,14], and the presence of
parasitism [15].
One environmental factor potentially impacting population
growth is the presence of stressors such as diatomaceous earth
(DE), which is mined from the fossilized remains of diatoms and is
widely used as a natural insecticide [16]. Consisting almost
entirely of SiO2, DE is believed to function by abrading or
adsorbing lipids from the epicuticle of insects, leading to
desiccation and death [17]. DE is considered relatively safe from
a human health standpoint [17], and its potential as an insecticide
has been studied on a variety of insect pests [1720]. Studies on
insects following DE application to whole grain kernels typically
report reduced production of progeny [2123]. By contrast, a
recent st (...truncated)