Abstract
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 2.1× 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 0.7× control values at low density and to 0.8× controls at high density, and also reduced egg cannibalism rates of adults to as little as 0.4× control values, but at high density only. Diatomaceous earth also reduced cannibalism by larvae on eggs to 0.3× 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.
Highlights
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
Pairwise comparisons (Fig. 2) indicated that net fecundity (Nt) under control conditions (0% diatomaceous earth (DE)) did not change over the course of the experiment, but that Nt at 2%, 4% or 8% DE was significantly higher than control for each census on days 15–30
The second aim of this study was achieved by failing to support the hypothesis that DE increased net fecundity by increasing oviposition – oviposition was reduced – and by supporting the hypothesis that DE acted by reducing egg cannibalism
Summary
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]. Females have a much higher rate of cannibalism on eggs than do males [6]. 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 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 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]
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