Protective geometry and reproductive anatomy as candidate determinants of clutch size variation in pentatomid bugs

Abstract

Many animals lay their eggs in clusters. Eggs on the periphery of clusters can be at higher risk of mortality. We asked whether the most commonly occurring clutch sizes in pentatomid bugs could result from geometrical arrangements that maximize the proportion of eggs in the cluster's interior. Although the most common clutch sizes do not correspond with geometric optimality, stink bugs do tend to lay clusters of eggs in shapes that protect increasing proportions of their offspring as clutch sizes increase. We also considered whether ovariole number, an aspect of reproductive anatomy that may be a fixed trait across many pentatomids, could explain observed distributions of clutch sizes. The most common clutch sizes across many species correspond with multiples of ovariole number. However, there are species with the same number of ovarioles that lay clutches of widely varying size, among which multiples of ovariole number are not over-represented. In pentatomid bugs, reproductive anatomy appears to be more important than egg mass geometry in determining clutch size uniformity. In addition, within this group of animals that has lost most of its variation in ovariole number, clutches with a broad range of shapes and sizes may still be laid.

Partial refuges from biological control due to intraspecific variation in protective host traits

“…For the other three host species tested, which had type O susceptibility curves, calculating mean survival over the full range of egg mass sizes tested resulted in more modest increases in predicted refuge sizes (Figure 5) as survival was often higher (albeit not significantly so) at lower and/or greater egg mass sizes (Figure 3). Our results provide some evidence, albeit relatively weak, that clutch size-dependent parasitism risk could, in principle, be among several biotic factors influencing clutch size evolution of some stink bug species (Abram et al, 2022).…”

“…We found mixed evidence for our original predictions that smaller and larger egg masses would have higher survival when exposed to T. japonicus . Smaller egg masses were rejected more often by T. japonicus only for two of the four host species, P. maculiventris and C. ligata , potentially because they were perceived by parasitoids to be of lower quality (i.e., would yield less offspring) than egg masses of their most closely associated host, H. halys , which usually lays 28 eggs (Abram et al, 2022). However, it is not clear why this trend was only observed for these two non‐target host species and not on H. halys or E. conspersus .…”

“…To measure baseline egg mortality in the absence of parasitism and ensure that egg mortality was not associated with egg mass size (or egg mass size manipulation), masses from each size category were F I G U R E 2 Egg mass size probability density distributions for the four stink bug species whose eggs were exposed to Trissolcus japonicus in this study. Data are from multiple pooled laboratory and field data sets from different research groups, compiled in the clutch size database in Abram et al (2022). Black curves are probability density functions using a Gaussian smoothing kernel with a smoothing bandwidth equal to 1.4 times the standard deviation of the smoothing kernel (set arbitrarily to maximize visual fit to distributions).…”

“…In this study, we tested whether intraspecific variation in the size of egg masses of target and nontarget hosts of T. japonicus could theoretically provide partial refuges from parasitism. Stink bugs lay their eggs in aggregated egg masses that vary inter‐ and intra‐specifically in clutch size (the number of eggs in each egg mass) (Abram et al, 2022). While clutch size is not technically a trait of each individual egg per se, it is a maternal trait (the product of a decision of how many eggs to lay in a clutch) as well as a trait of groups of eggs.…”

“…In this study, we tested whether intraspecific variation in the size of egg masses of target and nontarget hosts of T. japonicus could theoretically provide partial refuges from parasitism. Stink bugs lay their eggs in aggregated egg masses that vary inter-and intraspecifically in clutch size (the number of eggs in each egg mass) (Abram et al, 2022). While clutch size is not technically a trait of each individual egg per se, it is a T A B L E 1 Previous no-choice studies testing the host range of Trissolcus japonicus, with the number of nontarget (NT) Hemiptera species found to be susceptible (i.e., T. japonicus was shown to reproduce on and/or kill the species' eggs) and how many were tested (in addition to Halyomorpha halys acting as a control, unless otherwise indicated); the number of egg masses tested per NT species; whether the NT egg mass sizes were unmanipulated (i.e., naturally occurring sizes from insect colonies were used), held constant (manipulated to be the same or similar [~] size in every trial), or intentionally manipulated (intentionally made larger and smaller than naturally occurring egg masses, but encompassing the natural range); whether direct behavioral observations of parasitoids were conducted with videos or live observations; whether nonreproductive effects (NREs) of parasitoids on hosts were estimated by comparing mortality of exposed egg masses to controls; and arena type in which parasitoids were exposed to nontarget hosts.…”