Interpopulation variation in egg size in the winter cherry bug relative to oviposition site selection


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To investigate whether or not oviposition on substrates other than host plants (e.g., non-host plants, abiotic materials) would affect the evolution of egg size in phytophagous insects, we studied the winter cherry bug, Acanthocoris sordidus (Thunberg) (Heteroptera: Coreidae), as a model organism for its interpopulation variation in oviposition preference. The rate of oviposition off host plants is much higher in the Amami Island population than in either the Kyoto or Kochi populations. We compared egg size and number among the three local populations from Kyoto, Kochi, and Amami Island. In addition, to evaluate the adaptive significance of larger eggs for offspring in terms of searching for host plants, we examined the relationship between egg size and first-instar body size. We also searched for a relationship between egg size and starvation tolerance in the second instars because first instars can develop to second instars without food intake, and thus the substantial host-searching stage is the second instar, when females lay their eggs off host plants. Females from the Amami population produced fewer larger eggs than females from either the Kyoto or Kochi population. Regardless of the local population, the body size of first instars that emerged from larger eggs was larger, and the second instars originating from larger eggs had a higher starvation tolerance. The larger body size and higher starvation tolerance should enable nymphs to disperse further, which may enhance the probability of successfully reaching host plants. These results suggest that egg size in A. sordidus may be determined in relation to its oviposition habits to maximize reproductive success, resulting in interpopulation variation in egg size.