Generational variation in egg size and shape in Elasmostethus nubilus (Hemiptera: Acanthosomatidae) associated with different host plants

Abstract

In herbivorous insects with seasonal host switching, females are exposed to seasonal environmental heterogeneity arising from variation in oviposition host plants and abiotic conditions such as temperature. However, few studies have examined plastic variation in egg size, shape, and number among generations associated with different host plants. Elasmostethus nubilus (Dallas) (Hemiptera: Acanthosomatidae) is a bivoltine herbivorous shield bug that switches host plants between generations. In our study population, overwintered females (first generation) use Fatsia japonica (Thunberg) Decaisne and Planchon (Araliaceae) for breeding, whereas newly emerged adults that developed on this host plant (second generation) migrate to and breed on Dendropanax trifidus (Thunberg) Makino ex Hara (Araliaceae). We found that females of the first generation laid fewer but larger and more elongated eggs than those of the second generation, despite no significant difference in female body size. No phenotypic trade-off between egg size and clutch size was detected in either generation. Within each generation, we found no evidence of a positive allometric relationship between egg size and egg shape, as would be expected under constraints such as limitations imposed by female anatomy. We discuss potential factors underlying the observed intergenerational variation in maternal resource allocation and egg shape.