Oviposition site selection by herbivorous beetles: a comparison of two thistle feeders, Cassida rubiginosa and Henosepilachna niponica

In insects that feed on plants in both adult and larval stages, it is often difficult to distinguish oviposition preference from adult feeding preference, because oviposition can occur at or in proximity to feeding sites. In the present study, characteristics of oviposition site selection of two beetle species, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae) and Henosepilachna niponica (Lewis) (Coleoptera: Coccinellidae), were investigated in the field and laboratory, with particular attention to relationships with adult feeding sites. In the field, distances between adult feeding scars and egg masses differed for C . rubiginosa and H . niponica , with the former being very small and the latter averaging 24.6 cm. The same tendencies for the distances between adult feeding scars and egg masses of the two beetle species were confirmed in cages in which only female beetles were released. Cassida rubiginosa restricted egg laying to host plants in the field and to leaves in laboratory assays. On the other hand, H . niponica placed 8% of egg masses on plants adjacent to host plants in the field and often placed eggs on artificial substrates rather than leaf discs in laboratory assays. These results suggest that oviposition and female feeding sites are virtually inseparable in the case of C . rubiginosa , while H . niponica females do not necessarily keep to host plant leaves as oviposition substrates and they tend to oviposit at some distance from their feeding sites. Results are discussed in relation to proximate and ultimate causes of host selection behavior.