Oviposition cues for a specialist butterfly--plant chemistry and size.

The oviposition choice of an insect herbivore is based on a complex set of stimuli and responses. In this study, we examined the effect of plant secondary chemistry (the iridoid glycosides aucubin and catalpol) and aspects of size of the plant Plantago lanceolata, on the oviposition behavior of the specialist butterfly Melitaea cinxia. Iridoid glycosides are known to deter feeding or decrease the growth rate of generalist insect herbivores, but can act as oviposition cues and feeding stimulants for specialized herbivores. In a previous observational study of M. cinxia in the field, oviposition was associated with high levels of aucubin. However, this association could have been the cause (butterfly choice) or consequence (plant induction) of oviposition. We conducted a set of dual- and multiple-choice experiments in cages and in the field. In the cages, we found a positive association between the pre-oviposition level of aucubin and the number of ovipositions. The association reflects the butterfly oviposition selection rather than plant induction that follows oviposition. Our results also suggest a threshold concentration below which females do not distinguish between levels of iridoid glycosides. In the field, the size of the plant appeared to be a more important stimulus than iridoid glycoside content, with bigger plants receiving more oviposition than smaller plants, regardless of their secondary chemistry. Our results illustrate that the rank of a cue used for oviposition may be dependent on environmental context.

Optimum and maximum host sizes at parasitism for the endoparasitoid Hyposoter didymator (Hymenoptera: Ichneumonidae) differ greatly between two host species.

Host size is considered a reliable indicator of host quality and an important determinant of parasitoid fitness. Koinobiont parasitoids attack hosts that continue feeding and growing during parasitism. In contrast with hemolymph-feeding koinobionts, tissue-feeding koinobionts face not only a minimum host size for successful development but also a maximum host size, because consumption of the entire host is often necessary for successful egression. Here we study interactions between a generalist tissue-feeding larval endoparasitoid, Hyposoter didymator Thunberg (Hymenoptera: Ichneumonidae) and two of its natural hosts, Spodoptera exigua Hübner and Chrysodeixis chalcites Esper (Lepidoptera: Noctuidae). Larvae of C. chalcites are up to three times larger than corresponding instars of S. exigua and also attain much higher terminal masses before pupation. We hypothesized that the range of host instars suitable for successful parasitism by H. didymator would be much more restricted in the large host C. chalcites than in the smaller S. exigua. To test this hypothesis, we monitored development of H. didymator in all instars of both host species and measured survival, larval development time, and adult body mass of the parasitioid. In contrast with our predictions, C. chalcites was qualitatively superior to S. exigua in terms of the survival of parasitized hosts, the proportion of parasitoids able to complete development, and adult parasitoid size. However, in both hosts, the proportion of mature parasitoid larvae that successfully developed into adults was low at the largest host sizes. Our results suggest that qualitative, as well as quantitative, factors are important in the success of tissue-feeding parasitoids.