Influence of host density and population structure on egg production in the coccidophagous ladybird, Chilocorus nigritus F. (Coleoptera: Coccinellidae)
1 Studies on aphidophagous coccinellids have indicated that patch quality, and, in particular, the age structure of the prey or the phenological age of the plant, may play an important role in stimulating oviposition behaviour. However, little is known about the egg-laying tactics of coccidophagous species.
2 When restricted to a single, large colony of overlapping generations of their diaspid host Abgrallaspis cyanophylli (Signoret), adult females of the coccidophagous ladybird Chilocorus nigritus F. varied their egg production rate in a cyclic pattern that lasted for approximately 22 days. This information was used to generate hypotheses relating to eliciting cues for oviposition.
3 Two experiments were carried out in the absence of conspecific larvae to assess: (i) whether changes in host density at levels above those needed to sustain egg production in the females affected egg output and (ii) the effect of restricting beetles to various homogeneous prey population structures on daily egg production.
4 Changes in host density caused a significant but transient decline in egg production whereas a heterogeneous prey population elicited significantly higher levels of oviposition than homogeneous ones with similar host densities, irrespective of the growth stage of the prey.
5 Beetles were fed to satiation throughout the experiments but the data obtained suggest that cues for eliciting oviposition operated in a density-dependent fashion. The results of the study are used to discuss the possibility that visual and chemical cues are quantitatively used by ovipositing beetles to assess patch suitability.