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Host-specificity and its effect on mate choice in a plant-eating beetle
The beetle Trirhabda eriodictyonis lives on two shrubs with different plant defenses: Eriodictyon crassifolium has hairy leaves; E. trichocalyx has sticky leaves. The relationship between these plants and the leaf-eating beetles that depend on them has been unstudied until now. In choice tests, larvae and adults showed unexpected feeding preferences, with larvae from E. crassifolium showing no preference and those from E. trichocalyx preferring E. crassifolium. Adults all strongly preferred eating E. trichocalyx. Larvae and adults that I switched from E. trichocalyx to E. crassifolium died younger than beetles that I continued to feed the original host species. Mating trials showed that the only difference in preference involved males from E. trichocalyx, which were far more attractive to females on E. crassifolium than males on the same host. Finally, females laid more eggs if they ate E. trichocalyx than E. crassifolium, even if they had started life on the latter. It is clear that E. trichocalyx provides benefit to both males and females and these beetle populations are not differentiating based on host plants. Neither the differentiation hypothesis nor the preference-performance hypothesis are validated by this plant-insect interaction. Instead, it appears that the best explanation of this relationship is phylogenetic conservatism. The plant defenses, which appear dramatically different to humans, are unimportant to the beetles.