Abstract
The interactions between plant-eating insects and their hosts have shaped both the insects and the plants, driving evolution of plant defenses and insect specialization. The leaf beetle Trirhabda eriodictyonis (Chrysomelidae) lives on two shrubs with differing defenses: Eriodictyon crassifolium has hairy leaves, whereas E.trichocalyx has resinous leaves. We tested whether these beetles have differentiated onto the two host plants, and if not, whether the beetles prefer the better host plant and prefer mates who are from that host plant. In feeding tests, adult beetles strongly preferred eating E.trichocalyx regardless of which host they came from. In addition, females laid more eggs if they ate E.trichocalyx than E.crassifolium. So, E.trichocalyx is generally the better host. However, beetle mate preference was not in line with food choice. Males did not prefer to mate with females from E.trichocalyx. Females from E.crassifolium did prefer males from E.trichocalyx over males from E.crassifolium, but did not lay more eggs as a result of these matings. We conclude that the beetle populations we studied have not differentiated based on their host plants and may not have even adapted to the better host. Although to humans these host plant defenses differ dramatically, signs that they have caused evolution in the beetles are lacking. The case of T.eriodictyonis stands counter to many other studies that have seen the differentiation of ecotypes and/or adaptive coordination of an herbivore's life cycle based on host plant differences.
Highlights
Herbivorous insects rely on their host plant for nutrition, protection from predators, and as a locale for meeting mates
A significant Kruskal–Wallis test was followed by Dwass–Steel–Critchlow–Fligner pairwise comparisons. Adults reared on both plants preferred to eat the sticky E. trichocalyx (Table 1: adults from E. crassifolium P = 0.02; adults from E. trichocalyx P < 0.001)
Females feeding on E. trichocalyx showed no preference for males from one plant species or the other, but females living on hairy E. crassifolium were almost twice as likely to accept the mating advances of a male if he had lived and fed on sticky E. trichocalyx than if he had fed on E. crassifolium (Tukey-like comparison, Q = 3.635, four groups, P = 0.005)
Summary
Herbivorous insects rely on their host plant for nutrition, protection from predators, and as a locale for meeting mates. Numerous authors have celebrated how a shift between host plants can lead to a shift in mate preference as part of speciation (Bush 1969; Rice and Salt 1990; Feder et al.1994; Via 1999; Via et al 2000; Dres and Mallet 2002; Geiselhardt et al 2009; Downey and Nice 2011). When individuals prefer mates that have eaten the same plant they themselves are in the habit of eating, gene flow slows between the groups on the two host plants, and the populations begin to differentiate This idea is central to Thompson’s (2005) theory of the geographic mosaic of coevolution. It envisions that beetles on each host plant come to be adapted to their respective hosts in multiple ways as has been seen with apple maggots (Bush 1969), pea aphids (Via 1999; Via et al 2000), and checkerspot butterflies (McBride and Singer 2010)
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