Physiological Not Behavioral Adaptations of Leafminers to a Resistant Host Plant: A Natural Selection Experiment

Behavioral and physiological responses to a new or toxic host plant may materially affect the process of insect adaptation to that host plant. Critical to understanding how insects adapt to novel plants is observing how behavioral and physiological responses to the plant evolve in response to selection by the new host. Replicate populations of leafminers, Liriomyza trifolii Burgess, were maintained on a resistant cultivar of chrysanthemum, Chrysanthemum morifolium Ramat., for 10 generations. This cultivar was characterized by a gradient of increasing toxicity from the bottom to the top leaves of the plant. The number of offspring and larval survivorship on leaves at several heights of this plant were determined in 8 of the 10 generations. At the outset of the experiment, younger apical leaves were significantly more toxic to larvae than lower leaves but there were no significant differences among leaf positions in the number of eggs laid in each (i.e., there was no significant oviposition preference among a range of leaf heights and ages). Because of this gradient in toxicity a large increase in larval survivorship could be obtained either by shifting oviposition preference toward the less toxic lower leaves or by an increase in physiological tolerance at all leaf heights. Considerable increases in mean survivorship occurred over the course of the selection experiment, due entirely to an increase in mean larval survivorship, at all leaf heights, of ≈240%. In contrast, oviposition preference did not shift toward the less toxic lower leaves. After consideration of the variable results of other studies, I suggest that from a genetical perspective, neither behavioral nor physiological mechanisms for adaptation to a hostile host plant are necessarily 1st or most rapid.