Why does an herbivore crash when its food explodes? The perverse population dynamic of Lema daturaphila (Coleoptera: Chrysomelidae) on Physalis longifolia (Solanaceae)

Background/Question/Methods For a specialist herbivore, it is reasonable to expect that either 1) population size would follow the phenology of its host, increasing as its host increases in size, or 2) the herbivore would remain dormant until the host reaches substantial size. In northern Virginia, however, the Solanaceae specialist Lema daturaphila peaks early in the season when its primary host plant, Physalis longifolia, is relatively small and it often lays more eggs on a single plant than that plant can support. As its host grows larger in mid season, the herbivore declines precipitously, becoming rare, despite being multivoltine, when its food source is most plentiful. Through a series of manipulative experiments and population monitoring over 2 field seasons, we examined 4 factors that could potentially generate such a pattern: host plant switching, induced plant defense, predation, and parasitoidism

Results/Conclusions We found that 1) despite its name, Lema daturaphila preferred laying eggs on Physalis over Datura and did not switch hosts when Datura became abundant later in the season; 2) Plants that had been defoliated by Lema daturaphila did not become inferior hosts upon resprouting; 3) larval predation by generalist predators (primarily Reduviidae and Pentatomidae) increased greatly by late summer, such that most Lema larvae placed on Physalis plants late in the year got eaten, and 4) the tachinid parasitoid Myiopharus infernalus took an ever-increasing toll on larvae that survived the predators, such that by mid season nearly every larva reaching the last larval stage yielded a fly rather than a beetle. Despite this increasing pressure on Lema larvae, this may not be the most vulnerable stage of the life cycle. During the one summer it was monitored, 72% of eggs laid on plants never even reached the larval stage, but the cause of this mortality is not yet known. Taken together, it appears that the risks of food acquisition can greatly outweigh food abundance and impose a population dynamic that may at first appear perverse.