Larval host plant selection in the silver-spotted skipper (Epargyreus clarus)
Host plants for larval lepidopterans are commonly chosen by the mother insect. However, if larvae entirely consume or fall off of an initial host, they may need to find another. We asked whether larvae raised from hatching on one of three different host plants of unequal quality later discriminated amongst the three hosts. We also asked whether consumption of a new host, adopted for just the last larval instar, affected subsequent larval growth. Using Epargyreus clarus (Silver-spotted skipper), we raised larvae on one of three leguminous hosts: black locust (Robinia pseudoacacia), kudzu (Pueraria), and wisteria (Wisteria sinensis). Once at the 4th instar, we presented these larvae with a choice test of the same three hosts, meaning that one plant was the rearing host and the other two were novel. We also raised a cohort of E. clarus larvae on kudzu, and just prior to the molt to the 5th instar, switched half to Wisteria and left half on kudzu, in order to determine the effect of switching hosts late in larval life. In addition to measuring the weight change and days to pupation of these larvae, we also determined their growth efficiency on their assigned hosts.
E. clarus larvae showed an innate preference for their native host, Robinia. However, larvae raised on all three hosts were more likely to choose their natal host, meaning that induced preferences may also be at play. Few larvae chose Wisteria, which is known to be a poor-quality host. Larvae raised on Kudzu for their entire lives weighed more and reached pupation faster than those who were switched to Wisteria at the 5th instar. Our data suggest a difference in the nutritional value of the host plants that can affect larvae late in larval life, and that larvae do prefer their natal host plants. Interestingly, despite its apparently poor nutritional quality, adult females still lay eggs on Wisteria. The persistence of Wisteria as a host plant for E. clarus suggests that the host could be providing an alternate benefit, such as providing larvae with a space free from enemies.