Does anti‐parasitoid defense explain host‐plant selection by a polyphagous caterpillar?

While studies of tri‐trophic interactions have uncovered a variety of mechanisms influencing the dietary specialization of insect herbivores, such studies have neglected host‐plant selection by generalists. Here, we report an initial investigation on how host‐plant quality and a tachinid parasitoid interact to affect the survival and host‐plant selection by a polyphagous herbivore. This herbivore, Grammia geneura (Strecker) (Lepidoptera: Arctiidae), is a food‐mixing caterpillar that feeds preferentially on forbs. A previous study suggested that G. geneura might eat certain host species for reasons other than benefits of physiological utilization. We hypothesized that host‐plant mediated defenses could act against parasitoids, the major mortality agents of late instar G. geneura . Field observations indicated that caterpillars sometimes survived an attack by the parasitoid Exorista mella Walker (Diptera: Tachinidae) in nature. Laboratory experiments showed that the survival of parasitized caterpillars increased on acceptable but nutritionally inferior host‐plant species, indicating that anti‐parasitoid defense may explain host‐plant selection in this dietary generalist. We found no indication that host‐plant selection changed according to the parasitism status of individual caterpillars.