Size matters: Host-seeking ectoparasitoid larvae prefer larger hosts, but not the largest

Koinobiont parasitoids do not prevent host development immediately after the attack, hence the quality of their resource units may vary over time. Potential advantages exist, though less clear, of choosing a host of better initial nutritional quality. We studied the effect of host nutritional quality on Mallophora ruficauda Wiedemann (Diptera: Asilidae), a solitary, koinobiont ectoparasitoid that preferentially attacks larvae of Cyclocephala signaticollis Burmeister (Coleoptera: Scarabaeidae). In the field, lighter hosts were parasitized more often than heavier hosts, but sex did not explain parasitism patterns. Our experiments showed that parasitoid larvae did not differentially attack heavier or lighter hosts in either two- or four-choice selection experiments, but do show differential selection based on the size of the host. Experiments simulating a host patch showed that parasitoid larvae choose larger hosts, but at a finer scale, as the size of the hosts increased, larvae chose the smaller host in a dyad. These results suggest that M. ruficauda larvae may select hosts of higher nutritional quality (i.e., larger hosts) but that there is an upper limit where a trade-off between nutrient acquisition and survival (related to life expectancy or host defenses) may drive the larval decisions.