Microbial mediation of fruit fly–host plant interactions: is the host plant the “centre of activity”?

Abstract

Insects utilize resources in their environment with the aid of mutualistic or symbiotic mediation by microorganisms. Some insect species such as ants and termites often have complex ecological and evolutionary associations with their symbionts, while the nature and functional significance of such associations in non‐social insects is often unclear. In the Dacinae (Diptera: Tephritidae), specific Enterobacteriaceae (Erwiniaherbicola, Enterobactercloacae, Klebsiellaoxytoca) are believed to mediate interactions between the adult fruit flies and the larval host plant. This bacterial mediation is hypothesized as being integral to the larval host plant being the “centre of activity” of the fly. Using a non‐pest, monophagous fruit fly (Bactroceracacuminata [Hering]), we tested this hypothesis by manipulating the fruiting state of its larval host plant (Solanum mauritianum Scopoli) and subsequently assessing insect behaviour and phylloplane microflora on those hosts. On host plants that had never fruited, few flies or bacterial colonies were recorded, consistent with hypothesis expectations. On fruiting host plants or plants that had had their fruit removed, bacterial colonies were present; again consistent with expectation. However, few flies were recorded on fruit‐removed plants and all fly behaviours, other than resting or oviposition, were rare or absent on any hosts; inconsistent with expectation. The general pattern of results suggested that female flies coming to oviposit on fruiting hosts were spreading Enterobacteriaceae, but such spread was incidental and not part of some mutualistic interaction between fruit flies and bacteria.