Foraging behavior of a Dipteran leaf miner on exploited and unexploited hosts

The foraging behavior of females of the leaf miner, Agromyza frontella (Rondani), (Diptera: Agromyzidae) when encountering unexploited or exploited alfalfa plants was studied in large field cages and in laboratory bioassays. Females did not recognize any exploited leaflets before contacting them and did not distinguish between leaflets with an egg or first instar larva and unexploited leaflets, even after contact. Only one fly oviposited in leaflets which contained 80-120 nutrition holes, one late second or third instar larva or which were marked with an epideictic pheromone in field cages. In laboratory bioassays females oviposited less in leaflets containing a second or third instar larva or an empty larval mine than in unexploited ones. Females foraging on unexploited leaflets engaged in area-restricted search and 10 of 11 females remained on the test plant for the full 60 min of observation. However, females foraging on exploited plants were much more active, spent a greater proportion of their time searching for suitable hosts, had the highest rates of visitation to all above ground plant parts and emigrated to the cage walls before 60 min had elapsed. These quantitative measures of foraging behavior indicated that females ranked plants after landing on them in the following order: unexploited plants >plants marked with pheromone or with many nutrition holes >plants with late instar larvae. The order of host ranking by foragers was in general agreement with the suitability of the host plants for larval survival, development and reproduction, as estimated from previous laboratory studies.Females of A. frontella foraging on unexploited alfalfa plants fed and oviposited significantly more often in the upper apical leaflets than in the lower, older leaflets. However, the choice of feeding site by flies on exploited plants did not vary with leaflet position (age), indicating that females fed in order to sample leaflet quality and that females investigated lower (older) leaves after they discovered that the preferred upper leaves were occupied. These data suggest that high quality oviposition sites may be limiting for A. frontella females, which could explain why superparasitism of leaflets sometimes occurs in nature, even when unexploited sites are available.