PITCHER-PLANT MIDGES AND MOSQUITOS - A PROCESSING CHAIN COMMENSALISM

Larvae of the midge Metriocnemus knabi and of the mosquito Wyeomyia smithii are found only inside the water-filled leaves of the carnivorous pitcher plant, Sarracenia purpurea, where they feed on decaying invertebrate carcasses. I examined the interaction between the two species in a natural population in western Newfoundland, Canada. Experimental manipulations of rates of prey capture by pitcher-plant leaves indicated that both insect species are limited by carcass supply. However, the interaction between them is commensal rather than competitive. Midge growth was unaffected by experimental quadrupling of mosquito density in otherwise unmanipulated leaves. Mosquito growth, on the other hand, increased with midge density in both natural leaves and artificial leaf microcosms. This interaction is an example of a processing chain commensalism. Although both species feed on carcass material, they use it in different stages of decay: midges feed by chewing on solid material, while mosquitoes filter-feed on particles derived from the decaying matter. Consumption of particles by mosquitoes does not affect resource availability for midges, but feeding by midges does influence particle availability. In artificial leaf microcosms, high bacterial densities occurred sooner in artificial leaves with midges present than in identical leaves without them. Bacteria are a direct food source for mosquitoes, and high bacterial densities also indicate that other organic material is being comminuted, providing particles for mosquitoes and surface area for bacterial growth. Although midges remove some of the carcass resource that also limits mosquitoes, their net effect on mosquitoes is positive because they accelerate conversion of the remaining resource to particles.