Rates of Prey Consumption by Neotropical Robber Flies (Diptera: Asilidae)

Abstract

Adult robber flies are sit-and-wait predators that attempt aerial capture of flying insects. Estimates of daily rates of prey consumption per individual were combined with estimates of absolute population densities to obtain daily, populational rates of prey consumption for nine robber fly species in a lowland neotropical forest. Estimates of biomass consumed/day/ha/population varied from 23 to 1425 mg (dry weight) among the nine species, and total daily consumption for all species was approximately 4600 mg (dry weight)/ha. This latter value was relatively low compared to estimates available for other insectivorous taxa. Although many studies have measured consumption rates of insect herbivores (Teal 1962, Van Hook 1971, Reichle et al. 1973) or detritivores (Kowal & Crossley 1971, McBrayer & Reichle 1971, Santos & Whitford 1981), few studies provide estimates of prey consumption (i.e., ingestion) rates by insect predators. I am aware of only two studies in nonagricultural ecosystems that estimate prey biomass consumed per unit area of habitat per unit time by populations of predatory insects, and both of these concern populations of nymphal stoneflies (Plecoptera) in alpine streams of western North America. Siegfried and Knight (1976) reported an annual prey consumption estimate of 3.7 g/m2 (dry mass) for Calineuria californica in a Californian stream. Allan (1982) studied three stonefly species in Colorado and estimated total annual prey consumption by all species to be 3.0 g/m2 (dry mass). Both studies suggested that predatory stoneflies consume a significant fraction of the prey biomass available. Previous reports (Shurovnekov 1962, Joern & Rudd 1982) also suggest that robber flies have a major impact upon prey populations. However, the data collected do not allow calculation of consumption rates in terms of prey biomass taken per unit area and consequently can not be directly compared with consumption rates for other predatory species. Here I present estimates of daily prey consumption rates for populations of nine robber fly species in a lowland neotropical forest. Adults of all nine species are visual, sit-and-wait predators that attempt aerial capture of flying insect prey. Following a successful capture, individuals return to a perch and consume the liquid contents of the prey. Since attack rate, proportion of sucI Received 16 October 1984, revision accepted 16 April 1985. 2 Present address: Pacific Biomedical Research Center, University of Hawaii at Manoa, Honolulu, Hawaii 96822, U.S.A. 166 BIOTROPICA 18(2): 166-17