A preliminary study of the predator-prey relationships of watermites (Acari: Hydrachidia) and blackfly larvae (Diptera: Simuliidae)

The virtually worldwide occurrence of blackflies (Simuliidae) where flowing water exists, and their role as disease vectors of livestock and man, makes these haematophagous Diptera of considerable economic importance. Control of simuliid populations is usually achieved through the treatment of the larval stages with organophosphates. Biocontrol approaches have also focused on natural enemies of the immature stages. Recently, the migrobial insecticide Bacillus thuringiensis var. israelensis has shown considerable potential in this respect, with minimal effects on non-target organisms (Lacy & Undeen, 1987; Becker & Margalit, 1993). Adult simuliids oviposit either directly into the water, or onto some kind of substrate (stones, twigs, trailing vegetation, etc). The larvae hatch and drift some distance downstream before adhering to a suitable rock, or aquatic plant. Clean, fast-flowing, well-oxygenated water is preferred by larval simuliids, which filter fine organic particulate matter from the water through cephalic fans. The larvae go through 6-8 instars before pupating inside a tent-like cocoon. Upon emergence, the adult fly surfaces and crawls onto a dry substrate to allow the wings to expand and harden before flying off to mate, blood-feed, and oviposit (Crosskey 1990). With the exception of birds, e.g. dippers, predators of immature simuliids generally inhabit the same aquatic habitat as their prey. Fish are usually credited with the greatest fraction of simuliid predation and Trichoptera, Odonata, Ephemeroptera and Plecoptera species as the most important invertebrate predators. Chironomids and other dipteran species have frequently been recorded as feeding on Simulium spp., and cannibalism also occurs (Davies 1981). Perhaps the greatest, yet the most frequently overlooked of the simuliid predators, are waterrnites of the family Hydrachnidae (Hydrachnellae or Hydracarinae) which