EFFECTS OF BACILLUS-THURINGIENSIS VAR ISRAELENSIS ON NONTARGET BENTHIC INSECTS THROUGH DIRECT AND INDIRECT EXPOSURE

We investigated direct and indirect (food-chain) effects of Bacillus t huringiensis var. israelensis (B.t.i.) on selected nontarget benthic i nsects. Sixteen taxa of various Ephemeroptera, Plecoptera, Trichoptera , and Diptera were exposed to B.t.i. directly, and indirectly through ingesting B.t.i.-contaminated food (black flies and conditioned leaves ) in experiments that were conducted in artificial and natural streams . Lethal effects and sublethal effects (drift, feeding, growth, emerge nce success, and time to emergence) were measured up to 35 d following B.t.i. exposure. In addition, we measured larval detachment and decom position rates, following B.t.i. application, to determine how long B. t.i.-killed larvae remain available to potential consumers. We observe d no lethal or sublethal effects on most taxa following direct and ind irect B.t.i. exposure. Mortality was recorded with two species, Tipula abdominalis (Diptera:Tipulidae) and Arthroplea bipunctata (Ephemeropt era: Heptageniidae), when immatures were exposed to B.t.i. at dosages considerably higher (>50 x and 500 x, respectively) than recommended f ield rates. Acroneuria lycorias (Plecoptera:Perlidae) nymphs exposed t o a high B.t.i. dose (100 ppm for 120 min) drift at higher frequencies than nymphs not exposed. Predators consumed equal numbers of B.t.i.-k illed and live black flies, except for one trial where Isoperla signat a (Plecoptera:Perlodidae) ingested significantly fewer B.t.i.-killed b lack flies than live ones, and another trial where Ceratopsyche sparna (Trichoptera:Hydropsychidae) ingested significantly more B.t.i.-kille d than live larvae. Shredders switched to consuming black fly larvae o nce the black flies were killed with B.t.i. Siphlonurus rapidus (Ephem eroptera:Siphlonuridae) nymphs provisioned with B.t.i.-killed larval b lack flies attained a greater body mass over 7 d than those not so pro visioned. No change in adult emergence success or time to emergence wa s recorded after respective taxa were exposed to B.t.i. B.t.i.-killed black fly larvae remained attached to substrates in artificial streams up to 16 d, until decomposing beyond recognition. In summary, B.t.i. appears harmless to nontarget benthic insects, even when these organis ms are exposed to higher than recommended dosages, and when they consu me B.t.i.-killed black fly larvae.