Effects of Bacillus thuringiensis var. israelensis on Nontarget Benthic Insects through Direct and Indirect Exposure

Abstract

We investigated direct and indirect (food-chain) effects of Bacillus thuringiensis var. israelensis (B.t.i.) on selected nontarget benthic insects. 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, emergence success, and time to emergence) were measured up to 35 d following B.t.i. exposure. In addition, we measured larval detachment and decomposition rates, following B.t.i. application, to determine how long B.t.i.-killed larvae remain available to potential consumers. We observed no lethal or sublethal effects on most taxa following direct and indirect B.t.i. exposure. Mortality was recorded with two species, Tipula abdominalis (Diptera:Tipulidae) and Arthroplea bipunctata (Ephemeroptera: Heptageniidae), when immatures were exposed to B.t.i. at dosages considerably higher (>50× and 500×, respectively) than recommended field rates. Acroneuria lycorias (Plecoptera:Perlidae) nymphs exposed to a high B.t.i. dose (100 ppm for 120 min) drifted at higher frequencies than nymphs not exposed. Predators consumed equal numbers of B.t.i.-killed and live black flies, except for one trial where Isoperla signata (Plecoptera:Perlodidae) ingested significantly fewer B.t.i.-killed black flies than live ones, and another trial where Ceratopsyche sparna (Trichoptera:Hydropsychidae) ingested significantly more B.t.i.-killed than live larvae. Shredders switched to consuming black fly larvae once the black flies were killed with B.t.i. Siphlonurus rapidus (Ephemeroptera:Siphlonuridae) nymphs provisioned with B.t.i.-killed larval black flies attained a greater body mass over 7 d than those not so provisioned. No change in adult emergence success or time to emergence was 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 organisms are exposed to higher than recommended dosages, and when they consume B.t.i.-killed black fly larvae.