The effects of the duration of exposure on the toxicity of diflubenzuron, hexaflumuron and teflubenzuron to various stages of II instarSchistocerca gregaria

Abstract

Second-instar (II) nymphs of the Desert Locust, Schistocerca gregaria (Forsk.) were exposed to three benzoylphenyl ureas (BPUs), diflubenzuron, hexaflumuron and teflubenzuron. Nymphs were treated with precise doses by allowing them to ingest treated barley leaves at varying stages of the II instar. They were exposed to the same total quantity of active ingredient over one (Days 1, 2, 3 or 4), two (Days 1-2, 2-3 or 3-4) or four days (Days 1-4) of the four-day inter-moult period. The total amounts applied were 60 μg per nymph of diflubenzuron, 30 μg per nymph of hexaflumuron or 0.25 μg per nymph of teflubenzuron. The nymphs were then monitored for two moults after treatment until they reached the fourth (IV) instar, to observe both the acute and chronic effects of treatment. The timing of the exposure during the inter-moult period and the duration of exposure were both found to result in significantly different acute responses for each BPU. Treatment over one or two days showed that the closer to the moult II instars were treated, the greater the mortality. This indicated that locust pharate cuticle is predominantly synthesised late in the instar. Treatment over four days resulted in higher mortality than exposure to the same quantity of active ingredient over one or two days, suggesting that BPUs are highly toxic to locust nymphs but non-cumulative within their bodies. The timing of death was also significantly affected by both the timing and duration of treatment. A significant proportion of the mortality occurred after the first moult following treatment when nymphs were dosed on Day 1 and Day 2 with hexaflumuron and diflubenzuron respectively. Mortality following all other treatments occurred during the first moult after treatment. The duration of the II and third (III) instars were significantly prolonged in many cases following treatment with BPUs. The implications of increased mortality following prolonged exposure to BPUs and extended development periods are discussed in relation to the use of BPUs as barrier-sprayed insecticides for the control of mobile locust nymph populations.