THE EFFECTS OF THE DURATION OF EXPOSURE ON THE TOXICITY OF DIFLUBENZURON, HEXAFLUMURON AND TEFLUBENZURON TO VARIOUS STAGES OF II-INSTAR SCHISTOCERCA-GREGARIA

Second-instar (II) nymphs of the Desert Locust, Schistocerca gregaria (Forsk.) were exposed to three benzoylphenyl ureas (BPUs), diflubenzur on, hexaflumuron and teflubenzuron. Nymphs were treated with precise d oses by allowing them to ingest treated barley leaves at varying stage s of the II instar. They were exposed to the same total quantity of ac tive 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 tot al amounts applied were 60 mu g per nymph of diflubenzuron, 30 mu g pe r nymph of hexaflumuron or 0.25 mu g per nymph of teflubenzuron. The n ymphs were then monitored for two moults after treatment until they re ached the fourth (IV) instar, to observe both the acute and chronic ef fects of treatment. The timing of the exposure during the inter-moult period and the duration of exposure were both found to result in signi ficantly 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 o f active ingredient over one or two days, suggesting that BPUs are hig hly toxic to locust nymphs but non-cumulative within their bodies. The timing of death was also significantly affected by both the timing an d duration of treatment. A significant proportion of the mortality occ urred 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 mou lt after treatment. The duration of the II and third (III) instars wer e 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 mob ile locust nymph populations.