SPEED OF ACTION AND TOXICITY OF ACYLUREA INSECT GROWTH-REGULATORS AGAINST SPODOPTERA-EXEMPTA (WALK) AND SPODOPTERA-LITTORALIS (BOISD) LARVAE - EFFECT OF INTER-MOLT AGE

Ultra-low volume (ULV) spray bioassays at droplet densities (20-80 drops cm-2) corresponding to field deposits were used to determine the efficacy of two acylurea insect growth regulators, teflubenzuron ('Nomolt(R)') and flufenoxuron ('Cascade(R)') against three different age groups of third-instar Spodoptera exempta and Spodoptera littoralis larvae. While no response (mortality and/or abnormal moulting with impaired ability to feed) was observed with one-day-old larvae until 48 h after treatment, two-day-old larvae showed some response (27-93% abnormal moulting and 3-10% mortality when larvae were sprayed; 80-100% abnormal moulting and up to 83% mortality when larvae and plants were sprayed) after 24 h and up to 100% mortality at 120 h. Studies with non-feeding, pre-moult larvae showed that cuticular uptake of acylureas was sufficient to affect an appreciable proportion of larvae after 24 h, with up to 100 % mortality at 120 h. More detailed studies with pre-moult S. littoralis larvae showed that, while a few larvae which moulted between 1 and 5 h after treatment showed abnormal moulting to the L4 stage, a much larger proportion of larvae which moulted 6-20 h after treatment were affected. Previous studies with S. exempta have shown that different larval instars show similar levels of sensitivity to acylureas. The present work suggests that, while the age distribution within instars has little influence on the end-point mortality of acylureas, it can profoundly modify the speed of response, the majority of larvae within an instar being effectively controlled at the moult following treatment. The significance of these observations in relation to the crop protection activity of acylureas against field populations of Spodoptera larvae is discussed.