RESPONSE OF SPODOPTERA-EXEMPTA (WALK) LARVAE TO SIMULATED FIELD SPRAY APPLICATIONS OF ACYLUREA INSECT GROWTH-REGULATORS WITH OBSERVATIONS ON CUTICULAR UPTAKE OF ACYLUREAS

The acylurea insect growth regulators flufenoxuron ('Cascade(R)') and teflubenzuron ('Nomolt(R)') were applied to maize plants and/or larvae of the armyworm, Spodoptera exempta (Walk.) at volumes equivalent to medium volume (MV; 250 litre ha-1) and ultra-low volume (ULV; < 1.0 litre ha-1, VMD = 50 mum) sprays at rates (less-than-or-equal-to 10 g a.i. ha-1) at or below currently recommended field rates. The Egyptian cotton leafworm, S. littoralis was examined for comparative purposes on Chinese cabbage. One-day-old third-instar larvae (L3) of S. exempta sprayed (MV) with either compound (I and 10 g ha-1), and confined on similarly treated maize leaves for 48 h, showed a high level of response after 48 h (87-100 % effective mortality), with 100 % mortality after 120 h. Exposure of sprayed L3 to treated plants for 6 h was only slightly less effective. When only larvae were sprayed, the higher rate for both compounds gave 97 % mortality after 120 h. The response rate of one-day-old L3 S. littoralis was generally slower but was similar to that of S. exempta after 120 h. When plants and larvae were sprayed at relatively low densities (22-45 drops cm-2) with undiluted ULV formulations, L3 S. exempta showed a greater response to flufenoxuron (50 g litre-1) than to teflubenzuron (25 g litre-1) at 48 h but all larvae were effectively dead (93-100 % actual mortality) after 120 h. Similar results were obtained with L5 S. exempta. When only larvae were sprayed (23-25 drops cm-2), equivalent concentrations (25 g a.i. litre-1) of diluted flufenoxuron and cypermethrin ULV formulations gave 87 and 90 % mortality respectively with L3 S. exempta after 120 h, although the latter compound acted more rapidly. The diluted flufenoxuron was more toxic than its undiluted formulation at similar spray densities. L3 and L5 S. exempta and L3 S. littoralis showed a similar level of response to undiluted flufenoxuron and teflubenzuron. Little or no difference in toxicity was found for topically applied flufenoxuron and teflubenzuron against L3 S. exempta and S. littoralis. Accumulation of [C-14]flufenoxuron appeared to increase progressively over 6 h in both species, while that of [C-14]teflubenzuron reached a peak after 1 h. After 6 h, there were relatively greater amounts (> 2-fold) of radioactivity in flufenoxuron-treated than in teflubenzuron-treated larvae. Such differences may account for the greater speed of action observed with the former compound in some experiments. The potential of flufenoxuron and teflubenzuron for the control of S. exempta is discussed with particular regard to the significant contact activity and reasonably rapid response time (effective mortality at 48 h) shown by these compounds at field rates.