THE INTERACTIONS BETWEEN ROUTES OF EXPOSURE AND PHYSICOCHEMICAL PROPERTIES OF 4 WATER-DILUTABLE PERMETHRIN FORMULATIONS IN RELATION TO THEIR ACTIVITIES AGAINST TRICHOPLUSIA-NI (HUBNER)

A series of laboratory experiments were undertaken to compare the activities of four water-dilutable permethrin formulations against the cabbage looper Trichoplusia ni (Hubner). The formulations used were 'Pounce 3.2 EC', 'Ambush 2.0 EC' and two novel formulations (containing evaporation retardants) supplied, for comparison, by Roussel Uclaf Environmental Health Ltd, Berkhamsted, UK (formerly Wellcome Environmental Health Ltd). Bioassays were developed to investigate the activity of the insecticides via topical, residual and residual/dietary routes of exposure. The test insects were exposed to insecticide droplets, of 200 mum diameter, either directly or indirectly (on cabbage leaves or parafilm) using an on-demand droplet generator. Following exposure to insecticide, the test species were confined in petri dishes and mortality assessments were made at 24 h after treatment. The results were analyzed using probits. The results indicated that up to 15-fold differences in activity between formulations may occur when exposure is via residual contact on an artificial waxy surface (parafilm). The novel formulations were found to be far more active than the standard EC formulations. These differences in activity were significantly correlated with the dynamic surface tension and droplet spread upon cabbage leaves of the formulations. The results are discussed in terms of the bioavailability of pyrethroids upon waxy surfaces and also in terms of the potential for manipulating the pick-up of pesticides from plant surfaces.