Effects of Residue Type on Persistence and Biological Activity of Foliar Fenpropathrin

Abstract

Persistence characteristics and biological activity of total and subsurface residues of fenpropathrin applied as an emulsifiable concentrate to tomato and cotton seedlings were studied. Removal of the surface residue by artificial rain permitted a direct estimation of the activity of the subsurface residue. The decay time of the first 50% of fenpropathrin residue on tomato leaves varied with the concentration applied. Decay time was short (2.4 d) at 100 mg (AI)/Iiter to moderate (12 d) at 500 mg (AI)/liter. About 70-90% of the initial deposit existed as surface residue and was responsible for the variable chemical persistence. The half-life of the subsurface residue was 15-16 d for both concentrations. For both adults of a green leafhopper ( Asymmetrasca decedens (Paoli)) and larvae of the Egyptian cotton leafworm ( Spodoptera littoralis (Boisduval)), the toxicity of the subsurface fraction was significantly lower than that of the total residue. A large difference in the LC50 of the two residue types (LC50subsurface/LC50total = 10) suggested that the toxicity of the total residue against green leafhopper adults was due mainly to the surface component. Antifeeding effects of fenpropathrin on S. littoralis larvae appear to account for lower mortality rates of subsurface residue compared with total residue.