An impingement plate method to detect deposits of pyrethroid insecticides

Abstract

A silica gel impingement plate for monitoring pyrethroid deposits in environmentally sensitive areas is described. The plate is simple, commercially available, and inexpensive. A residue analysis method is given for deltamethrin deposits with 83.3% mean recovery and a minimum quantifiable limit of 0.005 μg/plate. Pyrethroid deposits are strongly adsorbed to the silica gel, which prevents wash‐off by rainfall and slows photodegradation. In two field experiments, deltamethrin was readily detected 3 weeks after direct spray applications at 6 and 10 g ai/ha. Deltamethrin dissipation on the plates was first‐order with a half‐life of 2.9 ‐ 3.7 d. Photoisomers of deltamethrin were also detected on the plates and the ratio of photoisomers to deltamethrin increased over time. This ratio will indicate the age of deposits in monitoring situations.