Microdroplet application for determination of comparative topical and residual efficacy of formulated permethrin to two populations of diamond back moth (plutella xylostella L.)

Abstract

AbstractAn on‐demand, uniform droplet generator was used to apply 120 μm diameter drops of formulated permethrin either directly onto 3rd‐instar Plutella xylostella, of two Ohio populations, or onto the abaxial surface of leaf discs (1‐1000 drops cm−2) to which those larvae were subsequently exposed. Incorporating a fluorescent tracer (Saturn Yellow GT‐17N) in the spray mixture confirmed that „direct hit”︁ larvae had received a topical dose from a single droplet. There was little difference in the LC50/LD50 for either population of P. xylostella when 24 and 48 h mortality assessments from direct hits were compared, but the LD50 of Celeryville larvae was 6‐8 times greater than that of Fremont larvae. Residual deposits of 120 μm drops containing approximately the LC50, based on the topical assays, revealed that a droplet density providing a dose per cm2 leaf surface of more than 1000 times the LD50 was required to cause median mortality. The efficiency of utilisation, based on the topical LD95 values, was 0.0006% and 0.072% for Fremont and Celeryville larvae, respectively.