Rainfastness of Insecticide Deposits on Loblolly Pine Foliage and the Efficacy of Adjuvants in Preventing Washoff

Abstract

The rainfastness of several insecticides on loblolly pine foliage was determined by using a laboratory rain chamber and by testing the foliage in bioassays with adult leaffooted pine seed bugs, Leptoglossus corculus (Say). The efficacy of eight commercial and two experimental adjuvants in preventing rainwashing of deposits of azinphosmethyl and phosmet from loblolly pine foliage was also determined. Deposits of azinphosmethyl from 0.2% (Al) simulated sprays made from emulsifiable concentrates (EC's), (Guthion 2S and 2L), were highly susceptible to rainwashing. Regression analysis of these data showed a significant linear relationship between arcsin percent mortality and rainfall amount in cm [y = 68.08 – 6.88/(Rainfall)]. Using the confidence intervals from the regression analysis, it was estimated that rainfall in excess of 1.42 cm would reduce mean seed bug mortality on treated foliage to an amount below 80% with 95% confidence, and likewise, rainfall in excess of 3.95 cm would reduce mean mortality to an amount below 50%. However, mortality of L. corculus on deposits of simulated azinphosmethyl 0.2% (Al) sprays made from the wettable powder (Guthion 50 WP) were virtually unaffected by 15 cm of rain. Spray deposits from permethrin (Pounce 3.2EC) at lower rates were moderately to highly resistant to rainwashing. Regression analyses perfomed on these data showed no significant relationship between mortality and rainfall amount. Most of the adjuvants were ineffective in preventing rainwashing of deposits of azinphosmethyl (EC) or phosmet (WP). Only two, Bio-Film and Exhalt 800, showed any promise.