Effect of application rates and abiotic factors on Steinernema carpocapsae for control of overwintering navel orangeworm (Lepidoptera: Pyralidae, Amyelois transitella) in pistachios

The effect of reduced application rate, soil temperature at shallow depth (⩾2.5 cm), and soil type on the efficacy of Steinernema carpocapsae against the navel orangeworm, Amyelois transitella, was evaluated in six field trials employing 1 m2 plots conducted from November 2003 through December 2004 in Madera and Kern Counties, California. Nematodes were applied at a concentration of 100,000 infective juveniles (IJs)/m2 (109/ha) in a volume of 187 ml water/m2 (1870 L/ha) with a post-application irrigation in all trials. Mortality ranged from 7.9 to 64.9% in successful trials and percent reduction in live larvae per plot was as high as 74.6%. Percent reduction and mortality were highly correlated (r2 = 0.78) and larval reduction typically was 10-11% greater than mortality for any treatment. In one trial, although nematode treatment significantly increased mortality compared to the controls, the treatment was deemed unsatisfactory because mortality was <15%. Soil temperature in this trial rose to 39 °C within 5 h after application. Nematodes failed in two other trials when soil temperature fell below freezing (minimum temperatures −3.0, −5.5 °C, respectively) several times in a 5-day period. We conclude that a commercially feasible application volume of 1870 L water/ha followed by post-application irrigation at this same rate was effective, and that soil maximum temperature at or below 32 °C during the first 24 h after application is necessary for treatment success.