The effect of reduced application rate, soil temperature at shallow depth (⩾2.5cm), and soil type on the efficacy of Steinernema carpocapsae against the navel orangeworm, Amyelois transitella, was evaluated in six field trials employing 1m 2 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)/m 2 (10 9 /ha) in a volume of 187mlwater/m 2 (1870L/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 (r 2 =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 5h 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 1870Lwater/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 24h after application is necessary for treatment success.