Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2005
Publication Date: 3/1/2006
Citation: Siegel, J.P., Lacey, L.A., Higbee, B.S., Noble, P.M., Fritts, R.J. 2006. Effect of application rates and abiotic factors on steinernema carpocapsae for control of overwintering navel orangeworm (lepidoptera: pyralidae, amyelois transitella) in pistachios. Biological Control. 36:324-330.
Interpretive Summary: The navel orangeworm, Amyelois transitella, is an important pest of California pistachios and almonds. Our previous research demonstrated that commercially produced nematodes that infect and kill insects could target the caterpillars of this moth that survive the winter inside fallen pistachios. If these nematodes could be applied using a commercially feasible amount of water then they could play a role in post-harvest sanitation of the orchards. We investigated the efficacy of these nematodes using a reduced amount of water in six small plot field studies, conducted between November 2003 and December 2004 in Madera and Kern Counties, California. In these studies we also investigated the effect of soil temperature on treatment success. In our previous study we did not recover all of the dead larvae and count them, therefore we understimated the impact of nematodes on navel orangeworm larvae. In this new study we used a different measure to assess treatment success, reduction in live larvae recovered per plot. This measure was highly correlated with mortality and also demonstrated that nematodes reduced the number of live larvae by as much as 75%. Our research also demonstrated that high soil temperatures above 90°F during the first 24 hours after nematode application were deleterious, and nighttime freezing temperatures also reduced treatment success. These observations will enable us to determine the optimum time to apply insect pathogenic nematodes, and combined with our demonstration that a commercially feasible application volume of 200 gallons of water per acre was successful, provide orchard managers with a new tool to manage navel orangeworm in pistachios.
Technical Abstract: The impact of reduced application rate and soil temperature at shallow depth (< 2.5 cm) on the efficacy of Steinernema carpocapsae against the navel orangeworm, Amyelois transitella, was evaluated in six field trials employing 1-square meter 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)/square meter (billion per ha) in a volume of 187 ml water/square meter with a post-application irrigation in all trials. Mortality ranged from 7.9-64.9% 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 percent reduction typically was 10-11% greater than mortality for any treatment. We conclude that percent reduction in live larvae per plot is a better measure to assess treatment efficacy than mortality because dead larvae were undercounted and this bias underestimated treatment success. In one trial, treatment was unsuccessful when soil temperature rose to 39°C within five hours after application. In the successful trials soil temperature did not exceed 32°C during the first 24 hours after application. Cold temperatures were also deleterious and nematodes failed in two trials when soil temperature fell below freezing (minimum temperatures -3.0, -5.5°C, respectively) during the first five days after application. We conclude that a commercially feasible application volume of 187 ml water/square meter (200 gal/ac) 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.