Postharvest survival of navel orangeworm assessed in pistachios

Authors: Siegel, Joel; Kuenen, Lodewyk; Higbee, Bradley

Submitted to: California Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/2007
Publication Date: 1/1/2008
Citation: Siegel, J.P., Kuenen, L.P., Higbee, B.S. 2008. Postharvest survival of navel orangeworm assessed in pistachios. California Agriculture. 62(1):30-35.

Interpretive Summary: Our research on the navel orangeworm (Amyelois transitella Walker) an important insect pest of pistachios and almonds, focused on its mortality after harvest and the availability of the unharvested nuts (mummies) in which it develops. Controlling this insect is troublesome, in part because mummies are difficult to manage by sanitation. We determined infestation rate and pistachio availability at four points in time during winter and spring (December, February, April and June), and calculated the decrease in the navel orangeworm population and availability of pistachios. The prevalence of navel orangeworm decreased 77% between December and February, and mortality was higher on the ground than in the trees. However, since very few mummies remained in the trees, we feel that sanitation strategies should concentrate on the ground. Initially, a very high percentage of the nuts on the ground could be used by the navel orangeworm, 71%, but by late spring useable nuts decreased to only 30.6% of the nuts recovered, and declined still further in June 2005 to 13% of the nuts. However, on a per acre basis, even at 13%, there are still tens of thousands of pistachios available for navel orangeworm oviposition. We conclude that the current sanitation regime for pistachios needs to be augmented.

Technical Abstract: This manuscript is an overview of our research on the overwintering mortality of the navel orangeworm (Amyelois transitella Walker) an important insect pest of pistachios and almonds. We also present data on the post harvest availability of split nut pistachios, in which it develops. Controlling this insect is troublesome, in part because unharvested nuts (mummies) are difficult to manage by sanitation. Current practices blow the mummies from the berm and till them into the soil, but a considerable number still remain available. We developed a novel technique to quantify the prevalence of both the overwintering population and split nut fraction of the mummies. Tens of thousands of mummies were collected on four dates between December and June, held at constant temperature, and adult emergence quantified. We also examined a subset of these mummies and determined the percentage of split nuts at each date. The prevalence of navel orangeworm decreased 77% between December and February, and this decrease, which we attribute to mortality, was higher on the ground than in the trees. Apparent mortality was even greater between February and April, but in this case the reduction was due to both mortality and emigration. This moth does not undergo diapause and adults emerge sporadically throughout the winter and early spring. The initial prevalence of split nuts on the ground after harvest was 71% and resulted from spillage. Nuts drop onto the ground continuously throughout the winter and spring and by mid winter most nuts are on the ground. In our June 2005 collection, only 13% of the nuts collected on the ground were split nuts. However, that still leaves tens of thousands of useable nuts per acre, and the next year the percentage of split nuts in our June sample was 43%. We conclude that the current sanitation practices need to be augmented, and are conducting research to evaluate ways to improve sanitation.