Establishment of treatment thresholds for Pacific spider mite (Tetranychus pacificus) based on the prevalence of its primary predator, sixspotted thrips (Scolothrips sexmaculatus)

Spider mites are a significant pest of almond production in the San Joaquin Valley, CA, USA. Treatment thresholds based on mite density and the presence or absence of biological control by phytoseiids have existed since the 1980’s.  However, the predominant predator of spider mites in California almond production has recently become the six-spotted thrips, Scolothrips sexmaculatus. Our research focused on using capture levels of S. sexmaculatus to improve the previous thresholds. We collected data on spider mites/leaf and S. sexmaculatus/sticky trap weekly. Ratios of S. sexmaculatus to spider mites were correlated to changes in mite density that occurred the following week. We found that mite density didn’t change where there was an average of 0.5 S. sexmaculatus/trap/week for every one spider mite/leaf. We recommend that information from presence-absence sampling for spider mites on leaves and capture rates of S. sexmaculatus on sticky traps could be used together to improve pest management decisions. Additionally, we developed a less precise, but more user-friendly model where absolute counts of S. sexmaculatus on sticky traps can be used to determine if orchards should be treated. Our data show, on average, that spider mite density remained unchanged for seven and 14 days if there were an average of five and three S. sexmaculatus/trap/week.