Field Biology of Scolothrips sexmaculatus (Thysanoptera: Thripidae) as a Predator of Tetranychus pacificus (Acari: Tetranychidae) in California Almonds.

Abstract

Biological control of tetranychid spider mites in California almonds has predominantly been associated with predatory mites in the family Phytoseiidae. However, changes in production practices, including shifts to more sustainable chemical control programs, have resulted in a trophic shift leading to the predominance of six-spotted thrips, Scolothrips sexmaculatus Pergande, as the primary mite predator. Over a 3-yr period, we studied S. sexmaculatus biology and relationship to Tetranychus pacificus McGregor in almond orchards in the southern San Joaquin Valley of California. Scolothrips sexmaculatus had two periods of activity in response to increases in T. pacificus density, one in the spring and one in the summer. Sex ratios were consistently skewed to more than 90% females captured on yellow sticky traps, with females producing eggs from May to October. Female thrips were larger than males, and both sexes were larger from June to September when temperatures were warmer and food was more abundant. In response to spider mite outbreaks, S. sexmaculatus increased from approximately 2/trap/wk to an average of 1,272/trap/wk over a 5-wk period, representing a population doubling time of 4.0 d. Over this time, we described trends in T. pacificus and S. sexmaculatus density that are consistent with population regulation through predator-prey relationships by a specialist predator.