Field evaluation of sixspotted thrips, Scolothrips sexmaculatus (Pergande), as a predator of Pacific spider mite, Tetranychus pacificus (McGregor), in California almonds

Abstract: Spider mites (Acari: Prostigmata: Tetranychidae: Tetranychus sp.) are significant pests of the more than 400,000 hectares of almonds grown in California, USA. Historically, the most commonly recognized predator of spider mites has been the phytoseiid Galendromus occidentalis (Nesbitt) (Acari: Mesostigmata: Phytoseiidae). However, over the past decade populations of this predator have diminished while populations of sixspotted thrips, Scolothrips sexmaculatus (Pergande) (Thysanoptera: Thripidae), have increased. During 2016 and 2017 we conducted a series of experiments to develop methods to monitor sixspotted thrips with sticky traps, evaluate the field biology of thrips, and measure the impact on spider mite populations. Sixspotted thrips was shown to be capable of providing complete biological control of Pacific spider mite, Tetranychus pacificus (McGregor), under field conditions in commercial almond orchards where thrips population doubling times were calculated to be 4.2 days. Data suggest that conservation biological control programs that avoid broad-spectrum insecticides for lepidopteran and heteropteran pests, in combination with the use of treatment thresholds that tolerate low mite populations as food for thrips, can result in excellent biological control of spider mites.