Can thrips-resistant onion plants and plastic mulches improve OMRI-Listed insecticides for controlling onion thrips?

Onion thrips (Thrips tabaci Lindeman) feeds on onion foliage, reducing photosynthesis and bulb yield. Organic management of onion thrips relies on insecticides, but their overuse may cause insecticide resistance. Our objective was to evaluate combinations of thrips-resistant cultivars and reflective mulch, with or without insecticides, to manage onion thrips in organic onions. Treatments included plastic mulch (reflective vs. white), cultivar (thrips-resistant ‘Rossa di Milano’ and B5336xB5351 vs. thrips-susceptible ‘Bradley’) and insecticide (spinosad + neem oil vs. untreated). Thrips densities were assessed weekly and bulbs graded and weighed at harvest. Reflective mulch reduced adult and larval densities in 2019 only by 17% and 6%, respectively. Insecticides further reduced thrips larvae in reflective mulch (2019 only). Resistant cultivars did not affect adult thrips densities but significantly reduced larval densities (‘Rossa di Milano’: 21% (2018), 32% (2019); B5336xB5351: 17% (2018)), compared to ‘Bradley’. Thrips were significantly lower in insecticide-treated plots (adults: 74% (2019); larvae: 40% (2018), 84% (2019)). In 2018, total bulb yield was highest in B5336xB5351 and equivalent to ‘Bradley’ in 2019, whereas total yield in ‘Rossa di Milano’ was lowest in both years; reflective mulch did not impact bulb yield. Total bulb yield was highest in insecticide-treated plots in both years (13% (2018), 23% (2019)). Future management of thrips in organic onions will require insecticides, but resistant cultivars and reflective mulch may provide additional benefits.