Interactions between weed cultivation and bactericides on thrips abundance, natural enemies, and disease severity in an organic onion system

In organic systems, insect pests and pathogens are a significant challenge to sustainable vegetable production in the southeast. Thrips (Thysanoptera) are the most economically damaging pest of onions and vector, Pantoea spp., the bacterial pathogen causing center rot disease, and weeds may harbor thrips and pathogens. We conducted an experiment to examine the interaction between weed management and organic bactericide applications on pest management, pathogen prevalence, and natural enemies. The experimental design was a randomized complete block plot design with four replications of each treatment combination. As predicted, thrips was the dominant herbivore, and we counted a total of 2703 in 2019 and 3005 in 2020 on sticky cards. Parasitoid wasps were the most abundant natural enemy found both years, (2840 and 4543, respectively). Thrips abundance was lowest in the hand-weeded plots (weed free) and highest in a low intensity mechanical tine-weeded treatment (2 passes post-transplanting) when combined with applications of Serenade. Disease severity was highest in weedy plots with no cultivation or bactericide application throughout the season. There was an overall positive correlation between thrips and parasitoid counts per sticky card, which indicates parasitoid activity in response to thrips. However, cultivation or bactericides did not influence natural enemy populations or diversity. High intensity mechanical treatments and hand-weeding will control for thrips and suppress pathogen. More research should be conducted using field trials to examine how common weed management techniques used by organic onion growers’ impact natural enemy communities.