Environmental conditions can indirectly affect passive pathogen spread by altering insect dispersal

Climate change impacts agriculture through shifts in regional environmental conditions, significantly altering insect vector and plant pathogen interactions. Plant pathogens that rely on insect vectors often have a positive density dependent relationship, with high vector abundance increasing pathogen spread. However, for pathogens that do not rely on insect vectors, the relationship can be more difficult to predict. Furthermore, environmental conditions, such as temperature and relative humidity, can indirectly alter pathogen spread by impacting vector reproduction or behavior. Therefore, studies examining the interactions between passively transmitted pathogens and potential insect vectors under varying environmental conditions are critical to understanding pathogen spread. Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is the main insect pest of onion (Allium cepa) and co-occurs with many pathogens, including Colletotrichum coccodes Wallr (Glomerellales: Glomerellaceae), a fungal pathogen of onion. We conducted two experiments to understand how different densities of onion thrips and environmental conditions affect pathogen spread. To first determine how onion thrips density affects pathogen spread, we released 0, 5, or 25 onion thrips on C. coccodes inoculated plants, that were symptomatic, and allowed them to move between the symptomatic and asymptomatic plants. Next, to determine the effects of environmental conditions on pathogen spread, we altered temperature and relative humidity and added either 0 or 5 onion thrips to the experiment. Onion thrips dispersal, damage distribution, and pathogen spread 13 days after onion thrips were released were assessed for each experiment. Our results indicate that C. coccodes was only disseminated in the presence of onion thrips and dispersal increased at the 25 onion thrips density. Pathogen spread also increased at 33°C compared with 22°C, regardless of relative humidity. Overall, our study highlights how changes in insect abundance, which can be directly affected by environmental conditions, can alter pathogen spread.