An integrated pest management (IPM) program is reliant on a thorough understanding of the biology and ecology of the target pest and incorporates multiple management tactics to reduce pest pressure. Management can be further complicated when multiple pest species, such as insect vectors and plant pathogens, interact in the field. Onion thrips (Thrips tabaci) are the main insect pest of onions and are well documented vectors of many different types of plant pathogens. The co-occurrence of onion thrips and facultative plant pathogens in the field is well documented, but it remains unclear how these pests interact, and whether the integration of multiple management tactics can provide synergistic control of these pests. This dissertation begins to fill that gap, exploring the relationship between onion thrips and Colletotrichum coccodes, a fungal pathogen of onion, and testing IPM strategies for multiple pest management. I first examined how C. coccodes influenced onion thrips behavior and physiology by conducting preference and performance experiments in the lab. By manipulating host plant health, I found that onion thrips can discriminate between multiple hosts and prefer hosts that were infected with C. coccodes compared to healthy hosts. Furthermore, adult onion thrips survived longer and reproduced at higher rates on plants infected with C. coccodes. Preference for an infected host can alter pathogen spread and increases in insect vector abundance often lead to more severe pathogen outbreaks. Therefore, I explored how onion thrips spread C. coccodes in the lab, examining density dependent and independent factors, such as temperature. At high temperatures, onion thrips reproduction increased, and they dispersed more often from an inoculated to a non-inoculated plant, leading to an increase in disease spread in the experiment. These results suggest that disease spread is likely density dependent, and that conditions favoring onion thrips outbreaks may also lead to more severe disease symptoms. Onion thrips and C. coccodes exhibit a mutually beneficial relationship; with onion thrips reproducing at higher rates on C. coccodes inoculated plants, and subsequently disease spread increased concurrently with increased vector abundance. I then focused on testing various IPM tactics in the field to reduce onion thrips and foliar fungal pathogens. Plastic mulches were tested as a non-chemical control method aimed at delaying onion thrips infestation in the field. I found that mulches are viable season-long alternative to conventional pesticides. Furthermore, the addition of conventional pesticides provided synergistic onion thrips control. Finally, I tested organic and conventional pesticide programs to develop a sustainable season-long control program for onion thrips and foliar fungal pathogens. Organic pesticides were not sufficient for onion thrips control, highlighting the need for alternative control methods and more sustainable conventional practices. To address this, I tested various action thresholds which reduced the number of pesticide applications throughout the season without compromising onion thrips control. This dissertation begins to fill the gaps in our existing knowledge of insect-plant-pathogen interactions and suggests that the role of insects as vectors of facultative pathogens should not be overlooked. The mutually beneficial relationship between these pests further highlights the need to develop a comprehensive IPM program that focuses on managing both pests rather than focusing on a single species. By testing several IPM tactics, this work offers new insight into multiple pest management strategies that could be implemented in the field.
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