The potential for evolution of resistance in Glasshouse whitefly (Trialeurodes vaporariorum) to biocontrol agents and the associated life history costs

Abstract

Understanding how pests evolve resistance to control is crucial for ensuring food security, yet resistance evolution to biocontrol is overlooked compared to pesticide resistance. In this thesis I investigate co-evolution between pests and their natural enemies in the context of biocontrol to explore how resistance to natural enemies impacts the efficiency and long term effectiveness of sustainable pest control. In chapter 3 I present the results of a lab experiment quantifying variation in resistance to parasitism in glasshouse whitefly (Trialeurodes vaporariorum) and its biocontrol agents Encarsia formosa, a parasitoid wasp, and a commercial fungal biopesticide containing Beauveria bassiana. I used these results to parameterise a simulation model predicting resistance evolution under different scenarios, varying crop type and biocontrol application regime. I found resistance to biocontrol agents in T. vaporariorum to be highly variable, and my simulations suggest resistance can readily evolve, but the speed of evolution depends on environmental context, in particular crop type. In chapter four I confirmed the results of the simulation with a six generation a selection experiment. Chapter 5 investigated life history trade-offs in Encarsia resistant whitefly. I assayed the fitness of T. vaporariorum that survived Encarsia attack in the lab. Surviving resistant whitefly had reduced fecundity, lower offspring survival, and slower development. These trade-offs imply that while whiteflies can survive parasitism, reduced population growth may constrain resistance evolution. Chapter 6 explores whether tank-mixing multiple biopesticides may be a viable option for pre-emptive management of resistance evolution and targeting multiple pest species at once. Using T. vaporariorum and the tomato leaf miner (Tuta absoluta) different biopesticides were applied alone or in combination. Mixing biopesticides showed no antagonistic or synergistic effects on pest mortality, suggesting control efficacy is not compromised by tank mixing and can enhance application efficiency when targeting multiple pest species. My thesis provides insight into the effectiveness of biological control strategies and the importance of understanding resistance evolution. Although resistance to biocontrol agents is less prevalent than to chemical pesticides, it remains vital for ensuring long-term efficacy.