This Mite be a Prickly Situation: The effects of predation by predatory mites on Sericothrips staphylinus and the impacts on its ability to control gorse

Abstract

Weed biological control agents are used based on the premise they are able to undergo rapid population increase in the introduced range due to release from predation by specialist predators in their native range, resulting in effective weed control. However, predation by generalist predators present in the introduced environment can decrease weed control efficacy. This thesis aims to determine the impact of predation by three generalist predatory mites (Amblydromalus limonicus, Amblyseius herbicolus, and Neoseiulus cucumeris) on Sericothrips staphylinus, a biological control agent for gorse (Ulex europaeus). The first part of the thesis demonstrates the ability of all three predatory mites to feed and reproduce on three S. staphylinus immature stages: 1st instar larvae, 2nd instar larvae, and prepupae. A. limonicus had the highest predation and oviposition rates across all three immature stages in the non-choice experiment and consumed the most of all three immature stages presented in the choice experiment, N. cucumeris consumed the least. The predation rate of all three predatory mites were highest when consuming 1st instar larvae and lowest when consuming prepupa. In the choice experiment, all three predatory mites had the highest prey preference for 1st instar larvae, with prepupa having the lowest preference index. In the second experiment, all three predatory mites exhibited a type 2 functional response when presented with fixed densities of S. staphylinus 1st instar larvae (2, 4, 8, 16, 24). A. limonicus had the highest attack rate, whereas A. herbicolus had the shortest handling time. N. cucumeris had the lowest attack rate and the longest handling time. The third experiment investigated the effects of A. limonicus, A. herbicolus, and N. cucumeris on S. staphylinuspopulations for 3 months in greenhouse conditions. The number of S. staphylinus in the A. limonicus and N. cucumeris treatments were significantly lower than in the control. A. herbicolus populations failed to establish and S. staphylinus numbers were significantly higher than treatments with A. limonicus and N. cucumeris. Plant growth was not significantly different amongst treatments. However, all treatments with S. staphylinus had significantly more plant damage than the control. This demonstrates that predation can reduce the efficacy of S. staphyinus as a biological control agent. Further studies are required for a comprehensive understanding of how generalist predators can impact a range of weed biological control agents. This information would be beneficial for determining the success of future biological control agents, and development of methods to minimise the predation impact.