Impacts of a weed biocontrol agent on recovery from water stress in a target and a non-target Hypericum species.
Biological control agents that impact on non-target native species have limited desirability. Aculus hyperici was introduced into Australia to help control the weed St John's wort Hypericum perforatum, despite indications from pre-release trials that this eriophyid mite could survive and reproduce on a non-target native species, Hypericum gramineum. We used field experiments to explore the probability of A. hyperici colonizing either H. gramineum or H. perforatum in sympatric populations of the alternative hosts, and to determine the impact of A. hyperici on the growth of field-grown H. gramineum. We used a glasshouse experiment to compare the impact of A. hyperici on the ability of each host to recover from water stress and to investigate how such recovery affects herbivore populations on the alternative hosts. As Australia is frequently affected by drought, this issue is critical to both the efficacy and safety of biological weed control. The probability of A. hyperici colonizing H. gramineum was significantly lower than the probability of it colonizing the target weed. Despite the ability of A. hyperici to infest H. gramineum, the mite had negligible impacts on all measured indices of growth and reproduction. During the water limitation experiment, the non-target host supported populations of A. hyperici throughout the experimental period, but population size was consistently low. In comparison, mite populations increased considerably on the target weed and were associated with significant decreases in growth. Water limitation compounded the adverse impacts of mites on the weed, but did not affect impacts on the non-target native. Despite the detrimental effects of mites on the growth of H. perforatum, we found little evidence that they impaired the ability of either H. perforatum or H. gramineum to recover from water stress. Synthesis and applications. Rigorous host-specificity testing is necessary to ensure that biological weed control agents do not impact on non-target species. Potentially, however, these tests may reject beneficial agents unnecessarily, because of low level feeding or reproduction on non-target species. Although host-specificity testing might indicate that non-target native species may host biocontrol agents, our results demonstrate that such non-target species are not, necessarily, damaged.