The effect of rainfall on the survivorship and establishment of a biocontrol agent.
Abstract
The weather is believed to affect the establishment of insect biological control agents in the field, in particular heavy rainfall immediately after release, but few studies have quantified this impact. A rain simulator was used to quantify the effect of rainfall duration and intensity on the post-release survivorship of a gorse biological control agent, the thrips Sericothrips staphylinus. This information was then used to predict the effect of rainfall on the establishment of thrips in the field. Rainfall significantly decreased the numbers of thrips remaining on the plants, but significantly increased the numbers of thrips found dead and missing. As rain duration increased, the numbers of thrips remaining on the plants decreased. However, the relationship was not linear: the majority of thrips were washed off the plants within 30 min of the 120-min experiment. Thrips survivorship was significantly affected by rainfall intensity (moderate, heavy and violent rainfall). Increasing rainfall intensity caused a significant decrease in the numbers of thrips remaining on the plants and significant increases in the numbers missing or found dead. The relationship between rainfall, thrips release size and their probability of establishment was determined by combining published data on their probability of establishment with the results of our experiments. Rainfall is likely to change the relationship between the release size and the probability of establishment. We advocate that biological control practitioners should be cognisant of weather conditions prior to any release. Control agents at risk from rain might be released in protective cages, while smaller sequential releases might be preferable to single large releases if adverse conditions are likely. This study demonstrates the value of controlled environments in understanding the effects of weather on biocontrol, and for invasion ecology more generally.