The effect of release size on the probability of establishment of biological control agents: Gorse thrips (Sericothrips staphylinus) released against gorse (Ulex europaeus) in New Zealand

Many biocontrol agents released against alien weeds and pests fail to establish in the field. Here, we ask whether better release strategies could improve the likelihood of successful establishment. A manipulative field experiment was used to investigate the relationship between the probability of establishment and the number of individuals released for a weed biocontrol agent. In this experiment, replicated releases of 10, 30, 90, 270 and 810 gorse thrips, Sericothrips staphylinus Haliday, were made on to isolated gorse bushes in New Zealand. The sampling efficiency was determined using a further experiment in which known numbers of thrips were released on to bushes. The data obtained showed that in approximately nine out of 10 release of 10 thrips, at least one thrips would be found. The thrips in the size of release experiment were sampled 1 year after their release. A higher proportion of the small releases became extinct during this time period: thrips were recovered from 100% of the releases of 270 and 810 thrips, but only from 33% of releases of 10, 30 and 90 thrips. Using gorse thrips as an example, a protocol was developed to determine the optimal release size for biocontrol agents. It is suggested that for a fixed number of insects available for release, smaller releases may increase the overall establishment rate. While a single large release can easily become extinct by chance, it is extremely unlikely that a large number of small releases will do so over the same time period. The optimal release size (i.e. that which maximizes the average number of successful establishments) for gorse thrips in New Zealand might be fewer than 100 thrips/release site. This contrasts with the current strategy in New Zealand of 1000 thrips/release and the suggestion in the biocontrol literature that large releases optimize establishment. Over 1 year of observation the thrips had no effect on gorse growth rate.