Revisiting release strategies in biological control of weeds: are we using enough releases?

The initial establishment of biocontrol agent populations is a critical step toward successful biocontrol. Theoretical approaches have revealed factors that determine the optimal release strategy (the combination of release size and number of releases), including the presence or absence of Allee effects in the agent population and its susceptibility to environmental variability. In this study, we take a more generalized empirical approach that may be more useful for guiding future releases. We analyzed release and establishment records for 74 species of biocontrol agents introduced against 31 weeds in the state of Oregon, U.S.A. Our main findings were that (1) establishment was not affected by release size over the range of release sizes typically used; (2) biocontrol agent species vary in how readily individual releases lead to establishment; and (3) biocontrol programs often use fewer initial releases than is optimal to obtain a high probability of overall establishment. The case of Prokelisia marginata (van Duzee), a planthopper introduced as a biocontrol agent for Spartina alterniflora Loisel (smooth cordgrass), is presented as an illustrative example of the benefits of using more releases.