A critical evaluation of augmentative biological control

The potential for using “augmentative” biological control (or “augmentation”) for suppressing arthropod pests has been recognized for many years. Nevertheless, augmentation is applied commercially in relatively few agricultural systems, particularly in the US. To address why this might be the case, we reviewed the literature on augmentative biological control and critically evaluate three questions. First, does augmentative biological control effectively suppress agricultural pests? Second, is augmentation cost effective? Third, what ecological factors limit the effectiveness of augmentation? We evaluated the effectiveness of augmentation by assessing whether pest densities were suppressed to specified target levels and by reviewing studies that explicitly compared augmentation with pesticide applications. Augmentation achieved target densities in about 15% of case studies and failed 64% of the time. Augmentation was also usually less effective than pesticide applications, though not always. In the evaluation of economics, augmentative releases were frequently more expensive than pesticides, although there were cases where augmentation was cost effective. Finally, 12 ecological factors were implicated as potential limits on the efficacy of augmentation. Unfavorable environmental conditions, compensatory mortality, enemy dispersal, host refuges from released natural enemies, and predation of released agents were most often suggested as ecological limits. Future research should seek to counteract ecological limits by combining different natural enemy species and/or by combining augmentative releases with low-risk pesticides. Use of low-risk insecticides and organic agricultural practices in particular provides new opportunities for augmentative biological control.