A critical evaluation of augmentative biological control

Abstract

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.

Introduction

The potential for using “augmentative” or “inundative” biological control to suppress arthropod pests has been recognized for many years (DeBach, 1964; Doutt and Hagen, 1949; King et al., 1985; Parella et al., 1992; Ridgway and Vinson, 1977;Stinner, 1977). Augmentative biological control (or “augmentation”) is simply the release of large numbers of insectary reared natural enemies with the goal of “augmenting” natural enemy populations or “inundating” pest populations with natural enemies. The use of augmentative releases might be appropriate, for example, if naturally occurring parasitoid or predator populations fail to colonize fields or orchards, or colonize too late in the season to provide effective control of the pest (e.g., Obrycki et al., 1997).

To our knowledge, Doutt and Hagen (1949) were the first researchers to experimentally apply this approach more than 50 years ago. These authors released green lacewings to control out-breaking mealybug populations in pear orchards. Since Doutt and Hagen’s pioneering study, augmentative biological control has been applied experimentally in a large number of pest systems (King et al., 1985; Parella et al., 1992; Ridgway and Vinson, 1977; Stinner, 1977). Sales of natural enemies for augmentation have also grown considerably in recent years (Cranshaw et al., 1996). Nevertheless, the use of augmentation on a commercial basis appears to be limited to a few systems, especially in the US (van Lenteren, 1988; van Lenteren et al., 1997).

One of the major stimuli for investigating the potential for augmentative biological control has been the drive to reduce a historic reliance on broad-spectrum pesticides for pest control. The trend has been to tighten regulations on pesticide use, with some broad-spectrum pesticides having their registrations withdrawn by governmental agencies. Augmentation might be implemented as a substitute for broad-spectrum pesticides if pest populations are sufficiently suppressed by the released natural enemies. van Lenteren (1988) argued that the first step in implementing augmentative biological control in greenhouses in the Netherlands has been to demonstrate to growers that augmentative releases are effective and comparable in cost to pesticide applications. Here, we critically evaluate the efficacy and cost of augmentative biological control by reviewing case studies in the published literature. The underlying question is whether augmentation can replace conventional pest management using broad-spectrum pesticides in US food production. We also evaluate ecological factors that may limit the effectiveness of augmentation, again by reviewing published studies. Based on the review, we comment on how future research and ongoing changes in agriculture may increase the implementation of augmentative biological control in the future.