Augmentative release trials with Metaphycus spp. (Hymenoptera: Encyrtidae) against citricola scale (Homoptera: Coccidae) in California's San Joaquin Valley

Abstract

In recent years, citricola scale, Coccus pseudomagnoliarum (Kuwana) (Homoptera: Coccidae), has re-emerged as an important pest of citrus in California's San Joaquin Valley. We seek a biological control solution to citricola scale's pest status as part of an evolving, ecologically based integrated pest management (IPM) program for citrus. We report on augmentative release thais against citricola scale involving 4 species of Metaphycus (Hymenoptera: Encyrtidae) M. flavus (Howard), M. helvolus (Compere), M. luteolus (Timberlake), and M. stanleyi Compere. We released these parasitoids against sleeve-caged citricola scales in a San Joaquin Valley citrus grove. Releases were made on 3 dates (hereinafter 'early', 'intermediate', and 'late' release dates), each date representing scales of different size, all between 1 and 1.5 mm in length. Two to 4 of the parasitoid species were released on any of the dates. Relative to a control (no parasitoid release), the M. flavus treatments (2 densities were released) provided the greatest degree of reduction in scale numbers in the early release (from >3,600 to <5 per cage), while M. helvolus had an intermediate effect. In the intermediate release, M. flavus again provided the greatest reduction in scale numbers (from >2,000 to <6 per cage), followed by M. luteolus, whereas M. stanleyi did not differ from the control. In the late release, all parasitoid treatments (M. flavus, M. helvolus, M. luteolus, and M. stanleyi) provided similar degrees of reduction in scale numbers (from ≃1,150 to ≃6 per cage) relative to the control. We discuss the relevance of citricola scale's size at the time of parasitoid release to the results obtained with each of the parasitoid treatments on each of the release dates. In addition, we discuss the perceived impact of hyperparasitism by a facultative autoparasitoid, Coccophagus lycimnia (Walker), on our results. We conclude that citricola scale's size at the time of release is particularly important for deciding which parasitoid (s) to release, and that M. flavus and M. luteolus are the most promising parasitoids, among those currently available to us, for further research toward developing augmentative biological control tactics against citricola scale.