Classical Biological Control of Codling Moth: the California Experience

Codling moth is a notorious fruit-boring pest that has extended its original distribution from the natural apple forests of Central Asia to cover all apple growing regions of the world. Having been discovered in California as early as 1872, codling moth has continued to be the dominant pest of apple, pear and walnut production causing extensive damage in the absence of insecticide treatment. In an effort to reduce reliance on insecticides, a classical biological control program was initiated in 1992. Following an initial survey for parasitoids of codling in Central Asia, three species were selected for importation and release in California; one larval parasitoid, Bassus rufipes (Braconidae) and two cocoon parasitoids Liotryphon caudatus and Mastrus ridibundus (Ichneumonidae). The outcome of releases made from 1993 to 2000 was that insufficient B. rufipes were released to gain establishment, L. caudatus established at least temporarily, and M. ridibundus became well established and continues to impact codling moth populations in the region. Although not a dramatic success in terms of the level of reduction of codling moth population densities, parasitism has played an important role in reducing the frequency of fruit and nut damage in orchards. This project provides an interesting example of what can be expected from parasitoid introductions against a notorious direct pest that belongs to a taxonomic family with a very poor history of success in the biological control record. INTRODUCTION California has a long history of classical biological control (henceforth referred to simply as biological control) originating with the successful control of the cottony cushion scale, Icerya purchasi Maskell (Homoptera: Margarodidae), as a pest of citrus in southern California in 1889 (Caltagirone and Doutt 1989). Since this first historical success, more than 100 years ago, California has been one of the most active regions of the world with regard to the pursuit of biological control solutions for invasive pests. There have also been numerous subsequent successes in California, including the suppression of invasive armored scales, whiteflies and mealybugs on citrus, as well as other notable cases such as olive scale and walnut aphid (Mills and Daane 2005). ____________________________ Classical Biological Control of Codling Moth: The California Experience Second International Symposium on Biological Control of Arthropods 127 The biological control record provides consistent evidence that homopteran pests have been the most successful targets for biological control and that lepidopteran pests have been more difficult to control through natural enemy importation (Greathead 1995; Mills 2000; 2005a). In addition, both Lloyd (1960a) and Gross (1991) have shown that projects against borers have been less successful than those against pests that have less of a physical refuge from parasitism. Thus, the codling moth Cydia pomonella (L.) (Lepidoptera: Tortricidae), a notorious fruit boring pest of pome fruit, walnuts, and some stone fruits, would appear to rank very low in terms of the chances for success as a target for biological control. However, as argued by Mills (2005b), it should not be neglected as a target, due to its economic importance in California and the fact that it is an invasive species in the western U.S., on an exotic crop plant, in a relatively undisturbed environment, and has a lower level of abundance in its region of origin in Central Asia. Codling moth first appeared in California in 1872 (Simpson 1903), and has since become a devastating pest of apples causing almost complete crop loss in the absence of effective management, and up to 40% loss of early-harvest pear cultivars and early-harvest walnut cultivars (Barnes 1991; Mills unpublished observations). As an invasive species it has extended its original distribution from the natural apple forests of Central Asia to cover all apple growing regions of the world, with the exception of eastern China and Japan (Mills 2005b). Here, I review the biological control project against codling moth in California that ran from 19922000, with an emphasis on the selection of parasitoids for introduction and the outcome of the parasitoid releases. SELECTING EFFECTIVE PARASITOIDS FOR INTRODUCTION Codling moth in California supports a small assemblage of indigenous parasitoids (Mills 2005b), including an egg parasitoid Trichogramma platneri Nagarkatti (Hymenoptera: Trichogrammatidae), a larval-prepupal parasitoid Macrocentrus ancylivorus Rowher (Hymenoptera: Braconidae), a cocoon parasitoid Mastrus carpocapsae (Cushman) (Hymenoptera: Ichneumonidae), and a pupal parasitoid Coccygomimus hesperus Townes (Hymenoptera: Ichneumonidae). It is also attacked by an egg-larval parasitoid Ascogaster quadridentata Wesmael (Hymenoptera: Braconidae) that was introduced into Washington State in the 1920s. In general, using corrugated cardboard bands to intercept codling moth larvae seeking cocooning sites on the trunk of orchard trees, parasitism of codling moth in California at the start of this project was low and typically less than 5% both in coastal and inland regions (Mills unpublished observations). However, egg parasitism by T. platneri frequently rose to 30-60% later in the season in unsprayed orchards, and parasitism of overwintering cocoons by M. carpocapsae was recorded to be as high at 23% in 1995 in one apple orchard on the Central Coast. In contrast, in Central Asia, codling moth supports a more diverse parasitoid assemblage (Fig. 1), including two hyperparasitoids Perilampus tristis Mayr (Hymenoptera: Perilampidae) and Dibrachys cavus (Walker) (Hymenoptera: Pteromalidae). Levels of parasitism were greater in this region (Mills 2005b), with a maximum of 33.3% recorded for Bassus rufipes (Nees) (Hymenoptera: Braconidae) and 43.9% for Mastrus ridibundus (Gravenhorst)