Entomopathogenic nematodes for control of codling moth (Lepidoptera: Tortricidae) in apple and pear orchards: Effect of nematode species and seasonal temperatures, adjuvants, application equipment, and post-application irrigation
Introduction
Codling moth (CM), Cydia pomonella (L.), is a global pest of apple, pear, and walnut (Barnes, 1991), and the principal pest of apple in the Pacific Northwest of the United States (Beers et al., 1993). In late spring, adults emerge and begin laying eggs when fruit is available. Neonate larvae enter the fruit and feed until full grown, then exit as fifth instars in search of cryptic habitats in which to spin their cocoons and pupate. In the Pacific Northwest there may be two to three generations per growing season depending upon weather (Beers et al., 1993).
A variety of broad spectrum insecticides are employed for CM control in conventional orchards during the growing season (Beers et al., 1993). Softer approaches that include the use of mating disruption (Calkins ans Faust, 2003) and the CM granulovirus (Arthurs and Lacey, 2004) are also employed. In temperate climates, CM overwinters in cryptic habitats as cocooned diapausing larvae. Thus, overwintering larvae represent the entire CM population in the fall, winter, and early spring. Their elimination or significant reduction at this stage would provide complete or substantial protection to fruit early in the following growing season. However, few interventions are employed for control of overwintering larvae. Cryptic habitats, such as those used by CM for their overwintering sites (under loose bark, in litter at the base of trees, in nearby woodpiles, fruit bins and the like) may also provide favorable environmental conditions for entomopathogenic nematodes (EPNs) (Begley, 1990, Koppenhöfer, 2000). Used under optimal conditions of warm temperatures and available free water, EPNs can be effective control agents of cocooned CM larvae in orchards (Kaya et al., 1984, Lacey et al., 2000, Nachtigall and Dickler, 1992, Sledzevskaya, 1987, Unruh and Lacey, 2001) and fruit bins (Cossentine et al., 2002, Lacey and Chauvin, 1999, Lacey et al., 2005). Studies by Kaya et al., 1984, Unruh and Lacey, 2001 elucidated the importance of moisture for control of CM by Steinernema carpocapsae in California and Washington, respectively. The habitat of overwintering CM and environmental conditions may vary from orchard to orchard and within orchards depending on the season and other factors. Further improvements of application methods and how irrigation systems and adjuvants can be used to enhance persistence and activity of EPNs will be necessary to provide growers with practical control options. Other factors such as the choice of nematode species that could influence the success or failure of EPNs in orchards have yet to be studied in detail. It was the objective of our research to investigate the effects of EPN species and seasonal temperature, addition of adjuvants, application method, and type of post-application irrigation on the efficacy of EPNs for control of cocooned CM larvae. Field trials were conducted over a 4-year period in orchards in eastern Washington State to elucidate the effect of these factors.
Section snippets
Materials and methods
Table 1 summarizes experimental information for studies conducted from September 1999 through October 2003 including application rates, type of orchard, EPN strains, and factors addressed in each experiment. Two procedures that are common in the experiments reported below are method of assessment of mortality using sentinel CM larvae and quality control bioassays of the EPNs tested.
Experiment 1. Nematode species and seasonal temperature
The efficacy of nematode applications (106 IJs/tree applied with a back pack sprayer) varied between species on two of the four treatment dates (P < 0.0001 in each case) (Fig. 1). In all cases control mortality remained ⩽5.3%. Late summer applications (September) were the most effective, resulting in 94.4% (S. carpocapsae) and 94.7% (S. feltiae) mortality in CM larvae (Fig. 1B). S. feltiae was also effective in the mid-October application, although there was a significant reduction of control for
Discussion
The main obstacles for successful CM control with EPNs are low temperatures and desiccation of IJs before they have penetrated the host’s cocoon. Applications that are made too late in the fall or too early in the spring when prevailing temperatures remain below the threshold of activity of the EPN species will be ineffective. CM pupae are also less susceptible to infection by EPNs than CM larvae (Lacey et al., 2005) emphasizing the need to target spring applications before larvae pupate. The
Conclusions
EPNs can provide effective control of overwintering CM when temperatures are 10–15 °C and higher (depending on nematode species) and moisture is maintained for several hours post-application. The use of a cold-active species, such as S. feltiae, will provide more flexibility on when orchards can be treated relative to less cold-active species such as S. carpocapsae. In our studies, moisture was adequately maintained by wetting trees before and after application of IJs using existing irrigation
Acknowledgments
We thank Jeff Upton, Ivan Campos, Richard Chauvin, Leon Ganuelos, Martha Marquez, and Laura Willett for technical support. We are grateful to our grower cooperators, Ron Wilcox in Wapato, Wade Smith in Quincy and Bill Pershall and Jim Nelson in Monitor and to Don Hostetter and Lisa Neven for review of the manuscript and their constructive comments and suggestions. We also thank Gerry Gefre and John Harvey for their help with field trials at the USDA-ARS-YARL experimental farm. We are grateful
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