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Effect of cover crop on apple leafroller populations, leafroller parasitism and selected arthropods in an orchard managed without insecticides Mullinix, Mark Kent


In Washington State the Pandemis leafroller, Pandemis pyrusana (Kerefoll) and obliquebanded leafroller, Choristoneura rosaceana (Harris) are serious pests in orchards in which mating disruption is used to control codling moth, Cydia pomenella. Resistance to organophosphate insecticides exacerbates the problem. Additionally, many question the dependence on insecticides and call for ecologically-based pest management. Parasitoid and predator conservation and augmentation via insecticide reduction and habitat manipulation are tactics proposed to achieve biological control. The objectives of these studies were to: 1) evaluate and compare development of leafroller populations and their biological control by parasitoids in an apple orchard with either a grass or alfalfa cover and in which no insecticides were used; 2) evaluate the influence of cover crops on the general orchard arthropod population in an orchard managed without insecticides; 3) evaluate the use of alfalfa as an orchard cover crop on fruit tree growth and development; and 4) conduct initial testing of strategies for augmenting the leafroller parasitoid Colpoclypeous florus. Experiments were conducted in an East Wenatchee, Washington apple orchard, over four years. Plots were approximately 0.5 ha in size and were sown to either grass cover or alfalfa. Insecticide applications were eliminated. In the absence of broad-spectrum insecticides leafroller populations initially rose to high levels and then dramatically declined. A granulovirus may have been primarily responsible for the decline. Leafroller parasitoids also contributed to leafroller biological control though not extensively. There were no differences in leafroller populations between ground cover treatments. In some instances parasitism was slightly greater in the alfalfa cover plots but this did not seem of any practical significance. Six species were identified in the parasitoid complex. No secondary arthropod pest achieved pest status in either ground cover during the study. Codling moth however became a serious problem in year four. Attempts to augment C. floras populations by seeding parasitized Ancylis comptana fragaria failed. Attempts to establish Xenotemna pallorana on the alfalfa cover to serve as an alternate host for C. floras were likewise unsuccessful. Alfalfa as a cover crop imposed no adverse effects on tree growth and development during the duration of these studies.

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