Parasitic Nematodes for Biological Control of Soil Insects
in Nursery Crops

Ralph Berry

Several species of root weevils cause serious economic loss in container-grown nursery crops, particularly azalea and rhododendron. This species complex includes black vine weevil and strawberry root weevil. Worldwide, the black vine weevil is the major root feeding pest on nursery and greenhouse plants and small fruit and over 140 plants have been identified as hosts of this species. Larvae of both species feed directly on the roots and plant crowns causing severe stunting and often death of plants. Adults feed on the foliage causing unsightly "notching" on the leaves thereby reducing plant value or making the plants unmarketable. In general, the larvae overwinter in the roots and often feed throughout the winter causing general decline of the plants. Adults emerge April through July and deposit eggs on the soil surface near the host plant. Eggs hatch into larvae that burrow in the soil and begin feeding on roots during late summer and fall and again in the early spring.

Entomopathogenic nematodes are obligate parasites of insects and the third-stage infective juveniles (IJs) are attracted to their hosts. The infective juveniles are capable of prolonged survival without feeding. On encountering an insect host they enter its mouth, anus or spiracles and penetrate the gut or tracheal walls. Once inside the insect host, the infective juveniles release symbiotic bacteria into the insect body and begin development. Growth of the bacteria in the insect is accompanied by the production of exo- and endotoxins that cause the death of the insect within a few days. The nematodes feed on the bacteria and insect tissues and complete their development in about 7 days depending on the temperature. The nematodes reproduce within the cadaver and infective juveniles escape into the soil to seek additional insect hosts. Entomopathogenic nematodes are safe to vertebrates, plants, and other non-target organisms, and are exempt from registration with the Environmental Protection Agency (EPA). They are easily applied using standard spray equipment and irrigation systems, and are compatible with many chemical pesticides. This combination of attributes has generated an intense interest in the development of these nematodes for biological control of several insect pests.

During a survey in Oregon in 1993, we isolated and described a new species of entomopathogenic nematode from the Oregon coast, Heterorhabditis marelatus (Liu and Berry 1996). Studies in the field and laboratory have shown that this new species is active at low soil temperatures (<14oC) which typically occur in the northwest during the early spring. We also have demonstrated in field and laboratory experiments that this new species controls the root weevil complex in strawberries and has high potential to control root weevils in container-grown azalea.

The results of this study indicate that H. marelatus provided excellent control of root weevil larvae at rates of 25, and 50 infective juveniles (IJs)/cm2 (equivalent to 4,500 and 9,000 IJs/6 inch ID pot) compared with the untreated controls. In fact, no live root weevils were found in any of the pots treated with H. marelatus. We found that H. marelatus persisted in the containers for 21 days, which is ample time for the nematodes to infect root weevil larvae.

Results of this research indicate that H. marelatus has high potential to provide excellent biological control of root weevils in container grown nursery crops. Even though there are no direct estimates of economic loss in nursery crops from root weevils, the Oregon nursery industries have ranked root weevil control as a high priority for several years. Damage caused by larvae feeding on plant roots causes severely stunted and weakened plants. Damaged or weak appearing plants cannot be sold and are usually discarded or held off the market until they can be restored to health. However, in most instances, plants damaged by root weevil larvae cannot be sold and are discarded. Also, consumers are reluctant to purchase damaged or weakened plants that have signs of adult weevil injury on the leaves and these plants also are unmarketable. Losses also are substantial when nursery plants infested with weevils cannot be shipped to domestic or foreign markets. The use of this new beneficial nematode species to control root weevils will significantly reduce the economic losses caused by these pests. Their use also will reduce the use of insecticides, which will help lessen some of the negative impacts attributed to the use insecticides, e.g., chemical residues, worker safety, and contamination of run-off water.

Ralph Berry
Department of Entomology
4012 Cordley Hall,
Oregon State University
Corvallis, OR 97331

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