Insect pathogenic nematodes were effective for controlling black vine weevil larvae in strawberry plantings. These beneficial nematodes provided less benefit against smaller root weevil species or white grubs, and did not have any effect in the field against lesion nematodes. Naturally occurring or native populations of nematodes (including a species new to science) were encountered in strawberry fields, making controlled studies difficult but providing benefit to growers. Growers should consider spring or late August application of these beneficial nematodes if there is greater than 50% of leaves with characteristic feeding notches at the end of harvest.
The goals were to develop management guidelines for application of beneficial nematodes based on research data, and to determine whether entomopathogenic nematode and/or Trichoderma harzianum applications would suppress plant-feeding nematode populations.
A. Replicated experiments
Replicated small plots of strawberry plants were established at the Valley Laboratory from plants collected from growers’ fields. Several separate experiments were conducted, including trials of Trichoderma harzianum for control of black root rot pathogens, trials of insect pathogenic nematodes for suppression of lesion nematodes, and tests of insect pathogenic nematodes for control of black vine weevil populations. The experiments were conducted as factorial designs to determine whether interactions between potential biological control agents were present. Due to the difficulties in working with insect pathogenic nematodes in small plots at the Valley Laboratory (beneficial nematodes were present in the untreated checks), the factorial design investigating species and timing of application was repeated in a grower’s field.
To identify the prevalence of lesion nematode, Rhizoctonia, and black vine weevil infestations in strawberry fields, a survey was conducted of 41 production fields. Paired samples were collected, one each from the margin of poor areas and one from adjacent relatively healthy areas (within a few rows of each other). Six paired subsamples were taken from each field, and later combined to form two composite samples (healthy vs. stunted), from which the nematodes were extracted and the presence of Rhizoctonia determined. The paired samples allowed us to assess the within-field impact of soil-dwelling pests and also the range of damage representing each field. Root weevil damage was assessed in these same fields by two methods, a random sample of 100 leaves to establish the percent of notched leaves, and a five-minute search under strawberry foliage and straw to confirm the identity of root weevil species involved and their numbers.
Growers were surveyed regarding their practices for managing lesion nematodes, Rhizoctonia, and black vine weevils. Growers participating with the field sampling were asked about the number of years these fields had been in production, estimated yield, and control practices for soilborne pests (fumigation, chemicals, rotation, etc). The pest populations were correlated with the growing practices. An additional paper survey sent to all Connecticut strawberry growers (on the extension mailing list) asked for information about growers’ perceptions of the presence and importance of soil-dwelling pests, their management practices, crop rotation, and years between rotations
Growers were encouraged to adopt the use of entomopathogenic nematodes where there were damaging black vine weevil populations. Soil samples were taken in a follow-up survey from these fields and from fields where nematodes had not been applied to compare their prevalence of insect pathogenic nematodes.
C. Economic analysis.
The economic benefit from applying beneficials was determined based on suppression of pests, models for yield improvement, application costs for nematodes, and a strawberry production accounting spreadsheet written by Cornell Cooperative Extension specialist Alison DeMarree.
Use of insect pathogenic nematodes in growers’ fields increased dramatically through the duration of this project. Prior to this work, one grower in Connecticut had applied insect pathogenic nematodes to his fields for controlling black vine weevils. In 2000, an additional four growers applied insect pathogenic nematodes in Connecticut. In 2001, the numbers of growers using nematodes in Connecticut was not monitored, but several growers in other New England states have expressed interest in or have adopted the use of beneficial nematodes in their fields.
In addition to 5 invited presentations at various growers’ meetings, we held an all-day workshop for growers on December 12, 2000, at the Valley Laboratory in Windsor, CT. Twenty-six growers attending the presentations earned six hours of pesticide applicator’s recertification credits. Subjects covered in this meeting included “Biology and Management of Root Weevils and Scarabs,” “The Economics of Root Feeding Pests,” “Biology of Insect Pathogenic Nematodes,” “Root Weevil Survey Results,” “Black Root Rot Survey Results”, “The Biology and Management of Black Root Rot”, and additional talks on disease and weed management.
The presentation for the New England Vegetable and Berry Growers’ Conference at Sturbridge, MA, on December 11, 2001, was published as a short article in their proceedings.
Impacts of Results/Outcomes
Neither insect pathogenic nematodes nor Trichoderma harzianum demonstrated any benefit in the field for suppression of root diseases. Management of lesion nematodes and Rhizoctonia fragariae, two components that can lead to black root rot, remains one of the great needs of strawberry growers.
The experiments conducted in this work unequivocally demonstrated that insect pathogenic nematodes can be an important component of the soil ecology in strawberry fields, and have special value in suppressing populations of black vine weevils. In one participating grower’s field, the population of weevils was reduced from approximately 15 per square foot (an extremely damaging population) to undetectable levels through the combined use of an effective foliar insecticide and application of insect pathogenic nematodes. At another farm black vine weevils used to be such a serious pest that plantings had to be plowed under after only one year of production. Due to the use of insect pathogenic nematodes, black vine weevils are now only a minor pest at this farm. Partly as a result of this work and our outreach efforts, strawberry growers throughout New England are now willing to use insect pathogenic nematodes for managing black vine weevil populations, and the reliance on foliar insecticide sprays has been reduced.
Areas needing additional study
We have not found satisfactory methods to manage lesion nematodes and Rhizoctonia fragariae, which lead to black root rot. We will continue our investigations to determine whether other inoculants might mitigate these pest problems. From the survey results, we know that when strawberry plants have an opportunity to quickly establish a healthy root system, they can remain productive for much longer than the typical 3-year rotation.
Manipulation of the crop environment should also be studied to enhance the effective biological control of soil dwelling insect pests with predatory ground beetles. The use of insect pathogenic nematodes with greater virulence to white grubs could reduce the need for imidacloprid, and could simultaneously control white grubs and root weevils.