Biological Control for Soil-Dwelling Insects - Diseases in Strawberries
Lesion nematodes, root rot fungi, and root weevils can destroy strawberry fields in a perennial cropping system. Tests of possible biological control organisms—entomopathogenic nematodes (EPNs) and Trichoderma harzianum (Rootshield)—found no suppression of lesion nematodes or root rot organisms in strawberries. Increased mortality of black vine weevil following application of EPNs was not detected in other tests. We attribute the low populations of root weevils in all plots, including untreated checks, to preexisting native or naturalized EPNs. Growers applying commercially produced EPNs have obtained excellent control of root weevils and likely have permanently established beneficial nematodes in their fields.
To determine whether EPN applications will suppress lesion nematode populations.
To determine the optimum application timing, rate, and species of EPN to reduce black vine weevil populations.
To determine whether Trichoderma harzianum will protect strawberry plants from black root rot caused by lesion nematodes and Rhizoctonia fragariae, and whether T. harzianum will interact with EPNs in preventing damage from black vine weevils, lesion nematodes, and R. fragariae.
Investigations on the ability of biological control organisms—EPNs or Trichoderma harzianum—to suppress lesion nematodes, root rot fungi, and black vine weevil followed two strategies. In the research trials, small plots were established at the Valley Lab or within a grower’s field, either with infested plants from a grower, or in an infested field. Factorial designs were used to determine the interactions between species or quantity of EPN applied and application timing, or between EPNs and T. harzianum, in their effects on pest populations. In surveys of growers’ fields, we evaluated their management program’s impact (including application of EPNs) on the presence and population levels of beneficial EPNs, lesion nematodes, root rot organisms, and black vine weevils.
Populations of plant parasitic nematodes (Pratylenchus and Meloidogyne spp.) were measured through standardized extraction and counting procedures. To extract nematodes, roots were washed free of soil, 2 g of which were placed in water on a shaker for one week. The resulting nematodes were then recovered through sugar centrifugation and counted. R. fragariae infection were quantified by direct isolation of the pathogen from washed root tissue on acidified water agar. Hyphal tips were then transferred to potato dextrose agar for identification. A line intersect method was used to evaluate strawberry root growth and percent root rot. Black vine weevil populations were determined by directly counting the number of larvae present in soil from individual plant samples or from a defined surface area.
Three years of evaluating EPNs and Trichoderma harzianum in small plots and in microplots found no suppression of lesion nematodes or root rot organisms in strawberries. Laboratory and greenhouse studies with Steinernema carpocapsae and S. feltiae determined that these nematodes or their byproducts temporarily immobilized free-living lesion nematodes, but this pest nematode became active again after 96 hours and was still able to infect plants.
Increased mortality of black vine weevil (relative to untreated checks) following application of EPNs was not detected in field tests. We attribute the low populations of root weevils in all plots, including untreated checks, to the presence of preexisting native or naturalized EPNs. Growers applying commercially produced EPNs have obtained excellent control of root weevils and likely have permanently established beneficial nematodes in their fields. Field surveys detected EPNs in six of eight fields, or 75 percent, where they had previously been applied; they persisted in one field for a year and a half after application. EPNs were detected in eight (17 percent) of an additional 47 fields where they had never been applied.
Impacts and Potential contributions
Based on information generated by this project, growers in Connecticut decreased their reliance on foliar sprays, and adopted insect pathogenic nematode application as the cornerstone for protecting strawberry fields from root weevils. In the previous two years, one grower had applied insect pathogenic nematodes to fields where black vine weevils were a problem. In 2000, five strawberry growers purchased and applied nematodes to treat fields affected by black vine weevils. We anticipate much broader acceptance of this approach following our educational meeting on December 12, 2000, which will be attended by growers from Connecticut, Massachusetts, New Hampshire, and Rhode Island
Reported November 2000