A comparison of entomopathogens for control of plum curculio indicated that the nematode Steinernema riobrave was most effective entomopathogen, but was only effective in sandy soils. Beauveria bassiana may be effective against plum curculio larvae in soils with less sand content, but efficacy remains variable year-to-year and may require micro-sprinkler irrigation.
Plum curculio is a key pest in Midwest tree fruit production. It is a small weevil that lays eggs in young fruit. The larvae develop inside the fruit, causing internal feeding damage, which leads to misshapen, rotten fruit. In conventional cherries, there is zero tolerance for larvae in fruit at harvest, leading to lost crops and a reduction in economic sustainability for farmers.
With conventional pesticides being phased out by the Food Quality Protection Act, newer chemistries, such as the neonicotinoids, insect growth regulators (IGR), pheromones, and sterol inhibitors are being explored as alternatives. These have shown some success in controlling plum curculio, but their use as an OP “replacement” is raising concern among growers with increased residue on finished products and questionable efficacy. The persistence of neonicotinoid and IGR residues provides low-level, long-term exposure that will likely raise potential for resistance development. In addition, these residues impact natural enemy diversity and abundance, reducing benefits from ecosystem services, a key concept in agricultural sustainability.
Biopesticides present reduced risk tactics for both conventional and organic producers. When implemented in conjunction with cultural controls targeting adult pests, the result is a sustainable pest management plan targeting multiple pest life stages.
Increasing interest in biopesticides in both the scientific and grower community is not extending into significantly higher adoption of biopesticides. Challenges including lower efficacy and environmental breakdown make utilization complex. Transitioning to biopesticides requires knowledge of biology, pest density, appropriate environmental conditions and precision in both application timing and rate. Successful completion of our outcomes would help familiarize farmers with these relationships through demonstration.
Of the biopesticides, entomopathogenic fungi (Beauveria and Metarhizium species) and entomopathogenic nematodes (Steinernema and Heterorhabditis species) are recognized for their ease of mass production for augmentative biocontrol. Once established, these fungi can propagate or hibernate, indicating the potential for a multi year impact.
Laboratory tests of both Beauveria and Metarhizium result in high plum curculio larval mortality (Tedders et al. 1982, Alston et al 2005). Infection in overwintered beetles (McGiffen and Meyer 1986, Lafleur et al. 1987) suggests that two windows of opportunity for soil biocontrol of plum curculio exist; summer generation soil life stages and adults in soil habitats surrounding orchards. A modification in formulation and application of hyphomycetes resulting in a granular product drilled into the soil has resulted in better and perhaps less expensive control of soil dwelling life stages of several insect pests (Keller 1992, Nankinga and Moore 2000, Cisneros 2000, Lewis et al. 2002, Dolci et al. 2006, Nelson et al. 2004). The unique method of entomopathogenic fungus delivery for control of the soil-dwelling life stages of weevils offers the opportunity to suppress a pest at a previously untargeted life stage, a major goal of higher level IPM programs throughout the world (Prokopy 1994).
Recently, a SARE grower demonstration trial approached pecan weevil problems with trunk Beauveria spray applications and Metarhizium formulated on fabric trunk bands. Similar adhesive bands impregnated with Beauveria are commercially registered in Japan for control of adult Asian Longhorned beetle on fruit tree trunks (Dubois, 2004). However, the cost of these fabric strips may be prohibitive and the method does not address the susceptible, previously untargeted life stages of weevils within the soil matrix.
Several years of SARE research in Maine found that foliar Beauveria applications for Colorado Potato Beetle (CPB) led to no increase in yield and cited high costs as a prohibitive factor for adoption into integrated potato management. Foliar (Wraight 2002, Poprawski 1997) and soil (Cantwell 1986, Gaugler 1989, Watt & LeBrun 1984) applications of Beauveria have been investigated with deviating, inconsistent results of population suppression.
Short-term outcomes primarily focus on developing the awareness of economic and ecological potential of entomopathogens in the organic tree fruit producing community for suppressing plum curculio populations.
Dissemination of our project results to current members of the entomopathogen industry will encourage investment in organic formulations for orchard use, resulting in ability of a core group of organic growers who have large enough farms to afford any associated risk to adopt the products. The initial adoption by a core group of organic tree fruit growers would lead to the intermediate outcome of broad adoption among all organic tree fruit growers. This would also spill over into minor adoption by conventional growers who are sustainable minded or have major problems controlling plum curculio with the cancellation of the organophosphate Guthion® (azinphosmethyl).
The experiment was conducted on two organic apple farms and three conventional tart cherry farms. Two of the conventional tart cherry farms were research stations (Clarksville Horticulture Research Station and Northwest Michigan Horticulture Research Station). The other three farms were privately owned.
The experiment was conducted ground cages by replacing undisturbed soil cores into pots sealed with a trap top. Ten plum curculio larvae collected from infested apples were placed on the soil surface, contained within ground cages within the tree dripline. Larvae were placed on the soil 10 or 5 days before, or 0, 5, 10, 15, or 20 days after entomopathogen application. There were eight cages of each entomopathogen x timing treatment combination. Entomopathogen treatments included two nematodes (S. riobrave and H. bacteriophora) at 1billion/ha or 4 billion/ha and the fungus B. bassiana at 5×1013 conidia/ha. Adults were collected and mean comparisons were conducted among treatments.
This work was a continuation of a three-year study of entomopathogens for plum curculio suppression. Steinernema riobrave was the only significantly effective entomopathogen against larvae and was significantly effective against larvae introduced 5 days before and on the date of pathogen application. This and previous research leads us to conclude that S. riobrave may only be successful in relatively sandy soils (80% sand in our study), while the fungi may be effective in heavier soils characteristic of organic farms but with irrigation. Both may perform best with micro-jet sprinklers, which can be inexpensively plugged into existing drip-irrigation tubing.
We also opened a dialogue with growers considering the use of fungicides as inhibitory to entomopathogen efficacy.
Educational & Outreach Activities
Pereault, R. (2008). Entomopathogenic fungi and nematodes for Michigan tree fruit management targeting plum curculio (Conotrachelus nenuphar). M.S. Thesis. Michigan State University, East Lansing, MI.
Pereault, R., Whalon, M.E., and Alston, D. (submitted). Entomopathogenic fungus and nematode field efficacy targeting last-instar plum curculio in Michigan cherry and apple. Environmental Entomology.
Great Lakes Fruit Vegetable & Farm Market Expo December 9-11, 2008: A poster was displayed for three days and a twenty-minute talk was delivered to growers at the Organic Tree Fruits Pest Management session.
Organic Tree Fruits Projects Field Day November 11, 2008: The work was discussed with growers attending this field day at one of the participating growers’ farms.
Entomological Society of America Annual Meeting November 17, 2008: Research findings were presented and discussed with a scientific audience.
It is anticipated that most organic growers, and less immediately IPM growers facing plum curculio damage, will find these entomopathogens more accessible, in regards to both dependability of efficacy and cost. Impact will be estimated by examining actions taken by entomopathogen producers (Novozymes, Laverlam, Becker Underwood, etc.) to develop the entomopathogen tactics into marketable organic formulations. More testing with irrigation and intensive recommendations are needed for growers to adopt these expensive alternative tools.
Cost of S. riobrave if $800/billion and 4billion/ha then $3200/ha or $1296/acre. Two applications per year means $2592/treated acre. Cost of B. bassiana if $100/qt at 4oz/1000sqft then $1089/treated acre. The total cost depends on the acreage treated. See the discussion in the “Farmer Adoption” section for ways to reduce acreage treated.
We reached 16 farmers at an on-farm field day hosted by one of the participating growers. We also reached farmers at the Great Lakes Fruit and Vegetable Expo, both with a poster and a 20-minute talk at the Organic Tree Fruit Pest Management session. Growers have access to reports through the following commodity groups: Michigan Apple Committee, Michigan Cherry Committee, and Michigan State Horticultural Society.
A “push-pull” strategy using kaolin clay and lures may effectively concentrate oviposition. We recommend that farmers use continuous kaolin clay coverage during the period of plum curculio oviposition. In addition, use Whalon modified tedders “pyramid” traps or screen traps accompanied with benzaldehyde and plum essence lures on the border rows, especially near wooded edges to monitor plum curculio populations. Monitoring should be practiced also by visual damage sampling by counting crescent-shaped plum curculio oviposition marks. The nematode S. riobrave may be applied at 4 billion per ha to the ground underneath oviposition-marked fruits during larval drop, but may only be effective in soils with 80% sand. Alternative hosts surrounding the orchard including feral apple and wild plum should be removed or treated.
A model indicating the degree-day based timings will be posted on http://www.enviroweather.msu.edu.
Areas needing additional study
Microhabitat modifications to keep soils moist to facilitate entomopathogen activity may reduce desiccation. A way to ameliorate variable moisture conditions would be to use an under-tree micro-jet sprinkler irrigation system to maintain moisture, as is used to control citrus root weevil This may be cost-effective for tree fruit growers that maintain drip irrigation lines, as the sprinklers are easily substituted into emitter holes. Organic formulations of S. riobrave and M. anisopliae remain to be tested.