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Funded Project

Funding Program: Regional IPM Competitive Grants - Northeastern

Project Title: Economic Decision-Making and Biological Management of European Corn Borer in Potatoes

Project Directors (PDs): Ellie Groden [1] James Dill [2] James D. Dwyer [3]

Lead State: ME Lead Organization: University of Maine

Undesignated Funding: $52,997

Start Date: Sep-01-1999 End Date: Aug-31-2001

Pests Involved: European corn borer

Site/Commodity: potatoes

Area of Emphasis: biocontrol, biological control

Summary: Potato is the most important horticultural crop in the Northeast. Heavy use of insecticides against Colorado potato beetle has been the standard practice and has also suppressed a secondary pest, the European corn borer. This changed abruptly in 1995 when imidacloprid was registered and rapidly replaced all other means of chemical control. Because this insecticide is ineffective against corn borer, economic problems with this pest have drastically increased in recent years, and wide-spread outbreaks occurred in the southern portion of Aroostook County, ME, in 1998. This pest is likely to continue to be a problem with the introduction of more specific management tactics for the primary pests, such as Bt transgenic potatoes. Growers have adopted a conservative insecticide program for managing corn borer, with little or no knowledge of the economic impact of this pest. The systemic action of soil-applied imidacloprid has increased the abundance of natural enemies of aphids and other secondary pests in potatoes, but the use of broad-spectrum foliars for control of corn borers negates this advantage. Because of these concerns, this pest/crop system was identified as a priority for IPM research in four Northeast states in 1995, and at least two additional states since then. This proposal addresses the following three objectives, findings from which will enable us to implement sound pest management programs for corn borer in the Northeast, and provide a biologically-based alternative for potato growers. 1. Determine the influence of timing of European corn borer infestion and varietal response of potatoes to damage and its effects on tuber yield. . The impact of corn borer damage on potato yield is unknown for the Northeast. We will conduct experiments using insecticide-manipulated and manually infested plots in Maine and Virginia to determine the damage/yield loss relationships and varietal susceptibility to corn borer injury. These relationships will enable us to compute a range of treatment thresholds that account for the variable yield and market prices experienced by Northeast potato growers. Such knowledge will result in more effective insecticide use and enhanced natural enemies populations. 2. Evaluate two types of pheromone traps to determine the most effective for predicting European corn borer incidence in commercial potatoes. Pheromone traps have been widely used in sweet corn IPM programs to detect corn borer flight activity as an indicator of when to initiate more time-consuming damage sampling in the crop. However, there are some conflicting reports on which design of pheromone trap is the most effective for monitoring corn borer in potatoes. We will evaluate two traps currently used for corn borer in the Northeast, to determine which is the better predictor of oviposition activity and larval damage in potatoes. 3. Evaluate the potential to manage European corn borers in potatoes with the fungal pathogen, Beauveria bassiana. B. bassiana is a natural pathogen of the corn borer which has been shown experimentally to suppress pest populations in corn. Given the feeding behavior of corn borer in potatoes and the favorable microenvironment for infection in this crop canopy, B. bassiana may be more effective at suppressing borer populations in potatoes. Furthermore, sublethal dosages of imidacloprid have been shown to synergize the pathogenicity of insect pathogenic fungi, including Beauveria bassiana. As imidacloprid is currently the most widely used insecticide in potatoes in the NE and it has been shown to have only sublethal effects on corn borers, there is potential for enhanced activity with this biological material. Objectives: 1. Determine the influence of timing of European corn borer infestion and varietal response of potatoes to damage and its effects on tuber yield. 2. Evaluate two types of pheromone traps to determine the most effective for predicting European corn borer incidence in commercial potatoes. 3. Evaluate the potential to manage European corn borers in potatoes with the fungal pathogen, Beauveria bassiana. Problem, Background and Justification Potato is the most important horticultural crop in the Northeast region with over 125,000 acres grown in 1997 with a gross value of over $230 million. Insect management on this crop has historically involved multiple applications of insecticides throughout the growing season to control the major pest, the Colorado potato beetle. Because of the heavy reliance on chemical control, the potato beetle has developed resistance to most classes of insecticides used in the Northeast (Forgash 1985, Roush et al. 1990). In the process of controlling potato beetles, foliar treatments have also suppressed the European corn borer (Ostrinia nubilalis Hubner), potato leafhopper, and aphids - secondary pests that have only occasionally been encountered. This scenario changed abruptly in 1995 when a new insecticide, imidacloprid (Admire® 2F), was registered and rapidly replaced all other means of chemical control. Imidacloprid is extremely effective against Colorado potato beetle as well as aphids but has virtually no activity against the European corn borer. In addition, genetically modified potatoes (NewLeaf) expressing the Bacillus thuringiensis tenebrionis endotoxin are currently used on commercial potato farms. This technology eliminates insecticide treatments for Colorado potato beetle but is totally ineffective against European corn borer, leafhoppers, and aphids. During the past three years, economic problems with European corn borer and leafhopper have drastically increased as the foliar insecticide use for Colorado potato beetle control declined with the adoption of imidacloprid and NewLeaf potatoes. Four Northeast states (MD, DE, NY, and NJ) reported European corn borers on potatoes as a high priority for IPM research on the 1995 statewide IPM needs assessment survey (VanKirk 1998). Since this time, European corn borer outbreaks have occurred in several other states including Massachusetts and Maine. In 1998, an outbreak in southern Arroostook County, ME, resulted in an average of 46% of the stalks in fields within the region infested. Because of the unpredictable risks associated with corn borer damage, growers have adopted a conservative program of applying broad spectrum insecticides at the first sign of moth activity. Timing of insecticide control relative to peak larval hatch is critical to achieve economic control. In 1995, many potato fields in MD experienced high levels of stem damage despite insecticide control because treatments were applied either too early or too late (Dively, personal communication). Furthermore, much of the potato acreage in the Northeast received insecticides in 1997 aimed at the European corn borer, yet infestations were considered below economic levels in many fields. Unnecessary and poorly-timed foliar treatments can be costly and also disruptive to natural enemies which have a major impact on insect pest populations. For the development of improved IPM programs on potato, the 1995 statewide needs assessment survey identified improved monitoring, establishment of economic thresholds, and biological management, as priority needs for research on this pest in potatoes (VanKirk 1998). European corn borer damages potato by tunneling in the stems. This causes the foliage to wilt and predisposes stems to breakage by wind and irrigation. Also, entrance holes made by older instars in the lower main stem provide infection courts for bacterial soft rot (Anderson et al 1981). Research in North Carolina has consistently shown that even high corn borer infestations have no effect on the yield of tubers in varieties Superior, Pungo, and Atlantic (Nault and Kennedy 1996, Nault 1994, Kennedy 1983). In 1995, yield losses of paired corn borer damaged and undamaged plants were assessed in Maryland potato fields and revealed variable reductions in tuber yields, suggesting that the economic impact of this pest may be influenced by variety and crop-pest phenology (Dively pers. comm.). Recent studies conducted by Nault in Virginia support this hypothesis that the timing of corn borer infestations influences the degree of crop loss (Nault pers. comm.). In North Carolina, European corn borer infestations occur near or during bloom, but severe injury is not observed until post-bloom during late May and early June when large larvae begin to tunnel. The lack of a yield loss due to corn borer damage in North Carolina may be related to the time at which borers injure the crop. The impact of corn borer damage on yield may be quite different in the Northeast, where corn borer infestations occur earlier in the development of tubers. Currently, no information exists on this relationship in areas outside of North Carolina, except one report from Prince Edward Island (Stewart 1992), where corn borer injury caused significant yield loss. Thus, the damage/yield loss relationships and varietal susceptibility to European corn borer injury in different geographic areas of the Northeast need to be explored. Such knowledge will lead to the development of realistic treatment thresholds and result in more effective management, and enhanced natural control. Monitoring programs for European corn borer are widely developed for sweet corn in the Northeast. As European corn borer egg masses and larvae are cryptic on the potato plant, the most effective monitoring utilize a combination of pheromone or light-trapping for adult moth activity and crop sampling for feeding damage (Handley and Dill 1996). Although pheromone traps have been inconsistant for predicting European corn borer damage in corn (Palaniswamy et al. 1990, Yu and Byers 1994, Ngollo et al. 1998), they have been effective for monitoring flight activity (Bartels et al. 1997) which is used to initiate degree-day models for egg hatch (Kelher et al. 1990). Ngollo et al. (1998) reported a strong correlation between moth capture with Scentry Heliothus® pheromone traps and cumulative European corn borer egg densities in sweet corn in Maine. Stewart (1994), however, found these traps ineffective in potatoes, and recommends the use of pheromone-baited water-pan traps for monitoring corn borer flight activity in potatoes. Blacklight traps, although extremely effective (Kennedy and Anderson 1980) for monitoring corn borer activity, are unsuitable for growers and costly for many pest management programs because of their lack of specificity and therefore, the resulting effort involved in sorting trap catch (Palaniswamy et al. 1990, Derrick et al. 1992). For effective timing of management tactics targeted against European corn borer larvae in potatoes, it is essential to determine an effective means of monitoring moth flight activity and its relationship to the timing of oviposition and larval development in this crop in the Northeast. The fungal pathogen, Beauveria bassiana, is one of the most commonly encountered pathogens of European corn borer and has been reported to cause natural epizootics in corn borer populations in corn (Lewis and Cossentaine 1986). This pathogen has also long been used experimentally to suppress populations in corn (Bartlett and Lefebvre 1934, Stirrett et al. 1937, Beall et al. 1939, York 1958, Riba 1984, Marcandier and Riba 1986 Lewis and Cossentaine 1986, Feng et al. 1988, Lewis and Bing 1991). We have recovered naturally occurring isolates of B. bassiana from both potato soils in Maine, Rhode Island, and Delaware (Groden, unpublished data), and in Colorado potato beetle populations in Maine. Foliar applications of B. bassiana conidia have also been an important component of a successful, experimental biocontrol program for potato beetles in northern Maine (Drummond and Groden 1996). Commercial formulations of B. bassiana are currently registered for potatoes (Mycotrol®, Mycotech Inc., Butte, MT), therefore, the opportunity exists to supplement natural populations of this pathogen for biological management of European corn borer. The efficacy of foliar applications of B. bassiana for management of corn borer will likely be favored on potatoes in comparison to corn. Corn borer egg masses are normally laid on the underside of the potato leaves close to the midrib. After hatching, neonates feed superficially on leaves and eventually bore into the petioles and smaller lateral stems of the plant. However, young larvae do not complete development in their first gallery. A larva normally moves further down on the main stem, producing 3 to 5 galleries with separate entrance holes during its development (Nault and Kennedy 1996b). The movement in and out of stems provide greater opportunity for contact with B. bassiana conidia applied to the foliage. Also, corn borer larvae infest potatoes in the mid season in Maine. Full canopy closure during this time creates a microenvironment which favors infection of this pathogen. In addition, the most widely used insecticide for management of the Colorado potato beetle in the Northeast is currently imidicloprid. Sublethal dosages of imidacloprid have been shown to synergize the pathogenicity of insect pathogenic fungi, including B. bassiana (Zeck et al. 1992, Boucias et al. 1996, Quintela and McCoy 1997, and Quintela 1996). We have found similar evidence for imidacloprid induced synergy of B. bassiana infection of Colorado potato larvae (Figure 1). Figure 1. Mortality of second instar Colorado potato beetle larvae exposed to sublethal doses of Beauveria bassiana (64conidia/insect), imidacloprid (0.0024 ug/cm2 of leaf surface), or a combination of both. As researchers have reported imidacloprid-induced stimulation of pathogen spore germination (Quintela et al. 1997), as well as, behavioral modifications which enhance infection (Quintela 1996, Boucias et al. 1996), it is likely that there are multiple mechanisms for synergy between this material and fungal pathogens. Sublethal effects of imidacloprid have been observed on European corn borer larvae (Dively, pers. comm.). Neonates feeding on diet-incorporated imidacloprid survived at concentrations up to 12 ppm (more than 20-fold higher than the LC50 of the Colorado potato beetle) but were slower to develop, gained less weight, and exhibited a greater tendency to disperse (Table 1). Given this, we hypothesize that synergistic interactions will be observed between imidacloprid and B. bassiana. Table 1. Lethal and sublethal effects on the survivorship and weight gain of neonate European corn borers after feeding for 10 days on artificial diet incorporated with imidacloprid. 1997 (Dively, unpublished data). Imidacloprid concentration (ppm)...Percent survival (mean +/- SEM)....Weight gain (mg) per larva (mean per SEM) 0....64.4 +/- 10.1....7.8 +/- 1.7 1.2....70.6 +/- 12.1....4.9 +/- 1.3 12....84.4 +/- 10.4....5.7 +/- 1.3 120....30.6 +/- 11.8....0.2 +/-0.1 1200....0.0.... ---- Means of 8 replicates - each consisting of four cohorts of five neonates each (total of 160 individuals tested per concentration). Imidacloprid and transgenic potato are currently providing very effective and relatively safe means of controlling Colorado potato beetle populations. As previously mentioned, their rapid adoption by potato growers in the region has brought about new pest problems but could also lead to enhanced biological control of these secondary pests. The objectives proposed for this project will address the impact of European corn borer on potato yield and assess the potential to manage this pest with foliar applications of B. bassiana. Findings from this research have the potential to lead us to development of IPM strategies that minimize foliar applications of conventional insecticides to imidacloprid-treated and transgenic potato fields. Such actions can have a spiraling effect because reductions in insecticide use will aid in the build-up of natural enemies, decrease selective pressures for resistance development, and reduce the overall insecticide load on the environment. Fewer applications will also improve farm worker safety and significantly reduce the risk of groundwater contamination -- an historic problem with pesticides used for Colorado potato beetle control. Outcomes and Impacts Summary from 2001 IPM Center report For years, growers controlled Colorado potato beetle (CPB) using insecticides that also suppressed a secondary potato pest, the European corn borer (ECB). But this approach changed abruptly in 1995 with the introduction of imidacloprid, which specifically targets CPB but is ineffective against corn borer. Since then, ECB infestation levels have reached 100 percent in some areas. Recently, growers in affected areas have applied one to three sprays each season for ECB control. This pest has been a priority for IPM research in the Northeast because we know so little about the damage it causes in potatoes or how to manage it. Eleanor Groden and colleagues are studying ECB's impact on potato yields and have found that low to moderate levels of infestation do not cause measurable crop loss. Their findings tell us that growers may be able to reduce pesticide applications by 75 percent without incurring economic damage. The savings in pesticide sprays represent a cost savings for farmers and will also diminish the risk of pests developing resistance to insecticides, protect beneficial insects that control plant pests, and reduce the overall insecticide load in the environment.

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