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Corn Earworm Research Update, and Midwest Pest Alert

Insect, Pest Fact Sheets

Corn Earworm Research Update, and Midwest Pest Alert

Bill Hutchison, Eric Burkness, Dept. of Entomology, University of Minnesota, St. Paul

(photo courtesy of USDA-ARS)

Thanks to many new cooperators throughout the Midwest Region, we have a much better idea of the status and extent of the current Corn Earworm (CEW) moth flight and prospects for management options this year. The impetus for this work has been an increasing concern for reduced efficacy of the synthetic pyrethroids to control the pest in sweet corn and other Midwest Vegetable crops (see below). Historically, the pyrethroid insecticides, in the absence of resistance, have been the most cost-effective treatment option for most growers and processing companies for the past 15 years. However, due to recent concerns about resistance (2003-2005), several industry, research and extension staff met in December 2005 and March 2006 to develop new cooperative monitoring and research objectives to address this concern. Subsequent to these discussions, proposals were submitted to several agencies. We very much appreciate the funding provided to date, including the following groups:

• NC IPM Center, Michigan State University and the University of Illinois
• Rapid Agricultural Response Fund, Univ. of Minn. Agric. Expt. Station
• Midwest Food Processors Assoc., Madison, WI
• Minn. Dept. of Agric., IPM Program, St. Paul, MN

In addition to these sources, many industry representatives and research/extension staff are contributing significant time and in-kind support. Because CEW is a highly mobile pest, and is known for long-distance migratory flights from the southern states (and likely Mexico), to the Midwest and Northeast (and Ontario), we have increased our communication and collaboration with research and extension staff in the southern U.S. We also plan to seek additional funding to address CEW research needs over this broader geographic range.  

CEW Moth Flight Status: Although consistently high trap catches have not yet been reported for all traps, there are several new “hot spots” that warrant concern, and indicate that insecticide treatments are now, or will soon be, necessary for newly silking sweet corn, and early fruiting/pod setting stages for other high-value crops (snap beans, tomatoes, peppers). We still anticipate the major CEW flight to be most intense, for southern Minnesota, beginning August 15^th-22^nd. This past week in Minnesota we have had Hartstack traps (wire-mesh) that exceeded 10/night (Le Sueur; T. Rabaey), and 20/night (Rosemount). In Iowa, the Polk Co. site (D. Mollenbeck) found trap catch increased to 25/night on 7/31 to over 100/night on 8/3. The Madison Co. IL site (east of St. Louis; R. Weinzierl) reported an increase to 50/night as of 7/27. These counts are all above our current “action” threshold of 4/night/trap (Hartstack trap) or 2/night (Scentry; nylon mesh trap), for average catch for a 2-week period, starting at first-silk. Based on recent research in Minnesota (2005, 2006), we are now using these thresholds for the two trap designs, as Scentry traps nearly always catch fewer moths on a given night. During the next few weeks, CEW moth flights in Minnesota, including historical comparisons, can be viewed at: http://www.vegedge.umn.edu/2006/moth.htm.

To view moth flight data throughout the Midwest Region, Ontario and subsequently, selected southern states, updates are available via a new Iowa State site (click on “Trap Summary”) at: http://www.ent.iastate.edu/trap/cornearworm/.

Concerns about Pyrethroid Resistance: We have two sources of data that have prompted our concern about increasing or variable levels of resistance to the pyrethroid insecticides. These include a) efficacy results from traditional small-plot trials (e.g., 2- or 4-row plots, treated by ground application equipment; replicated 3-4 times), and b) follow-up results with Adult Vial Test (AVT) assays, where adults are placed singly in small vials treated with 5 or 10 ug of cypermerthrin (an older pyrethroid, but used for long-term resistance monitoring comparisons). One example of the trend in more variable or reduced efficacy of two commonly used pyrethroid insecticides, are shown below from our small-plot trials conducted annually in Minnesota (Rosemount, Dakota Co.). These results also reflect the efficacy on final late-instar (> one-inch size) larvae. Results with all instars combined typically result in lower efficacy results; however, it is primarily the late-instars that cause most of damage and are the primary concern as a contaminant in processing sweet corn or green beans.

To illustrate the variability in pyrethroid efficacy against CEW, at multiple Midwest sites, we have included a summary of the 2005 data provided by several colleagues (Table 2). These data were also collected from late-planted sweet corn plots, using similar protocols. Data were provided by Rick Weinzierl (IL), Rick Foster (IN), Bryan Jensen (WI), and Tom Rabaey (Green Giant/General Mills; Le Sueur, MN ). Although our MN trial indicated reasonably good control, efficacy was much lower or more variable in other states, particularly under “high pressure” situations (see UTCs).

One caveat about small-plot data, is that such treatments primarily reflect the impact of the insecticide on eggs and larvae. Pyrethroids are known to have some activity on eggs (ovicidal) as well as larvicide efficacy, on several moth species. In large commercial fields, treated by aerial application, there is an additional benefit of direct mortality to adult moths in the field, as well as eggs and larvae. Thus, the results in Table 1 should be viewed as one “resistance monitoring tool,” giving us an indication of what can happen in larger fields, but not a direct connection to the efficacy we expect in large fields. Fortunately, to date, we have not had that many reports of crop failures in commercial fields, and the added effect of adult control may be one explanation.

Having said that, CEW Adult Vial Test (AVT) assays that have been conducted in both Southern states (since 1985) and Midwestern states (since 2003), thanks to Dr. Roger Leonard (LSU), tend to confirm the trends we have observed in the small plot trials (Tables 1-2). For example in Louisiana, Dr. Leonard found that, using the standard rate of 5ug cypermethrin/vial, CEW Survival has increased from about 2% in 1988-1990, to 30-50% from 2002-2006 (see also Table 4). For data collected in MN, WI, IN, IL and Ontario, since 2003, we have recorded survival estimates of 20-60%; we have also found variable but high levels of survival at the 10ug rate. The Midwest and Ontario data were based on late-season larval collections from each state, then follow-up adult assays at Dr. Leonard's lab. These data, as well as small plot data have been provided by excellent cooperation from: Rick Weinzierl (IL), Rick Foster (IN), Bryan Jensen (WI), Tom Rabaey (General Mills, MN) and Brian Flood ( Del Monte, IL ). More complete reports will be available later this year.

2006 Pyrethroid Resistance Updates from Potential “Source” Populations: As of July 31, we now have within-season resistance monitoring results from Texas (provided by Dr. Patricia Pietrantonio and Roy Parker) as well as Louisiana (Dr. Roger Leonard and students). Each study used the pyrethroid cypermethrin and two different resistance monitoring methods, but the trends are similar. In Texas , a multi-dose Adult Vial Test (AVT) was used to estimate the LC-50 and LC-90, the concentrations necessary to kill 50 and 90% of a given CEW population, respectively. These values are compared to a known susceptible population, to obtain a Resistance Ratio (RR). If the RR=1, the test population is equally susceptible to the known susceptible strain. For the past 3 months of 2006, for CEW collected in Nueces Co. (Corpus Christie; southeast, TX), these CEW show consistent, and fairly high levels of resistance.

In Louisiana, a single-dose AVT was used for two of the most common concentrations, of 5 and 10 ug of cypermethrin/vial. These results show a general increase in survival by CEW moths as the season progressed, and lower but consistent survival at the 10 ug level. To keep these results in perspective, when the AVT assay was first developed and used in the 1980's, the 10ug dose was considered discriminatory for resistance; i.e., there should be zero survival (100% mortality) at this level.

In brief, another way to interpret the data from both states, is that the LC-50 results from Texas show an average of 50% survival at concentrations of 2.9-3.5 ug/vial, and in Louisiana, the researchers found 24-57% survival at the 5ug/vial concentration, as of July 2006.

Again, these are results from potential source populations of CEW that are now migrating in large numbers to the Midwest . Source populations for a given production area can vary from year to year, as can pyrethroid use and selection pressure in a given source state. This is why monitoring on a regular basis is so important, to allow us to fine-tune or adjust treatment and spray programs as necessary, as the season progresses. For example, unlike the Southern U.S. results so far, the one early estimate we have from the Midwest region is Kearney, NE. Thanks to Len Dobbins (FMC) and S. Madden (Monsanto), the first AVT results in a northern state this year (July 17-21) actually show 0.0% Survival at the 10 ug cypermethrin rate. This may very well change for other Midwest sites. However, this indicates that the source moths for NE, may have originated from an area with less pyrethroid use, and may also provide good news for the Nebraska production areas. In summary, all results show that we need to continue to monitor to verify whether or not this trend holds, or also occurs farther east.

Treatment, Management Options: In light of the resistance monitoring results, and the risk of damage by CEW on late-planted, high-value crops, we are providing one significant change this year, particularly for fresh-market growers of sweet corn, green beans, tomatoes and peppers. We also emphasize that in addition to CEW being a contaminate concern in fresh-market sweet corn, green beans, or tomatoes, growers should be aware that large larvae (full size; >1”) in sweet or field corn, on average damage about 13 kernels/worm. This level of damage can result in significant economic losses in terms of direct yield loss for both sweet corn, and seed corn production fields (dent corn). Given the damage potential, and concern for resistance, the following recommendations also reflect the LACK of alternative treatment options, beyond the pyrethroids, for most crops at risk. By crops at risk, I am referring to all high value vegetable crops, and seed corn fields, that either have late planting dates (mid-late June, Midwest) and/or expected harvest dates in mid-late August to Sept.

Briefly, and currently in Minnesota , we are recommending that growers use the MAXIMUM rate of labeled pyrethroid insecticides for the 1^st and/or 2^nd spray on a given crop. In part, this is due to our small-plot trial results (Table 2), where the more typical (medium) pyrethroid rates show variable efficacy. Although this will increase costs, it may be necessary for several reasons. Another suggestion is to reduce the spray interval between the first two sprays (e.g., 4-5 days max.), in high-pressure CEW areas. As we have discussed before, the timing of the 1^st spray is most critical for overall management, to prevent (minimize) the establishment of larvae within the ear (fruit); it is the early eggs and subsequent larvae that establish 21-25 days before harvest that have the time necessary to reach maximum size (>1”) at harvest. A good general rule of thumb is that most of the larval pests grow in length about 1 mm/day (avg. summer temperatures); so, for larvae that have reached approx. 25 mm at harvest (ca. 1”), they likely hatched about 25 days earlier (see Foster and Flood (2005), Midwest Insect Management guide; Sweet corn chapter for more info.).

Final Notes: If you use the maximum pyrethroid rate per spray, you must check the label for the maximum, allowable active ingredient allowed for the crop. For example, the maximum rate for Capture/spray would allow only two sprays of Capture on sweet corn. After that, alternative pyrethroids or other insecticides should be used. In terms of alternatives, the only other option (which also increases cost) is to tank-mix a pyrethroid with Lannate. Lannate primarily provides additional ovicidal control, and some control of adult moths. However it has very short residual activity (< 48 hrs). Our analysis of previous studies indicates it can provide an additional 10-20% control. Lannate also tends to provide additive control only, with no evidence of a synergistic effect with the pyrethroids.

For fresh-market producers using Bt sweet corn hybrids (e.g., Syngenta Seeds), the Bt hybrids will provide a high level of control for CEW, but not 100% control. In high-pressure situations, at least one pyrethroid spray should also be used (again during early silk; 21 days before harvest).

For organic growers, the Entrust formulation of spinosad (Dow AgroSciences) will provide reasonable control of CEW; however, most studies show that this will only provide consistent control under low-moderate pest pressure.

Finally, for all sweet corn growers, you may still have European corn borer and/or Corn Rootworm beetles (chewing silks) to contend with. If your timing is correct (1^st sprays at early silk), the pyrethroids will continue to provide excellent ECB control, and should provide adequate control from CRW beetles. For several reasons, we have observed increasing, severe pressure by CRW beetles in clipping silks, and thus precluding optimal kernel fill. We will provide a summary of recent research on the timing of silk clipping, for fresh-market and processing sweet corn hybrids, in a future issue of the MN Fruit and Vegetable IPM News.

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Co-Editors: Bill Hutchison (hutch002@umn.edu), Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (woldx018@umn.edu), Department of Entomology, University of Minnesota

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jean Ciborowski, 651-201-6217, jeanne.ciborowski@state.mn.us, MDA, 625 Robert St. North, St. Paul, MN 55155-2538. You can access the Newsletter at the U of MN web site in htm format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf format at: http://www.mda.state.mn.us/ipm/ipmnews/

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