In this Issue:

VEGETABLE NEWS

Summary of Corn Earworm and European Corn Borer Dynamics in 2007

Insecticide Efficacy for Corn Earworm: Small-Plot Trial 2007

BERRY NEWS

The Benefits of Frost

Coming Attractions

APPLE NEWS

2007 Trap Catches for Codling Moth and Dogwood Borer at 5 Apple Orchards Monitored by the MDA

Minnesota Department of Agriculture’s 2007 Exotic Apple Insect Pest Survey

To Our Readers

Order: 2007 Minnesota Vegetable Guide

Insect, Pest Fact Sheets

Summary of Corn Earworm and European Corn Borer Dynamics in 2007

Bill Hutchison & Eric Burkness, Dept. of Entomology, St. Paul, Minn.

The following summary contrasts some unique dynamics occurring among two of the primary Midwest insect pests of sweet and field corn, and in large part reflect one key difference in the life cycle of each pest.  Corn earworm (CEW) is not able to overwinter in Minnesota (north of the 40^th latitude), whereas the European corn borer (ECB) readily overwinters in Minnesota and as far north as Canada.  Despite the inability to overwinter in Minnesota, CEW moths are able to consistently migrate from the southern U.S. each year, and cause considerable damage to late-season crops, such as sweet corn and seed corn, that are silking during these flight periods.  On the other hand, although ECB reside locally, their moth flights and subsequent larval infestations have been relatively low, compared to the outbreak years of the 1990s.  Here, we review what we observed at selected locations in Minnesota during 2007, and compare these results to recent years. 

Corn Earworm:  As in previous years, migrating CEW moths did not arrive in significant numbers this year, until late July to early August (Fig. 1).  Unlike previous years, this was the highest moth flight we have documented in the past 3 years, and possibly for the past 10 years. As shown, Blue Earth, MN, has been a fairly consistent “hot spot” for CEW flights.  However, given the fact that our current “action threshold” for CEW moth flights (via wire-mesh pheromone traps) is only 5 Moths/Night/Trap for 3 consecutive nights, during the corn silking phase, these moth flight counts are outstanding or horrendous, depending upon your perspective.  In addition, the flights at Blue Earth usually begin to pick up a bit sooner than our more northern MN locations, and thus serves as an early warning indicator for growers further north. In addition, using tools such as the new national “PestWatch” web-based Map (see reference below) developed by colleagues at Penn State University, watching the moth flights build in Kansas, Oklahoma, Nebraska and Iowa, can also serve as early indicators of moth flight potential in MN as well. 

Despite some variation in the intensity of CEW moth flights, it is useful to review the consistency of the timing and magnitude of larval infestations, at harvest in sweet corn ears (Table 1). For example, at Rosemount from 2005-2007 in untreated plots, larval infestations for sweet corn harvested before August 8^th were very low to nil.  Although this is not surprising to those who have been active in the business, this does highlight the advantage of harvesting as much early sweet corn as possible, via earlier hybrids, earlier planting dates, etc.  After this date, the data confirm a gradual increase in larval infestations that track the moth flights each year.  In addition to larval infestations, there is also the concern of lost kernels, which average from 10-14/ear for each large larva that survives to the last instar.  Examples of this impact are shown in the companion article regarding insecticide efficacy. The results from the 2007 efficacy trial are also in agreement with results from the past 4 years, that most of the standard pyrethroid rates, in small plot trials, at the timings shown (4 or more sprays during silking), are not providing acceptable control, particularly for fresh-market sweet corn.  The use of Bt sweet corn is still effective, but under high-pressure situations as in 2007, will also require at least one insecticide spray to minimize larval infestations; as Bt sweet corn (BT-11 event; Cry1Ab toxin) does not provide a true “high-dose” for CEW, it is common to see “stunted” yet surviving CEW (typically <1/4” length) in the tips of Bt ears at harvest. In the future, hopefully, some new insecticides, with alternative modes of action, will also be available.

European Corn Borer:  For most MN locations, the ECB moth flights were again fairly low. One important exception to this trend was the St. James site, where the 1^st generation was over 200/night and a likely univoltine peak of ca. 280/night on July 7^th (Fig. 2). Also, near Owatonna, the 2^nd generation flight peaked at 395/night on August 2^nd (not shown).  At Rosemount, our peak flight of 70/night on August 2^nd, did not translate to high larval populations in nearby untreated sweet corn (Table 2). 

ECB larval data, again from untreated sweet corn plantings at Rosemount, MN, 2005-2007, illustrate several points.  First, although late-season infestation pressure is relatively low compared to the 1990 outbreak years, these infestation levels are still high enough to cause concern among growers, and require careful management (via insecticide or Bt sweet corn), particularly for the late harvest dates. Secondly, although state-wide ECB pressure has been reduced since the adoption of Bt field corn (e.g., current statewide Bt corn use for 2007 is approx. 54%, for ECB, Bt toxins; NASS), we continue to have areas in the state where Bt corn use is less than the state average, and ECB populations will still be high enough in these areas (such as Rosemount) to warrant monitoring and possible insecticidal control; ECB remains a concern here due to its tendency to bore into the side, butt and tips of ears, whereas CEW is almost exclusively a tip feeder.  Finally, a close review of the 2006 data shows, that even for Rosemount, in some years, we continue to have low levels of univoltine ECB, which results in July infestations at this latitude, and thus requires careful monitoring for the univoltine moth flights in southern Minn., as well as west-central and northwestern Minnesota. 

In summary, 2007 has been another very interesting year. In addition to these results, we obtained new data on the dynamics of corn rootworm damage to sweet corn, including root and silk clipping injury.  We also plan to summarize Western bean cutworm flights for southern Minnesota, which were generally low. As per recent years, we did not observe any WBC larvae in untreated sweet corn at Rosemount, nor in commercial sweet corn in areas near Rosemount and Hampton (Dakota, Goodhue Counties). These results, as well as additional ECB and CEW data, will be summarized at upcoming meetings this winter. 

Thanks again to ALL of the excellent cooperators who assisted in collecting moth flight, and resistance monitoring data during 2007!

For More Information:

ZEA-MAP Source Site, for Pyrethroid Resistance Monitoring Updates; Midwest:
http://www.vegedge.umn.edu/ZeaMap/zeamap.htm

PestWatch, National Map for Monitoring CEW Flights:
http://www.pestwatch.psu.edu/sweetcorn/tool/tool.html

2006 CEW-Pyrethroid Resistance Symposium (July 2007; Plant Health Progress)
http://www.plantmanagementnetwork.org/pub/php/symposium/hzea/

Return to index

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 Jeanne Ciborowski, 651-201-6217, jeanne.ciborowski@state.mn.us , MDA, 625 Robert St. North, St. Paul, MN  55155.  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: www.mda.state.mn.us/plants/pestmanagement/ipm/ipmnews.htm

Partial funding for this publication is provided through partnership agreements with the Minnesota Fruit and Vegetable Growers Association (MFVGA) and the United States Department of Agriculture – Risk Management Agency (RMA).  These institutions are equal opportunity providers.

DISCLAIMER

Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current manufacturer directions.

Last Revised September, 2007 by woldx018@umn.edu
The University, including the Minnesota Extension Service, is an equal opportunity educator and employer. Privacy Policy
©1999-2007 Minnesota Extension Service, University of Minnesota. All rights reserved. Contact copyright@extension.umn.edu for information on reproduction or use of this material.