In this Issue:

VEGETABLE NEWS

Review and Recommendations for Late-Season Corn Earworm Control in Sweet Corn—2004

Corn Leaf Aphid Update from Illinois

Vegetable Insect Update

APPLE NEWS

A Stella is Born: How a Minnesota Apple Grower Developed an Apple of His Own

Apple Scab Infections

Degree Day Accumulations

Weekly Trap Counts

Insect, Pest Fact Sheets

Review and Recommendations for Late-Season Corn Earworm Control in Sweet Corn—2004

Bill Hutchison and Eric Burkness, University of Minnesota, St. Paul

Thus far, it looks like it could be a “below normal” year for European corn borer (ECB). That’s the good news;...now, the troubling news about Corn earworm (CEW). Sweet corn is most attractive to CEW Moths during the late tassel, and early silking stages. Although CEW moths can potentially lay eggs just about anywhere near the upper half of corn plants, the moths prefer to lay most of their eggs on fresh silks. Following egg hatch, the young (1/8 inch in length) larva will rapidly move down the silk channel to begin feeding on the kernels. Once larvae are feeding within the protection of the husk, it is virtually impossible to control the larvae with insecticide sprays. As silks begin to turn brown, and as ears mature, moths will move to nearby sweet corn that is just beginning to silk. CEW will also lay eggs in snap bean, tomato and pepper fields, but sweet corn is usually the preferred host, when given a choice.

Given the damage potential for CEW in sweet corn, action thresholds are very low. For both fresh and processing sweet corn, insecticide sprays should begin when 4% of the plants have eggs or young larvae present on the silks. Unlike ECB, CEW eggs are laid singly, and easy to see if you have good eyesight (or have a 10X hand lens). However, because it is difficult to search for CEW eggs, at low densities, many growers and consultants use the following trap-based, moth-catch thresholds. Egg hatch rate is also based highly dependent upon temperature. The following thresholds have been adapted from Ohio State and Cornell University recommendations.

Insecticide Choice, Selection, and Potential for CEW Resistance:

CEW is more difficult to control than the ECB, and not all of the labeled insecticides, or pyrethroids, work as well on CEW. The following pyrethroids are generally effective against CEW (see Table). Additional considerations will include Pre-harvest interval (PHI), Re-entry interval (REI) and cost.

However, we now must consider the possibility of CEW resistance to pyrethroids. As noted last fall (Sept. 12, 2003 Newsletter, http://www.vegedge.umn.edu/mnvegnew/vol5/912cew.htm), following our analysis of multiple insecticide trials in MN and WI, we are concerned that there has been a gradual buildup of pyrethroid resistance in CEW. Last year, in both MN and WI trials, we observed a low level of CEW control (33-45% in small-plot, replicated trials). Industry cooperators also confirmed commercial fields with ca. 10% ear infestations with late-instar CEW. Larvae collected from MN, WI and IL, and tested by Dr. Roger Leonard (Louisiana State Univ.), all confirmed various levels of resistance or tolerance to the test pyrethroid (cypermethrin). Fortunately, the resistance levels were variable, but the levels are also similar to those recently reported in LA, TX and other southern states. As most of the Upper Midwest summer infestations of CEW originate in the southern U.S., the selection for resistance appears to be fairly stable in certain segments of the southern U.S. (e.g., cotton/soybean/corn areas). Subsequently, moths emerging from southern crops in late summer, which then migrate to northern states, bring the genes for resistance with them. For a additional information from this work, please see the N.C. Branch ESA Web Abstract, March 2004 at : http://esa.ent.iastate.edu/confreg/?gridaction=viewonepresentation&year=2004&presnum=150&q=Hutchison.

Because of the 2003 results, we have planned to conduct multiple trials this year to assess the potential of new tank-mixes with pyrethroids, or alternative insecticides. As this work is underway, we cannot yet offer recommendations. Given the list of possible, currently labeled options, and based on previous experience in the south (cotton), we are recommending that at least for the first, most critical spray this year, that growers consider tank-mixing Lannate (e.g., at ¾ of full rate) with one of the pyrethroids (mid to full rate). Lannate is a good ovicidal material, but has very short residual activity. However, for 2004, we are recommending that this material be tank-mixed with one of the pyrethroids for added egg and early instar larval activity. Based on research in cotton, Lannate is also known to provide some adult control.

Notes: The pre-harvest interval (PHI) is not a problem for any product. However, the re-entry (REI) should be considered. All pyrethroids (Asana, Ambush, Baythroid, Capture, Pounce, Warrior, Mustang) and Lannate are Restricted Use Pesticides (RUP). Sevin (carbaryl), SpinTor and Bt products are the few non-RUPs labeled for sweet corn. SpinTor and Bt provide good control of ECB, but less consistent control of CEW.
Asana is good for CEW control; however, Asana does not provide consistent ECB control.
**Also note the MAXIMUM product/acre/field that can be applied per year. Read labels carefully.
**For more information: Refer to the 2004 Midwest Vegetable Guide at: http://www.entm.purdue.edu/entomology/ext/targets/ID/index.htm

Bt Sweet Corn: If fresh-market growers are using Bt Sweet Corn (e.g., Rogers/Syngenta hybrids), they should see about 90% control of CEW (as well as 99-100% control of ECB), without any insecticide applications. Sprays for CEW control will only be needed if moth flights remain heavy throughout the year. If needed for CEW, Bt sweet corn should only need to be treated once, e.g., anytime between 20-25% silk. One spray at early to mid silk may also be necessary to control corn rootworm beetles feeding (clipping) on fresh silks. Both northern and western corn rootworm beetles are usually controlled in non-Bt sweet corn with conventional insecticide sprays. Bt sweet corn (current Bt events) will not control adult corn rootworm beetles.

Organic growers: Although some parasitic wasps can be used for ECB control, parasitic wasps are less effective on CEW. The best cost-effective options are to use a Bt product (Bacillus thuringiensis), such as Dipel, and/or an organic-certified pyrethrum product, such as “Pyganic” (again, starting at first silk). Check with your organic-certification group regarding which Bt and pyrethrum products are certified organic. Dipel, alone, will often provide effective ECB control. However, for CEW, I would consider a pyrethrum product. Pyrethrum products typically have less residual activity than synthetic pyrethroids, and must also be applied on a 3-5 day schedule, depending on max. field temperatures. *See also federal organic certification information in: August 23, 2002 newsletter at: http://www.vegedge.umn.edu/mnvegnew/vol4/823org.htm

For related articles on CEW control in sweet corn, and Recent Research Reports, please refer to previous issues at the Newsletter site, and type corn earworm in the Search box, at: http://www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm

ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@umn.edu
Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Division,jeanne.ciborowski@state.mn.us
Suzanne Wold-Burkness, Department of Entomology, University of Minnesota, woldx018@umn.edu

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., Tuesday to Jeanne Ciborowski, 651-297-3217, jeanne.ciborowski@state.mn.us , MDA, 90 W. Plato Blvd., St. Paul, MN  55107-2094.  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/biocon/fruitreports/default.htm

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

DISCLAIMER

References 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 August 5, 2004.
The University, including the Minnesota Extension Service, is an equal opportunity educator and employer. ©1999-2004 Minnesota Extension Service, University of Minnesota. All rights reserved. Contact copyright@extension.umn.edu for information on reproduction or use of this material.