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Rod Youngman
Professor and Extension Specialist
Department of Entomology
Virginia Tech, Blacksburg, VA 24061-0319
Despite the near ideal weather conditions over the past several days, I don't need to tell you that this year's extremely wet spring has caused serious delays in corn planting across the state. If that wasn't enough, the combination of excessive rainfall, unseasonably cool temperatures, and extensive cloud cover has caused the plant growth in many timely-planted cornfields to be way behind where they should be for this time of year; especially when compared to years with more favorable growing conditions.
So what does this mean for insect pests this summer?
Without raising undue concern, you should be aware that all late-planted corn (i.e., after May 15th, as well as all earlier-planted fields whose current growth is more representative of late-planted corn) is potentially at high risk to European corn borer (ECB) and fall armyworm (FAW) infestations beginning about the middle of July and continuing into September.
What management options are available to control these pests?
There's no question that the best option for any cornfield being planted now is to plant a Bt hybrid for stalk boring (mainly ECB larvae beginning with the 3rd-instar stage) and leaf feeding (larvae of both ECB [1st-3rd instar stage] and FAW) insects. A second option is to direct a granular or liquid insecticide application over the top of the corn rows in order to get enough insecticide on the leaf surface and into the leaf-stalk junction to ensure contact with the pest. It is critical the application be made after the majority of ECB eggs have hatched, but before the larvae have begun tunneling into the stalk. Insecticides are no longer effective once the insect has tunneled into the stalk. The second option also is effective against FAW larvae throughout its immature development period because the larvae do not tunnel into the stalk. Because of this, FAW larvae are much more exposed to the insecticide during application. Whenever possible, however, it's always best to time insecticide applications at younger larvae rather than older larvae, because younger larvae are easier to kill and will have consumed less leaf material than older larvae.
How do I best time the application if an insecticide is my only option?
For ECB, refer to the scouting guidelines for second generation eggs on p. 151 of the Pest Management Guide for Field Crops 2003 (VCE publication 456-016). As of this writing, my best estimate to begin ECB egg mass scouting on a weekly basis would be the week of July 7th. Realize that this calendar estimate is highly dependent on environmental conditions. Also, if you plan to spray without scouting, you need to realize you run the risk of seriously reducing the effectiveness of the application because of improper timing. For FAW, scouting and subsequent timing of insecticide applications are easier and generally more effective because of the insect's exposed habit. For details on FAW scouting and management, please refer to p. 159 of the Pest Management Guide for Field Crops.
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