Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
Congratulations to John Comegys for correctly identifying damage as cutworm and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!
Cutworm
By David Owens, Extension Entomologist
Corn planting is at full speed, and early season insect pest pressure is not far behind. The above photo is an example of small larval cutworm feeding injury to field corn. Small larvae cannot cut plants, they will chew holes through the whorl that, as the leaves unfold, leave a characteristic, symmetrical shot hole pattern. Cutworms can be a significant corn pest. Older larvae can bore through larger plants causing dead heart and cutting plants off at the ground level.
In general, there has been a trend towards decreasing cutworm activity, though not as strong as the trend seen with European corn borer. How can we manage cutworm, and what do the holes mean? First, many of the Bt varieties have suppressive efficacy on early instar cutworms. Please reference the Handy Bt trait table here: https://lubbock.tamu.edu/files/2018/01/BtTraitTableJan2018.pdf. There are traits that do not have cutworm efficacy, so know what you have! DoublePro, TriplePro, and Yieldgard corn do not have traits that are effective on cutworm. If you see this type of injury in fields planted without an effective trait, pay special attention to the field and be ready to treat it with a pyrethroid if you see 5% damaged plants and you find larvae. But what about those fields that do have one of the traits that does have suppression activity (Cry1F or Vip3A)? The Bt traits are stomach active toxins, and specific to target insects or groups, meaning they need to be ingested and there is specificity. Rootworm traits only work on rootworms, not on wireworms, caterpillars, or grubs. Larvae may feed on plants for short period of time before dying. If you see this type of injury in a field with Cry1F and Vip3A corn, monitor, preferably within a few days. It also means that traits are most effective on small larvae that require less of a dose. We are seeing large dingy and black cutworms out right now in some cover crops. These will be done with their life cycle by the time the field is planted and seed germinating. However, cover crops are attractive to ovipositing moths, so if a field is planted soon after burndown or into a green cover, pay attention to stand. A few holes in leaves will alert you to a population, but if that is all you see in consecutive field visits, the traits have probably done their work. My colleagues in western states will caution that if initial populations are REALLY high, the traits may need to be supplemented with additional control. It also pays to scout a field. There are seedling corn pests that are not managed by any Bt trait. If you see feeding injury, try to find the culprit as best as possible.
Supplemental control can be in the form of a non-neonic seed treatment. Right now, there is chlorantraniliprole, trade name Lumivia (same active ingredient as Prevathon and Coragen). It is rated by Auburn and University of Tennessee entomologists Dr. Kathy Flanders and Dr. Scott Steward as having good efficacy for cutworm. Neonics do not. You can find their efficacy charts here: http://blogs.ext.vt.edu/ag-pest-advisory/files/2014/10/Field-corn-insecticide-seed-treatment-chart.pdf and here: http://www.utcrops.com/cotton/cotton_insects/pubs/PB1768-Corn.pdf.
Additional control can be achieved with a pyrethroid. What scenarios are at greater risk for cutworm? LATE planted fields that have living weeds/cover within a week of or two of planting. Pay special attention to these fields and to fields planted green. As we wrote recently, there is a cost/benefit trade off with high seed treatment rates and a pyrethroid in furrow/over the soil. The closer you are to planting, the more likely a pyrethroid at burndown will hit an insect pest. However, if you are not planting right away, pyrethroids break down over time. Should another seedling pest move in, the original spray will not be effective. If your concern is soil pests such as wireworm, put the pyrethroid in the furrow. The pyrethroid should still have efficacy on cutworm.