Corn and soybean planting and crop progress in our area is way behind the 10-15 year averages, yet we are fortunate compared to western and northwestern Ohio. The late planting and smaller corn and soybean plants that we see in fields provide some good feeding opportunities for several pests, so growers are advised to scout their fields. In particular, watch out for black cutworm damage, slug feeding and armyworm feeding damage. Crop scouts with our Wayne County Extension integrated pest management (IPM) scouting program have noted feeding from all three of these pests in Wayne County fields in recent weeks.
Frequent rainfall events prevented early season weed control on crop fields. As a result, many fields had excellent growth of chickweed, henbit and purple deadnettle. These winter annual weeds, especially chickweed are attractive to spring flights of the black cutworm (BCW) moths as they lay their eggs. After weeds are tilled in preparation for planting or die following an herbicide application, larvae move to the developing corn crop to feed. Corn is most susceptible to BCW larvae feeding damage from emergence until the V-4 growth stage. BCW larvae pass through seven instar or growth development stages and do most of their damage between the fourth and seventh instar stage. Small holes in the leaf, often spaced linearly across the leaf is one symptom of BCW. As larvae reach the fourth and later instar stages they are capable of cutting plants off near the soil level or tunneling into the plant, causing it to wilt and die. Scouting involves checking 20 plants in five random locations for every 25 acres of field. Rescue treatment is warranted under the following conditions: 1) 3% or more or the plants have been cut or tunneled, 2) corn is at the V-4 or younger growth stage and 3) the BCW larvae are an inch or less in length.
Slugs tend to be a problem pest primarily in no-till fields. They prefer cool, moist conditions and no-till crop residue provides good slug habitat. Slug eggs generally hatch in mid to later May in our area of the state and young juvenile slug feeding damage can be observed within a couple of weeks of hatching. Slugs have rasping mouthparts and visible feeding symptoms on corn is windowpane damage and, as slugs increase in size, leaf shredding. On young soybean plants, slugs feeding causes craters in the cotyledons and ragged holes in leaves on older plants.
The younger a corn or soybean plant is, the more susceptible it is to slug feeding damage. By the time corn reaches the V-4 to V-5 growth stage, slug feeding is no longer a concern. In fact, research by John Tooker at Penn State indicates that although slug feeding damage on corn can look severe, it rarely has an impact on corn yield. At an Ohio Agribusiness Association conference presentation, I heard John Tooker say that the best advice is probably to walk away from slug damage on a corn field and not over react. Slug feeding damage on soybeans is another story.
Unlike corn, whose growing point remains below ground until the V-5 growth stage, the growing point of soybeans is above the soil surface with the emergence of the cotyledons. Therefore, any slug feeding damage to the cotyledons and/or newly emerging leaves is directly damaging the growing point and when severe enough can kill the plant. Late planted soybeans combined with no-till creates a higher risk situation for damage by slugs. If those soybeans are planted into wet field conditions and the seed furrow is not closed, this creates an additional risk as slugs can use this open seed furrow as a highway march down the row, feeding on germinating and newly emerging soybeans.
Wayne County IPM scouts have found some small, isolated incidences of armyworm feeding on corn. At this point, very slight damage and not a concern. However, a recent Purdue Pest and Crop newsletter advised growers to be diligent in scouting corn fields, again especially young corn, in the next few weeks. No-till corn and corn planted near or adjacent to grass cover crops, particularly cereal rye, are most at risk. Armyworm feeding damage creates a ragged appearance to the corn plant. As larvae number and size increase, feeding severity can increase to a point where only the midrib of the plant remains. Rescue treatment may be warranted if more than 50% of the plants in a field exhibit feeding damage and larvae are less than one and a quarter inch in length.
Risk of feeding damage from black cutworm, slugs and armyworms is increased due to our delayed corn and soybean-planting season. Growers should be scouting their fields to make sure these pests do not cause stand loss and/or affect yield potential. For more information about these pests, scouting procedures or treatment thresholds, contact the Wayne County Extension office at 330-264-8722.
Rory Lewandowski is an OSU Extension Agriculture & Natural Resources Educator and may be reached at 330-264-8722.
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