Illinois Corn, Soybeans: Scouting for Early Season Pests After Late Start

It goes without saying that this spring has been a challenge. With extreme planting delays throughout the state, crop development is well behind normal expectations, while insect pest populations have continued to progress.

In addition, the tight schedule we have faced has forced planting into less than ideal conditions in terms of both soil moisture and weed control, which can have consequences for insect pest management.

There are a few pests in particular to target during early season scouting this season:

True armyworm, black cutworm, variegated cutworm

These insect pests are more likely to be a problem in later planted fields, especially where late burndown herbicide applications allowed weed cover to build up (unfortunately, an all too common occurrence this season).

All three usually develop on weedy plant species, then move to corn or soybean when their weedy hosts mature or are killed with a herbicide; armyworms are more of a concern where there are dense populations of grasses, while black and variegated cutworms have a wider host range that includes legumes and other broadleaf plants in addition to grasses.

While all of these can cause defoliation, the cutworm species can reduce stand directly when their feeding “cuts” the plant close to the ground. (Note: be sure to follow Kelly Estes on Twitter [@ILPestSurvey] for periodical updates on moth flights for true armyworm, black cutworm, and other pests).

Image of two variegated cutworm larvae

Fig. 1. Variegated cutworm larvae from a heavily damaged soybean field. The overall color varies quite a bit from brown to blue or gray, but look for yellow or white markings along the back (Photo: Victoria Kleczewski, Growmark)

image of a true armyworm larva

Fig. 2. A true armyworm larvae. Note the light colored bands on the side, the net-like pattern on the head, and the dark bands on the prolegs. Photo: Frank Peairs, Colorado State University, Bugwood.org

True armyworm larvae in wheat

Fig. 3. Several different developmental stages of true armyworm larvae in wheat. (Photo: Robert Bellm)

Slugs

Slugs are primarily an issue in no-till or conservation tillage fields which have a lot of residue and moisture. The wet conditions that favor slug damage can also lead to problems with seed slot closure, which exacerbates slug damage by allowing them to feed on the developing plant as the seed germinates.

Unfortunately we do not have a good rescue treatment for slugs in soybean in Illinois. The best management strategy is to plant into a warm, dry seedbed (not always an option this season), and tillage is the best control we have available.

Slug in an open seed slot

Fig. 4. A slug in a seed slot left open due to wet planting conditions. (Photo: Nick Seiter)

Slug damage to a soybean cotyledon

Fig. 5. Slug damage to a soybean cotyledon (Photo: Jennifer Woodyard, University of Illinois Extension)

Bean leaf beetle

Bean leaf beetle feeding is often noticed on soybean fields that are among the earliest planted in the state; when there are relatively few acres that have emerged, the highly mobile beetles are concentrated in those few fields. Usually this damage is mostly cosmetic, as soybeans are excellent at overcoming early defoliation.

The economic threshold for defoliation of soybeans prior to bloom in Illinois is an average of 30% of leaf tissue removed with the defoliator still present in the field.

Bean leaf beetle on seedling soybean

Fig. 6. A bean leaf beetle and its feeding damage on a young soybean plant (Photo: Nick Seiter)

Scouting is necessary to determine both the necessity and timing of an insecticide application for these insect pests. We want to avoid “revenge sprays” that occur after the insect has either progressed through its life cycle (in the case of the caterpillar pests) or moved along to another field (bean leaf beetles) and is no longer damaging the crop.

As always, feel free to contact me if you are seeing anything unusual in the field related to insect management. Here is hoping for improved conditions as the season moves forward.


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