Scouting for Early Season Pests in Corn and Soybean After a 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.

 

Contact: Nick Seiter nseiter@illinois.edu Twitter: @nick_seiter

Research Assistant Professor, Field Crop Entomology

 


Slug Management in Illinois Field Crops

Authors: Nick Seiter, Talon Becker, and Nathan Johanning

Slugs can be a difficult pest to manage when conditions are favorable for them, which has been the case often (particularly in southern Illinois) over the last couple of years. These mollusks can damage both corn and soybean early in the season, along with a variety of other crops; however, they have the potential to be especially problematic in soybean, where they can kill the cotyledons and ultimately reduce stands. There are a few management points to consider for slugs in field crops:

  • Monitor slugs before planting to estimate the severity of the problem. Slugs can be monitored by inspecting residue, or by creating artificial shelters (made from shingles or other flat materials placed in the field to create a dark, damp environment) and inspecting them periodically before planting and during early stand establishment.1

    A slug found under a shingle trap placed in a field prior to planting in southern Illinois. Photo: Talon Becker.

  • Cool, wet weather during stand establishment results in greater slug problems. Slugs require a moist environment to survive, and they perform best when conditions are wet. Cooler temperatures extend soil drying time and delay plant development, leaving seedlings vulnerable to slug feeding damage for a longer period of time. Discussions with several CCAs in southern Illinois highlighted the fact that, while slug damage is a fairly normal occurrence on a small scale in most years, particularly in no-till fields, the mild winter of 2017 followed by wet and cool conditions in the spring after many acres had already been planted likely contributed to the greater incidence of slug damage last season. It appears that soybeans were most affected last season in southern Illinois, with several thousand acres of replanting reported.
  • Reduced tillage and/or certain cover crop systems can lead to larger slug populations. Higher levels of residue retain water and provide harborage for slugs, resulting in an increased probability of slugs reaching damaging levels. Reports from southern Illinois indicate that most problem fields last spring had a cereal rye cover crop that had not been terminated before producing excessive growth, creating a favorable environment for slugs. It is important to manage residue in cover cropped fields, particularly if they are no-till. If you had a problem with slugs last year, or have found concerning levels under your “shingle traps” in the field, make it a priority to terminate the cover crop before too much above ground biomass has accumulated (generally less than 1 ft. of growth). Cover croppers may also consider decreasing their seeding rate or planting a cover crop mix which includes species that winter-kill along with their favorite over-wintering species.
  • Avoid open seed furrows. When planter closing wheels fail to seal the furrow, the resulting trench provides an ideal environment for slugs and allows them to consume developing cotyledons as the seed germinates.
  • Chemical control options are limited. Slugs are not insects, and insecticides do not provide effective control. There are slug-specific baits available, but they tend to be expensive. Note that several formulations of the active ingredient metaldehyde (e.g., Deadline®) are labeled for use in corn, but this molluscicide is not currently labeled for soybean in Illinois.

Ultimately, the most reliable management tactic for slugs is to plant into a warm, dry seed bed, which is not always an option. However, by understanding conditions which are likely to lead to slug problems, you can be better prepared to address them when and where they occur.

Correspondence:

Nick Seiter: nseiter@illinois.edu – Research Assistant Professor, Field Crop Entomologist, University of Illinois Department of Crop Sciences

Talon Becker: tbecker2@illinois.edu – Extension Educator, Agriculture and Natural Resources, Illinois Extension

Nathan Johanning: njohann@illinois.edu – Extension Educator, Agriculture and Natural Resources, Illinois Extension

1 Douglas, M. R. and Tooker, J. F. 2012. Journal of Integrated Pest Management 3(1) DOI: http://dx.doi.org/10.1603/IPM11023


Insect Snapshots from the Field

Just a quick overview of some insect issues presenting themselves recently.

True Armyworm

Lots of reports of armyworm being found in wheat and corn. With reports of wheat harvest starting/getting close, reports of armyworm in corn seem to be taking over. I’ve seen a range of larvae stages from 2nd-4th instars. Injury to the whorl and ragged leaf margins is usually noticed around field margins first.  Armyworm larvae are night feeders and will usually spend the day in soil cracks, under dirt clods, or in the whorl. Control may be justified when 25% of the plants are being damaged. Things to consider: hot spots in the field and also size of the larvae. Larvae greater than 1 ¼” will have completed most of their feeding.

IMG_5699a

Armyworm larvae (Photo courtesy of Stephanie Porter)

 

Corn Rootworm

Reports of lightning bugs in the central part of the state along with degree day accumulations suggest rootworm hatch is underway.

CRW June 9

Corn Earworm

We picked up our first corn earworm moth last week. To date, we’ve trapped a total of 10 moths in Champaign. We expect those numbers to slowly pick up over the next couple of weeks.

 

Corn earworm moth (Photo courtesy of University of Maine)

Corn earworm moth
(Photo courtesy of University of Maine)

Fall Armyworm

A few fall armyworm moths have been picked up in our traps here in Champaign. We expect those numbers to increase in the next few weeks as well.

 

European Corn Borer

We have yet to pick up any moths in our trap in Champaign. We also have been sampling action sites (dense stands of tall grasses along roadsides ditches or waterways) by making 100 sweeps with a standard insect sweep net. To date, no moths have been recovered in any of the samples statewide.

 

European corn borer  (Photo courtesy of Marlin Rice)

European corn borer
(Photo courtesy of Marlin Rice)

Stalk Borer

A few reports of stalk borer in corn. These younger larvae did not have the distinct purple “saddle” that we often use to ID these insects. Young larvae will be light brown with a narrow white stripe running down its back from head to tail. There will be a similar white stripe on each side of the body that is interrupted by a purplish-brown band that circles the front third of the body. As larvae get bigger they move from the grass hosts they are quickly outgrowing to larger hosts, such as corn.

Stalk borers will attack corn in 2 different ways:

  • Burrow into stalks at ground level and chew upwards through the center of the plant. Wilted leaves will be the first obvious symptom with this injury along with the potential for some plants to buckle at ground level.
  • Some larvae may crawl to the top and feed down through the rolled leaves into the stalks. Ragged leaves, along with frass on the leaves may allude to this type of feeding which may also cause wilting of the top half of the plant.

 

Stink Bugs

I’ve also gotten reports of stink bug injury in southeast Illinois. Stink bug can cause three different types of injury – tillering, stunted plants, and may even kill small seedlings. Signs of stink bug injury will include oval holes where they have inserted their needlelike mouthparts.  The stink bug sticks the base of the plant or their leaves with their small needle-like mouth. When they do this, they are causing these plants to die as a small seedling, produce stunted plants- such as misshapen ears instead of tassels- or suckering- the production of tillers form the base of damaged plants. Most often, you see this pest in fields that are no-tillage, near wooded areas, or conventional fields as well as along field edges.

 

Slugs

Slug injury has been reported across much of southeast Illinois as well and as far north at Livingston county in both corn and soybeans. Slugs are generally a problem in no-till fields; especially if they are late-planted. Slugs take advantage of the smaller plants and as the plants grow, they quickly outpace the slugs. Feeding generally occurs on the lower parts of the plants. Symptoms may resemble that of the corn flea beetle (narrow, irregular tracks or scarring), but the presence of a slime trail will be indicative of the presence of slugs.

 

Black Cutworm

Once again, we received sustained flights across much of Illinois. Several counties saw repeated significant flights. This map gives you a good indication of projected cutting dates, but several counties like Champaign and Lee saw 4-5 significant flights, so the bottom line is that some of this late planted corn will still be susceptible for BCW injury. Speaking of BCW injury, I have had a few reports of feeding around the state. No indication of anyone area being more severe than another. There is still a lot of small corn across the state that could be susceptible.

BCW May 26

Centipedes/Millipedes

I’ve also fielded a few call on centipedes and millipedes last week, though with the warm, dry weather, I don’t suspect to hear much more about these.

 

 

Information compiled with the assistance of Kaela Miller, University of Illinois Agriculture Communications.