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

 


Insect Trapping Update:Week Ending May 11, 2018

Black cutworm

Black cutworm moth flights continue across much of Illinois. Several counties reported second -and even third- significant moth flights (Madison, Champaign, and Lee). Several counties had near significant flights (Piatt and Coles). It is important to remember that lack of a reported significant flight and subsequent projected cutting dates does not take black cutworm out of the equation in your area. I have had reported of 1st-2nd instar feeding and even a report of 3rd-4th instar cutting in a couple areas of the state. These reports have been isolated, and under recommended thresholds for rescue treatments.

Black cutworm projected cutting dates.

True Armyworm

While spring storms and southern winds brought black cutworm moths to many areas of Illinois, that is not the case for true armyworm. Trap counts remained low across the state for a second week in a row.  

 

Weekly Moth Total

(May 5-May 11)

Northern (Lee County) 2
West Central (Warren County) 4
East Central (Champaign County) 5
Southern (Madison) 0

 

Corn Rootworm

We remain several weeks from corn rootworm hatch in Illinois. Soil temperatures are warm and degree-day accumulations reflect totals that are slightly ahead of the historical average. We will continue to monitor degree-days and egg hatch.

Alfalfa Weevil

No reports of alfalfa weevil feeding have made their way to the office this week, but varying life stages are possible across the state. A quick refresher on biology, injury and management can be found on the alfalfa weevil factsheet.

Corn Earworm

While we won’t begin trapping for corn earworm for a couple of weeks, Purdue University reports the first 2018 moth catch of the year in Indiana. At this time of year, the catch has little significance on field crops, but is a reminder that spring is truly here and we will be transitioning for our early season insect pests to summer insect pests soon.

 


Impressive Moth Flights across Midwest

Impressive moth flights have not only kept the Illinois insect monitoring network cooperators busy, but neighboring states are reporting lots of black cutworm and true armyworm moth activity as well. The current forecast and planting progress has raised questions about the potential for these pests in the coming weeks.

With the assistance of University of Illinois Extension educators, producers and industry volunteers, nearly 60 trap sites have been established across Illinois. Captures of both black cutworm and true armyworm have been common across the state. Several counties have reported significant flights (nine or more moths caught over a 2-day period). In fact, several counties have repeated significant flights.

BCW Apr28

Illinois black cutworm projected potential cutting dates based on degree day accumulations.

 

 

As shared in previous Bulletin articles, more complete information about the biology, life cycle, and management of black cutworms, a fact sheet is available from the Department of Crop Sciences, UIUC. Provided below is a brief overview of some key life cycle and management facts concerning black cutworms.

  • Black cutworm moths are strong migratory insects with northward flights commonly observed from Gulf States into the Midwest from March through May.
  • Moths are attracted to fields heavily infested with weeds such as chickweed, shepherd’s purse, peppergrass, and yellow rocket.
  • Late tillage and planting tends to increase the susceptibility of fields to black cutworm infestations.
  • Cutting of corn plants begins when larvae reach the 4th instar — with a single cutworm cutting an average of 3 to 4 plants during its larval development.
  • Cutting tends to occur most often during nights or on dark overcast days.
  • Fields at greatest risk to cutting and economic damage are in the 1-to-4 leaf stage of plant development.
  • An early warning sign of potential economic damage includes small pinhole feeding injury in leaves (caused by the first 3 instars).
  • Producers are encouraged to look for early signs of leaf feeding as a potential indicator of cutting, rather than waiting for cutting to take place.
  • Don’t assume that all Bt hybrids offer the same level of cutworm protection. Plants in the 1- to 4-leaf stage are most susceptible to cutting.
  • Cutting of plants earlier than these projected cutting dates is possible — localized intense flights may have occurred and were not picked up by our volunteers.
  • A nominal threshold of 3% cutting of plants has traditionally been used as a point at which growers should consider a rescue treatment.
  • Not all Bt hybrids offer adequate protection against black cutworm damage. Growers should consult the Handy Bt trait table prepared by Dr. Chris DiFonzo at Michigan State University to determine the level of protection provided by their chosen Bt hybrid.

 

Switching our focus to true armyworm, this insect has also been very active this spring. Illinois has seen steady flights across the state with numbers slowing down only within the last week. Christian Krupke, Purdue, summarized the potential threat very well in a recent article. Remember, true armyworms prefer to lay eggs in grass covered areas. Wheat as well as corn planted into cover crops will be a prime target. The Handy Bt trait table above is also a great reference for hybrids that may offer some protection, but once again, don’t assume it’s 100% full-proof. With significant infestations, some damage may be observed before the Bt-proteins will suppress the feeding.

The take-home message today? Be vigilant with your fields this spring. The mild winter and warm spring certainly got things rolling. This upcoming cool and wet period may slow things down, but both black cutworm and true armyworm moths have been abundant up until this point (and they pick back up again). Please feel free to share updates from the field by email (kcook8@illinois.edu) or twitter (https://twitter.com/ILPestSurvey).