Statewide Corn and Soybean Survey Indicate Lower Insect Populations in 2019

The Illinois Statewide Corn and Soybean Insect Survey has been occurred in eight of the last nine years (2011, 2013–2019). These surveys have been conducted with the goal of estimating densities of common insect pests in corn and soybean cropping systems. In 2019, 40 counties representing all nine crop reporting districts were surveyed, with five corn and five soybean fields surveyed in each county.

 

Within the soybean fields surveyed, 100 sweeps were performed on both the exterior of the field (outer 2 rows) and interior (at least 12 rows beyond the field edge) using a 38-cm diameter sweep net. The insects collected in sweep samples were identified and counted to provide an estimate of the number of insects per 100 sweeps (Tables 1 and 2).

A common question during the growing season was, “How would insect populations respond to the severe cold events from the 2018/2019 winter following by the record breaking precipitation in the spring?” A very simple answer? Not well. For the most part, insect numbers were lower when compared with our 2018 survey.

 

While Japanese beetle populations were trending higher statewide in 2018, district averages declined everywhere with the exception of the East crop reporting district. High averages in both Iroquois and Livingston counties pulled the district average up. Growers in western and northwestern Illinois were happy to see lower numbers after extremely high Japanese beetles present in 2018 (Figure 1).

Figure 1. Average number of Japanese beetles per 100 sweeps (2019 Statewide Soybean Survey).

Included for the first time in the soybean survey, was the Dectes stem borer. While present in Illinois for many years, recently this pest, this insect pest has been garnering attention from soybean growers in southern Illinois for the past couple of years. Soybean sweeps did confirm higher numbers in the southern part of the state, particularly in the southeast, but was present at low levels in other districts as well (Figure 2).

Figure 2. Average number of Dectes Stem Borer per 100 sweeps (2019 Statewide Soybean Survey).

As expected with the very wet spring, western corn rootworms populations remained very low in 2019. In addition to sweep samples in soybeans (Figure 3), cornfields were sampled for western corn rootworm by counting the number of beetles on 20 consecutive plants beyond the end rows of a given field—a beetle per plant average was calculated for each field.  Despite lower statewide averages, there are local areas where populations were higher. This was especially evident in Iroquois and Livingston county soybean fields that were also contending with Japanese beetles. In corn, Christian, Sangamon and Greene counties each had a few fields with higher western corn rootworm numbers compared to others in those respective districts.

 

Figure 3. Average number of western corn rootworm per 100 sweeps (2019 Statewide Soybean Survey).

Funding for survey activities was provided by the USDA National Institute of Food and Agriculture. This survey would not be possible without the hard work and contributions of many people, including Cooperative Agriculture Pest Survey Program interns Evan Cropek, Calli Robinson, Jacob Styan, Carson Robinson, Morgan Rothermel, and Mitch Clodfelter.

 

 

 

 

 

 

 

 


Insect monitoring in soybean: what to look for during pod fill

At this point in the season, most of our insect monitoring efforts are focused on soybean. There are several pests that can damage soybean during pod fill, and proper scouting is necessary to identify and, occasionally, control these insects. While not an exhaustive list, these are some of the insects and insect relatives to be on the lookout for as the growing season winds down.

Stink bugs. Stink bug (Fig. 1) feeding during pod fill (particularly R5- R6) can reduce soybean yield and quality. These insects feed directly on the developing seeds, resulting in wrinkling, shriveling, and discoloration in addition to reductions in yield. This damage can be compounded by pathogens and weather; note also that pathogen and weather-related quality issues can sometimes be confused for stink bug damage. Unfortunately, many fields in Illinois are not thoroughly scouted for insects during pod fill, and infestations sometimes go unnoticed until the crop is graded at the elevator.

Image of brown marmorated stink bug adult in soybean

Fig. 1. While not the most common stink bug in soybean, the invasive brown marmorated stink bug has been found more frequently in Illinois over the last several years.

Image of sweep net sampling in soybean

Fig. 2. Using a sweep net to sample insects in soybean

The most effective way to scout for these insects is with a sweep net (my personal favorite) or a drop cloth. (Both of these methods are also effective for many other soybean pests). A sweep net (Fig. 2) is swung through the canopy perpendicular to the rows a set number of times (usually 25 “sweeps” per sample). With a drop cloth, a small section of row (usually 3 feet) is shaken vigorously over a cloth, and the insects that are dislodged from the soybean canopy are counted. An insecticide application is warranted if you meet or exceed the economic threshold, which is 9 per 25 sweeps with a sweep net or 1 per row foot using a drop cloth. Note that the window of residual activity provided by insecticides for stink bug control is short (generally < 1 week); therefore, preventative applications targeted to a certain growth stage are unlikely to be effective. The most effective applications are those that are made only when (and if) a damaging population occurs –a rare event in Illinois.

Spider mites. Unfortunately, we have struggled with drought stress in parts of Illinois in recent weeks. While not a problem every year (and not an insect), spider mites often become an issue when soybeans are drought stressed. Spider mite feeding causes yellow to brown discoloration of soybean foliage (Fig. 3), and can result in severe stress to the plant. The infestations often (but not always) begin at field edges. Closely examining infested foliage will reveal the mites and the webbing that they produce. Shaking the mites onto a white piece of paper and/or using a hand lens might be necessary, as they are quite small.

Image of spider mite damage in a field with close-up

Fig. 3. Soybean foliage discolored from spider mite damage; inset shows a close-up of the underside of a damaged leaf, which has a “sandblasted” appearance.

Dectes stem borer. This insect caused some unexpected damage in southern Illinois in 2018. The stem borer larva feeds on the pith inside the soybean stem. As the plant matures, the larvae can girdle stems which leads to lodging, especially in situations where harvest is delayed. While we do not have an economic threshold for this insect (or a reliable way to control the larvae with insecticides), infested fields can be identified and, where possible, prioritized for earlier harvest to reduce their potential for lodging. Adult dectes stem borers are gray, long horned beetles that can be found using a sweep net or drop cloth. The first sign of infestation by the larvae is usually “flagging” of petioles in which a dectes larva has been feeding (Fig. 4). The larvae themselves can be observed by slicing the stem in half (Fig. 5).

Image of damage to soybean petiole from dectes stem borer

Fig. 4. “Flagging” of a dying petiole that has been fed on by a dectes stem borer larva (image: Scott Stewart, University of Tennessee).

Image of dectes stem borer larva inside of a soybean stem

Fig. 5. A dectes stem borer larva inside of a soybean stem.

Bean leaf beetle. Bean leaf beetle adults (Fig. 6) tend to be both the first and the last defoliating pest to enter soybean fields. As with other defoliating insects, the decision of whether or not to treat should be made based on (1) the level of defoliation in the field (the economic threshold post-bloom in Illinois is 20% defoliation) and (2) the continued presence of the pest within the field. Estimate the overall percent defoliation by collecting individual leaflets throughout the field. There are now several smarphone apps available that can help you to “calibrate” your defoliation estimation skills (e.g., BioLeaf Foliar Analysis for Android) (Fig. 7).

Image of a bean leaf beetle adult

Fig. 6. A bean leaf beetle feeding on seedling soybean foliage.

Image of defoliated soybean leaf

Fig. 7. A partially defoliated soybean leaflet measured by a smartphone app; approximately 25% of the leaf area has been removed by insect feeding.

As always, if you have any questions or are seeing anything unusual in the field, don’t hesitate to contact me. Happy scouting!

Author contact:

Nick Seiter | nseiter@illinois.edu | 217.300.7199


Dectes Stem Borer and Lodged Soybeans

Many soybean growers have had problems with lodging at harvest this year. The primary culprit for this (as for many of our woes this fall) was the extended period of unfavorable weather that we have suffered. However, in parts of southern Illinois damage by the dectes stem borer contributed to this problem.

The adult dectes stem borer (Figure 1) is a “long-horned” beetle that can often be found in soybean and on other plants. The adult female chews a hole into the surface of the plant (usually at the petiole), and lays her eggs in the pith. Upon hatching, the larva (Figure 2) tunnels throughout the stem and feeds on the pith. Feeding in the petioles often results in individual petioles wilting or falling completely off of the plant, which is usually the first sign of an infestation. As the plant nears maturity, the larva moves into the base of the stem where it will spend the winter. As awful as the bored-out stem of a soybean plant looks after being attacked by dectes stem borer (Figure 3), economic losses only occur if this damage leads to lodging. When preparing to overwinter, larvae will often girdle the base of the stem, causing the plant to break off and leading to harvest difficulties and reduced yield (Figure 4). This insect has one generation per year, with the adults usually emerging from soybean residue beginning in late June in Illinois to start the cycle again.

Adult dectes stem borer

Figure 1. An adult dectes stem borer at rest.

 

Larva of the dectes stem borer

Figure 2. A larva of the dectes stem borer removed from a soybean stem

Dectes stem borer larva in soybean stem

Figure 3. Damage to the inside of a soybean stem caused by a dectes stem borer larva

Soybean lodging caused by dectes stem borer

Figure 4. Lodged soybeans due to dectes stem borer feeding (Photo: Eric Alinger, Dupont Pioneer)

Insecticides are generally not recommended for control because the larvae are protected within the stem and the adults lay eggs over a long period of time in the summer (approximately mid-July through August in Illinois). While there appear to be some differences in varietal susceptibility, these differences are not well documented, and to my knowledge no soybean varieties have been characterized as resistant to dectes stem borer. However, there are some cultural management options available to producers:

  • Monitoring. While there is no economic threshold established, finding adults at higher than average numbers will be the first indication of a problem. Note wilting or broken-off petioles, and split soybean stems toward the end of the season to gauge the level of infestation. In addition, examine soybean stems in lodged areas to determine if dectes stem borer was part of the problem. If you have never done so before and you are in southern Illinois, the results might surprise you.
  • Timely harvest. Obviously, we would harvest on time every year in every field if we could. However, if you note fields that are infested with dectes stem borer, put those fields as early as possible on the priority list to reduce the potential for lodging.
  • Soybean stubble. Destroying or burying soybean stubble in the fall reduces dectes numbers locally, but the adults readily move from their overwintering sites to surrounding fields. Areas with a lot of no-till production are likely to have more issues with dectes stem borer.
  • Alternate hosts. Dectes stem borers feed on several other host plants, including sunflowers and giant ragweed. Areas with high populations of these plants could have higher populations of dectes stem borers as well. (As if you needed another reason to kill giant ragweed).

Contact:

Nick Seiter nseiter@illinois.edu (217) 300-7199

Research Assistant Professor, Field Crop Entomology

University of Illinois Department of Crop Sciences

7/26/19 – corrected to clarify reason for shedding of petioles