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


And the Survey Says…

Figure 1. What pests were most prevalent in Illinois corn and soybeans in 2018? The survey says…

 

For those that attended Agronomy Day this past August, the title and above graphic may look familiar. As field and research season winds down, we’re able to finish collecting and summarizing data. One of our biggest summer projects is the annual corn and soybean survey. While some of that information was shared at Agronomy Day, the complete results are summarized below.

As a recap, this survey has been carried out across the state for several years (2011, 2013–2018). In 2018, 40 counties representing all nine crop reporting districts were surveyed, with five corn and five soybean fields surveyed in each county. These surveys have been conducted with the goal of estimating densities of common insect pests. The estimates provided in this article should not be considered a substitute for scouting individual fields and making informed pest management decisions—even areas of the state that appear to be at low risk could have contained fields with high densities of a given insect pest.

Figure 2. Average number of Japanese beetles per 100 sweeps.

As I’ve talked with growers throughout the summer, in their opinion, the top insect pest of 2018 is the Japanese beetle. And both the survey results and I agree.

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).

Japanese beetle populations were higher statewide compared to 2017. Western Illinois saw record numbers last year and populations stayed high in 2018. The highest Japanese beetle populations remained in western Illinois, but numbers increased dramatically in the northwest as well (from 54 beetles per 100 sweeps to 175).

Table 1. Average number of insects per 100 sweeps on the edge of the field.

 

Table 2. Average number of insects per 100 sweeps on the interior of the field.

Western corn rootworms are always a concern, but populations have been very low in recent years. In addition to sweep samples in soybeans, 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. A mild winter followed by favorable conditions at egg hatch and adult emergence helped the small populations from 2016 gain some traction in 2017 (Table 3). However, per plant averages were lower in all districts again in 2018. Populations were variable. Many fields had low to nonexistent populations, but there were fields with higher numbers. It is important to remember, fields are randomly selected. We have no knowledge of insect management strategies that are used – soil insecticides, transgenics, or foliar applications.

Table 3 Mean number of western corn rootworm beetles per plant in corn by crop reporting district and year.

As we’ve seen repeatedly, grape colaspis populations are highly variable. Despite having reports of sporadic larval injury in the spring, adult populations were lower in 2018 compared to last year. We did see more stinkbugs as well as green cloverworms and soybean loopers statewide. While the majority of the stink bugs are green and brown, we did not pick up any of the southern species like red banded and redshouldered stink bugs in the survey. Brown marmorated stink bug was found for the first time in soybean field sweeps in several counties, though.

 

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. I would like to thank Illinois Cooperative Agriculture Pest Survey Program interns Evan Cropek, Hannah Hires, Calli Robinson, and Cale Sementi as well as Department of Crop Science intern Matt Mote.


Increased Insect Densities Reflected in Annual Corn and Soybean Survey

 

Thirty-six counties representing the nine crop reporting districts were surveyed at the end of July/beginning of August as part of our annual statewide corn and soybean survey. The surveys were performed by sampling five corn and five soybean fields per county. For the past several years (2011, 2013–2017), surveys in corn and soybean fields have been conducted with the goal of estimating densities of common insect pests. The estimates provided in this article should not be considered a substitute for scouting individual fields and making informed pest management decisions—even areas of the state that appear to be at low risk could have contained fields with high densities of a given insect pest.

Crop Reporting Districts

Figure 1. Illinois crop reporting districts surveyed during 2017 annual corn and soybean insect survey.

 

Western corn rootworm beetles were sampled in cornfields 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. A mild winter followed by favorable conditions at egg hatch and adult emergence helped the small populations from 2016 gain some traction in 2017 (Table 1). Per plant averages are up compared to recent years, though looking at the big picture, these numbers are still considered low. The district average from the northeast (1.95 beetles per plant), was affected by a single field in LaSalle county that average 7 beetles per plant which leads to a very important point to consider with this survey. Fields are randomly selected. We have no knowledge of insect management strategies that are used – soil insecticides, transgenics, or foliar applications.

Table1

Within an adjacent soybean field, 50 or 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 2 and 3).

Table2

 

Table3

The number of western corn rootworm adults in soybean fields throughout the state was low as well. The greatest number of beetles in soybeans occurred in McLean County, 8.00 beetles per 100 sweeps. All other counties sampled had fewer than 5 beetles per 100 sweeps (range of 0 to 2.4 per 100 sweeps).

Japanese beetles continued to increase in number from 2016 in the western part of Illinois. Both Fulton and McDonough counties recorded over 200 beetles per 100 sweeps in several fields, with their county averages of 525 and 340 beetles per 100 interior sweeps, respectively. Undoubtedly some of the highest numbers I’ve seen in this survey.

Overall, grape colaspis numbers were higher in several districts. This follows earlier reports during the growing season of grape colaspis feeding in soybeans. Unfortunately, there is no direct correlation between grape colaspis presence in soybeans and potential for larval injury in corn the following year. Numbers continue to be variable for this insect, but were high in the east southeast counties and should bear watching in 2018.

Stink bug injury in soybeans continues to make news in the southern states. We saw little damage caused by stink bugs in this survey, though numbers were slightly higher than past years. We continue to monitor for potential spread of not only the southern species like red banded and redshouldered stink bugs, but also the spread of brown marmorated stink bug as it gets its foothold here in Illinois.

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. I would like to thank Illinois Cooperative Agriculture Pest Survey Program interns Evan Cropek, Ryan Pavolka, Emma Sementi, Jacob Styan and Hannah Hires as well as Department of Crop Science interns Lacie Butler, Kaela Miller, and Matt Mote.