Western Minnesota soybean IPM survey results as of July 26

Angie Peltier, UMN Extension crops educator and Anthony Hanson, UMN Extension IPM educator

Crop scouts are out and about in western Minnesota soybean fields this summer scouting for pests and disease symptoms through a Minnesota Soybean Research & Promotion Council sponsored scouting program. Last week’s field visits have provided indications about what pests and diseases are becoming more prevalent in western Minnesota soybean fields. As planting dates and soybean varieties varied throughout the state, what was observed in scouted fields may differ significantly from what is ailing the plants in your own fields. There is no substitute for scouting your own soybean fields. Joint maps from University of Minnesota and North Dakota State University scouts can be viewed here from previous weeks and into August for soybean and wheat pests.

Soybean aphid

Figure 1. Low-level soybean aphid
infestation on a single leaf. Photo:
Robert Koch and Bruce Potter, UMN.

The percentage of plants infested with soybean aphids (Figure 1) ranged from 0 to 100, with some fields in northwest and southwest Minnesota and southeast North Dakota reaching 100% incidence (Figure 2). As winged soybean aphids leave buckthorn to fly to soybean plants in spring, the earliest planted fields with the largest plants are at particular risk for early infestation.

While the number of soybean aphids per plant were still relatively low as of last Friday (Figure 3), during the growing season, soybean aphids are born pregnant and when exposed to moderate temperatures (70-77 degrees) and low relative humidity can double in population in as few as 3 days. It is for these reasons that routine scouting of soybean fields is a must to both avoid losses to yield potential by waiting until treatment thresholds have long-since passed and to avoid unnecessary treatments before the treatment threshold has been reached. It is also important to understand the population dynamics in the field you are scouting. For example, is it later in the growing season and while your field is near the treatment threshold, you see more soybean aphids that have wings than don’t? Scouting once more a few days later will help you to determine whether those winged aphids took flight to find ‘greener pastures’ and a spray isn’t needed.

Figure 2. Soybean aphid incidence. Map provided by NDSU Extension IPM program.

Figure 3. Average soybean number per plant at each location. Map: NDSU Extension IPM Program.

Soybean aphid populations have reached the treatment threshold when all three of the following criteria have been met:

• The average aphid count is greater than 250 soybean aphids per plant
• More than 80% of plants are infested with soybean aphids
• The aphid population is increasing

Remember that the 250 aphid threshold is still very conservative, and detectable plant damage has not yet occurred. At this point, growers will typically have about 5-7 days before aphids reach the economic injury level (i.e., the break-even point for return on investment from insecticide) of approximately 670 aphids.

Should your crop reach the treatment threshold, while insecticides in the pyrethroid class of insecticides are still likely to be ineffective due to resistance, there are still several effective insecticide options, for example, Sivanto, Sefina and Transform and 10 chlorpyrifos-containing insecticides labeled for use in Minnesota in 2024.

For more information about the soybean aphid click here. Click here for more information about scouting for soybean aphid.

Bean leaf beetle

Figure 4. Bean leaf beetle on a soybean leaf. Note
that while the color of bean leaf beetles can vary
considerably, the small black triangle toward the
top of wing coverings stay the same.
Photo: Angie Peltier.

Scouts observed low levels of defoliation injury caused by bean leaf beetles and other defoliators (Figure 4), particularly south of Polk County in northwest Minnesota (Figure 5). However, overall incidence of bean leaf beetles themselves was low (Figure 6). The treatment threshold for insects that defoliate soybeans has been reached when the pest is still present and the severity of feeding injury is 30% or greater in vegetative plants or 20% or greater in plants that have reached reproductive growth stages. During this period of poor commodity prices, it is all the more important to make sure that each input will at least pay for itself. 

Figure 5. Average defoliation from bean leaf beetles and other defoliators. Map: NDSU Extension IPM Program.

Figure 6. Total number of bean leaf beetles per 50 sweeps. Map: NDSU Extension IPM Program.

Keep in mind that these defoliation thresholds for treatments are an average of the severity of defoliation throughout the entire canopy, not just the lower leaves, or the leaves in the mid-canopy or leaves toward the top of a plant and not just one spot in the field. 

To estimate percent defoliation, select multiple plants located in multiple areas of each soybean field, collect a leaf from the lower, middle and upper canopy of each of those plants and estimate percent defoliation for each leaf. One can 'train their eye' to better estimate soybean defoliation severity by taking the "Soybean Insect Defoliation Training" quiz until comfortable.

For more information about bean leaf beetles and treatment thresholds, visit this UMN Extension webpage.

Two-spotted spider mites

Figure 7. Two-spotted spider mites (red arrows)
and their eggs (blue arrows) on the bottom of a
soybean leaf. Photo: Angie Peltier.

As the summer has turned a bit drier in recent weeks than in the first couple of months this spring, two-spotted spider mite (TSSM) infestations are starting to be detected on field edges (Figures 7 & 8). As the food quality of the plants along field edges on which they begin the spring deteriorates as plants near maturity, TSSM enter crop fields in search of a better source of food. TSSM infestations are favored by hot, dry field conditions.

Figure 8. Presence (red triangles) or absence (black dots) of two-spotted spider mites on edge of scouted soybean fields. Map: NDSU Extension IPM Program.

For more information about TSSM, visit this UMN Extension webpage.

Foliage feeding caterpillars

Foliage-feeding caterpillars have been found on fewer than 11% of scouted plants (Figure 9). There are several foliar-feeding caterpillars than can injure soybeans in Minnesota, including green cloverworm, thistle caterpillar, alfalfa looper and soybean looper.

Figure 9. Percentage of plants infested with foliage-feeding caterpillars. Map: NDSU Extension IPM Program.

Treatment thresholds are reached during soybean reproductive growth stages when the pest is still present and at least 20% leaf injury is observed throughout the entire canopy. This is regardless of the combination of defoliators present. While green cloverworms have been observed in northwest Minnesota this year, overall defoliation severity remains below 11% (Figure 10).

Figure 10. Percentage defoliation caused by foliage-feeding caterpillars. Map: NDSU Extension IPM Program.

For more information about foliage-feeding caterpillars, here is an article written last year as many fields neared treatment thresholds in northwest Minnesota.

Soybean diseases

While scouts observed plenty of fields of soybeans suffering from symptoms caused by iron deficiency this spring and other fields with evidence of herbicide injury, foliar soybean diseases with a biological cause were slow to develop.

Phyllosticta leaf spot 

Figure 11. Soybean plant exhibiting symptoms
 of Phyllosticta leaf spot. Photo: Angie Peltier

In west-central Minnesota, scouts visited multiple soybean fields with a disease called Phyllosticta leaf spot (Figure 11). It is yet undetermined what has increased the incidence of this disease: soybean varieties more susceptible to the pathogen, pathogen populations better able to be seed transmitted or survive in residue from previously infected crops or a combination of weather conditions that has favored disease occurrence – likely a combination factors. If you think you have symptoms of Phyllosticta leaf spot on your soybeans please consider making a submission to Digital Crop Doc as we are interested in getting a better understanding of how widespread this disease is throughout Minnesota.

For more information about Phyllosticta leaf spot, check out this article detailing how to tell the difference between this disease and frogeye leaf spot.

Cercospora leaf blight

Figure 12. Cercospora leaf blight on Crookston
soybeans in 2018. Photo: Angie Peltier

Cercopsora leaf blight (CLB) can be found most years with high relative humidity as temperatures warm and soybeans reach reproductive growth stages (Figure 12). Scouts have observed up to 100% incidence of CLB in some surveyed fields (Figure 13). For those that grow sugarbeets, know that the Cercospora species (Cercospora kikuchii) that causes leaf blight in soybeans is a different species than the species that causes disease in sugarbeet.

Figure 13. The percentage of plants exhibiting symptoms of CLB. Map: NDSU Extension IPM Program.

The fungus that causes CLB colonizes leaf tissue and produces a light activated toxin and so symptoms of CLB tend to be most severe on the leaves at the top of the plant and can eventually lead to defoliation. CLB is an economically important disease of soybeans in the southern US. Please also let us know whether you have a severe CLB infection in your soybeans that results in defoliation by making a submission to Digital Crop Doc.

More information about CLB can be found here.

Other diseases observed

Symptoms of Septoria brown spot, alfalfa mosaic virus, frogeye leaf spot and bacterial blight have also been observed in some scouted fields.