Defoliators reaching treatment thresholds in some NW MN soybeans

The first painted lady butterfly (Figure 1) made its way into my side yard (8 miles east of Crookston) in early June.

Painted lady butterfly. Photo credit: Whitney Crenshaw, Colorado State University, Bugwood.org

Pretty butterflies to some, to those that have battled their thistle caterpillar (Figure 2) offspring they are anything but.

Figure 2. Thistle caterpillar in a leaf that it rolled to provide protection (Crookston, MN, 2019).

Recently I have heard from trusted crop advisors in Mahnomen, Marshall and Norman Counties that in certain client fields, injury caused by both thistle caterpillars and green cloverworms have reached treatment thresholds.

During the middle of August in 2019, Extension entomologist Bob Koch, wrote an article that I quote here,

"To obtain an estimate of the level of defoliation for a field:

1. Select at least ten plants (more for larger fields) spread throughout the field.
2. From each plant, select a leaf from the top, middle and bottom third of the plant.
3. Use the reference below (Visual Guide for Estimation of Soybean Defoliation) to estimate percent defoliation for each leaf. Average the percent defoliation across the three leaves from each plant and then across the multiple plants to obtain the average percent defoliation per field.
4. The average percent defoliation per field can be compared to treatment thresholds for the decision about pest control".

A soybean defoliation injury training tool is available online here to help folks to train their eyes to be able to impartially pick out treatment threshold levels of defoliation.

"By using this method you ensure that the estimate of defoliation is representative of the whole canopy. Furthermore, this should help overcome the tendency of many people to overestimate percent defoliation for a canopy.

Treatment thresholds for defoliation from any combination of defoliating insects are 30% defoliation in pre-bloom growth stages and 20% defoliation from bloom to pod fill growth stages. When thresholds are exceeded, labeled rates of foliar insecticides can be used to protect soybean yield from losses due to defoliating insects. Follow directions on the product label. However, prior to treating a field, it is important to ensure the pests are still present. If the pests are no longer present (or no longer in feeding life stages), it makes little sense to treat the field.

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