Late-Season Scouting for Grasshoppers in Soybeans and Alfalfa

By Anthony Hanson and Ian MacRae et.al

Grasshopper populations have become more noticeable this year, especially along field edges, pastures, and CRP acres. Due to the hot dry conditions, population densities earlier in the year were similar to what we’d typically see in August in some areas, which has led to sustained or even higher populations as we now move towards the end of the growing season. Earlier this summer, some parts of the state, such as northwestern Minnesota, did have treatable levels of grasshoppers in small grains. While grasshoppers are making their presence known this year, presence alone does not necessarily mean they are causing economically significant damage in soybeans or forages (Fig. 1). Insecticide options may also be limited as harvest season approaches.

Figure 1. Differential grasshopper on soybean. Photo: Anthony Hanson

There are multiple species of grasshoppers in Minnesota that can commonly be found in field crops (Table 1). Immature grasshoppers (called nymphs) of all these species can be found in May and June, but egg-laying adults can occur at different times of year. Across all species, adults typically prefer to lay eggs in untilled soils, such as grasslands, and as populations build, grasshoppers will move into adjoining crop fields. Red-legged and two-striped grasshoppers will oviposit in alfalfa and soybeans, sometimes well into the field.  However, if defoliation and pod feeding are below economic levels, it is not a good practice to treat grasshoppers to prevent egg laying. When we’ve had August-like conditions throughout much of the summer, higher numbers of grasshoppers and feeding damage is expected as these conditions are ideal for grasshopper populations to build.

Table 1. Summer lifecycle of major cropland grasshoppers

Soybeans

Soybeans can tolerate some defoliation before yield is affected enough to make an insecticide treatment a consideration. Grasshopper leaf feeding is most common, but pod feeding can affect yield much more directly and is less tolerable.

To scout for grasshoppers in soybeans, examine at least 10 plants randomly throughout the field to estimate defoliation. In reproductive stage soybeans, insecticide treatment should be considered if more that 20% of the leaf material across the whole plant is defoliated (Fig. 2), which is a metric used across damage across all leaf feeding insects that may also be present in the field at the same time. If feeding damage is present on more than 10% of pods.

Figure 2. Defoliation estimates for a single trifoliate soybean leaf. When scouting, be sure to average defoliation across the entire plant. A single leaf could have 40% defoliation as depicted above, but the remainder of the plant could have 5% defoliation or less where treatment would not be warranted. Image: Robert Koch, UMN.

For more guidance on determining soybean defoliation percentages and decision-making, visit: https://extension.umn.edu/soybean-pest-management/grasshoppers-soybean

Alfalfa

The third cutting of alfalfa is mostly wrapped up in the state, but growers may be concerned about keeping stands healthy for fall growth, especially after walking through clouds of grasshoppers. To scout for grasshoppers visually in alfalfa fields, thresholds are calculated as the population per yd² (Table 2). When sampling within the field, be sure to get a representative count of the whole field. While walking through the field, visualize a 1 ft² area (imagine a large kitchen tile) at a point ahead of you. When you reach the 1ft² , count the number of grasshoppers moving or feeding within it (most easily done by moving your foot over the area and counting the number of grasshoppers that hop away). Count a minimum of 20 samples, calculate the mean number of grasshoppers per ft² and multiply by 9 to arrive at the population per yd² . Action thresholds vary for adult and nymphs and whether you are scouting the margin or inside the fields.

Table 2. Count-based grasshopper thresholds suited for alfalfa fields.

Control methods

Many of the insecticides commonly used for other soybean and alfalfa insects, such as pyrethroids and organophosphates, are labeled for grasshopper control. Large adult grasshoppers can be difficult to control, so use appropriate labeled insecticide rates. Especially as some soybean fields reach maturity, check the pre-harvest interval (PHI) on any insecticides that are being considered. Common PHIs can range between three to four weeks in soybean, so for some growers, the treatment window is narrowing. As plants mature, grasshoppers may also move out of soybeans in search of more palatable food sources.

Watch soybean and alfalfa fields with high populations next spring as well. Cold, wet weather next spring could solve a grasshopper problem for you.