7/21/2022 | Volume 25 issue 13
This newsletter and the advice herein are free. You usually get what you pay for.
Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather.
As of 7/18, the SWROC is at 1371 GDD and 6.46 inches of precipitation since May 1. This compares to historical averages of 12182 GDD and 9.66 inches or precipitation. Corn entering reproductive stages and R2 to R3 stage soybeans are waiting for rain.
Two dry springs have favored the survival of small grasshopper nymphs, and now grasshoppers are becoming more evident in SW Minnesota. Most of these have been red-legged grasshopper nymphs with some differential grasshopper nymphs mixed in.
Cold, wet springs are detrimental to newly hatched nymphs that need to feed soon after hatching. During wet weather, grasshoppers are often controlled by several diseases. As grasshopper populations increase, expect their predators to increase also. Blister beetle larvae prey on grasshopper eggs but adults in hay are toxic to livestock, particularly horses.
Grasshoppers lay eggs in the late summer, preferring un-tilled soil. Red-legged grasshoppers often lay eggs in alfalfa. Both red-legged and differential can be found within soybean fields and may lay eggs there. Focus your initial scouting efforts on no-till fields, alfalfa ground, road ditches, and edges of grasslands. Large nymphs and adults often migrate from areas where they hatched into crops.
How many grasshoppers are too many?
In soybeans, grasshopper feeding injury can be combined with other defoliators. Treat reproductive stage soybeans if insects are still present and whole plant defoliation reaches 20%. Treat if pod feeding reaches 10% and treat aggressively if grasshoppers start to clip pods. For more information see grasshoppers on soybean.
You can use an action threshold based on grasshopper numbers too. The following Action thresholds based on numbers have been around for a long time, work for multiple crops, and work well.
- Estimate numbers emerging in 20 one-square-foot areas ahead of you as you walk.
- Calculate the average and multiply by 9 to obtain the number/square yard.
- Alternative: Four 180-degree sweeps with a 15-inch sweep net ~ 1 yd2
- Treat when grasshoppers reach the threatening level ( > 30-45 nymphs, 8-14 adults) within the field.
- When nymph numbers are severe (> 60) in adjacent non-crop, treat to prevent movement to cropland. Use an insecticide labeled for both areas.
- Large nymphs and adults are mobile and harder to kill. Enlarge treatment areas into the crop.
- Use the higher rates of labeled insecticides for large hoppers.
- Hot weather reduces the efficacy of many insecticides.
Root feeding larvae are still present, and rootworms are beginning to be more obvious. We have 1st instar to adult western corn rootworms at the SWROC and root damage is still increasing.
Scouting for corn rootworm beetles should begin in fields. Both whole-plant counts and yellow sticky traps have a place in rootworm management.
Red-headed flea beetles are causing concern in some fields.
Feeding usually is not economic and defoliation remains in the lower canopy. If flea beetles move to silks, include any silk clipping with that by rootworm beetles. I wouldn’t worry unless silks are pruned to less than ½ inch exposed and pollination is not complete.
Red-headed flea beetles also feed on soybean foliage.