1. Why did we see high populations of grasshoppers in Montana in 2020? Favorable weather conditions have promoted grasshopper pest populations. In 2019, a cool wet spring delayed hatching which resulted in high percentage hatch survival rates. In addition, vegetation was highly productive in 2019 especially sweet clover. As a result, grasshopper populations grew without being noticed or managed.
2. Why should I be concerned in 2021? A dry summer in 2020 and an open fall promoted a high egg lay which will result in large hatches this spring. If a drought continues in Chouteau County, populations will continue to increase. Grasshoppers are a boom and bust species and thrive with arid conditions.
3. Will a wet spring break the cycle? The answer is yes. Prime conditions for decreasing populations begin with warm, early springs followed by a hot period followed by a minimum of one week of cloudy wet weather. Warm early springs promote embryo development. A hot period in early spring promotes hatching and one week of cloudy wet weather promotes fungal pathogens on grasshoppers.
4. When should I start monitoring for grasshoppers? I would begin on May 1st. The grasshoppers of most concern ( Two-stripped, Migratory and Packard grasshoppers) all hatch close to May 15. Another monitoring rule of thumb is that embryos will continue development when the soil temperature rises between 50-55 degrees Fahrenheit. Eggs will typically hatch 14 days later.
5. I saw grasshoppers way before the soil temperature reached 50-55 degrees. Should I be concerned? The answer is most likely no. Catch the grasshopper and determine if it has wings.
Winged grasshoppers are adults that have overwintered and will not cause any harm to the crop.
6. How do I monitor for grasshoppers? Visualize a square-foot area from a distance and count the number of grasshoppers jumping out. Divide the number of grasshoppers by the number of sites and multiply by nine to estimate density per square yard. Be sure to completely disturb the square foot area because 1st and 2nd instar nymphs often will not jump.
7. Is it true that the migratory grasshopper is often the most harmful species to cereals? Yes, it can travel very long distances, destroy seedlings, defoliate crops throughout the growing season and clip cereal grain heads before harvest.
8. How do I tell a harmful grasshopper from a non-harmful grasshopper? Harmful grasshoppers are spurthroated which means they have a spine on the throat. Slantfaced grasshoppers have slanted faces that are pointed in profile or they have disproportionately large heads.
9. What would be a good step-by-step management order if grasshoppers continue to exceed economic threshold levels? First, spray grass borders and neighboring rangeland at beginning of egg hatch using Dimilin. Second, use Dimilin on field borders (a minimum of 150 feet). Lastly, spray contact insecticides if populations exceed economic threshold. Be sure to rotate insecticide groups to prevent resistance. Remember, Dimilin can be mixed with the knockdown insecticide to provide 30 total days of crop protection.
10. Where can I find more information for grasshopper management in cropland including economic thresholds? Information is available at https://chouteau.msuextension.org/agriculture/grasshoppers/index.html
Montana State University, U.S. Department of Agriculture and Montana Counties Cooperating MSU Extension is an equal opportunity/affirmative action provider of educational outreach.
