July 5, 2007
There have been reports from numerous locations in the Red River Valley of small grasshoppers both in CRP, ditches and roadsides, says Ian MacRae, University of Minnesota extension entomologist, Northwest Research & Outreach Center, Crookston, Minn.
"I haven't heard reports of grasshoppers in crops yet," he says
But in case the small hoppers turn out to be one of the five species of grasshoppers that can cause damage to crop he recommends that you begin scouting now to determine infestation levels.
Twostriped, Migratory, Clearwinged, Redlegged and Differential grasshoppers pose a threat to crops, MacRase says, because readily feed on a variety of plant species and will switch from one crop to another when the first either becomes less palatable (e.g. drying wheat) or when the first is harvested.
Because all five species prefer to lay eggs in no-tilled soil, infestations generally start at field edges and then progress into the field.
Scout the field margins first, MacRae advises. If there are no grasshoppers in field margins, it is unlikely they are in the middle of the field. The exception is that some species will lay eggs in soybeans, alfalfa, and dry bean fields. Also at risk are no-till fields, where grasshoppers prefer to lay eggs.
To scout for grasshoppers, walk along field edges. Picture a 1 square foot area in front of you. As you approach, count the number of grasshoppers that hop out of that area. Repeat the process approximately 20 times through the area you are checking. Multiply the average number of grasshoppers you see by 9 to get the average number of grasshoppers per square yard.
The thresholds for spraying are:
Nymphs - 50-75 per square yard at the field margin and 15-25 per square yard within the field
Adults - 10-20 per square yard. at the margin and 8 per square yard within the field
For more information on grasshopper management, see nwroc.umn.edu/ent/redent.html.
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