Dry conditions the past few years have reduced, natural grasshopper population controls that thrive in warm, humid weather. While recent rains may have helped slow number growth, grasshopper populations may become an issue for pasture and hay crops as the summer continues if dry weather returns.
Control begins with scouting to determine if insecticides are economically useful. I can’t give you an exact economic threshold because variables like the value of the forage and the growth stage of both forage and grasshopper are factors that need consideration. For a very rough start, if the grasshopper population in an established alfalfa field are higher than 5 hoppers per square yard throughout the field or 15 hoppers per square yard in field margins, insecticides should be considered. Newly planted fields may need treatment if the grasshopper population is even lower. Economic thresholds for grasshopper densities in pastures or rangeland vary from 8 to 40 grasshoppers per square yard.
Grasshoppers consume up to 50% of their body weight every day in forage. A rate of just 2.7 grasshoppers per square yard equals 12,971 grasshoppers per acre. An acre with 69.7 grasshoppers per square yard equates to the consumption of forage by one cow per day.
To scout, randomly select a point several feet away and visualize a one-square-foot area around that point. When first learning this method, practice with a measured square-foot area to improve your ability to visualize the counting area. Walk toward this point while watching this square-foot area and count the number of grasshoppers in or jumping out of the area. Repeat this procedure 18 times and divide the total number of grasshoppers by two. This will give you the number of grasshoppers per square yard (9 square feet).
Counting sites should be 50-75 feet apart and randomly chosen. Just after egg hatch, when grasshoppers are small, they will be difficult to see and underestimating the true hopper density is common.
Around many alfalfa fields, grasshoppers have just started moving in from the field margins. Treating just the outside 150 feet or so may be sufficient in these situations. However, if the entire field already is infested, it usually is best to first harvest the alfalfa and then apply insecticide to protect the regrowth.
For hay ground, we can reduce the cost and amount of insecticide used when treating an entire field by bringing the grasshoppers to a common area we can focus our application on. Harvest the hay but leave several small, uncut strips across the field. The remaining grasshoppers will quickly congregate in these strips, enabling you to just treat these smaller areas.
Another option to reduce herbicide use in both pasture and hay ground is the Reduced Agent/Area Treatment (RAAT) method, pioneered by the University of Wyoming. This chemical control strategy utilizes the insecticide Dimilin which interferes with the molting process of grasshoppers. Dimilin is applied in alternating strips, reducing application costs by 50 to 60% and reducing the amount of insecticide used by 65 to 70%, compared to conventional broadcast treatments. The RAAT system provides up to 85% control, depending upon rate of growth of the forage, the size of the grasshoppers and the coverage obtained. Corn or canola oil used as a carrier in grasshopper treatments can increase effectiveness as these are grasshopper feeding attractants.
Another treatment option is the use of insecticide bait. Baits are best used on rangeland with short, dry vegetation. Some grasshopper species will not feed on baits, so knowing what species you are trying to control is important. Uniform distribution of bait and re-application if the bait no longer is attractive to the grasshoppers is important. Attractiveness of the bait will be substantially reduced by rain or heavy dew.
Remember to carefully read and follow all label directions and be especially careful to avoid injuring bee and other important pollinating insects. Finally, those producers with ground close to or in the sandhills region may also be within the range of the American Burying Beetle, a federally threatened species, and should contact the U.S. Fish and Wildlife Service before using insecticidal grasshopper control methods.
-Ben Beckman is a beef systems Extension Educator serving the counties of Antelope, Cedar, Knox, Madison and Pierce. He is based out of the Cedar County Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: ben.beckman@unl.edu