Summer heat brings insect pressure in alfalfa stands. One pest that you may see increasing in numbers are grasshoppers.
With dry conditions statewide, natural grasshopper population controls that thrive in warm, humid weather are not as effective and the growing nymphs are seeking out high energy food as they mature. If conditions stay dry, they will only get worse as the year continues and control may be needed.
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 alfalfa and the growth stage of both alfalfa and grasshopper are factors that need consideration. Still, if the grasshopper population in an established field is 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.
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.
With second cutting occurring or happening shortly, 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 alfalfa 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.
If growing alfalfa treatment is needed, the Reduced Agent/Area Treatment method, pioneered by the University of Wyoming has been proven an effective control technique while reducing herbicide applied. To use, apply 8 oz. of carbaryl per acre in 100 ft. swaths alternating with 100 ft. untreated swaths in between, or 4 oz. per acre of malathyion in 100 ft. swaths alternating with 25 ft. untreated swaths in between.
Remember to carefully read and follow all label directions and be especially careful to avoid injuring bee and other important pollinating insects.
If you have many grasshoppers in your alfalfa, control them soon. As they grow larger, they’ll only get worse and control may be less effective.
-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: firstname.lastname@example.org.