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Grasshopper outbreaks expected

By Assefa Gebre-Amlak and Frank Peairs

Colorado State University Extension

The 2011 grasshopper hazard map, based on the 2010 survey of adult grasshopper activity conducted by USDA's Animal and Plant Health Inspection Service, Plant Protection and Quarantine, shows a greater than normal risk for outbreaks of these pests in some eastern Colorado counties. This map may be viewed at www.sidney.ars.usda.gov/grasshopper/Extras/map10.htm. This website contains a large amount of useful grasshopper information, in addition to the hazard maps.

Higher risk of grasshopper infestation is predicted for Yuma, Logan, Morgan and Washington counties in northeastern Colorado. Other counties with more localized spots of high risk include Kit Carson, Weld, Phillips, Sedgwick, Lincoln, Cheyenne, Kiowa, Prowers, Bent and Otero.

Weather conditions will determine how much of the outbreak potential will be realized. For example, cool wet conditions after hatch can result in enough mortality in immature grasshoppers to prevent an outbreak. In addition, if adequate moisture is available, forage regrowth will offset much of the grasshopper damage.

Most grasshopper outbreaks occur when drought conditions are prevalent.

There are over 100 different species of grasshoppers in Colorado but only about a dozen of these are considered important on rangeland, and five species of these cause most problems on crops.

Grasshoppers are important on rangeland because they compete with cattle for forage. The forage consumption of a grasshopper infestation averaging eight per square yard over a 10-acre area is roughly equivalent to that of an individual cow.

Landowners in high-risk areas should start monitoring grasshopper populations in rangeland soon after grasshoppers hatch, primarily during late May and June. Early scouting is important because treatments are most effective when grasshoppers are small. The goal of scouting is to get an estimate of grasshoppers per square yard, as well as their stage of development.

Economic threshold for grasshoppers on rangeland: The simple economic threshold for grasshoppers in rangeland is 15 to 20 grasshopper nymphs per square yard. This number should result in eight to 10 adult grasshoppers per square yard. However, the economic importance of an infestation is affected by such factors as range condition, cattle prices, and treatment costs. CARMA is a computer program that allows the landowners to include these factors in their treatment decisions. CARMA is available at the same website as the hazard map mentioned earlier.

Treatment options for grasshopper management are based on the Reduced Agent and Area Treatment strategy, which alternates untreated swaths and swaths treated with reduced chemical rates. Using lower rates and leaving untreated areas reduces treatment costs by as much as 50 percent and preserves biological control. Grasshoppers move constantly, insuring that they will enter a treated swath and that levels of control will be only slightly lower than complete coverage applications. Large infestations can be treated aerially with malathion, carbaryl or diflubenzuron (Dimilin). Smaller infestations can be controlled with RAAT treatments applied aerially or with all-terrain vehicles appropriately equipped to apply carbaryl or diflubenzuron. All-terrain vehicles also can be used for spot treatments of egg-laying sites such as pastures, ditches, and untilled field margins. Grasshopper nymphs tend to remain concentrated in their hatching areas for some time after they emerge, where the application of an approved insecticide can provide effective and economical control of localized infestations.

In Yuma County, Yuma County Pest Control District has been working with county landowners and aerial applicators to coordinate rangeland grasshopper management; to participate in this treatment program, contact the pest district at 970-848-2509. To find out if there is a treatment program in your counties check with your local county Extension office.

For more information on grasshopper survey results and technical assistance in rangeland grasshoppers' management, you may contact USDA APHIS Colorado office at 303-371-3355.

Strategies for managing grasshoppers in cropland are somewhat different. Recommendations for specific crops can be found in the High Plains Integrated Pest Management Guide, www.highplainsipm.org.

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