2019 grasshopper populations and risk of infestation and damage in Colorado
Colorado State University Extension
According to the 2018 USDA APHIS adult grasshopper counts, http://www.aphis.usda.gov/plant_health/plant_pest_info/grasshopper/…/hazard.pdf, there were low populations of grasshoppers in Colorado last year with the exception of some areas of a low to moderate risk of rangeland infestations in southeastern counties this year.
The rest of Colorado had much lower counts of the insect and no risk of grasshopper infestations and damage expected in 2019.
We encourage ranchers and producers to monitor grasshopper situations in your area in those counties with moderate risk of the hazard. Generally, grasshoppers have one generation per year. Eggs are deposited in the ground in the fall. The eggs hatch in the spring and summer (late May through early June) and hatch is dependent on soil temperature, which differs for different species.
Weather conditions will determine how much of the damage potential will be realized in those areas with light to moderate populations of grasshoppers. Most grasshopper outbreaks are associated with several years of dry conditions.
The simple economic threshold for grasshoppers in rangeland is 15-20 grasshopper nymphs per square yard. This number is equivalent to eight to 10 adult grasshoppers per square yard. However, the economic importance of an infestation is affected by such factors as grasshopper species, 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 results in untreated swaths and swaths treated with reduced chemical rates. Using lower rates and leaving untreated areas reduces treatment costs by as much as 50% and preserves biological control. Grasshoppers move constantly, insuring that they will enter a treated swath and that levels of control will be similar to 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 (https://wiki.bugwood.org/HPIPM:Main_Page) appropriately equipped to apply carbaryl or diflubenzuron. See labels for grazing restrictions.
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.
Dimilin (diflubenzuron) treatment for grasshoppers should be applied in second to third instar stages because this growth regulator insecticide will not control adults. Strategies for managing grasshoppers in cropland are somewhat different. Recommendations for specific crops can be found in the High Plains Integrated Pest Management Guide, (https://wiki.bugwood.org/HPIPM:Main_Page). ❖
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