March 9, 2012

The risk of grasshopper infestation in western Nebraska rangelands was high in 2011, but overall there was a decrease in average adult numbers. Subsequently, 2012 will have a reduced probability of outbreaks in many areas. Average adult grasshopper numbers (per square yard) dropped from 17.8 in 2010 to 15.6 in 2011.

Grasshopper numbers are responsive to environmental conditions; however, species differ in their response to precipitation, plant communities, soil characteristics, and microclimate. A wet spring and rains at time of grasshopper hatching and development (mid-May to June) improve rangeland forage quality and reduce the loss from grasshopper infestation. Ranchers and range managers should be prepared to monitor grasshopper densities during these times.

The risk category in 2012 is based on 2011 surveys conducted by the U.S. Department of Agriculture's Animal and Plant Health Inspection Service. Numbers of adult grasshoppers last year are an index of the number of eggs laid, which is the overwintering stage of the most damaging species. Most grasshopper species are not considered rangeland pests and many have been shown to increase plant production under certain circumstances. Of over 100 grasshopper species present in Nebraska, species of concern for 2012 include Ageneotettix deorum, Phoetaliotes nebrascensis, and Melanoplus angustipennis, which comprised 30%, 11.5%, and 7.9% of the total adult numbers from 2011, respectively. Ranchers should look for elevated numbers of these species.

Factors Affecting Grasshopper Numbers

The potential for high grasshopper density will be determined largely by two factors:

Food Availability. Immediately after grasshoppers hatch from their eggs, they have few fat reserves and are vulnerable to cool, wet weather. If they can't feed readily during these early stages, high mortality will result. Since grasshoppers hatch over an extended period, only some of the hatch may be affected; however, this mortality can be significant enough to reduce heavy populations below threshold levels in many areas. Grasshoppers feed based on the need for a limiting nutrient, mainly nitrogen, and can compensate by supplementary feeding on a plant with high nutrient content. Proper range management practices are key, as grasshopper densities can increase significantly on disturbed rangeland as a result of overgrazing. Additionally, the amount of time since overgrazing and disturbance affects grasshopper feeding preference. Early hatching species are often better able to survive on disturbed rangeland because annual plants are dominant during the spring, while perennial grasses are more suited for sustaining populations later in the year.

Rainfall. In areas with ample rainfall, especially in mid-May, fewer problems will materialize because of the increased grass growth, resulting in less pressure for grass. Additionally, wet weather improves conditions for pathogens (e.g., fungi) that attack grasshoppers. Dry conditions that limit grass growth and depress insect pathogens result in a greater value for the available forage and a greater need to manage grasshopper populations. Healthier, more vigorous grass growth equals fewer grasshopper problems. Additionally, bare soil improves conditions for oviposition (egg laying), and may result in higher populations the following year.

Benefits of Using RAATs

If grasshopper populations readily persist through the early hatching period and dry conditions limit grass growth, there likely will be widespread areas with serious grasshopper infestations, and control may need to be considered.

The recommended control method for rangeland grasshoppers is the reduced agent/area treatments, or RAATs program. The RAATs program has been widely used and ranchers have been satisfied with the control levels they've seen. RAATs consists of spraying a swath and leaving a swath untreated so that only half the treatment block is sprayed, reducing treatment costs.

Any of the three insecticides registered for rangeland grasshopper control can be used, but Dimilin has been used almost exclusively with this program in Nebraska. The longer residual of Dimilin (21-28 days) allows time for grasshoppers to move from the untreated areas into the treated area and contact the insecticide.

The overall effectiveness of control may be reduced slightly with this method, but the cost will be reduced by 50% or more. A major cost determinant for using RAATs is the size of the treatment block – larger blocks are much more efficient for applicators to treat. If treatments are warranted, ranchers are urged to work together to treat larger areas to increase the efficiency and reduce the cost of treatments.

Jeffrey Bradshaw
Extension Entomologist, Panhandle REC
Sean Whipple
Postdoctoral Research Associate, Panhandle REC
Robert Wright
Extension Entomologist, Lincoln