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Weed & Pest

Extension Education: Rangeland Grasshoppers: More Damaging in Drought

Grasshoppers are serious rangeland pests in the 17 western states of the U.S. where on average they destroy 25 percent of rangeland forage at an estimated value of $1 billion per year. In Wyoming, there are more than 100 different grasshopper species, and about a dozen of them are recurrent economic pests. 

All our grasshoppers are native and are an essential component of a healthy rangeland ecosystem. Normally, they stimulate plant growth, participate in nutrient cycling and provide food for many prairie animals and birds. Usually, grasshopper populations are regulated by weather and natural enemies, which keep their densities at low non-economic levels. However, from time to time, these control mechanisms fail, and grasshoppers produce devastating outbreaks. 

Densities of only 40 adults per square yard can equal 150 pounds of grasshoppers per acre over vast areas of rangeland. Since every grasshopper eats its own weight of green vegetation daily, the six-legged pests become fierce competitors for nutritional resources with livestock and wildlife. To save the rangeland forage, infestations require large-scale control treatments, which involve significant economic and environmental costs. 

Ideal conditions

What are the conditions that contribute to grasshopper population build-up? In Wyoming, grasshoppers spend three-quarters of their life cycle in the soil, in the form of eggs. Hatching for most species usually starts in May, after which the immature grasshoppers, or nymphs, develop during four to six weeks. During this period, hot and dry weather is very favorable for grasshopper growth and survival. It speeds up the grasshopper growth and precludes the development of diseases. 

Studies have found that the bulk of the grasshopper population, up to 90 percent, perishes during the first 10 days after hatching. At this time, inclement weather events such as heavy rainstorms or night frosts can cause mass losses of tiny grasshopper nymphs. But if the weather is hot and dry, the nymph survival rate increases sharply. 

During drought, rangeland vegetation becomes sparse and forage quality diminishes. Recent findings show that grasshoppers selectively seek plants with poor nutritional content, or low nitrogen. 

Hot and dry weather also increases the fecundity of females and prolongs their reproductive period. Normally, the adult grasshoppers live for a couple of months, but warm and dry weather in late summer and fall will increase this period, allowing the females to produce more eggs per pod and put additional pods into the soil.

Human contributions

It is important to note that we, as managers, can contribute to the grasshopper problem ourselves. Poor grazing practices and frequent use of old style, broad-spectrum insecticides to control grasshoppers may lead to more frequent grasshopper infestations. Drought conditions will only worsen the situation. 

Rather than trying to extinguish the “grasshopper fire,” every effort should be made towards preventing the outbreaks by practicing good grazing practices, such as leaving more residual forage to help cool and humidify the soil they hatch from. 

Addressing outbreaks

What can we do if preventative management fails, and we face millions of voracious grasshoppers gobbling the precious rangeland forage and making our livestock starve?

Grasshopper infestations can be controlled with reasonable economic cost and with minimum harm to the environment. 

During the last 18 years, University of Wyoming entomologists have developed, refined and delivered to clients an Integrated Pest Management (IPM) method of Reduced Agent and Area Treatments (RAATs) for rangeland grasshoppers. RAATs are a strategy in which the rate of insecticide is reduced from traditional levels and untreated swaths, or refuges, are alternated with treated swaths. RAATs work through a combination of chemical control whereby grasshoppers are killed in treated swaths and conservation biological control, where more grasshopper natural enemies survive in untreated swaths and attack grasshoppers there. 

The 2010 grasshopper outbreak in Wyoming proved the efficacy of the RAATs method. 

This outbreak was the worst in 25 years, and 5,903,616 acres of rangeland were protected in Wyoming using the RAATs method. Had ranchers used the traditional, blanket application of insecticides labeled for grasshoppers at conventional high rates, the entire program would have cost $21.8 million. RAATs effectively reduced pest grasshopper densities back below the economic level, but the resulting cost was only $7.4 million, or only $1.25 per protected acre! This means savings of $14.4 million to Wyoming agriculture, allowing Wyoming agriculturists to survive the severe pest outbreak and maintain the viability of their operations without harming the environment – even under extremely dry conditions.