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Weed & Pest

Grasshopper control

Riverton — Grasshopper management is an ongoing concern to many producers in Wyoming. University of Wyoming assistant entomologist Scott Schell says part of the management problem is lumping the approximately 120 species in Wyoming into one plan.
“In a specific habitat a grasshopper population will consist of five to 20 species with one to four of those species accounting for over 50 percent of the total population,” explained Schell during the Fremont County Farm and Ranch Days, held mid-February in Riverton, where he discussed grasshoppers and recommended control methods.
Most grasshoppers live and die in the vicinity where they hatch, he said. However, some nymphs will march several miles to find food when necessary. Other species can fly long distances and their migratory nature causes lots of problems from year to year. Last summer grasshoppers traveled over 280 miles from South Dakota into Wyoming. Understanding the differences in species could lead to more advanced and effective treatment methods, he explained.
“Producers can treat theirs and have others fly in. They are a difficult pest who are mixed feeders,” stated Schell. Adding that they will consume forbs and broadleaves first, then grasses, then evergreens and just about anything else that grows. In some locations grasshoppers have completely wiped out noxious weed populations.
Grasshoppers produce one new generation each year. Females lay eggs primarily in the soil and produce one to four egg pods with five to 40 eggs per pod on average. Schell noted that some species are much more prolific and that most can lay eggs in the hardest, nastiest soils available.
“Females secrete a foam-like substance when laying eggs into soil that protects and insulates them. The foam is a defense mechanism,” explained Schell.
Upon hatching the grasshoppers go through simple metamorphosis five times. Each stage is known as an instar and each instar lasts about a week until they reach adulthood. Their total life span is about three months and they mate eight to 10 days after becoming adults.
The most critical stage for control is during the first two instars, with the idea that the sooner the better after eggs have hatched. As eggs they are so small they have a microclimate and can tolerate cold temperatures. Upon reaching adulthood they are more mobile and resistant to control methods.
Hatching weather is very important, said Schell. Grasshopper eggs only last one year, so they must hatch. An accumulation of degree-days above a certain soil temperature triggers the hatch. For the Clear Wing species found in Wyoming, that temperature threshold is 50 degrees. “It’s all about the week when peak hatch occurs,” said Schell.   
During this time a hard rain could drown the hatchlings. Snow or severe cold could freeze them or lack of green plant life could starve them. Schell explained the theory that the cold, wet June of 2009 didn’t kill the grasshoppers because the soil temperatures were too low for them to hatch. They just waited until it warmed up later in the year. He also commented that a wet year in Wyoming would be considered a dry year in other parts of the west that also deal with grasshoppers.
“If they were sensitive they probably would have gone extinct long ago. They are definitely hardy and adaptable,” stated Schell.
Recommended control methods include Reduced Agent Area Treatments (RAAT), which involve applying insecticide to a percentage of an area. Schell recommends applying Dimilin 2L to 50 percent of an area with vegetable oil as the adjuvant.
Vegetable oil increases control because it contains fatty acids. Grasshoppers eat their dead to obtain these fatty acids and the vegetable oil causes them to key in on grasshoppers that were killed with Dimilin, which will also kill those feeding.
Dimilin is very safe for humans and other wildlife compared to other insecticides, such as Malathion and Carbaryl. According to Schell it is 70 to 80 times less hazardous to birds and several thousand times less hazardous to fish.
The insecticide has about a 20 day residual and does allow for tank mixing with some other chemicals. Producers can likely mix it with their weed spray when re-seeding or doing other chemical applications. It is relatively expensive but has a lower application level.
“Every year enough grasshopper eggs are produced to cause an outbreak the following year if a high percentage survive,” stated Schell. “It is difficult to control them because they aren’t a consistent pest. The infrequency of outbreaks hurts response and monitoring efforts.”
While some spider species, the blister beetle, Wind scorpion and Robber fly are all natural enemies of the grasshopper; they can’t keep up with them during an outbreak year. The faster development in an ideal year makes the grasshopper less susceptible to disease and natural enemies. A slide where a Wolf spider was on a fencepost surrounded by grasshoppers was shown to the audience. “He was too full to eat another one,” stated Schell.
Grasshoppers do play a major role in aboveground grassland biomes and are part of the diet of many prairie animals, including young sage grouse. “We don’t want to eradicate grasshoppers, just bring them back down under the economic threshold.” Schell added that part of the appeal of a RAAT control method is it doesn’t kill every grasshopper. The untreated swathes provide food to the natural grasshopper enemies but kill any that fly or march into a treated zone.
As producers prepare for another year with the grasshopper, an early control plan is key, said Schell. Weather plays an important, unpredictable role and can’t be counted on to eliminate populations. While management control methods don’t guarantee results, they are on option that can work.
Heather Hamilton is editor of the Wyoming Livestock Roundup and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..