Weed & Pest
UW entomologist encourages early management of grasshopper populationsWritten by Natasha Wheeler
Worland – “I think grasshoppers are one of the most important and most frequent pests on small acreages for range and forage production,” remarked University of Wyoming (UW) Assistant Extension Entomologist Scott Schell at WESTI Ag Days in Worland on Feb. 20.
There are approximately 120 species of grasshoppers in Wyoming, and there are enough differences between them that it is important to identify species that may be found in certain regions.
“In Wyoming, we have about a dozen species that are serious pests. Some of those are only pests in rangeland, others are a multi-threat, and some of them are more of a threat in croplands or gardens and horticulture with ornamental plants,” Schell explained.
As an individual insect, a grasshopper generally has very little impact, but high numbers can cause significant damage. An adult two-striped grasshopper, for example, can waste six times as much forage as it eats, and it can eat its weight daily in green vegetation.
“If there are 30 two-striped grasshoppers per square yard, which is not necessarily a very heavy infestation, that’s 200 pounds of grasshoppers per acre,” he stated. “That’s like having sheep on every acre of our field, eating their weight daily and wasting six times more than they eat.”
Although the two-striped grasshopper is one of the more destructive species, other species can also make an important impact on an operation.
Some grasshopper species overwinter as nymphs, but a majority of species spend the winter as eggs, buried within the top 1.5 inches of soil. Typically, the eggs hatch in the spring, coinciding with the emergence of new green vegetation so they have plenty of food to eat.
“They start out looking like miniature adults, and after growing stages, they have to molt because they have a hard exoskeleton. Most of them have about four or five molts before becoming adults,” he explained.
In most species, full-sized adults grow wings and then begin to lay eggs in the soil.
“Usually, it’s not recommended to treat at that point. If we do treatments, we want to try to do it early to prevent damage, growth and any egg laying for the next year,” he said.
Grasshoppers are generally most vulnerable during their first stage of life after hatching, and bad weather or other events can significantly impact their populations during that time.
“That’s probably the reason we have some years that are good and bad with grasshoppers,” he noted.
Some species of grasshopper are capable of traveling great distances. The insects have been observed marching seven miles to find food, and airplanes have encountered them at 8,000 to 10,000 feet of elevation.
In the 1930s, grasshoppers were documented flying up to 180 miles from South Dakota to Wyoming.
However, Schell noted, “Most years, grasshoppers die where they hatch.”
Large seasonal populations are more likely due to mortality rates than migration. Experts estimate that in any given year, there are enough eggs deposited in the soil to support an outbreak the following spring.
“For example, for one species found in the Bighorn Basin, 40 grasshoppers can produce up to 900 eggs per square yard,” he comments, adding that the insects prefer to deposit their eggs along irrigation ditches where the ground has not been cultivated or sub-irrigated.
“If we have grasshoppers originating in crop areas, a lot of times they’ll be in the corners, along the roadsides, fence rows, and all of those places that are not sub-irrigated where the grasshoppers have had successful development in the soil and hatched,” he described.
This can mean grasshoppers are sometimes more prevalent in no-till or minimum till systems, where equipment has not disrupted the soil and damaged the eggs.
“Protecting the field from the grasshoppers before we get into a field is the most important thing because it’s so much cheaper and easier to treat an infestation when they’re out of the field,” he continued.
Treatment options labeled for crop use are typically much more expensive than non-crop treatments, and Schell suggested preventing incidents that require the use of chemicals if possible.
Different species of grasshoppers can also hatch over a period of 52 days, so multiple treatments may sometimes be necessary.
“Because grasshoppers have prolonged hatching periods, we may need to spray again. Scout the area to see if a hatch is going on, and if a landowner needs to, they should spray again. The products can start to wear off after a few weeks,” he remarked.
New vegetation will also be untreated and preferable to the insects.
“Applying early is critical,” he stated.
Schell also reminded producers to read all product labels, follow directions closely and to wear appropriate protective equipment when applying pesticides.
“We want to determine how bad the infestation is, what stage the grasshoppers are in, what species they are and what threat they may pose to our broadleaf crops or forages,” he said.