Current Edition

current edition

Weed & Pest

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.

Weed scientists often discuss the importance of managing weeds to maintain or improve rangelands for livestock production, wildlife habitat or other ecosystem services. These implications may make intuitive sense because we have heard them repeatedly, but the details of how or whyweeds are able to have these impacts are often not discussed as frequently.

Carrying capacity

Weed infestations can reduce carrying capacity of rangelands by using resources that desirable plants need. 

Many of the problematic rangeland weeds are highly competitive against desirable plant species for limited resources. Light, soil nutrients and water are needed by plants to grow and reproduce. When one of these resources is limited in availability and neighboring plant species each need the resources, competition takes place. 

For multiple reasons, weeds are often better able to obtain these resources than native plants. Some potential reasons for the superior competitive ability of weeds are lack of grazing pressure from herbivores or insects, lack of pathogens leading to “healthy” plants, and an evolutionary history that resulted in highly competitive populations. Another potential explanation is allelopathy – where weeds produce chemicals that inhibit the growth of neighboring plants. This plant “chemical warfare” is often cited as a primary explanation for weeds forming very dense stands to the exclusion of other species, but the science to support widespread allelopathy in weeds is inconsistent. 

Regardless of the mechanism for competitive ability, the result for livestock producers is similar – increased proportion of non-palatable weeds in a management unit is often associated with a decrease in available forage. Several research studies suggest that only moderately dense infestations of weeds such as leafy spurge or Russian knapweed can greatly reduce grass production. As these weeds spread across pastures, increasing their area, total available forage in the pasture decreases. 

Ranchers facing this situation that do not manage their weeds may experience reduced weight gains in the pasture, or they may need to reduce the stocking rate in the infested pasture. Maintaining historical, pre-weed stocking rates may increase grazing pressure beyond sustainable levels in the non-infested portions of the pasture, thereby increasing susceptibility to further spread of weeds.

Rotational grazing

Weeds may limit flexibility in rotational grazing systems. 

Many current grazing best management practices are related to rotational or rest-rotation grazing strategies – try not to graze the same pasture at the same time every year, target approximately 50 percent utilization for plant health, limit the number of times a grass plant is grazed in the same year, etc. Most of these practices, with the exception of moderate utilization, are achieved by manipulating livestock distribution throughout the year using fences, water sources, herding or other methods.  Logistical constraints, such as shipping corrals or calving barns, also affect how a grazing rotation is implemented. 

Another factor that may limit which pastures can be used at a specific time of year is the presence of specific weeds. 

Toxic plants like larkspur and death camas have a higher probability of causing livestock deaths when they are palatable to livestock, usually in the early spring. Lupines can cause birth defects if bred cattle consume them during the middle of the first trimester. 

Direct losses associated with poisonous plants can cause significant economic impact to a livestock producer. Managing aroundthese toxic windows in plants limits, or precludes, the use of those pastures during dangerous times of year and also reduces the number of timing options to graze other pastures in the same grazing system. 

Nutritional value

Other weeds may alter the seasonality of nutritious forage. Heavy infestations of early-maturing weeds, such as cheatgrass or possibly bulbous bluegrass, may reduce the time that palatable green forage is available in a pasture. Such plants use early spring moisture to complete their life cycle in a short time period, and in so doing, reduce soil moisture for plants that may stay greener longer into the year. This effect is particularly evident in years with limited precipitation in later spring to early summer.

The impacts of invasive weeds in rangeland systems are varied and manifold. Thinking through how weeds may affect grazing decisions will enable land managers to better develop a strategic approach to their rangeland weed challenges.

Brian A. Mealor is an Assistant Professor and Weed Extension Specialist at the University of Wyoming and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it.. More information about ongoing weed science research at the University of Wyoming can be found at


“Managing pests and invasive species is everyone’s business,” said Maui County, Hawaii Soil and Water Conservation Service District Director Mae Nakahota. 

Nakahota and other conservation professionals across the West presented for the Western Governors’ Association webinar series on invasive species management. Each speaker presented on different issues affecting their area, as well as the importance of cooperation amongst parties.

Cover crops 

“As conservationists, we have often pushed cover crops without really paying attention to some of the negative impacts they can have,” said Nakahota. “These cover crops created the perfect home for semi slugs.” 

Nakahota explained the semi slug is very small and has a structure on its back that resembles a backpack. The slugs can carry a number of diseases including rat lungworm disease. 

“Rat lungworm disease originates from rats and is passed to amphibians such as frogs and slugs and then onto humans,” said Nakahota. “Recently, 60 people became ill after consuming a beverage left outside that had a slug crawl into it.”

“The islands have been highlighted as a source of the worms,” Nakahota said. “Cover crops need to be sprayed as an absolute no- amphibian zone.”

Purpose gone wrong

We also have to remember not all invasives start as an accident. Some species are brought in for a specific purpose and get out of control. 

“After massive pasture die-off, we brought in glycerin to help, and it got completely out of control,” said Nakahota. “What appears to be okay today may not always be okay tomorrow or in the distant future.” 

“The axis deer was brought to the islands as a gift to the King of Hawaii and has wreaked havoc ever since,” explained Nakahota. “The deer were in very limited numbers at first but became very popular among hunters, and their numbers have exploded.” 

She noted the deer compete with native species and livestock and have caused massive damage to watersheds. 

“The deer not only physically damaged the watersheds and caused erosion, they also spread disease by defecating in the water,” said Nakahota.

“We have to have boots on the ground and be proactive when it comes to any non-native species that we introduce purposely or is brought in accidentally,” said Nakahota.  

Collaboration is key 

In Oregon, the Clackamas River Invasive Species Partnership (CRISP) is using their numerous partnerships with local organizations to fight the ever-growing issue of garlic mustard, said Lindsey Karr, program coordinator.

“CRISP meets twice per year to discuss concerns regarding invasive species within the area,” said Karr. “Through this collaboration, we are able to discuss successes and failures with control and set priorities for controlling garlic mustard.”

Aside from trading ideas and information, the group is also able to secure more funds to aid in the control of invasive species, according to Karr. 

She explained the group took the watershed approach and divided the area into smaller zones and set habitat priorities. 

“We want to focus on areas that have good habitat and human recreation interest,” she noted. “Human activity can be a major source of spreading for species such as garlic mustard.”

“The trickiest part of this area is there are a lot of island,” said Karr. “Ownership is difficult to determine, and management history has been problematic.” 

She explained through their partnership they collaborated with local partners to figure out ownership of islands and who and how they had been treated. Local partners in the area donated use of a boat, enabling CRISP to treat invasive on the islands.

Analysis and treatment

“When we looked at garlic mustard, we found that 23 percent of the infestations outside our area were within the floodplain, and 87 percent were within the area were in the floodplain,” Karr pointed out. 

“What this shows is the advanced survivability of the seeds,” she said. “They do really well with disturbances and can spread around in a river setting.” 

Karr explained this analysis allowed for their team to create an educated plan to go about controlling the plant. 

“We spray in the spring in the fall,” she explained. “In the spring, we do two sprays and try to catch the plants before they seed.” 

She continued, “The plant flowers in its first year and in the second year is a rosette. We also utilize pulling, which allows for greater flexibility with the weather and to catch any plants that missed the initial spray.” 

She noted they would also spray in the fall because it is useful for carpets of seedlings or those mixed in with native annual seeds. 

“By treating in the fall, we allow native annuals to go to seed so they can regenerate it makes spring treatments lighter, as well,” according to Karr.

Callie Hanson is the assistant editor of the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..

One of the questions University of Nebraska Range and Forage Specialist Mitch Stephenson gets quite often is what is and isn’t considered a weed. 

Stephenson tells producers they should consider any plant growing where it is unwanted or a plant having a negative value within a management system as a weed. 

Identifying weeds

Noxious, native and exotic plants, as well as poisonous plants, can all be considered weeds, he says. 

Noxious weeds are designated by the agricultural authority as one that is injurious to agricultural or horticultural crops, natural habitats, ecosystems, humans or livestock, he says.

Poisonous plants are “plants containing naturally occurring toxic compounds that when consumed by livestock, can cause biochemical or physiological changes resulting in death or reduced performance.”

Native weeds can alter the landscape in a way that is undesirable in terms of forage production, wildlife habitat, aesthetics or ecosystem function. Exotic weeds originate outside North America and are often highly invasive and out-compete native vegetation.


Stephenson tells producers the best defense for controlling weeds is developing a good offense, especially if it is done before the plants put on a seed set. 

“I would create a good strategy and define what my objectives are. It will help control my time and costs,” he explains. “Monitoring results is also important.”

He recommends recording changes over time, in pasture and rangeland, with photos, instead of relying on the eye to remember.

Noxious weeds, like Leafy Spurge, musk and Canadian thistle, Eastern Red Cedar and Diffuse and Spotted knapweed should also be controlled. 

Stephenson says some management options are herbicides, mechanical methods like mowing or cutting it down, fire, targeted grazing and bio-control. 

Using herbicides

“Herbicides can provide the most direct and complete control of invasive species,” Stephenson explains. He also tells producers about the “Nebraska Herbicide Guide,” which provides an overview of what herbicides are available and most effective. 

He says recommendations of when to spray may differ based on whether the weed is an annual or perennial. Producers should also keep in mind that most herbicides have a zero- to 14-day grazing restriction. 

“A lot of times, we treat when we can, but to get the most bang for the buck, we should treat when its most effective,” he says.

Limiting seed set

Mowing or cutting can also be effective to limit the seed set, Stephenson says. 

“It is most beneficial when applied with other treatments, like mowing, just before the seed sets in late spring, and then spraying herbicide later in the fall,” he says. 

If the plant is a perennial, meaning it has a lifespan of more than three years, management differs, as well as recommendations of when to spray and how to control it. Stephenson says the most effective time to control biennials is when they are in the rosette stage, which is late fall or early spring. 

Some native plants are able to compete with noxious weeds, as long as moisture is adequate. In the case of the native forb western ragweed, Stephenson says one study showed, as the density increased from five to 41 tillers, scientists observed more grass had developed in north central Oklahoma. 

Another study shows western ragweed would need to produce more than 1,000 tillers an acre to affect grass production on the clay uplands in Kansas. 

“Western ragweed is not a strong competitor in the presence of vigorous grasses,” he says.

The problem with using herbicides on forbs like western ragweed is most herbicides are non-specific, Stephenson explains. 

“If we spray western ragweed, it can also impact other native forbs. Annual forbs can serve an important purpose. They provide ground cover and allow grass to establish,” he notes. 

If producers are using herbicides to control invasive plants, Stephenson recommends focusing on localized areas or spot spraying to avoid non-targeted species.

Target grazing 

Even well-managed rangelands can have some species of weeds invade them, he says. One of the most common is cheatgrass or Downy brome. 

“Cheatgrass actually has high nutritional value early in the growing season,” Stephenson says, noting a current study underway looking at how often cattle will select cheatgrass early in the growing season, and if native perennials are harmed by trying to graze the cheatgrass before it matures. “We want to see how much they select cheatgrass, compared to native perennial forage.” 

In the first year of the study, the cattle were turned out April 20 to start grazing, and then they stopped for about a week. “

There was a cold snap, the cheatgrass stopped growing, and they had already grazed it out.

  “Then, we hit a flush and the amount of cheatgrass in their diet was substantial,” he says. 

The ongoing study will continue this spring.

Targeted grazing can be key to controlling some invasive species, like cheatgrass and leafy spurge.

“The key is picking the right animal,” Stephenson says. 

Producers also need to consider what season of grazing will give them the most benefit, as well as duration and intensity. Target grazing can benefit forage quality for livestock, since many weeds are high in nutrient quality, he says. Targeted grazing can also offset some of the cost by adding value. 

He explains, “We may have to stock those areas pretty heavily, because, in a case like cheatgrass, it can get away from us if you don’t have enough head to keep up with it.”

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..

Casper – Across the western United States, landowners, government agencies and local weed and pest organizations alike are working to control the ecological changes that are occurring due to cheatgrass invasion.

Natrona County Weed and Pest Supervisor Brian Connely explains that the control strategies they use now will have a significant impact on what the Wyoming landscape will look like for future generations.

“We’re managing the land for our generation and for the next. After that, they can make their own decisions,” says Connely.


Bromus tectorum, commonly known as cheatgrass, is an invasive weed species from Eurasia. The annual plant is extremely aggressive and thrives in disturbed areas including fire, overgrazing, recreation and construction sites.

While it is extremely prevalent in the western United States, the plant has very limited use for wildlife and livestock grazing.

“Cheatgrass is only palatable for two to four weeks out of the year in the spring,” says Connely.

A variety of mechanical, chemical and biological control measures can be used in managing cheatgrass invasion and are typically used in combination. However, eradication of cheatgrass can be extremely difficult once it is established.

Diligence is essential in cheatgrass eradication, as seeds can remain viable in the soil for several years, depending on the soil.

“The literature says that the seeds remain viable in our soil anywhere from one to six years, with a caveat of nine years,” explains Connely.

While cheatgrass is not a state-designated noxious weed in Wyoming, it is a county-declared weed for many counties.

Changing ecosystems

Connely explains that the primary habitat in Natrona County is sagebrush steppe, and 66 percent of the nation’s pronghorn live within 200 miles of Casper.

Natural fires typically occur every 40 to 60 years in a sagebrush steppe habitat. However, cheatgrass invasion changes the fire interval to every five years.

“As the cheatgrass gets denser, the area becomes very fire prone. That time period in our ecosystem is right in time for summer thunderstorms,” he comments.

While the fire destroys all of the plant matter above ground, the cheatgrass duff on the ground creates insulation for a seedbank, protecting it from the fast-moving flames.

Cheatgrass also has an advantage over native grasses in using post-fire nutrients for rapid growth.

“After the fire, the cheatgrass is able to utilize nitrogen very well. Native grasses are not accustomed to nitrogen because there’s not much decomposition, resulting in low nitrogen in the native soil,” says Connely.

Because of the rapid growth and recovery of the cheatgrass compared to the native plants, the ecosystem changes to primarily cheatgrass and maintains the frequent fire interval.

“Our environment can’t do that,” stresses Connely.

Herbicide use

Using an integrative approach to cheatgrass control can be extremely effective, notes Connely.

“Chemical control, when used judiciously and efficiently, can be a great tool, but it is not a solution,” he says.

Those involved in control efforts are able to use the herbicide imazap, which is known by the brand name Plateau. Plateau is a pre-emergent herbicide.

“Before germination, in the late fall, we spray it on the ground. It can halt germination for 2.5 growing seasons,” Connely explains. “When released from competition, our native grass community will respond and thrive.”

The process uses six to eight ounces of herbicide per acre, depending on the soil type.

Prices for aerial treatment vary somewhat from year to year, but it costs approximately $16 per acre in Natrona County.

“That’s $9 for the pilot and $7 for the chemical,” says Connely.

Joint effort

“Cheatgrass is everyone’s problem because it changes everything we need and love out of this landscape,” emphasizes Connely. “It is important that people be able to recognize it as ‘my problem.’”

According to Connely, educating the public about ecosystem sustainability and health is essential to cheatgrass control. Noxious weed management is not well understood by many, requiring instruction on the complexity of the issue.

“Many people can’t tell whether the ecosystem is healthy or not, and they simply want an easy fix,” says Connely.

Cheatgrass control is a joint effort between landowners, government agencies, local weed and pest offices and other stakeholders.

“Local weed and pest agencies develop decision making priorities with the Bureau of Land Management, Natural Resource Conservation Service, Wyoming Game and Fish Department and the landowner,” he continues.

Future treatments

Research is currently being done on using Pseudomonas fluorescens as a bioherbicide. It is a naturally occurring, cold-loving bacteria that targets cheatgrass roots during spring growth.

“They are looking at bacteria as another management tool, not as a cure,” emphasizes Connely.

Like any other treatment option, there are periods of time that the bacteria would not be effective in controlling cheatgrass growth. However, Connely is hopeful that it will be a useful tool to use in combination with current treatments.

“If the treatments can be overlapped with chemical herbicides and other treatments and the ineffective period isn’t the same, maybe there’s a synergy there,” he concludes.

Emilee Gibb is editor of Wyoming Livestock Roundup and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..