Grasshopper Management and Control in Montana2022 Spring
by Marni Rolston
who is the Crop Insect Diagnostician at the Schutter Lab
Grasshoppers are a normal part of the Montana landscape. They can be a source of high-protein food for beneficial insects, fish, birds and mammals, and some grasshopper species even prefer to eat certain weeds. Approximately 70 grasshopper species occur in the state, and of those, only about two dozen are considered pests of rangeland or crops. In a typical year when there’s plenty of moisture, seasonal populations of grasshoppers remove approximately 20% of rangeland vegetation, and feed sporadically on crops, but their damage isn’t very destructive because plants are able to compensate for feeding injury by regrowing.
However, grasshopper populations are cyclical, and when outbreaks occur, rangeland infestations may cause significant economic losses for producers by reducing forage available for their livestock, and farmers may see their entire crop decimated. The USDA develops “Grasshopper Hazard Maps” each year, using survey data from the previous summer to predict grasshopper abundance for the upcoming year. The 2022 Hazard Map for Montana predicts grasshopper numbers this summer that are higher than normal (Figure 1.).
Montana’s three most important grasshopper pests are the migratory (Melanoplus sanguinipes), clear-winged (Camnula pellucida) and two-striped grasshoppers (Melanoplus bivittatus). These species prefer habitats such as stubble fields, ditches and weedy pastures that have plenty of bare, dry ground mixed with grasses and forbs. This type of habitat provides the perfect location for grasshoppers looking to maintain optimal body temperatures and to lay their eggs. Grasshoppers are ectothermic, which means they can’t internally regulate their body temperatures and rely instead on the external environment for heat exchange. Open, sunny locations allow them to warm up, so they can feed, reproduce, avoid predators, and grow more quickly. Droughts exacerbate these conditions, resulting in grasshopper outbreaks. However, the onset of cool, wet, spring weather can abruptly end grasshopper outbreaks by slowing down their rate of growth and increasing the prevalence of diseases.
Adult grasshoppers are the most damaging stage because they’re larger, eat more, and can quickly fly from one area to another. Nymphs can’t fly, but if they hatch in areas without suitable or sufficient food, they can move, at times in swarms, by crawling and hopping. Grasshopper control is most effective for nymphs since they aren’t as mobile and are more susceptible to insecticides.
Cultural controls for grasshopper management are best implemented as part of an overall Integrated Pest Management strategy when populations are seasonal. However, during outbreak years these strategies may be most effective if they’re combined with other management approaches. On rangeland, grazing regimes that occasionally remove cattle from the landscape allow vegetation to regrow, which in turn creates more shaded, less desirable habitat for grasshoppers. In crops, farmers can use summer fallow as a trap crop, which attracts egg-laying females. Tilling the following spring reduces the emergence of nymphs. Farmers can also modify seeding regimes so crops aren’t as susceptible to grasshopper damage. Early spring seeding allows for the early establishment and vigorous growth of plants, so grasshopper feeding isn’t as harmful. In winter wheat, delayed fall seeding may be helpful, unless there’s a late frost. Also, avoid planting near fields with late-maturing crops, because the green plant cover may attract hungry adults, which will also be looking for a good place to lay eggs. Finally, in years where high grasshopper numbers are anticipated, doubling the seeding rate along field borders may slow migration into crops.
Damaging grasshopper species can reach outbreak levels and cause serious economic losses, especially when accompanied by a drought. On rangeland, treatment may be justified when there are 15-20 nymphs/yd² or 10 adults/yd². This amount of feeding damage is equivalent to losing 200-500 pounds of forage/acre, in part because grass stems and blades are cut, reducing seed stock and impacting grass production. The University of Wyoming has developed a grasshopper treatment strategy called Reduced Agent and Aerial Treatments (RAATs) (Figure 2.). This “skip-pass” approach can be applied by ground or air, and it uses lower rates of insecticide while achieving 80 to 95% control (compared to 85-99% control with complete blanket coverage at the full insecticide rate) at a lower cost.
In the spring and early summer, diflubenzuron is typically used for RAAT applications because it’s effective against nymphs, and is less toxic to nontarget animals and the environment. This chemical is an insect growth regulator that prevents immature insects from molting, so the best time to apply it is after most grasshopper eggs have hatched and before adults are present. Later in the season, when most grasshoppers are adults, carbaryl and malathion are more effective.
Grasshoppers often move into cropland from surrounding grassy areas that are drying out. Before treating, it’s important to know the economic thresholds for both nymphs and adults (Figure 3). Crop protection is typically achieved by applying a border treatment of insecticide to prevent grasshoppers from entering. A border width of 150 feet may be adequate, but if grasshopper densities are severe, spraying up to ¼ mile around the edge of the field may be justified. Many different insecticides are registered for grasshopper control in crops, including pyrethroids. If the field is surrounded by rangeland, using RAATs may be the best course of action. Insecticide baits can also be used, but they aren’t recommended for grasshopper densities higher than 25/yd². These treatment guidelines work for many Montana spring crops, including spring wheat, dry beans and alfalfa.
For winter wheat, the combination of young plants emerging in an arid landscape with large populations of adult grasshoppers is a recipe for disaster. It’s important to be much more proactive when protecting winter wheat, especially when grasshopper numbers are high. Use the lowest treatment thresholds (Safe Rating) in Figure 3, and apply insecticides at the higher end of the label rate. Finally, the timing of treatment is critical. It’s best to spray just before the crop emerges. If it’s applied too early, there’s not enough residual, and if it’s applied too late, feeding damage has already occurred.
These active ingredients may not be all inclusive and endorsement of products is not intended. Read the pesticide product label for legal use of products on various crops.