By Sharla Sackman
Prairie County MSU Extension Agent
2011 was an interesting grasshopper year for Montana. Record spring rains in May and June did have a negative impact on early hatching grasshoppers. However, there are areas where the grasshoppers simply hatched later and the summer weather allowed them to reach maturity. The wet spring allowed for substantial grass/forage production, which may have buffered some of the impact, despite the high grasshopper densities in parts of the state. In addition, Montana had a fairly long, mild fall, which enabled the adult grasshopper populations to continue to lay eggs for quite a while. What this means, is only that there were likely a lot of eggs laid in 2011. How many eggs emerge and how many of those nymphs reach maturity will be determined primarily by this spring’s weather patterns.
More than 70 species of grasshoppers can be found in Montana the Northern Great Plains Region but only one to two dozen of these species become economic pests of rangeland and crops. Grasshopper populations tend to increase over a 2-4 year period and a few species tend to make up the majority of the outbreak.
Most grasshoppers complete their life cycle (egg, nymph, adult) during a single season. Eggs hatch in the spring and the juveniles go through a series of molts to become adults later in the summer. Grasshoppers develop to adults by incomplete metamorphosis, meaning the juveniles resemble the adults in appearance. Nymphs can be distinguished by their wing buds that increase in size after each molt, only becoming fully developed wings and functional for flying in the adult stage.
Fifteen to 20 grasshopper nymphs per square yard in rangeland or pastures is considered the economic threshold. This number is considered to equate to eight to 10 adults. However, the economic threshold can be modified by weather conditions. If moisture is adequate regrowth of the consumed or destroyed rangeland vegetation may offset the damage.
In spring wheat, 8-14 per square yard within the field, or 21-40 along the edge of the field, is considered to be the economic threshold for treatment. Newly seeded winter wheat can be particularly susceptible to damage since it emerges later in the season when many of the grasshoppers are larger adults. In winter wheat, 3-7 grasshoppers per square yard within the field, or 11-20 along the edge of the field, is considered to be the economic threshold for treatment. When present at very high populations, grasshoppers can be difficult to control since they can continue to migrate into a crop field. Crop protection typically involves boarder protection with an insecticide.
Chemical controls vary widely by habitat and crop. Nymphal grasshoppers can be safely controlled in many situations using Dimilin, an insect growth regulator with low mammalian toxicity and low non-target effects. Dimilin is not effective against adult grasshoppers. Both carbaryl (several formulations of Sevin available) and malathion are available for later season applications, once grasshoppers have reached the adult stage. However, their effectiveness is directly related to residual activity of the product because of the movement and re-infestation potential of adult grasshoppers (residual: carbaryl, 14 days; malathion, 1 day). Read labels carefully for specific information.
In rangeland Reduced Area and Agent Treatment Strategies (RAATs) can be used. The RAATs program developed at the University of Wyoming is an economical method of treating rangeland for grasshoppers in which both the insecticide rate of insecticide and acreage treated is reduced by alternating untreated and treated swaths. There are savings both in the acreage treated and in the amount of product used. Pesticides in treated swaths kill grasshoppers directly and also provide control as hoppers move from treated to untreated swaths. Predators and parasites are preserved in untreated swaths so that they can suppress grasshopper populations.
A long-term strategy to reduce frequency and intensity of grasshopper outbreaks was developed by Dr. Jerry Onsager, USDA, ARS, retired. He examined twice-over rotational grazing compared with traditional season- long grazing for grasshopper outbreak control. He found in rotational grazing that nymphs develop slower, survival rates by stage of growth were lower, fewer adults were produced, and they appeared later in the season.
If you would like more information on grasshoppers, please contact the Prairie County Extension Office at 635-2121.
Published May 4, 2012