Grasshoppers: Will This Be a Big Year?
This article originally appeared in the July 2005 Ag News and Views newsletter.
Grasshoppers are considered an intermittent problem in Oklahoma and North Texas pastures. However, when they are present in large numbers, the damage can be severe. Before grasshoppers develop into a problem, producers need to determine a treatment threshold and plan of action for their operations. To set the threshold, consider the developmental stages of the grasshoppers and crop, value of the crop, weather conditions and cost of control.
Although there are many grasshopper species, only five cause most of the damage in our area. These are the differential, migratory, two-striped, red-legged and Packard grasshoppers. Their life cycles consist of the egg stage (where most over-winter), five to six instar stages and the adult stage.
During late summer and fall, eggs are deposited up to 2 inches below the soil surface in fallow fields, ditches, fencerows and other weedy areas. The eggs hatch in spring or early summer, depending on soil temperature. The nymphs will feed on tender weeds and grasses near the hatching sites until they reach adulthood in 40 to 60 days, when most develop wings and become highly mobile.
Mating and egg laying activity begins within three weeks of reaching adulthood. The adults may live up to two more months, depending on the weather, with daily dry matter consumption of 30 percent to 250 percent of their body weight. A general rule of thumb is that 30 pounds of grasshoppers is equivalent to a 600-pound steer in terms of dry matter consumption.
Determining the economic threshold for grasshopper treatment is almost as much art as science. The potential for an economic grasshopper infestation begins with the number of eggs laid the previous fall. Optimum conditions for emergence and survival of the nymphs are warm, dry conditions with sparse vegetation that allow adequate soil warming. Once the hatch begins, weather conditions and the amount of available forage are the primary factors in determining population development.
If drought or overgrazed conditions are prevalent, then forage production is decreased, and competition for available forage will reduce the number of grasshoppers that can be tolerated. A well-managed, healthy crop can usually tolerate significantly higher numbers. Infestation is usually measured in average number of grasshoppers per square yard in both field margins and interiors. Published treatment thresholds range from three to 40 per square yard depending on crop, utilization requirements and where the measurements were taken.
Grasshoppers can be suppressed by both chemical means and natural mortality. Birds, other animals and insects utilize large numbers of grasshoppers as an important food source. The naturally occurring fungus Entomophthora grylli can also cause high mortality. Grasshoppers infected with this fungus will grasp the top of a plant, extend their back legs and die in this position. Fungal spores then develop in their bodies, become airborne and infect other grasshoppers.
The fungus is most effective in warm, humid conditions. Always scout for grasshoppers in this condition prior to making a chemical application. Early chemical control may be beneficial to hatching sites such as field borders and fencerows during the nymph stages. Later in the season, if the treatment threshold is met, broadcast applications or RAAT (reduced agent and area treatment) programs may be required. RAAT is an integrated pest management based program with a reduced pesticide rate applied by alternating treated swaths with untreated swaths. Due to the mobility of adult grasshoppers, effective late-season chemical control is often difficult to obtain.
An excellent source of information on grasshoppers, including decision-making aids for identification, threshold development and treatment options is at http://www.sidney.ars.usda.gov/grasshopper/Handbook/TOC.htm.
At the time of this writing, we have been experiencing the warm, dry conditions that are ideal for grasshopper development. We can only hope that by the time you read this, the rains have come, everything is green and lush and grasshoppers won't develop into a severe problem.