Grasshoppers In Abundance

By PR Extension Office

Grasshoppers are hatching in large numbers, according to MSU Extension Powder River County Agent Mary Rumph.“Reports are coming in from several areas throughout the county,” said Rumph. “Unfortunately, a wide scale rangeland treatment program through APHIS is unlikely in the next few weeks,” she added.

“I encourage landowners to scout their fields to determine population densities and to be prepared to protect crops if needed. APHIS will hopefully have scouts in our area this week to determine what species are present,” added Rumph. The number of insects that require treatment to prevent economic damage can depend on factors such as the weather, crop health and crop stage. For example, damage to rangeland by the same density of grasshoppers is more severe during drought periods. Fewer grasshoppers can cause economic damage to crops if they are feeding at sensitive developmental times. In some cases, grasshoppers in spring wheat crops can clip the developing heads, for example. Therefore, treatment thresholds should be used as a sound guide tempered with experience and observations of damage. General economic thresholds for range and cropland are listed below:

Rangeland: A total of 15-20 grasshopper nymphs per square yard is considered an economic threshold for treatment. At these densities grasshoppers can result in 200 to 500 pounds of lost forage per acre of rangeland, depending on their duration and conditions such as precipitation. Reduced Agent and Area Treatment strategies (RAATs) can be used to control grasshoppers in rangeland. USDA research has demonstrated that RAATs, a “skip pass” approach that also uses lower rates of insecticide, can achieve 80 to 95% control (compared to 85-99% control with complete blanket coverage at the full insecticide rate) at a lower cost. RAATs can be applied by ground or air. The insecticide dimilin is most commonly used in large-scale grasshopper spray operations. Dimilin is an insect growth regulator that is only effective against juvenile insects that are molting. It is NOT effective against adult insects that no longer molt, so timing is critical. Ideally the majority of grasshoppers should be in the 3rd instar stage during treatment. Commercial honeybee colonies are often placed on ranchland. A benefit of dimilin’s mode of action is that it is relatively safe to adult honeybees. Other labeled insecticides for rangeland are Sevin XLR Plus and Malathion.

Spring Grains & Alfalfa: Grasshoppers are notorious for their ability to move into cropland from surrounding grassy areas; cropland surrounded by grass is particularly at risk. The treatment threshold for immature nymphs in spring grains and alfalfa is 50-75 in field margin; 30 to 45 in field/sq. yard. The threshold for adults is 21-40 in field margin; and above 15 in the fields.

Crop protection is typically achieved by applying a border treatment of insecticide to keep the grasshoppers from entering the crop. A border width of 150 feet surrounding the crop may be adequate for control, but if grasshopper densities are high, control may require up to a 1/4 mile border treatment where ground applied RAATs can be considered. Under extreme pressure, control may be difficult and multiple border treatments may be required. Border areas and crop margins should be monitored after treatment to ensure that grasshoppers do not re-enter the field. Insecticide baits can also be effectively used but USDA research has found that the effectiveness of insecticidal baits can depend on grasshopper densities. Baits are not recommended when grasshopper densities are higher than 30-40 per square yard.

Commonly used insecticides for grasshopper control in small grains include Mustang Max, Baythroid XL, chlorpyrifors, Cobalt, dimethoate, gamma cyhalothrin (Nexide/Fentrol), and lambda cyhalothrin (Deman, Karate, Warrior).

Additional information is available from the Powder River Extension Office at 436-2424 or on the USDA ARS Sidney grasshopper website, http://www.sidney.ars.usda.gov/grasshopper/. The High Plains IPM Guide website, http://wiki.bugwood.org/HPIPM, provides detailed information on sampling, thresholds and management.