The past few years have seen below-normal precipitation in many parts of Nebraska, which could potentially lead to an increase in grasshopper populations. The 2023 Rangeland Grasshopper Hazard Map highlights ten counties in Nebraska where fall adult grasshopper numbers averaged over 15 per square yard. These numbers, based on the 2022 adult survey, serve as an indicator of potential grasshopper issues for the summer of 2023. Economic thresholds for grasshopper densities in rangeland range from 8 to 40 grasshoppers per square yard, influenced by factors such as control product costs, projected forage yield, and the value of the forage.
Grasshoppers are voracious feeders, consuming up to 50% of their body weight in forage every day. For instance, a density of just 2.7 grasshoppers per square yard translates to 12,971 grasshoppers per acre. Similarly, an acre with 69.7 grasshoppers per square yard would consume forage equivalent to that of one cow in a day.
Grasshopper outbreaks are most common in areas with less than 30 inches of annual rainfall, which includes the western two-thirds of Nebraska. Consecutive years of drought and below-normal precipitation have created conditions conducive to grasshopper outbreaks. Monitoring grasshopper numbers is crucial to determining the appropriate action required.
The square foot method is an effective way to assess grasshopper density. It involves sampling multiple sites approximately 50 to 75 feet apart. Select a point, visualize a one-foot square area around it, and count the grasshoppers within or jumping out of that area. Repeat this process 18 times, tally the total number of grasshoppers, and divide by 2 to determine the number of grasshoppers per square yard.
The life cycle of grasshoppers consists of three stages: egg, nymphal, and adult. Most species overwinter as eggs, remaining dormant until the ground temperature reaches 50° to 55° Fahrenheit. Newly hatched grasshoppers resemble miniature adults but lack wings and developed sex organs. They molt several times as they grow, with the average being five molts. The rate of nymphal development is influenced by temperature and food quality, with most species reaching the adult stage 30 to 50 days after hatching.
While approximately 50 grasshopper species are found on Nebraska rangelands, fewer than 10 species typically reach economic levels and cause 95% of the damage. Grasshoppers defoliate grass by direct feeding on leaf and stem tissue and by cutting off leaves or stems.
To address grasshopper populations, an integrated pest management (IPM) program called Reduced Agent and Area Treatment (RAAT) has been developed. This strategy utilizes the insecticide Dimilin, which disrupts the molting process of grasshoppers. Dimilin is applied in alternating strips, reducing application costs by 50 to 60% and minimizing the amount of insecticide used by 65 to 70% compared to conventional broadcast treatments. The RAAT system provides up to 85% control, depending on forage growth rate, grasshopper size, and coverage achieved.