South Dakota grasshopper survey: What to expect in 2014
SDSU Entomology Field Specialist
Grasshoppers are one of the most difficult and challenging insect pests in western South Dakota. They are generalist feeders with a wide host range and have the potential to cause severe destruction when huge numbers coincide with drought conditions. Approximately 70 different species are known to occur in South Dakota, of which about 10 species are considered pests of field as well as forage crops. Annual grasshopper surveys are conducted by a USDA agency, Animal and Plant Health Inspection Service (APHIS), over thousands of acres of range and forest lands to estimate population levels and determine the need for suppression programs. Populations of grasshoppers that trigger the need for a suppression program are normally on a case-by-case basis. Suppression programs are carried out based on the threat of potential damage and severe destruction of forage for livestock and wildlife, reduction of wildlife habitat, soil erosion and the threat of crop damage and yield loss resulting from migrating grasshoppers. The goal of APHIS suppression programs is to reduce grasshopper populations to acceptable levels in order to protect rangeland ecosystems and/ or cropland adjacent to rangeland (Bruce Helbig,USDA APHIS, PPQ).
APHIS conducts their surveys each spring starting mid-May by counting nymphs, and continues to the end of August with the survey of adult grasshoppers. Sampling for nymphal populations is carried out using sweep nets and making 40 sweeps at each site (one per township). The density of grasshoppers per square yard are calculated by dividing the total number of grasshoppers from 40 sweeps by 10.
The adult grasshopper survey utilizes a visual count. Grasshoppers are counted at 18 one-square foot sites, and the total number for the 18 sites is divided by two for the number per square yard.
Although the annual surveys are intended to estimate the population levels in the current year, they also enable us to forecast future population levels and identify risk zones for the following spring and summer. The predictions will be based on adult grasshopper numbers that have the potential to reproduce and lay their eggs in the soil prior to the winter. Climatic conditions that prevail after the survey and into the following spring are also taken into account. Grasshoppers have one generation per year and tend to lay their eggs in the soil during late fall. Their eggs overwinter in a state of diapause and new generation nymphs start hatching when spring temperatures hit 50° F. The adult grasshopper survey from 2013 shows that majority of the counties in western South Dakota were in low risk zones with few isolated areas in medium to high risk zones. Counties with areas that had threshold-level populations include Fall River, Bennett, Perkins, Corson, Meade, and Jackson. However, the early October blizzard last fall led to unusually wet conditions for prolonged periods, which could have detrimental effects on grasshopper eggs in the soil. High moisture favors disease-causing natural fungi and bacteria to flare up and kill many soil-dwelling insects. Since the majority of the counties in Western South Dakota were in the path of the blizzard, the 2014 outlook for grasshoppers and insect populations in general could be promising, and we may be dealing with lower than usual infestation levels. However, it is important to note that natural enemies which feed on pest insects will be equally affected by the blizzard. Lack of natural control early in the season will favor rapid multiplication of pest insects, leading to severe outbreaks. So, it is always necessary for growers and ranchers to do frequent and early scouting to identify the pests and deal with them efficiently.
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