Figure 1. A pasture that appeared relatively green until closer inspection revealed areas of grasshopper feeding. Credit: A.Varenhorst.
We have been monitoring grasshopper populations in the Eastern part of the state throughout the summer. Initially populations appeared to be relatively low, but we are now observing and also receiving reports of grasshopper populations that are at thresholds or have greatly exceeded them. Grasshopper activity tends to be more noticeable during dry conditions because they move from grassy areas into crops. However, the majority of observations we have made are in pastures.
Making the decision to manage grasshoppers in pastures can be difficult due to determining the value and amount of forage that is being removed. The pastures we observed have noticeable reductions in grass stands, but may still have a green appearance. This deception is being caused by other non-grass forages that are also present in the mix (Figure 1). Close inspection to potentially injured pastures reveals that there is limited if any regrowth and the grass appears to be turning brown.
The threshold for grasshopper populations that was established by the United States Department of Agriculture is 8 grasshopper adults per square yard. Adult grasshoppers have fully formed wings that cover the abdomen, while the immature nymphs will only have wingpads and most of the abdomen will be visible. The two methods for determining if populations in your pasture are at or above this threshold are to: 1) stand in an area and estimate a square yard around yourself and count the number of grasshoppers that are moving within the area, or 2) use a sweep net and capture grasshoppers in four pendulum swings (approximately one square yard of area).
Sampling & Identification
When sampling grasshoppers, examine 3 to 5 locations within the pasture. Grasshopper populations can vary greatly from one area of a pasture to another, and the overall impact of the feeding should be considered. Once the grasshoppers are counted from those locations calculate the average to determine if they exceed the 8 adults grasshoppers per square yard threshold. It is also important to determine the species of grasshoppers that are present as some are more of a threat to pastures and crops than others. The two species that we have been observing the most in Eastern South Dakota are the redlegged grasshopper (Figure 2) and the differential grasshopper (Figure 3). In our samples, we are still observing quite a few nymphs, which can be difficult to identify to species.
|Fig. 2. Redlegged grasshopper adult. Note the fully formed wings. Credit: A. Varenhorst
||Fig. 3. Differential grasshopper adult. Credit: A. Varenhorst
If thresholds are exceeded an insecticide that is labeled for use in pastures and also for grasshoppers should be selected. Table 1 has a list of labeled insecticides and their recommended rates. Remember to always read and follow the label directions.
Table 1. Insecticides labeled for grasshopper management in range and pastures.*
|Insecticide||Active Ingredient||Rate/Acre||Harvest/Grazing Interval|
|Sevin XLR||Carbaryl||32-48 oz||14 days|
|Malathion 57E||Malathion||32 oz||1 day|
|6-10 oz||0 day|
|Baythroid XL||Beta-cyfluthrin||1.6-1.9 oz||7 days for harvest none
|Warrior II||Lambda-cyhalothrin||1.3-1.9 oz||7 days for harvest none
|Mustang Maxx||Zeta-cypermethrin||2.8-4.0 oz||0 day|
|*This list is not meant to be comprehensive.|