Anticipating Grasshopper Damage Ahead of Grazing Season04 March 2015
Ranchers are reminded to consider the dangers of grasshoppers to rangeland vegetation ahead of the spring grazing season.
Rangeland specialist at South Dakota State University (SDSU), Roger Gates, writes that developing plans to allocate the livestock feed available from pasture resources is essential to optimize pasture use and minimize associated costs.
Useful plans are made in advance and anticipate adjustments that may be needed to respond to changes during the growing season.
Modifying the intensity (e.g. animals per pasture) and duration (grazing days per pasture) will most often be dictated by growing conditions, primarily rainfall. Anticipating more severe conditions (i.e. drought, flood, hail, fire) requires contingency plans, and having them in place may reduce the impact of strong emotions associated with disaster conditions.
When grasshopper populations reach high densities, the resulting impact on pasture vegetation can be devastating, writes the SDSU expert.
The USDA-Animal and Plant Health Inspection Service personnel conduct adult grasshopper surveys annually in late summer and fall. These surveys provide an “early warning” about the potential for grasshopper infestations in the following growing season.
An iGrow Fact Sheet, Grasshopper Outlook on Rangelands: 2015 reports findings from the most recent survey. Contingency plans responding to grasshoppers might include pesticide control.
Decisions should be made based on the value of the feed resource and the cost of the control measures.
Survey results suggested a low probability of severe grasshopper damage in 2015. However, that is not a guarantee and a carefully planned contingency response will remain valuable and can easily be updated should conditions change.
Value Estimation of Grasshopper Control
The cost of applying an insecticide would need to be less than the values shown (tables 1, 2) for the treatment to be cost effective, says the South Dakota Grasshopper Outlook for 2015.
It adds that the cost of controlling grasshoppers on rangeland given a population density of 25 hoppers per square yard at $32 per AUM, given the assumptions above, needs to be less than $15.07 per acre to be profitable.
The cost of controlling grasshoppers in alfalfa fields at 25 hoppers per square yard at $100 per ton, given the assumptions above, needs to be less than $18.79 per acre. If not, the value of forage saved would be less than the cost to control the grasshoppers, states the SDSU outlook.
The difference in cost returns between grasshopper control on rangeland versus alfalfa cropland is that total forage production per acre on rangeland is usually less than cropland. Widespread application on rangeland is more economical with government costshare arrangements and/or strip/hot spot applications or by using the Reduced Agent and Area Treatment Grasshopper Outlook on Rangelands: 2015 Recent positive results using RAAT applications will increase the use of this treatment procedure for cost share programs.
An approach that may limit grasshopper development on rangeland is to manage pastures to minimize bare ground and maximize shading. North Dakota research demonstrated that pastures that were grazed rotationally had less bare ground and more shading, which resulted in lower grasshopper populations (Onsager 2000).
The temperature differences between bare ground and shading can be significant. Differences have been found in some cases to exceed 30 degrees F. Grasshoppers, because they are cold blooded, use these temperature differences to regulate their body temperature. They can move in and out of warm spots to maintain an optimum temperature.
Optimizing body temperature increases rates of digestion and reproduction, accelerating population growth. Designing grazing management to reduce grasshopper outbreaks exclusively may not be productive.
Outbreaks are difficult to predict. However, grazing management that is beneficial to other grassland functions, which may also diminish the optimal environment for grasshopper multiplication, ought to be given consideration.
More than 90 species of grasshoppers occur in South Dakota. Eradication is neither possible nor prudent.
Grasshopper Treatment Thresholds
The High Plains IPM guide suggests that control applied when nymph populations reach 15 to 20 per square yard (which equates to 8 to 10 adult grasshoppers). Insecticides for use against grasshoppers on rangeland include:
- diflubenzuron * diflubenzuron is an insect growth regulator; as such, to be effective it should be applied when the predominate grasshopper stage is 3rd instar.
It is not effective on adult grasshoppers. Recommendations for grasshopper management and control in western South Dakota can also be found in the High Plains Integrated Pest Management Guide (HPIPM).