Manage alfalfa weevil by using degree days to implement a sheep grazing program

by Hayes Goosey - Montana State University

Background

Alfalfa is produced on approximately 1.8 million acres in Montana and is the foremost forage crop in many high, semiarid, intermountain states. Two biological stressors (insects and weeds) combined with poor field management are primarily responsible for reduced alfalfa production. In the U.S. alone, arthropods cause an estimated $260 million loss to alfalfa with the alfalfa weevil, Hypera postica Gyllenhal, being the most damaging phytophagous pest in the United States.

Montana sheep producers often rely on fall regrowth of alfalfa as a source of winter pasture. Alfalfa regrowth also is utilized as overwintering habitat by the adult weevil, which hibernates in leaf litter or around plant crowns. In colder northern states, such as Montana, temperatures restrict winter activity and so no oviposition occurs resulting in only one generation of weevils per year.

Temperature controls the developmental rate of many organisms. Invertebrate animals such as alfalfa weevil require a certain amount of heat to develop from one point in their life cycle to another. This measure of accumulated heat is known as physiological time and is often expressed and approximated in degree day units. Degree days are the accumulated product of time and temperature between lower and upper developmental thresholds which vary by species. One degree day is one 24 hr period with the temperature above the lower developmental threshold by one degree. Using degree days to predict when Integrated Pest Management Programs should start and stop is a common practice and on-line calculators which use regional temperature data make it easy for producers to estimate local degree days.

Purpose Statement

The objective of this research was to develop a temperature based grazing model which keeps alfalfa weevil larvae below the economic threshold in the crop but does not reduce alfalfa growth rates. The grazing model is preventative and is implemented in the spring before and during the crop green-up period.

Conclusions

In general, as stocking rate and degree day accumulation of grazing increased, weevil larvae and relative plant age decreased however, the growth rate of the alfalfa was not altered.

When this grazing model is used, field populations of weevil larvae respond in a predictable fashion. This relationship is presented in Fig. 1. You’ll notice that during the alfalfa growing season alfalfa weevil populations never exceeded the economic threshold of 1.5 to 2 per stem. This is because spring grazing is preventative and impacts population so that they are unable to reach economic levels prior to harvest.

Grazing alfalfa in this fashion decreased NDF, ADF, TDN, and Yield and increased CP (Table 1). This is because grazed plants are younger and digestible and nutritious but are generally smaller and yield less. However when as illustrated by Fig. 2, grazed alfalfa only requires three to five days to yield equal to ungrazed alfalfa. These types of grazing systems over yield traditional ungrazed alfalfa production. By delaying harvest by three to five days, the same tonnage and quality of hay is harvested and baled. However by spring grazing to manage alfalfa weevil, sheep consume one half to one ton per acre which increase the total yield of the field over harvesting for baled hay alone. Alfalfa weevils hibernate in northern latitudes above approximately 42° N so and we expect this model to only work in similar environments.

Applications

To manage alfalfa weevil, the recommendation from this research is to stock alfalfa fields in the spring prior to with rates between 102 and 236 sheep days/acre. Sheep should be allowed to graze to a minimum of 106 and maximum of 150 degree days before removal. The minimum number ensures that grazing has proceeded far enough into the spring to manage the weevil and the maximum is the point where grazing must be stopped to ensure that the model predicting equal stem biomass and yields is accurate.

To calculate for you regional area follow these instructions:

• Follow this link to a regional degree day calculator: http://uspest.org/wea/
• In the U.S. map click on Montana (or the state of your choice)
• A ‘Montana Weather Models’ window appears
• Click on ‘All Available Networks’ following the Table interface label
• A new page is loaded
• Find ‘Thresholds’ at the top of the page and enter:
  • 9 in the lower box
  • 31 in the upper box
  • click on the °C box at the far right.
    • It is critical to enter the threshold values in degrees Celsius other wise the degree calculations will be wrong.
• Scroll down the page and find weather station in your county by looking at the second column which lists counties in Montana.
  • **Note** There may be several weather stations in you county so find the closest one to your location.
  • To find which station is closest to you, click on the name of the weather station in the first column.
  • A second page will appear with graphs and other information including a map with balloon markers of weather stations.
  • On the map you can use the + and – buttons to zoom in or out.
  • Once you locate the station closest to you, hover you curser over it and a window will appear with information on that station. Write down the information
• Hit the ‘back’ button on you browser so that you end up on the main page with all the weather station listed.
• Use the station code listed in 6th column to locate that station.
  • Once you have found it, double check that you threshold values at the top of the page are still entered correctly (lower = 9, upper = 31).
• To calculate current year degree days, click the 2010 circle for the station of your choice.
• A new window appears with several columns of numbers.
• The first two columns are the month and day. For example:
  • 1 1 is January 1st
  • 2 1 is February
• The far right column is cumulative degree days
  • You will use these to implement the grazing program
• To manage alfalfa weevil, the recommendation from this research is to stock alfalfa fields in the spring prior to 34 cumulative degree days with rates between 102 and 236 sheep days/acre.
• Sheep should be allowed to graze to a minimum of 106 and maximum of 150 degree days before removal
• It is recommended that you calculate DD every morning to estimate where your grazing program is at in relation to alfalfa weevil management needs.
• Also, be sure to visually monitor your fields to assess the grazing impacts.

Table 1. Grazing treatment alfalfa MSW, NDF, ADF, CP, TDN, and Yield combined over two study years.

Means in columns followed by the same letter grouping are not significantly different (P>0.05); least significant difference analysis (Proc Mixed; SAS Institute 2002).

¹Mean Stage by Weight: Higher values indicate more mature plants.

²TDN (% of DM) = 82.38-(0.7515 x ADF%).

³Yield: Tons/Acre.

&sup4;Standard error of the least squared means/least significant difference analysis.

Special thanks to Bob, Marie, and Ben Lehfeldt and John and Nina Baucus for their help and contributions during this project.

This project and research was funded by the USDA Western Region SARE grants program: Project No SW07-013.