Alfalfa weevil is the first pest to hit alfalfa fields and is an annual occurrence as they overwinter as adults. Fortunately, the alfalfa weevil only produces one generation per year and once first harvest passes, little damage from alfalfa weevil will occur the remainder of the year.
“It is amazing how insect pests can zero in on a field and seem to come out of nowhere,” says James Rogers, NDSU Extension forage crops production specialist at the North Central Research Extension Center in Minot. “Several years ago, I planted a field to alfalfa that had not been in alfalfa production for several years, and it was the only field of alfalfa for several miles. Sure enough, alfalfa weevils found it and control measures were called for.”
Alfalfa weevil requires heat units for egg hatch to begin and growth development to occur. The heaviest damage to alfalfa is due to larvae feeding at the third and fourth instar stages of development when the larvae have reached maturity. Heavy damage to alfalfa will result in the leaves of plants having a frosted appearance with numerous holes from larvae feeding.
The best practice for alfalfa weevil management is scouting, says Rogers. Scouting should take place now through the first cutting.
To scout an alfalfa field:
Walk the field in an “M” pattern and stop at five locations along the route. At each stop, collect six alfalfa stems and shake the stems into a bucket. Measure the height of the alfalfa and notice any flowering or bud development. Cover the entire field over the course of your route. At the end of the route, you will have collected thirty stems. Record the average plant height and count the total number of larvae found and average number per stem. To determine the economic threshold for control using a pesticide, it is best to determine the market value of the hay and the cost of control. Alfalfa crop market value is the major driver in determining the economic threshold for control.
In general, as crop value increases, the number of larvae it takes to justify control decreases. This is modified by plant height (maturity) and control ($/acre) costs.
Source: ndsu.edu
Photo Credit: GettyImages-0shut0
Categories: North Dakota, Crops, Alfalfa