North Dakota agriculture extension specialists are encouraging producers to manage alfalfa weevils using temperature-based predictions.
Phillip Glogoza, an entomologist with North Dakota State University’s extension service, said early and persistent scouting is the key to effective control.
Temperature, or degree day, models indicate that producers need to be on the lookout now for hatching larvae.
The degree day system allows entomologists to predict larval hatching within about two to three days.
Alfalfa weevil development occurs within a range of temperatures. Glogoza said thresholds are determined experimentally and are different for each insect species. Egg hatch for alfalfa weevil begins after 300 degree days have accumulated in the spring, a level now reached in most of Manitoba and Saskatchewan.
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Alfalfa weevil larvae are yellow to light green and legless. When mature, larvae are plump, 10 millimetres long and green with a white stripe down the back.
Once weevils are found, Glogoza recommended monitoring alfalfa tip-feeding injury to assess infestation levels and damage. He suggested the following technique:
Select 50-100 alfalfa stems (10 to 20 randomly selected stems from each of five locations) and examine for signs of feeding damage in the leaf buds and growing tip leaves.
Divide the number of stems with recent tip injury by the total stems collected, convert to a percent, and compare with the threshold.
Thresholds are dependent upon weather conditions. Growers suffer the most evident weevil damage during times of drought. In contrast, rapidly growing plants that receive sufficient moisture have the ability to tolerate greater larval infestations.
The alfalfa weevil threshold before the first cutting is 35 percent of the plants suffering from feeding damage in a weak stand. In vigorous stands, it is 40 percent, two live larvae per stem, or both.
When the threshold is exceeded, treatment options include early cutting and insecticides. Insecticide application delays cutting because of the post treatment interval and can lead to economic loss if quality of the hay declines with delayed harvest.
Fortunately, Glogoza said alfalfa weevil generally do the most damage around normal cutting time, which means early cutting can prevent substantial damage.
Roger Ashley, an area extension specialist at the Dickinson Research Extension Center, said cutting before full bloom is advantageous because alfalfa quality and yield suitable for beef cattle peaks when just one in 10 stems is in bloom.
Thus, cutting early to combat weevils can also be beneficial in terms of tonnage and value.
Weevils can survive the first cutting, so it is important to assess the need for management after cutting.
Glogoza suggested producers monitor regrowth for potential stubble infestations, particularly beneath windrows.
After the hay has been picked up, sample the stubble and early regrowth in 20, one sq. foot samples, four chosen randomly from five locations. When regrowth after harvest is sufficiently tall, go back to monitoring tip injury.
After the first cutting, the threshold in stubble is eight or more larvae per sq. foot and six per sq. foot on sandy soil. If the larvae suppress regrowth, this indicates that the threshold has been exceeded.
Insecticides are more cost effective at this time, because producers may be able to restrict treatment to the areas where the swaths were laying and where larvae concentrate.
