Scouting for alfalfa weevil larvae has traditionally started when 300 degree days have accumulated in your area, but the recommendation is going to change. The reason is research recently completed by Todd DeGooyer, a graduate student in the Department of Entomology at Iowa State University. He monitored weevil populations from Ames to the Missouri state line during the past three years. DeGooyer found that alfalfa weevils in southern Iowa were laying some of their eggs in the fall and these were starting to develop before winter. The result was that eggs were hatching earlier, especially in southern Iowa, before 300 degree days had accumulated the following spring.
The recommendation is now to begin scouting at 200 degree days (base 48) in fields south of Interstate 80 and at 250 degree days north of the interstate. These degree days reflect the average first occurrence of weevil larvae during the previous three years. The new degree day recommendations will make scouting more reliable.
Examine the degree day chart (base 48 column) on the last page of the newsletter to determine which areas of the state have accumulated 200 or more degree days. These are the areas which can now be scouted. In fact, Marco Buske, field specialist in crops, reported on May 3 that larvae were being found in Page County in southwestern Iowa.
Scouting has traditionally involved collecting a 30-stem sample and examining the stems for larvae. When stems are collected, take care not to snap them too hard or the larvae can be knocked off. Grabbing the tip of the plant in one hand and breaking the stem with the other hand, or cutting with a knife, is an extremely effective method of collecting all the larvae. Large larvae can be counted by beating the stems inside a five-gallon bucket. Small larvae that have recently hatched are not easily dislodged so the terminals must be pulled apart to accurately count them.
Alfalfa weevil larvae typically have a very dark head (almost black) and are pale green in color with a white stripe down the center of the back. Early feeding will be evident as very small, pin-sized holes in the leaflets. As the larvae get larger, feeding can defoliate the upper leaflets (see photo).
Consider managing the population in a field when at least two to three larvae are present per stem, and stem tips show feeding injury (25 percent of plants with injury on short stems, 40 percent of plants with injury on tall stems). Populations of less than two larvae per stem do not cause economic yield losses.
Management options include cutting the alfalfa or applying an insecticide if an economically damaging population exists. If the field can be cut within five days after scouting determines a problem, this is probably the best option. If cutting is not an option within five days, the field may need to be sprayed with an insecticide to prevent yield loss.