Insects are cold-blooded, which means that their development is dependent on external temperature. This allows them to grow quickly during warm days but development slows down during cold days. Because of this, degree day models are used to predict insect emergence and their development for sampling and to make timely management decisions. Overwintering insects need a minimum temperature (base temperature) to break dormancy and resume development. Growing degree days (GDD) are counted starting on the day it hits the base temperature and are calculated summing up the number of degrees that exceed the base temperature per day using the following formula:
(Max Temperature + Minimum Temperature)/2 - Base Temperature = Daily GDD
Alfalfa weevils are one of the early season insects whose base temperature was determined as 48° F. The weevil activity is generally seen when the degree days accumulate to 190. According to Laura Edwards, our Climate Field Specialist, most of southern and west-central South Dakota has warmed up to the point of seeing possible alfalfa weevil activity as of April 29. North and north-east South Dakota is still cold and falls below 190 and 100 GDD respectively (as seen in Figure 1).
Above: Figure 1. Accumulated growing degree days base 48° F
Scouting for weevils was conducted in alfalfa fields in the Newell and Nisland area on April 22 in three fields spaced 10 miles apart. Alfalfa had sprouted and growth was 1-2 inches in length. No weevils were found, as the scouted area was cold and had not yet reached 190 GDD. However, predators such as ladybird beetles and Nabids (damsel bugs) were spotted in one of the fields. Considering the high temperatures in the past couple of days, GDDs accumulate rapidly and the majority of areas may exceed the 190 degree day threshold by next week. Frequent and early scouting is recommended to keep track of the weevils.
How to Scout
Above: Figure 2. Adult alfalfa weevil.
Photo by: Patrick Beauzay, NDSU
Alfalfa weevils survive winters in South Dakota as adults in protected sites such as shelter belts, field boundaries, and ditches under plant debris. With warming spring temperatures, adult weevils emerge from their overwintering sites, mate, and lay eggs inside alfalfa stems (Figure 2). Early in the season, check for adult weevils under the plant debris in field borders, shelter belts, and ditches. In the field, check at the base of the plants and under the debris for active adults. Burning the field borders (as seen in Figure 3) to get rid of the debris is an effective technique to kill the adult weevils emerging from overwintering sites.
Above: Figure 3. Burning the edges of an alfalfa field bear Nisland.
Photo by: Anitha Chirumamilla
Sampling for weevils should start early in spring once 190 GDD are accumulated and continue at least once a week. Sweep net sampling is not recommended for alfalfa weevil larvae and the best method is to use the “bucket method”. For the bucket method, walk at least 20 paces inside the field, and randomly collect 30 stems while walking in a ‘W’ or ‘U’ pattern. Place the stems into a pale colored bucket, clipping them at the soil line. On a flat surface, beat four-five stems at a time against the side of the bucket to dislodge the larvae. Check folded leaves with pinhole damage, as the larvae may be inside (Figure 4). Count the number of larvae, then randomly select 10 of the 30 stems and measure them to the nearest inch to determine the average stem length as well. Use the table below, developed by the Extension faculty at the University of Kentucky, to guide your decision about pesticide applications.
Above: Figure 4: Alfalfa weevil larva.
Photo by: Ada Szczepaniec, former SDSU Extension Entomology Specialist
|Number of larvae / 30 stems||27||67||100||130|
|Number of larvae / 30 stems||15||19||20|
|Number of larvae / 30 stems||37||60||83||105||135|
|Number of larvae / 30 stems||82||105|
|Number of larvae / 30 stems||52||64||72||80|
- If number of larvae is greater than the number in the table for the average height of stems: apply a long residual insecticide early in the season and a medium to short residual insecticide as the crop nears harvesting or harvest if alfalfa is in the 30% bud stage or greater;
- If number of larvae is less than the table value, sample again in 2 days.
- Source: Townsend, L. Alfalfa weevil field sampling program. University of Kentucky, Extension. Entfact-127.
Early cutting of fields when possible is recommended to manage alfalfa weevils, and regular scouting is crucial in making sustainable management decisions. Grazing by sheep early in the spring or late in the fall is found to reduce the infestation levels as sheep directly consume or trample alfalfa weevil adults, larvae, and/or eggs located on or inside alfalfa plants. Natural enemies such as parasitic wasps, predators and pathogens play a major role in lowering the severity of alfalfa weevil infestations.
Mere presence of these pests in the fields does not warrant pesticide applications. The general threshold (and least precise) is to treat if there are 1.5 to 2 larvae per stem or 30-40% of tips are damaged by the weevils and early harvest is more than one week away. Alfalfa weevil insecticide treatments are detrimental to honey bees, so it is important to follow all pesticide label precautions concerning bee safety. The list of insecticides available to control alfalfa weevil is available at High Plains Integrated Pest Management Guide.