By Samantha Daniel and Shannon Sand et.al
Since the first cutting of alfalfa has been completed in much of the state, Nebraska producers may be tempted to believe our troubles with alfalfa weevil are over. Unfortunately, this may not be the case. Alfalfa weevil feeding on regrowth has been reported in some fields, so scouting is highly recommended, particularly if you notice that regrowth doesn’t begin within four to five days after cutting.
Alfalfa plants usually suffer the most injury and economic damage before the first cutting; however, feeding on regrowth can have impacts as well. While larvae cause the most damage, adults can also cause damage by feeding along leaf margins and leaving irregular notches. Adult beetles are roughly 5mm in length with a blunt snout and a brown body covered in golden hairs. Larvae are 1.5mm to 8.5mm long with a black head, wrinkled green body, and white stripe along the back and will curl into a C-shape when disturbed.
Scouting for alfalfa weevil is simple and can help mitigate economic losses. Randomly select five sites across the field and collect six stems at each site, cutting the stems at ground level. Beat the stems into a deep-sided white bucket and count the number of larvae to determine the average number per stem. Economic thresholds for alfalfa weevil can vary greatly from an average of one to seven larvae per stem, depending on a variety of factors.
While there are several insecticides labeled for alfalfa weevil management, some Nebraska weevil populations may be losing susceptibility to the pyrethroid lambda-cyhalothrin. Therefore, if you believe you have an infestation above threshold, contact your county extension office for further guidance.
Source : unl.edu