Alfalfa weevil larval hatch occurred on schedule over the first two weeks of May, in areas of Wisconsin that have reached and exceeded the 300 alfalfa weevil degree-days (base 48F) necessary for egg hatch. Reports have come to me between Thursday May 10^th and Wednesday May 16^th from southeastern, southwestern and western Wisconsin.

 

To date, the common theme in the reports I have received of alfalfa weevil meeting or exceeding threshold, is that damage is being noted on lighter, sandy soils. These soils warm up faster accumulating alfalfa weevil degree days more quickly. Reports from Tom Novak, Crop Consultant –Total Crop Management LLC; Tim Bender, Crops/Soils Agent –UW Extension Jefferson Co.; and Steve Huntzicker, Agriculture Agent – UW Extension LaCrosse Co. all noted that as they inspected heavier soils alfalfa weevil feeding was well below threshold or not yet noted.

 

Regardless of soil type, alfalfa weevil scouting will be very important for the remainder of first crop, and second crop regrowth. Remember, third and fourth larval instars do the most feeding and can cause significant defoliation if populations go unnoticed.  

 

For details on alfalfa weevil scouting, degree-day accumulations, biological control, economic threshold, first crop harvest cultural control, and insecticide treatment recommendations please refer to the May 3rd Wisconsin Crop Manager article titled "Alfalfa Weevil Will be Active Soon" http://ipcm.wisc.edu/WCMNews/tabid/53/EntryID/255/Default.aspx

 

The WCM article link above contains an alfalfa weevil degree day accumulation map and is a convenient way to update yourself on weevil larval instar in your area. This article is a reminder to scout fields, confirm presence of alfalfa weevil larvae and quantify percent tip feeding damage.

 

Scout alfalfa fields by collecting groups of 10 plants at 5 random locations throughout the field (50 random stems total). Count all stems that show signs of tip feeding damage and divide that number by 50 to determine percentage tip feeding. Management action is recommended when 40% or more of the stems show signs of alfalfa weevil larval feeding. If alfalfa weevil threshold (40% feeding) is reached within 7-10 days of planned harvest, cutting is recommended. If threshold is reached more than 7-10 days from scheduled cut, insecticide application is recommended. Please visit the May 3rd WCM article link above for detailed management recommendations.

 

Thursday May 10^th – Monday May 14^th: Tom Novak reported two farms south of Fort Atkinson with 40% to 80-90% tip feeding levels reached in a dozen fields over the two farms. Fields are more than 7-10 days out from scheduled harvest. Insecticide treatment was recommended and applied as these fields had reached or greatly exceeded the 40% tip feeding threshold. In addition several alfalfa fields north of Elkhorn required treatment (approximately 120 acres). Scouting data revealed these fields at 60% feeding, with two fields at 90-100% feeding on stem samples taken from throughout the field. Also, south of East Troy and only on the higher/rolling type fields with light silt loams, feeding levels were around threshold 40%, but there were a significant number of plants with tiny weevils on them that didn't show feeding yet.

Wednesday, May 16^th. Steve Huntzicker reported two fields from LaCrosse County located north of Holman and West of Mindoro close to 60% feeding based on his thorough field sample of 50 stems. Although alfalfa weevil thresholds are based on percent tip feeding of the 50 stem sample; and not number of weevil larvae per plant, Steve did also report easily fining two larvae per plant on infested plants in his sample. 

 

The following report provides the most recent update available from the WI DATCP Pest Survey, Friday May 11^th. http://pestbulletin.wi.gov/pests.jsp?categoryid=6&articleid=656&issueid=79

 

Alfalfa weevil - Surveys in Rock and Walworth counties revealed high numbers of first and second instar larvae as well as evidence of tip feeding injury. Counts of larvae hatched from overwintered eggs range from 9-23 per 25 sweeps, while adult weevils average 2 per 25 sweeps. With most alfalfa fields still days away from flowering, it appears damage to first crop hay is a distinct possibility. Under the present weather conditions, individual females may deposit roughly 60-70 eggs per night. Look for larval populations to surge in the next week.