Watch For Early Alfalfa Weevil Damage

There was a resurgence of alfalfa weevil damage in some parts of Kentucky in 2016, so it is reasonable to be prepared for higher-than-normal feeding on the first cutting in 2017. In addition, the mild winter sets the stage for an earlier-than-normal appearance of this key pest occurring at first cutting.

Insect Development

Alfalfa weevils fly to alfalfa fields in the fall and remain there over the winter. They are active only when air temperatures climb above 48oF. Usually, this means that females do not begin to lay eggs in live and dead alfalfa stems until spring. Our unseasonably warm winter should mean that many eggs were laid much earlier than normal. Consequently, tip-feeding by the larvae is likely to appear earlier, too. Not only can this catch alfalfa growers off-guard, but plants are smaller and more susceptible to damage by fewer weevils.

Degree Day Accumulations as a Tool for Predicting Alfalfa Weevil Damage

Figure 1. Tip feeding by alfalfa weevils may appear early this season. (Photo: Lee Townsend, UK)

Figure 1. Tip feeding by alfalfa weevils may appear early this season. (Photo: Lee Townsend, UK)

Characteristic tip-feeding damage (Figure 1) from spring-laid eggs can be expected when 190 degree days (base temperature 48 degrees F) have accumulated. The table below compares degree day accumulations from January 1 through February 20 for 2016 and 2017 at three locations in the state. In addition, the table shows the date in which 190 degree days had accumulated in 2016.

2016

2017

 

 

Alfalfa weevil degree days

Jan 1 to Feb 20

Date for 190 degree days

Alfalfa weevil degree days

Jan 1 to Feb 20

Princeton

76 March 13 166

Lexington

58 March 14 115

Somerset

66 March 12 167

The 2017 accumulation is well ahead of last year’s pace and continued high temperatures should push accumulations to 190 degree days by late this week.

Field sampling is the most reliable way to assess the need for alfalfa weevil control (see Alfalfa Weevil Field Sampling Program, ENTFACT-127).  The use of degree day accumulations, average alfalfa stem length, and numbers of weevil larvae on 30 stems collected randomly in the field allows accurate management decisions to be made.The 2017 accumulation is well ahead of last year’s pace and the continued high temperatures should push accumulations to 190 dd by late this week.

 

By Lee Townsend, Extension Entomologist

 

 

Posted in Forages