Tale of Two Weevils

Alfalfa weevils (AW) are the key pest of the first alfalfa cutting in Kentucky. These green, legless larvae (Figure 1) initially chew small “pin holes” in developing tip foliage (Figure 2).

Figure 1. Alfalfa weevil larva (Photo: Oregon State University)

Figure 1. Alfalfa weevil larva (Photo: Oregon State University)

Figure 2. Alfalfa weevil feeding damage. (Photo: LeeTownsend, UK)

Figure 2. Alfalfa weevil feeding damage. (Photo: LeeTownsend, UK)

The clover leaf weevil (CLW) is a second species (larva similar are similar in appearance, Figure 3) that also feeds on the first cutting. CLW damage is minor compared to that of the AW, so control of CLW is seldom justified. However, CLW feeding can cause some confusion when diagnosing early chewing damage to alfalfa.

Figure 3. Clover leaf weevil larva (Photo: Oregon State University)

Figure 3. Clover leaf weevil larva (Photo: Oregon State University)

Distinguishing Between Larvae

The table below compares the two weevils at a glance. Typically, feeding by CLW is not an issue. Suspect CLW where feeding damage appears up and down plants but no larvae are found; CLW feed at night and hide during the day. Follow-up by checking duff around alfalfa crowns. CLW spend winter as partly grown larvae so they will tend to be larger than AW larvae early in the season.

Feature

Alfalfa weevil larva (AW)

Clover leaf weevil larva (CLW)

Head color Black Light brown
Body Green with white back stripe Green, white back stripe with red or pink border
Feeding site Tip foliage Not limited to tip
Day In tip foliage In debris around crowns
Night In tip foliage On plant

Temperature & Alfalfa Weevils

How might a cold snap affect the early appearance of alfalfa weevils? AW are active, feeding, and developing as long as temperatures are above 48oF. However, as with many early season insects, they can tolerate the erratic temperatures that can occur.

Supercooling (stopping ice formation) is a means some insects can use to survive very low temperatures. Eggs are particularly hardy. Supercooling temperatures of 5-day and 10-day old eggs are -7.4oF and -10.8oF, respectively. Comparable temperatures for the four larval stages are -1.7 oF, 2.8 oF, 14.3 oF, and 17.2 oF, respectively. The unseasonably warm spell may increase these some, but it is clear that this weevil is no lightweight when it comes to cold tolerance.

Assessing the Need for Treatment

Just looking for feeding holes or using a sweep net will indicate the presence of weevil larvae in fields, but stem sampling will give a more accurate assessment of the situation. Using degree day accumulations, average alfalfa stem length, and numbers of AW grubs per 30 tips is a reliable way to assess the need for treatment. The sampling process and charts to aid in decision making are available in Alfalfa Weevil Field Sampling Program (EntFacts 127).

 

By Lee Townsend, Extension Entomologist

 

 

Posted in Forages