Alfalfa Weevil – The Next Step

Alfalfa weevils have been very damaging in some fields this year (Figure 1). While Individual larvae feed for about 3 weeks, a wide window for egg hatch this year has resulted in an extended season. Unfortunately, additional damage is still possible as larvae pupate and adults emerge.

Most alfalfa weevil larvae are completing their development. Taking a page from the caterpillar manual, mature weevil larvae spin coarse silk cocoons in which to pupate (Figure 2). Cocoons can be found in stem tips or in leaf litter on the ground. An alfalfa weevil emerges about 7 to 10 days in the pupal state (Figure 3).

Figure 1. Undamaged weeds are the only green foliage in this alfalfa field following significant alfalfa weevil feeding (Photo: M. Baxter).

Figure 2. Mature alfalfa weevil larva in a cocoon that has been partially removed (Photo: Lee Townsend, UK).

Figure 3. Alfalfa weevil pupa in cocoon (Photo: Lee Townsend, UK)

With much of larval feeding damage over, harvest can give alfalfa plants a chance for regrowth. However, the weevil can still inflict some serious damage. With very good curing conditions, mortality of larvae present at harvest can be high. Many will die of starvation or exposure if the alfalfa cures quickly and the crop is baled without delay. Larvae under windrows can survive if it rains or curing conditions delay curing and baling.

Scout to Determine Need for Management

Weevils that survive to the pupal stage and emerge as adults may congregate under windrows to feed on crown regrowth. There may be enough adults present to justify an insecticide application. Watch fields carefully for the normal green‐up that indicates active re‐growth. Surviving larvae may feed on developing leaves and new adults can cause “notch‐like” feeding holes on leaves, which gives foliage a feathery appearance.

Use the table below to evaluate larval numbers and plant height to determine if control is needed. Treatment may be justified if there is feeding on 50% or more of the crowns or re‐growth is prevented for 3 to 6 days. Use adequate water (15 gallons per acre or more) if an insecticide application is necessary.

Table 1. Assessing alfalfa weevil larvae on first cutting re‐growth (5 to 7 days after harvest)

Plant height

2”

4”

6”

8” or taller

Action

 

number of larvae per 30 stems*

 

greater than 20 greater than 33 greater than 47 greater than 60 Apply short residual insecticide
17 to 20 17 to 32 23 to 46 23 to 59 Sample again in 2 days if larvae per 30 stems is in this range.

*If numbers are below this level, no treatment should be if necessary.

 

By Lee Townsend, Extension Entomologist

 

 

Posted in Forages