Scout Alfalfa Fields Now for Worm Infestation

Spraying for alfalfa weevils can rarely be avoided.

Published on: May 4, 2009

Now is the time to be scouting for alfalfa weevils, says Barton County extension agent Rick Snell.

Barton and the counties surrounding it are the "Golden Belt" of alfalfa production in Kansas and about 75 percent of the crop is non-irrigated. Alfalfa weevils can cost producers up to a ton per acre of hay production in the early stages and adults can totally wipe out the stand.

Snell warns that it is rare that a grower can get away without spraying for weevils. Possible non-chemical treatments include removal of stem material by grazing of burning.

Kansas State University has also done studies on such controls and have learned that parasitic wasps and fungal diseases that affect the weevils can sometimes be effective in limiting damage.

Snell advises however that the careful scouting for weevils and chemical treatment work best.

Here's his advice on scouting:

  • Determine the height of the average plant or at least establish a height range.
  • Go to each corner of your field and one or two places out in the middle to sample five or six places. Cut off 5 to 10 stems in each area at ground level and shake them into a bucket.
  • Count all the weevils and divide by the number of stems.
  • When hay prices are good, like now, you should probably spray 5-to-8 inch alfalfa when you have an average of 2 worms per stem. When you get into 11 to 14 inch hay, where it likely is by now, spray at 2.5 weevils per stem. At 17-20 inches spray for 3 weevils per stem. If it is past 20 inches, cut it early and monitor to see if you need to spray the stubble later.

The worms you are looking for are yellowish-green to dark green, black-headed and legless. They have a distinctive white stripe down the middle of their back. Even when mature, they are only about 1/4 of an inch long.

The larvae eat the leaves and foliar damage is not obvious at first. Young larvae create scattered pin holes in the top leaves and developing buds. As the hatching weevil larvae increase in size and number, leaf damage increases. Leaf loss results in a reduction of quantity and quality of hay. Most damage occurs on the first cutting unless they come in late.

However, carryover losses can occur when there are high infestations.

Scout your fields now! However, don´t wait too long to pull the trigger on spraying. Also be on the alert for pea aphids. Many predators, such as lady beetles, were killed by the spring freeze so pea aphids are plentiful.

For more information, pick up a free copy of the K-State bulletin, Alfalfa Insect Management 2009 at your local extension office or get it online at www.oznet.ksu.edu/library/ENTML2/MF809.pdf. It has a summary of the treatment thresholds and the labeled insecticides.