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Alfalfa weevils, pea aphids are moving in on crops

Staff writer

Area farmers are beginning to spray alfalfa fields to kill weevils and aphids that attack plants and destroy crops if not controlled.

The alfalfa weevil was introduced into the U.S. from southern Europe. First discovered in 1904 in Utah, it now is present in all 48 mainland states.

Alfalfa weevil adults are one-fourth inch, brown snout beetles with a distinctive dark, narrow stripe down their backs. They insert yellow oval eggs in alfalfa stems.

Following egg hatch, small legless yellowish-green larvae emerge. Each larva has a conspicuous black head and is approximately 3/8 inch when mature.

Transformation to adult stage is passed in a loosely-woven white cocoon about the size of a pea.

Some egg-laying may occur in November before the onset of cold weather and during the winter, when temperatures permit.

Generally, weevils become active in late March or early April. Female weevils insert eggs in clusters of two-25 inside alfalfa stems.

Larvae become noticeable in April and are readily observable in early June. They feed for three to four weeks, depending on the quality of the alfalfa and the temperature.

Larvae molt or shed their skins three times. Following the last molt, they spin cocoons on plants or in curled-up leaves that have fallen to the ground.

The pupil stage lasts one to two weeks. Upon emergence, adults feed for a week or two, and then move to sheltered areas to spend an inactive summer.

It is likely that all three life stages can be found in an alfalfa field at any given time.

When colder weather arrives, adult weevils seek out the crowns of alfalfa plants or the protection of wooded areas or vegetation provided in ditch banks or fence rows to overwinter.

Pea aphids are large, green insects with long legs and antennae. They prefer cool temperatures and can be present at the same time as alfalfa weevils.

The aphids inject a toxin into the stem of the plant that retards growth, reduces yields and can kill it. They also reduce feed value by promoting a black, sooty mold that grows on the honeydew they excrete. The mold makes the hay less palatable to livestock.

Damage is most severe on short plants.

Management

It is important to scout for pest activity early in the growing season.

Kevin Suderman of Crop Production Center in Hillsboro said Monday he has been scouting farmers’ fields for three weeks.

“There was a setback with the last freezes but activity has picked up since then,” he said.

He noted that pea aphids are more prevalent than weevils this year, but the same insecticide works on both.

Randy Vogel of Marion has 80 acres of alfalfa that were sprayed Thursday. He said he has learned from past experience to keep an eye out for pests in his alfalfa. Spraying fields for weevil seems to have become a yearly occurrence during the past 30 years or so, he said. The only exception was 1996, when spraying was unnecessary.

“The key is not to let it go too far,” he said. “I can see weevils falling off plants when walking through a field, and I can tell it’s being affected.”

He also spotted aphids in new alfalfa fields, with areas where the plants didn’t show growth.

There are biological predators of weevils and aphids, such as several species of beetles and green lacewings. However, it appears these have been overwhelmed and no longer are in sufficient numbers to control the pests.

Last modified April 23, 2009

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