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Published June 16, 2014, 10:12 AM

Scout for pests

Alfalfa fields in west South Dakota are looking green and healthy with little or no pest insect activity.

By: SDSU Extension Service,

BROOKINGS, S.D. — Alfalfa fields in west South Dakota are looking green and healthy with little or no pest insect activity. But Anitha Chirumamilla, South Dakota State University Extension entomology field specialist, warns alfalfa growers not to be complacent when it comes to scouting fields.

“Except for few spots in the northeast, much of the state has warmed up enough for alfalfa weevils to become active,” Chirumamilla says, encouraging alfalfa producers to monitor.

She adds that monitoring for weevils is being carried out every 10 days in alfalfa fields in Butte and Haakon counties.

“There are signs of weevil activity beginning in these areas. It looks like the on-off cold weather had slowed down the activity of insects along with the growth of plants,” she says.

Alfalfa butterflies

Besides spotting weevil activity in isolated fields, several yellow butterflies, called alfalfa butterflies, were seen flying in alfalfa and weeds on field edges. While these butterflies are pretty, Chirumamilla says it is important to know that their larvae, known as alfalfa caterpillars, feed on alfalfa leaves.

The butterflies are pale to bright yellow or orange in color with black wing margins and two dark spots on the front wings. The caterpillars are velvety green with a white stripe running on each side of the body. Unlike alfalfa weevil larva, alfalfa caterpillars feed on entire leaves including the veins and mid-rib.

“Larvae are very sensitive to touch and roll-up when disturbed or dislodged from the plant,” she says. “The best time to see them actively feeding is early in the morning.”

“Scouting for these caterpillars should be done using a sweep net, and chemical spraying should be considered only if the number of larvae exceeds 10 per sweep,” Chirumamilla says.

Lygus bug

The other insect that was seen actively running in alfalfa fields was a lygus bug, commonly known as a tarnished plant bug. “It is a true bug about 0.1 to 0.25 inches in length, oval-shaped, and green to brown in color,” Chirumamilla says.

She says it can be easily recognized by a distinctive triangle on the back and large bulging eyes. Nymphs are green and similar to adults but lack wings. They look like large aphids from a distance. Both nymphs and adults of lygus bugs have piercing and sucking mouthparts, allowing them to feed on plant sap. The saliva of these bugs also causes necrosis of plant tissue at the feeding spot. Lygus species are severe pests in seed alfalfa and many other field crops, but not a serious concern in forage alfalfa.

Chirumamilla says niether the alfalfa caterpillar nor lygus bug are considered serious pests of forage alfalfa.

“Presence of these might actually help feeding the natural predator and parasitoid populations,” she says. “However, alfalfa caterpillars can be damaging in case of high populations. Scouting frequently with a sweep net and keeping an eye on insect species and populations is always a safe and secure practice.”

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