Released: May 31, 2001

Insects Could Be Culprit In Causing Crop Damage

MANHATTAN, Kan. – Kansas alfalfa growers have largely finished their first cutting of the season, but in some instances fields are not "greening up" as they should after cutting.

"If fields are not greening up, check for damage by alfalfa weevil larvae, alfalfa weevil adults or variegated cutworms," said Kansas State University entomologist Randy Higgins.

If growers find large numbers of alfalfa weevil adults in their fields, it is likely they’ll find evidence where the adults have fed heavily on the stems, ‘barking’ them as the outer layers of cells are removed, said Higgins, who is the state entomology leader with K-State Research and Extension.

Although data regarding chemical control for adult alfalfa weevils is scarce, Furadan 4F, Imidan and some products containing methyl parathion mention such control, he said. Warrior T and Baythroid have labels for alfalfa weevil control on alfalfa but do not differentiate between larvae and adults.

Treatment appears to be most effective on already-cut fields, as the foliage will intercept some of the spray on uncut fields.

"Another insect that can keep alfalfa from greening up is the variegated cutworm," Higgins said. "Unlike many pests, this is a climbing cutworm that has four or five white or yellow, somewhat diamond-shaped spots on the top center of the back of the larva. These worms frequently hide under alfalfa crowns and in the leaf duff, coming out to feed heavily on new growth."

Large numbers of variegated cutworms can be "quite devastating," he said.

He recommends that farmers treat a field when more than two variegated cutworms per square foot are present after the hay has been cut – if most are not expected to pupate in the next three to four days. Windrows should be baled and the bales removed before the field is sprayed.

Higgins said recommended products against variegated cutworms on alfalfa include Lorsban 4E, Baythroid 2, Lannate, Pounce 3.2EC, Ambush 2E, and Warrior T.

"Also, be sure to pay attention to pre-harvest waiting intervals since they vary widely among products and rates," he added.

In some cases, there are so many adult alfalfa weevils in untreated alfalfa fields that they are moving into nearby soybean fields and causing extensive damage, Higgins said. High numbers of hornworms also have been reported in some Kansas soybean fields, as well as in sunflowers, sorghum, and corn. Hornworms have a large pointed projection at the top rear of the caterpillar.

"Large numbers of hornworms appear to be present in several no-till fields that had mustard and possibly other weeds present," the scientist said. "Burndown herbicides just before planting eliminated their normal food source and they are moving over to feed on newly-emerging crops."

Under special provisions of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), companies are allowed to make their registered products available for controlling unexpected pests developing on crops for which their insecticides are registered. FMC Corp. now recommends Pounce 3.2EC against alfalfa weevil adults, hornworms and true armyworm on soybeans and against hornworms on field corn.

Capture 2E and Furadan 4F have new recommendations permitting their use against hornworms on corn. Furadan 4F also now can be used against hornworms on sunflower plus hornworms, alfalfa weevil adults, and soybean aphids on soybeans under these company supported recommendations.

Similarly, Syngenta has issued new Section 2(ee) recommendations supporting Warrior and Warrior T for hornworm and adult alfalfa weevil control in corn, sorghum for grain, soybeans and sunflowers in Kansas, Nebraska, Missouri and Iowa only. Higgins said producers should refer to the specific product FIFRA Section 2(ee) recommendation for rates, waiting intervals, approved locations, and other restrictions. In addition, all restrictions and use guidelines listed on the full product label must be followed.

Higgins said he’d also received reports of webworms on weeds in some no-till soybean fields in north central Kansas. The larvae were reported to be 1/4 to 3/8 inch long, had distinctive black spots with one or more hairs growing from each spot, and were associated with obvious webbing. Webworm larvae also move backward rapidly when touched on the head.

"They may cycle through and be gone by the time the soybeans emerge or they may still be around, so watching the situation is important," he said.

For more information on insect management, interested persons may contact the local Extension office or visit the Website http://www.oznet.ksu.edu/entomology/extension .

Brand names appearing in this article are used for product identification. No endorsement is intended, nor is criticism of similar products not mentioned.

-30-

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Mary Lou Peter, Communications Specialist
mlpeter@oznet.ksu.edu 

K-State Research& Extension News