HPIPM:Webworm Alfalfa Looper Armyworm
Authors: Sue Blodgett and Frank B. Peairs[1]
Identification (and life cycle/seasonal history)
Webworms tie up leaves with silken thread, creating a web which protects the larvae while feeding. Webbed leaves are often visible in the alfalfa terminals. Fields that are heavily infested have visible webs in the alfalfa canopy, producing a light gray or "frosted" appearance. Leaves inside the webbings are consumed leaving the tougher leaf veins. Larvae grow to slightly over an inch in length, greenish or occasionally brownish orange, with six black spots (tubercles) on each segment, and a stiff hair extending from each tubercle. The larvae quickly crawl backwards and may escape from the webbing when disturbed.
Loopers are greenish caterpillars, tapered from back to front. Because of the arrangement of legs (only 3 prolegs) they walk with a typical looping motion. Usually two generations per year of this pest occur in Colorado.
Yellow striped, fall and true armyworms have been detected in the High Plains region. These pests occasionally reach damaging numbers. The caterpillars are usually black, with two prominent stripes and many narrow bright ones on each side. At maturity they reach approximately 1.5 to 2 inches long. Eggs are laid in clusters on the upper side of leaves and covered with a gray, cottony material. Eggs hatch in a few days and larvae reach full size in 2 to 3 weeks. Larvae pupate on or just under the soil surface. Adults are brown moths that primarily fly at night but may be encountered flying up as you walk through the field.
Management
Early cutting may give satisfactory control because the larvae cannot survive on dried alfalfa forage. Alfalfa stubble and the new regrowth, especially under the windrows, should be scouted within 2 to 3 days after cutting to make sure webworms are not feeding on regrowth. Insecticide applications are rarely warranted in the High Plains region.
Product List for Webworms, Loopers and Armyworms
| Insecticide | Product per Acre (Fl oz. or oz. product) | Preharvest Interval, remarks |
|---|---|---|
| FastacR,1,2 | 2.2 - 3.8 | 3 days. 12 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. No more than 3.8 fl oz per cutting, 11.4 fl oz per season. |
| Bacillus thuringiensis2 | See labels | 0 days. Use of a spreader-sticker is recommended |
| Baythroid XLR,1 | 0.8-1.6 (looper) 1.6-2.8 (webworm, armyworm) | 7 days. 12 hr REI. Extremely Hazardous to Bees! Maximum of 2.8 oz/ A applied per cutting and total 11.2 oz/A applied per season. Do not apply to seed alfalfa. Do not apply to mixed alfalfa-grass stands. 2.0-2.8 oz rate is recommended for yellowstriped armyworm. |
| carbaryl1,2 | See labels | 7 days. 12 hr REI. Most formulations are Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Sevin XLR+ is safe for bees if applied at <1.5 lbs ai/acre when no bees are in the field. Do not apply more than once per cutting |
| Prevathon | 14-20 | 0 days. 4 hour REI. 1 application per cutting, do not apply more than 0.2 lb ai/acre/crop. |
| chlorpyrifos 4ER,1,2 | See labels | 14 days (1 pt), 21 days (over 1 pt rate). 24 hr REI. Do not make more than 4 applications/year or one application per cutting. Extremely Hazardous to Bees! Do not apply when bees are present. Minor phytotoxicity may occur on rapidly growing foliage. |
| chlorpyrifos + gamma cyhalothrinR,1,2 | see labels | See labels for preharvest intervals and specific use restrictions. 24 hr REI. Extremely Hazardous to Bees! Do not apply when bees are present. See labels for additional restrictions for individual active ingredients. |
| Cobalt AdvancedR,1 | 11-26 | 21 days. 14 days to graze. 24 hour REI. See labels for additional restrictions for individual active ingredients. Extremely Hazardous to Bees! |
| Grandevo | 1-3 lbs | 0 days. |
| cyfluthrinR,1,2 | See labels | 7 days. 12 hr REI. Extremely Hazardous to Bees! Maximum of 0.05 lb a.i./acre applied per cutting and total 0.2 lb ai/acre applied per season. Do not apply to seed alfalfa. |
| Belt1 | 2 - 4 | 0 days. 12 hour REI. No more than 4 fl oz product/cutting or 12 fl oz product/season. |
| gamma cyhalothrinR,1,2 | See labels | 1 day PHI forage, 7 day PHI hay. Extremely Hazardous to Bees! Do not apply more than 0.03 lb ai/A per cutting. Do not apply more than 0.12 lb ai/A per season. |
| lambda cyhalothrinR,1,2 | See labels | 1 day PHI forage, 7 day PHI hay. 24 hr REI. Extremely Hazardous to Bees! Do not apply when bees are present. Do not apply more than 0.03 lb ai/A per cutting. Do not apply more than 0.12 lb ai/A per season. |
| BesiegeR | 6 - 9 | 1 day for forage, 7 days for hay. 24 hr REI. Make applications when bees are not actively foraging by applying during early morning or evening hours. See labels for additional restrictions for individual active ingredients. Do not apply more than 9 fl oz product per cutting or 31 fl oz per season. |
| methomylR,1,2 | see labels | 0 days, 7 days grazing or feeding, 48 hrs REI. Extremely Hazardous to Bees! Do not apply more than 3.6 lbs ai/A/crop or make more than 10 applications/crop. |
| permethrinR,1,2 | See labels | 0 days. When rates greater than 0.1 lb ai then 14 day. Do not apply more than 0.2 lb ai per cutting. Highly toxic to bees. Do not apply when bees are present. |
| Intrepid 2F | 4-8 | 3 days. 4 hr REI. Do not apply more than 32 fl oz (0.5 lb ai) per acre per year, No more than 1 application per cutting. |
| Coragen | 3.5 - 5.0 (looper only) | 14 days. 4 hour REI. See labels. |
| zeta cypermethrinR,1,2 | See labels | 3 days cutting, grazing. 7 days harvest. 12 hr REI. Extremely Hazardous to Bees! Maximum of 4.0 oz per cutting, 12.0 oz per season. |
| StallionR,1 | 5.0 - 11.75 | 7 days. 24 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. See labels for additional restrictions for individual active ingredients. No more than 11.75 oz per cutting, 35.25 oz per season. |
| RRestricted use pesticide 1Labeled for chemigation 2Generic active ingredient, several formulations | ||
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.
