Alfalfa  

Alfalfa Caterpillar

Sue Blodgett, revision

Michael J. Brewer, Frank B. Peairs & Jay D. Donahue

Alfalfa Caterpillar

The alfalfa caterpillar (also called "alfalfa butterfly") is a sporadic warm weather pest. Natural enemies often maintain this insect below economic levels most years.

Identification (and life cycle/seasonal history)

Adult alfalfa caterpillars (alfalfa butterflies) are medium-sized, with yellow or whitish wings that are solid on the top surface and bordered by black on the lower surface. The wingspan is about two inches. When they are seen flying about, they have probably just emerged from the field where they wintered as pupae. Females lay eggs singly onto alfalfa under one foot high. In about one week, the eggs hatch, and the larvae grow rapidly. They grow to one inch long at maturity about three weeks later. Larvae are green, covered with white hairs, and the larger larvae have thin white stripes along the sides of their bodies. Alfalfa caterpillars are favored by hot dry weather, abnormally slowly growing crops and by low densities of natural enemies.

Plant Response and Damage

Alfalfa caterpillars consume the leaves whole, whereas armyworms skeletonize the leaves. Damage is worst in newly planted fields, where the plants are too small to withstand much defoliation.

Management Approaches

Cultural Methods

In established fields, early harvest avoids serious damage from dense caterpillar populations.

Host Plant Resistance

None, although mature plants are able to withstand a great deal of defoliation.

Biological Controls

Preservation of natural enemies may keep alfalfa caterpillar from becoming economic most years. Besides general predators such as spiders, parasitic wasps and pathogens specific to this pest are common. Bacillus thuringiensis kurstaki is also active against alfalfa caterpillar (the larval stages only). Diseased and parasitized larvae tend to be abnormally colored, and are not as active. Natural enemies are preserved by avoiding unnecessary insecticide applications and by practicing non-chemical controls, such as early harvesting.

Chemical Control

Sampling/Surveying/Timing of sampling

Fields should be checked twice monthly, with more frequent monitoring after the adults are seen flying, especially when the plants are young and weather is hot and dry. Alfalfa caterpillars are best monitored with a sweep net. Diseased and parasitized larvae in high numbers compared to normal larvae indicate good control by natural enemies, so the threshold given below may be adjusted upwards, thus avoiding unnecessary insecticide applications.

Economic thresholds

An average of one caterpillar per two plants is the threshold for chemical control. However, as noted above, many diseased or parasitized caterpillars indicate natural enemy populations are present, so insecticide use may disrupt future control. Bacillus thuringiensis kurstaki should be considered because only butterfly and moth larvae are killed, preserving alfalfa caterpillar's natural enemies.

Product List for Alfalfa Caterpillar:
Insecticide	Product per acre  (Fl oz. or oz. product)	Preharvest Interval, remarks
Bacillus thuringiensis2	See labels	0 days.  Use of a spreader-sticker is recommended
Baythroid 2E,XLR,1	1.6 – 2.8	7 days. 12 hr REI. Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Maximum of 2.8 oz per acre applied per cutting.  Total 11.2 oz/A applied per season.
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.
chlorpyrifos 4ER1,2	16 - 32	14 days (1 pt), 21 days (over 1 pt rate). 24 hr REI. Do not make more than 4 applications/year or more than one application per cutting. Extremely Hazardous to Bees! Do not apply when bees are present. Minor phytotoxicity may occur on rapidly growing foliage.
CobaltR1	13-26	7 days after application of 13 fl oz, 14 days after application of 26 fl oz, 21 days after application of rates above 26 fl oz per acre. Extremely Hazardous to Bees! Do not apply when bees are present.
lambda cyhalothrinR,1,2	1.92 – 3.20	7 days for hay. 24 hr REI.  Extremely Hazardous to Bees!  Do not apply to alfalfa in bloom. Do not apply more than 0.24 pt /A per cutting.  Do not apply more than 0.96 pt/A per season.  Advisable to move bees during application and allow 3 (low rate) or 5 (high rate) days before re-introduction of bees
LannateR SP, LV	8-16 (SP) 12-24 (LV)	7 days to graze or feeding. 48 hr REI. Do not apply more than 3.6 lbs ai /season. Do not make more than 10 applications per crop. Extremely Hazardous to Bees!  Do not apply to alfalfa in bloom.
malathion2	See labels	0 days. 12 hr REI.  Not effective below 65oF. Apply to blooming alfalfa only in evening or early morning when bees are not working.
Mustang Max ECR,1	2.24 – 4.0	3 days.  12 hr REI. Extremely Hazardous to Bees!  Do not apply to alfalfa in bloom. No more than 4 oz per cutting, 12 oz per season
permethrinR,1,2	See labels	1 day if less than 0.1 lb ai/A. 14 days at rates greater than 0.1 lb ai. 12 hr REI. Do not apply more than 0.2 lb ai per cutting.  Extremely Hazardous to Bees!  Do not apply to alfalfa in bloom.
ProaxisR,1	1.92 – 3.20	7 days for hay. 24 hr REI. Extremely Hazardous to Bees!  Do not apply to alfalfa in bloom. Do not apply more than 0.24 pt /A per cutting.  Do not apply more than 0.96 pt/A per season.  Advisable to move bees during application and allow 3 (low rate) or 5 (high rate) days before re-introduction of bees
Steward1	6.7 – 11.3	7 days. 12 hrs REI.  Do not apply more than 45 oz/A. Minimum interval between treatments is 5 days. Not for seed alfalfa.  Can be applied with reasonable safety to bees.
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.

Categories: Alfalfa, Insects, Alfalfa caterpillar

Date: 5/20/2008