Description of the Pest
Western yellowstriped armyworm may be abundant in alfalfa fields in the Central Valley from June to early September.
The caterpillar is usually black, with two prominent stripes and many narrow bright ones on each side. At maturity it is 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.
There are at least five generations per year in the low desert and four generations in the Central Valley.
Damage
Armyworms skeletonize leaves, leaving veins largely intact.
Management
Armyworms are frequently controlled by natural enemies and are more or less cyclic, occurring in large numbers only every few years. Early harvest, border cutting and biological control are important management methods that prevent damage from armyworms.
Biological Control
Natural enemies can provide good control of armyworms in many fields. Predators include bigeyed bugs, spiders, minute pirate bugs, damsel bugs, and lacewings. The parasitic wasp, Hyposoter exiguae, is believed to be the most important of at least 10 parasites attacking this pest. Sample for parasitism by pulling the heads from older caterpillars and squeezing the body contents out toward the head end. Hyposoter larvae are a light, translucent green color. Viral diseases can also be important.
Cultural Control
Fields may be cut to avoid damage.
Organically Acceptable Methods
Biological and cultural control methods, as well as sprays of Bacillus thuringiensis (e.g., Xentari), are acceptable for use on an organically certified crop.
Monitoring and Treatment Decisions
(View photos for identification of caterpillars)
In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars; monitoring can be discontinued after September. Divide each field into 4 sections and take 5 sweeps per section with a 15-inch diameter sweep net, for a total of 20 sweeps. For information on sampling, see SAMPLING WITH A SWEEP NET.
Combine monitoring of armyworms with monitoring for alfalfa caterpillars and leafhoppers as described in ALFALFA CATERPILLAR AND ARMYWORM MONITORING. Count and record the number of healthy and parasitized caterpillars caught in your sweep net on a monitoring form .
If cutting is not practical or not scheduled soon after monitoring, apply a pesticide if there is an average of:
- 10 or more nonparasitized alfalfa caterpillars per sweep,
- 15 or more nonparasitized armyworms per sweep, or
- 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.
Pesticides and Natural Enemy Releases
Not all registered pesticides are listed. The following are ranked by their IPM value, with the most effective and least harmful to natural enemies, honey bees, and the environment listed at the top of the table. When choosing a pesticide, consider information related to water and air quality, resistance management, and the pesticide's properties and application timing. Use PestManage to compare summarized management options for different pests in the same crop. Always carefully read the label of the product being used and take all necessary precautions when handling pesticides.
| Rank | Active ingredient | Example trade name | Group | Group Order | MoA 1 | Amount per acre | REI (hours) ‡ | PHI (days) ‡ | Comments | Selectivity 2 | Bees 3 | Predatory mites 4 | Predators 5 | Parasitoids 5 | Residue duration 6 | Leaching(fish) 7 | Adsorbed runoff(fish) 8 | Solution runoff(human) 9 | Leaching(human) 10 | Solution runoff(human) 11 | Last updated 12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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|
XenTari # | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
11A |
0.5–2 lb | 4 | 0 | Apply when larvae are small (in first or... | narrow | low | low | low | short | — | — | — | — | — | 04/2024 | ||
|
|
Vantacor | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
28 |
1.2–2.5 fl oz | 4 | 0 | Make no more than one application per cu... | narrow | low | low | low/moderate | short | — | — | — | — | — | 04/2024 | ||
|
|
Steward EC | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
22A |
9.2–11.3 fl oz | 12 | 7 | Make no more than one application per cu... | narrow | — | low | low | moderate | low | low | intermediate | very low | low | 04/2024 | ||
|
|
Intrepid 2F | Application Timing Varies (See UC IPM Pest Management Guidelines and Label) | 8 |
18 |
4–8 fl oz | 4 | See label | Make no more than one application per cu... | narrow | low | low | low | short | very low | low | low | very low | low | 04/2024 |
Legend
- ‡ a b Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
- # ↩ Acceptable for use on certified organic crops. Check with your certifier to confirm before application.
- 1 ↩ Group numbers for insecticides and miticides are assigned by the Insecticide Resistance Action Committee (IRAC). Insecticides with unknown modes of action are assigned mode-of-action group numbers (MoAs) that begin with UN. Rotate pesticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B.
- 2 ↩ Range of insect and mite groups affected by a pesticide. Broad means the pesticide affects most groups of insects and mites; narrow means the pesticide affects only a few specific groups.
- 3 ↩ Risk of harm to honey bees. For more information, see Bee Precaution Pesticide Ratings.
- 4 ↩ Risk of harm to predatory mites. Toxicities are generally to western predatory mite, Galendromus occidentalis. Where differences have been measured in toxicity of the pesticide-resistant strain versus the native strain, these are listed as pesticide-resistant strain or native strain.
- 5 a b Risk of harm to parasitoids and general predators. Toxicities are averages of reported effects and should be used only as a general guide. Actual toxicity of a specific insecticide depends on factors including the application rate, environmental conditions, and the life stage and species of a parasitoid or predator.
- 6 ↩ Length of time residue affects natural enemies. Short means hours to days; moderate means days to 2 weeks; and long means many weeks or months.
- 7 ↩ Risk of harm to fish from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 8 ↩ Risk of harm to fish from adsorbed runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 9 ↩ Risk of harm to fish from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 10 ↩ Risk of harm to humans from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 11 ↩ Risk of harm to humans from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 12 ↩ Date information was last updated in the UC IPM Pest Management Guidelines.
UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430
L.D. Godfrey, Entomology, UC Davis
E.T. Natwick (emeritus), UC Cooperative Extension Imperial County
P.B. Goodell (emeritus), UC IPM and Kearney Agricultural Research and Extension Center, Parlier
R.F. Long (emeritus), UC Cooperative Extension Yolo County
V.M. Barlow, UC Cooperative Extension Riverside County
I.M. Grettenberger, Entomology, UC Davis
M.M. Leinfelder-Miles, UC Cooperative Extension San Joaquin, Contra Costa, Sacramento, Solano, and Yolo counties
M.D. Rethwisch, UCCE Riverside Co. (Blythe)
Acknowledgement for Contributions to Invertebrates
D.R. Haviland, UC IPM and UC Cooperative Extension Kern County
C.G. Summers (emeritus), Entomology, UC Davis and Kearney Agricultural Research and Extension Center, Parlier
