Description of the Pest
The adult beet armyworm is a small, mottled gray- or dusky-winged moth. The moths are nocturnal (fly mostly at night) but may be seen by taking sweep net samples in the field.
Females deposit pale greenish or pinkish, striated eggs on the upper side of the alfalfa leaves in small or large masses covered with white cottony material. The eggs hatch in a few days, and the tiny caterpillars begin feeding on the plant. Heavy feeding on the tips of plant stems can cause flagging as terminal leaves turn white. The smooth-skinned caterpillars become full grown in about 2 to 3 weeks and are about 1.25 inches long. They range from olive green to almost black in color down the middle of the back with a yellow stripe on each side of the body.
Armyworms are common pests in the Central Valley and desert valleys from June through September. There are at least 5 generations per year in the low desert and four in the Central Valley. The final generation may overwinter as large larvae or pupae.
Damage
Armyworms skeletonize foliage, leaving veins largely intact. First- and second- instar larvae tend to feed in clusters around the egg mass from which they hatch. This frequently causes a tattered appearance to the terminals. This whitish appearance caused by the feeding is known as "whitecaps" and can be seen across a field. As the larvae mature and move to more stems, the areas of "whitecaps" tend to coalesce and the entire field takes on a tattered look.
Management
Populations of 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 components of a management program that will 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, assassin bugs, and lacewings.
The parasitic wasp, Hyposoter exiguae, is 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 of armyworms are also important natural control agents under certain conditions of temperature and humidity. Diseased caterpillars first appear yellowish and limp. After death they hang from plants as shapeless forms oozing the disintegrated body contents.
Cultural Control
Border-strip harvesting is a useful method for preserving natural enemies because it helps retain parasite larvae in the field. For more details, see BORDER-STRIP HARVESTING. Early cutting will give satisfactory control if the infestation appears late in the cutting cycle.
Organically Acceptable Methods
Biological and cultural controls, as well as sprays of Bacillus thuringiensis, are acceptable for use on an organically certified crop.
Monitoring and Treatment Decisions
In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars and continue through fall. 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 caterpillar 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 |
6.7–11.3 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 |
Label rates | 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
