Description of the Pest
Eggs of the beet armyworm are deposited in masses that are covered with a white cottony material. The larvae are about 1.25 inches long when full grown. They are mottled olive green to almost black. Eggs of the western yellowstriped armyworm are similar to those of the beet armyworm. The egg masses are larger and covered by a gray cottony material. The larvae attain a length of 1.5 to 2.0 inches, are black with a prominent yellowish stripe and several narrow bright stripes on each side of the body. An intense black spot is usually visible on each side of the first legless segment behind the head.
Armyworms skeletonize leaves when feeding in colonies shortly after hatching. Larvae that are half grown or more will feed singly on leaves and bean pods. Damaged pods will have holes in the pods and beans.
While armyworms may be present anytime from June through September, numbers are usually most damaging in late summer. Insecticide applications will be most effective if applied when larvae are young.
Common natural enemies of armyworms include the parasitoid, Hyposoter exiguae, which lays an egg that hatches into a green larva inside the parasitized armyworm. To determine if caterpillars are parasitized, pull worms (at least 0.5 inch long) apart to see if light green parasitic wasp larvae are inside. Additionally, there are numerous general predators that attack armyworms, including assassin bugs, damsel bugs, and spiders. Plant habitat that attracts natural enemies and avoid use of broad-spectrum pesticides known to harm natural enemies.
Organically Acceptable Methods
Use biological control, Bacillus thuringiensis, and the Entrust formulation of spinosad for an organically certified crop.
Monitoring and Treatment Decisions
Start inspecting for armyworm damage during the vegetative growth stage. Continue looking during the flower bud to bloom and pod fill stages. Specific treatment thresholds have not been established for armyworms on beans. Insecticide applications are seldom necessary.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees , and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Intrepid 2F)||4.0–16 fl oz||4||7|
|MODE-OF-ACTION GROUP NUMBER1: 18|
|COMMENTS: Early season applications: 4 to 8 fl oz; Mid- to late-season applications: 8 to 16 fl oz.|
|(Coragen)||3.5–5.0 fl oz||4||1|
|MODE-OF-ACTION GROUP NUMBER1: 28|
|(Steward EC)||6.7–11.3 fl oz||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 22A|
|D.||BACILLUS THURINGIENSIS ssp. AIZAWAI#|
|MODE-OF-ACTION GROUP NUMBER1: 11A|
|COMMENTS: Control is maximized by thorough good coverage and by making applications when larvae are small.|
|(Success)||4.0–6 fl oz||4||28|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|COMMENTS: Do not apply more than 12 oz per acre per season of Success or 3.75 oz per acre per season of Entrust. For blackeyes, apply with oil. Spinosad is not recommended as an effective control of western yellowstriped armyworms in California.|
|**||Mix with sufficient water to obtain full coverage.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|#||Acceptable for use on organically grown produce.|
|1||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; pesticides with a 1B group number should be alternated with pesticides that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|