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UC Pest Management Guidelines


Western yellowstriped armyworm on a damaged bean leaf.

Dry Beans

Armyworms

Scientific names:
Beet armyworm: Spodoptera exigua
Western yellowstriped armyworm: Spodoptera praefica

(Reviewed 8/07, updated 12/08)

In this Guideline:


DESCRIPTION OF THE PESTS

Eggs of the beet armyworm are deposited in a mass that is 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 ones on each side of the body. An intense black spot is usually visible on each side of the first legless segment behind the head.

DAMAGE

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 pod and beans.

MANAGEMENT

While armyworms may be present anytime from June through September, populations are usually most damaging in late summer. In those rare instances where control measures are required, the beet armyworm is more difficult to control than the western yellowstriped armyworm. Insecticide applications will be most effective if applied against small larvae.

Biological Control
Common natural enemies of armyworms include the parasite Hyposoter exiguae and numerous general predators including assassin bugs, damsel bugs, and spiders.

Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis are organically acceptable.

Monitoring and Treatment Decisions
Start inspecting for armyworm damage along with other pests and their damage during the vegetative growth period. Continue looking during the flower bud to bloom and pod fill periods.

Specific treatment thresholds have not been established for armyworms on beans and treatment is seldom necessary.

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact.
 
A. ACEPHATE
  (Orthene) 75 SP 0.67 lb 24 14
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: May result in mite outbreaks. Highly toxic to honey bees; do not apply when bees are present. Ground or air application.
 
B. METHOMYL*
  (Lannate SP) 0.5–1 lb 48 14
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Highly toxic to honey bees; do not apply when bees are present. Do not apply more than 4.5 lb a.i./acre/crop.
 
C. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (various products) Label rates 4 0
  MODE OF ACTION GROUP NUMBER1: 11.B1
  COMMENTS: Does not destroy natural enemies of corn earworm. Control is maximized by thorough coverage and by making applications when larvae are small.
 
D. CARBARYL*
  (Sevin) 80S 1.25–1.875 lb 12 see comments
  (Sevin) XLR Plus 1–1.5 qt 12 see comments
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: For use on western yellowstriped armyworm. May result in outbreak of spider mites. Do not use on lentils in California. XLR Plus formulation is the least toxic to honey bees when direct application to bees is avoided and the spray residues have dried. Apply from late evening to early morning when bees are not foraging. Do not apply within 14 days of grazing or harvest for forage, within 3 days of harvest of fresh beans or peas, and within 21 days of harvest of dried beans, peas, seed, or hay.
 
E. LAMBDA-CYHALOTHRIN*
  (Warrior with Zeon) 3.84 oz 24 see comments
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: May cause outbreaks of mites. Preharvest interval (P.H.I.) is 7 days for succulent shelled or edible podded crops and 21 days for dried shelled crops.
 
F. ZETA-CYPERMETHRIN*
  (Mustang) 4.3 oz 12 see comments
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: May cause outbreaks of mites. Preharvest interval (P.H.I.) is 1 day for succulent shelled or edible podded crops and 21 days for dried shelled crops.
 
** Mix with sufficient water to obtain full coverage.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on organically grown produce.
1 Rotate chemicals 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; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Dry Beans
UC ANR Publication 3446
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
R. F. Long, UC Cooperative Extension, Yolo County

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