How to Manage Pests

UC Pest Management Guidelines

Cucurbits

Armyworms

Scientific Names:
Beet Armyworm: Spodoptera exigua
Western Yellowstriped Armyworm: Spodoptera praefica
Yellowstriped Armyworm: Spodoptera ornithogalli

(Reviewed 12/09, updated 6/12)

In this Guideline:


DESCRIPTION OF THE PEST

Beet armyworm. Larvae are usually dull green and have wavy, light-colored stripes running lengthwise down the back and broader stripes on each side. Eggs are laid in a mass covered with hairlike scales.

Western yellowstriped armyworm. Females lay eggs in clusters covered with a gray, cottony material. Larvae measure about 1.5 inches (3.8 cm) long when fully grown. They are usually black with one prominent stripe over many narrow bright ones on each side of its body. The head is brown with a netted pattern. There is also a large black spot above the first abdominal spiracle.

Yellowstriped armyworm. Larvae of the yellowstriped armyworm are almost black, with two prominent and many fine, bright yellow stripes on the side.

DAMAGE

Primarily a foliage feeder, armyworms will also attack fruit, creating single or closely grouped circular or irregular holes on the surface. In many cases, feeding is superficial and little loss would result if not for decay organisms that enter wounds and rot fruit. The beet armyworm caterpillars occasionally develop inside the fruit. Yellowstriped armyworm does not enter the fruit and infestations are most severe from July to mid-September.

MANAGEMENT

Yellowstriped armyworm is not a serious pest every year but is very destructive on occasion. Armyworms tend to build up in alfalfa and weedy areas around the field and migrate from these areas when cut. Armyworms only need to be controlled if it is feeding on the crop. Keep crop residue and weeds in field and surrounding areas to a minimum to lessen the attraction of the field.

Biological Control
Many natural enemies attack armyworms. Among the most common parasites are the wasps, Hyposoter exiguae and Chelonus insularis, and the tachinid fly, Lespesia archippivora. Armyworms can easily be checked for the presence of Hyposoter exiguae by pulling the larva apart and looking for the parasite larvae. Viral diseases also kill significant numbers.

Organically Acceptable Methods

Sanitation in the field and surrounding areas along with biological control and sprays of Bacillus thuringiensis or the Entrust formulation of spinosad are acceptable to use in an organically certified crop.

Monitoring and Treatment Decisions

Beet armyworm may be present in and around the field feeding on bindweed and little mallow (malva).

  1. Start monitoring weeds and crop foliage for larvae just after transplanting or when seedlings emerge.
  2. If young instars are found, consider treating with low-impact products such as Bacillus thuringiensis, methoxyfenozide (Intrepid), or spinosad (Entrust). For yellowstriped armyworm, the older stages can be treated with methomyl. Treat the crop only if there is severe defoliation prior to fruiting. No thresholds have been established.
Common name Amount per acre R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
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. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. SPINETORAM
  (Radiant SC) 5–10 fl oz 4 see comments
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: PHI for cucumbers is 1 day and for other cucurbits 3 days.
 
B. SPINOSAD
  (Entrust)# 1.25–2.5 oz 4 see comments
  (Success) 4–8 fl oz 4 see comments
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Time spray to target eggs at hatch or small larvae. Do not apply more than 9 oz Entrust or 29 fl oz of Success/acre/season. PHI for cucumbers is 1 day and for other cucurbits 3 days.
 
C. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 28
 
D. METHOXYFENOZIDE
  (Intrepid 2F) 4–10 fl oz 4 3
  MODE OF ACTION GROUP NUMBER1: 18A
  COMMENTS: Time spray to target eggs and small larvae.
 
E. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  MODE OF ACTION GROUP NUMBER1: 11.B1
  (various products) Label rates 4 0
  COMMENTS: Use to control small armyworms only (first and second instar) when populations are light and full coverage sprays are applied. Repeat treatment as necessary. If mature larvae or heavy populations are present, use another material.
 
F. INDOXACARB
  (Avaunt) 3.5-6.0 oz 12 3
  MODE OF ACTION GROUP NUMBER1: 22
 
G. METHOMYL*
  (Lannate 90SP) 0.5–1 lb 48 see comments
  (Lannate LV) 1.5–3 pt 48 see comments
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: For use on cucumbers, melons and summer squash only. PHI is 1 day when 0.5 lb or less for 90SP or 1.5 pt or less for LV formulations is used; when more than 0.5 lb (90SP) or 1.5 pt (LV) is used, PHI is 3 days.
 
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.
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/.
# Acceptable for use on organically grown produce.
* Permit required from county agricultural commissioner for purchase or use.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445

Insects and Mites

  • E. T. Natwick, UC Cooperative Extension, Imperial County
  • J. J. Stapleton, UC IPM Program, Kearney Agricultultural Center, Parlier
  • C. S. Stoddard, UC Cooperative Extension, Merced & Madera counties
Acknowledgment for contributions to Insects and Mites:
  • R. L. Coviello, UC Cooperative Extension, Fresno County
  • L. D. Godfrey, Entomology, UC Davis
  • C. B. Fouche, UC Cooperative Extension, San Joaquin County
  • C. G. Summers, Entomology, UC Davis/Kearney Agricultural Center, Parlier
  • J. B. LeBoeuf, AgriData Sensing, Inc., Fresno
  • M. Murray, UC Cooperative Extension, Colusa/Glenn counties

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