How to Manage Pests

UC Pest Management Guidelines

Cotton

Beet Armyworm

Scientific Name: Spodoptera exigua

(Reviewed 5/13, updated 5/13)

In this Guideline:


Description of the Pest

Beet armyworm larvae are 1 inch long when fully grown. They are usually dull green but color can vary from pale to dark green with wavy, light-colored stripes running down the back and a broader pale stripe along each side. They usually have a dark spot on each side of the body above the second pair of true legs. Eggs are laid in clusters. These clusters are covered with dirty white, hairlike scales, forming a cottony covering. Adult beet armyworms are mottled gray and brown moths with a wingspan of a little over 1 inch. There are three to five generations a year. The pupa is the overwintering stage, but all stages may be present throughout the year in the low desert agricultural production valleys of southeastern California.

Damage

Beet armyworm destroys seedlings, terminals of young plants, and squares and small bolls during early July. Early season infestations may develop on weeds and move to cotton when weeds are controlled, destroying seedling cotton or the terminals of older plants. As cotton plants grow, young larvae skeletonize leaves and bracts, often spinning loose webbing over the feeding site. Older larvae chew irregular holes in leaves and also feed on squares, flowers, and bolls. Square damage by the beet armyworm differs from bollworm damage in that the surrounding bracts and foliage are often damaged by the beet armyworm but not by bollworm. The loss of a majority of squares and bolls during July or August may reduce yield or delay maturity by delaying fruit set. Severe defoliation may cause crop loss as well.

Management

In addition to cotton, beet armyworms feed on alfalfa, vegetables, sugarbeets, beans, and weeds such as pigweeds (Amaranthus spp.) and nettleleaf goosefoot (Chenopodium murale). In occasional years, there may be widespread outbreaks when favorable weather allows exceptionally large populations to build up early in the season on alternate hosts. Damaging populations may also occur where insecticides applied for other pests reduce natural enemy populations. Watch for beet armyworm on adjacent crops and on weeds in and around the field. If many larvae are present on weeds while cotton plants are small, it may be worthwhile to use an insecticide to kill them before destroying the weeds. Otherwise, they could move to the seedlings and cause stand loss. Treatment of a limited area, such as a strip at the edge of the field, is usually successful. When selecting an insecticide from a group of effective products, always select the insecticide that is least harmful to natural enemies.

Biological Control

Many predators and parasites combine to substantially maintain armyworm populations at low levels. Predators include assassin bugs, 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; other parasitic wasps include Trichogramma spp. And Cotesia marginiventris. Virus and bacterial diseases can also be important. Insecticide sprays for other pests may disrupt natural control.

Cultural Control

A recently developed transgenic cotton, Bollguard II, offers suppression of a broader range of caterpillars, such as beet armyworms, cotton bollworm, pink bollworm, and tobacco budworm, than earlier Bt varieties (Bollguard I).

Organically Acceptable Methods

Biological control and applications of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically grown cotton.

Resistance

To manage insecticide resistance in beet armyworm, limit the total number of sprays of each insecticide. The best way to do this is to practice the basic principles of IPM:

  • Monitor pests and maximize the use of biological and cultural controls.
  • Spray only when pest numbers warrant an application.
  • Use the most selective insecticides first to conserve natural enemy populations so they help with the control of pests.
  • Do not use insecticides with the same mode of action number on successive beet armyworm generations.

The following table provides insecticide resistance management guidelines.

Insecticide Resistance Management Guidelines for Beet Armyworm.
Insecticide Class Mode of Action1 Early Season April to Mid-June Mid-season Mid-June through July Late Season August through Sept.
Bacillus thuringiensis
ssp. Aizawai
11 various products various products NA
insect growth regulators 18
15
Intrepid2
Diamond
NA Intrepid2
organophosphate 1B NA Lorsban, Curacron Lorsban3
carbamate 1A NA Lannate, Larvin Lannate
miscellaneous 28
22A
5
Belt
Steward
Success
Belt
Steward
Success3
NA
pyrethroid4 3 NA NA Brigade, Asana
Do not use the same product or class of insecticide in succession.
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/.
2 Do not use more than twice per season or on successive generations.
3 If not used previously.
4 Pyrethroids are suggested only late in season to avoid aphid and spider mite disruptions.
NA  Not applicable.
Monitoring and Treatment Decisions

Early in the season, plants can sustain up to 50% loss of leaf surface without affecting yield. During the fruiting period, only 20 to 25% of the leaf surface can be lost without yield loss. After this period, up to 50% loss of leaf surface can again be tolerated. If beet armyworms build up on weeds in areas adjacent to the crop, consider treating a strip at the edge of the field to prevent entry into cotton.

When taking sweep net samples for lygus bug, also look for beet armyworm egg masses. The egg masses are covered with grayish white, hairlike scales and are laid on upper leaf surfaces in the upper plant canopy, but below the terminal area. Also watch for clusters of small, greenish caterpillars that feed in groups in leaf folds that are webbed together. To determine the actual number of caterpillars present, lay a 40-inch square piece of canvas between the rows and vigorously shake an arm's length of plants from one row onto the canvas. Count the number of armyworms on the canvas. There is no set treatment threshold for beet armyworm; it is up to the grower, based on past history and overall crop conditions, to determine if beet armyworm is causing significant economic losses to justify a treatment.

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, also 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. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (various products) Label rates 4 0
  SELECTIVITY: High
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Short, (Natural Enemies) Short
  MODE OF ACTION GROUP NUMBER1: 11
  COMMENTS: Does not disrupt natural enemies.
 
B. METHOXYFENOZIDE
  (Intrepid 2F) 4–10 fl oz 4 14
  SELECTIVITY: High
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Moderate
  MODE OF ACTION GROUP NUMBER1: 18
  COMMENTS: An insect growth regulator.
 
C. CHLORANTRANILIPROLE
  (Coragen) 3.5–7.0 fl oz 4 21
  SELECTIVITY: High
  PERSISTENCE: Pest: Unknown NE:2 Short
  MODE OF ACTION GROUP NUMBER1: 28
 
D. FLUBENDIAMIDE
  (Belt SC) 2 oz 12 28
  SELECTIVITY: High
  PERSISTENCE: unknown
  MODE OF ACTION GROUP NUMBER1: 28
  COMMENTS: A newer material; impact on beneficials not yet determined. Highly toxic to honey bees.
 
E. SPINOSAD
  (Entrust)# 1.25–2 oz 4 28
  (Success) 4–6 fl oz 4 28
  SELECTIVITY: High
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Short
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Use of Success allowed under a supplemental 24(c) registration.
 
F. INDOXACARB
  (Steward) Label rates 12 14
  SELECTIVITY: Moderate
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Moderate
  MODE OF ACTION GROUP NUMBER1: 22A
  COMMENTS: An oxadiazine.
 
G. DIFLUBENZURON*
  (Dimilin 25W) Label rates 12 14
  SELECTIVITY: High
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Short
  MODE OF ACTION GROUP NUMBER1: 15
  COMMENTS: An insect growth regulator. Do not exceed 6 applications or 24 oz per season.
 
H. NOVALURON
  (Diamond 0.83EC) 6–12 fl oz 12 30
  SELECTIVITY: Moderate to High
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Moderate
  MODE OF ACTION GROUP NUMBER1: 15
  COMMENTS: A chitin inhibitor.
 
I. CHLORPYRIFOS*
  (Lorsban Advanced) 1.5–2 pt 24 14
  SELECTIVITY: Moderate
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Short
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: An organophosphate. Do not graze or feed trash to livestock. Avoid drift and tailwater runoff into surface waters.
 
J. PROPHENOFOS*
  (Curacron 8E) 0.5–1 pt 48–72 14
  SELECTIVITY: Moderate
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Short
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: An organophosphate. Use low rate for light to moderate infestations, and high rate for heavy infestations.
 
K. THIODICARB*
  (Larvin 3.2) 24–36 fl oz 48 28
  SELECTIVITY: Moderate
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Moderate, (Natural Enemies) Moderate
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: A carbamate. Apply before bolls begin to open. Do not exceed 1.8 lb a.i./acre/season.
 
L. BIFENTHRIN*
  (Brigade 2EC) 3.8–6.4 fl oz 12 14
  SELECTIVITY: Low
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Long, (Natural Enemies) Long
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: A pyrethroid. Apply in a minimum of 5 gal water/acre with ground equipment or 1 gal/acre by air. When applying by air, 1 qt of emulsified oil may be substituted for 1 qt water in the finished spray. May also be applied in refined vegetable oil. Do not apply more than 0.3 lb a.i./acre/season or make more than 3 applications/season. Do not graze livestock in treated areas or cut treated crops for feed.
 
M. ESFENVALERATE*
  (Asana XL) 5.8–9.6 fl oz 12 21
  SELECTIVITY: Low
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Long, (Natural Enemies) Moderate
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: A pyrethroid. Do not graze or feed trash to livestock. Very destructive to natural enemies; can result in buildup of spider mites, and is not recommended in San Joaquin Valley. Do not exceed 0.5 lb a.i./acre/season.
 
N. METHOMYL*
  (Lannate SP) 0.12–0.25 lb 72 15
  SELECTIVITY: Low
  PERSISTENCE: Pest: Short NE:2 Short
  PERSISTENCE: (Pest) Short, (Natural Enemies) Short
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: A carbamate. Kills eggs and larvae. Use may redden cotton. According to the label, do not make more than 8 applications/season. Do not graze or feed cotton trash to livestock.
 
** Mix with sufficient water to provide complete coverage.
Preharvest interval. Do not apply within this many days of harvest.
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on organically produced cotton.
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: Cotton
UC ANR Publication 3444

Insects and Mites

  • L. D. Godfrey, Entomology, UC Davis
  • P. B. Goodell, UC IPM Program and Kearney Agricultural Research and Extension Center, Parlier
  • E. T. Natwick, UC Cooperative Extension - Desert Research and Extension Center, Imperial County
  • D.R. Haviland, UC Cooperative Extension, Kern County and UC IPM Program
  • V. M. Barlow, UC Cooperative Extension, Riverside County and UC IPM Program
Acknowledgment for contributions to Insects and Mites:
  • E. E. Grafton-Cardwell, Kearney Agricultural Research and Extension Center, Parlier
  • N. C. Toscano, Entomology, UC Riverside

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