How to Manage Pests

UC Pest Management Guidelines


Beet Armyworm

Scientific name: Spodoptera exigua

(Reviewed 12/13, updated 12/13, corrected 10/16)

In this Guideline:

Description of the Pest

Beet armyworms are a widespread pest in California found in tomato fields every year. In some areas beet armyworm may be the most important caterpillar attacking tomato.

Eggs are laid on leaves in clusters covered with hairlike scales left by the female moth; there may be more than 100 eggs per cluster, but usually there are fewer. Newly hatched larvae feed together on foliage near the egg cluster and gradually disperse as they grow. Older larvae feed on leaves and fruit. Larvae usually are dull green with many fine, wavy, light-colored stripes down the back and a broader stripe along each side; they usually have a dark spot on the side of the thorax above the second true leg. The color varies, however, and the spot is absent in a proportion of some populations. The pupa is similar to that of the tomato fruitworm; it pupates in a depression made on or pocket just below the soil surface. The adult moth is mottled gray and brown with a wingspan of about 1 inch. The life cycle takes about a month in warm weather, and there are three to five generations a year.


Beet armyworm attacks both foliage and fruit, creating single or closely grouped circular or irregular holes. In processing tomatoes, fruit feeding is often shallow and superficial as most wounds eventually dry. Little loss would result from feeding damage when the processing pack is for paste or juice uses. However, loss is more significant when decay organisms directly enter wounds and rot the fruit, or if feces or the caterpillar remain in the fruit. Damage is problematic for whole pack or diced uses. Check with the processor for acceptable levels of armyworm-scarred fruit. In fresh market tomatoes, the presence of such holes results in unmarketable fruit. The caterpillars occasionally develop inside the fruit, causing damage similar to that of the tomato fruitworm, and may feed on floral buds causing buds to abort.


Beet armyworms are sometimes kept under control by natural enemies and a polyhedrosis virus. Use the UC fruit sampling procedure below to determine need for treatment.

Biological Control

A nuclear polyhedrosis virus often reduces populations in fall and winter. Hyposoter exiguae, a small wasp, is the most important parasite of beet armyworm. General predators such as bigeyed bugs and minute pirate bugs feed on eggs.

Organically Acceptable Methods

Biological control and sprays of the Entrust formulation of spinosad and Bacillus thuringiensis ssp. aizawai are acceptable for use in an organically certified crop.

Monitoring and Treatment Decisions
Processing tomatoes

In processing tomatoes, begin sampling when fruit has reached 1 inch or more in diameter. Treatment is not necessary prior to this size as the damaged fruit will fall from the plant and yield loss will be minor. Pick at least 100 fruit at random while walking through the field, being careful not to select red fruit when the majority of fruit are green. If damaged fruit are found, determine the amount of damage present and the size and species of the worms. Count fruit as damaged if it has any hole deeper than 0.1 inch (2.5 mm), if the hole is contaminated with feces, or if any larvae are present in the fruit. The treatment threshold is 3.25% damaged fruit. A sequential sampling technique is available (PDF) to help reduce the number of samples required to reach a treatment decision.

Fresh market tomatoes

In fresh market tomatoes, begin sampling when fruit appears. Pheromone traps are useful for determining when major flights occur, but not for predicting damage. A 5-minute timed search is useful in determining the need for treatment. On average, if one or more larvae or egg masses are found in 5 minutes, treatments may be justified. Picking large numbers of fruit each week and assessing percent damage may not be economically feasible. Ground applications provide maximum effectiveness of the pesticide.

Common name Amount per acre** R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
The following materials 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.
  (Coragen) 3.5–5 fl oz 4 1
  COMMENTS: Can be applied as foliar spray or by drip chemigation. Read label for treatment intervals.
  (Intrepid 2F) 8–16 fl oz 4 1
  COMMENTS: Low toxicity to beneficials. Apply at the beginning of egg hatch. When traps indicate moth flights have begun, sample leaves for eggs. Treat when eggs are first detected.
  (Radiant SC) 5–10 fl oz 4 1
  COMMENTS: Apply as a foliar spray.
  (Entrust)# 1.25–2.5 fl oz 4 1
  (Success) 4–8 fl oz 4 1
  COMMENTS: Use higher rate for larger worms and heavy infestations. Best control is achieved when aimed at newly hatched larvae and coverage is thorough. Less toxic to natural enemies than many other choices. For resistance management, do not apply more than 0.45 lb a.i./acre per season.
  (various products) Label rates 4 0
  COMMENTS: This insecticide is most effective against newly hatched larvae, so proper treatment timing is essential. This insecticide is also somewhat effective on other worm pests.
  (Rimon 0.83EC) 9–12 fl oz 12 1
  COMMENTS: Apply at egg hatch to the second instar. Use higher rates when larvae are large or foliage canopy is tall or dense.
  (Proclaim) 2.4–4.8 oz 12 7
  (Avaunt) 3.5 oz 12 3
  (Lannate SP) 0.5–1 lb 48 1
  COMMENTS: Will also control fruitworm, yellowstriped armyworm, cutworms, and cabbage looper. Primary use of methomyl should be if older larvae, which are difficult to control with other insecticides, are present. Some resistance has been documented. Do not use if psyllids are in the field as carbamates tend to promote development of their populations; also if leafminers are present, it may cause outbreaks by destroying their natural enemies. Do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
  (Asana XL) 5.8–9.6 fl oz 12 1
  COMMENTS: Use only when pest numbers are high close to harvest. Some resistance has been documented. May cause outbreaks of Liriomyza spp. leafminers and tomato russet mite. In some areas where tomatoes are grown, resistance to this material is a problem. Do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
  (Danitol 2.4EC) 10.66 fl oz 24 3
  COMMENTS: Use only when pest numbers are high close to harvest. May cause outbreaks of Liriomyza spp. leafminers and tomato russet mites.
** See label for dilution rates.
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 until harvest can take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
* 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




[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470

Insects and Mites

E. T. Natwick, UC Cooperative Extension, Imperial County
C. S. Stoddard, UC Cooperative Extension, Merced and Madera counties
F. G. Zalom, Entomology, UC Davis
J. T. Trumble, Entomology, UC Riverside
G. Miyao, UC Cooperative Extension, Solano and Yolo counties
J. J. Stapleton, UC IPM Program and Kearney Agricultural Center, Parlier (false chinch bug)
Acknowledgments for contributions to Insects and Mites:
C. G. Summers, Entomology, UC Davis and Kearney Agricultural Center, Parlier
C. F. Fouche, UC Cooperative Extension, San Joaquin County
N. C. Toscano, Entomology, UC Riverside

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