How to Manage Pests
UC Pest Management Guidelines
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.
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
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 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.
UC IPM Pest Management Guidelines:
Insects and Mites
E. T. Natwick, UC Cooperative Extension, Imperial County
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