How to Manage Pests
UC Pest Management Guidelines
Western Yellowstriped Armyworm
Scientific name: Spodoptera praefica
(Reviewed 12/13, updated 12/13)
In this Guideline:
Description of the Pest
Larvae of the western yellowstriped armyworm are almost black, with two prominent and many fine, bright yellow stripes on the side. This insect is not a serious pest of tomato every year, but is occasionally destructive. Infestations in tomatoes often originate from moths or caterpillars that migrate from alfalfa fields when the alfalfa is cut or from beans and other crops when they are harvested or dry out.
This pest feeds on both foliage and fruit. It rarely bores deeply into the fruit, but eats on the surface, causing irregular holes. Infestations are typically sporadic occurring in some years, and when a problem, most severe from July to mid-September.
Monitor yellowstriped armyworms along with other caterpillars when fruit reaches 1 inch. Biological control agents such as Hyposoter exiguae may keep populations under control. Treat when the thresholds are exceeded.
Organically Acceptable Methods
Sprays of Bacillus thuringiensis ssp. aizawai and the Entrust formulation of spinosad are acceptable for use in organically certified crops.
Monitoring and Treatment Decisions
In processing tomatoes, begin sampling fruit when it has reached 1 inch (2.5 cm) or more in diameter. Treatment is not necessary before this size because the damaged fruit will fall from the plant and will be replaced by more fruit. 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 (armyworms and fruitworms). 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. Record observations on a monitoring form . The treatment threshold is 3.25% damaged fruit. A sequential sampling technique is available in the online version of this guideline to help reduce the number of samples required to reach a treatment decision. In general, this pest is larger and thus more difficult to control than beet armyworm.
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: Tomato
Insects and Mites
E. T. Natwick, UC Cooperative Extension, Imperial County
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