How to Manage Pests

UC Pest Management Guidelines

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 very destructive on occasion. 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.

DAMAGE

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 most severe from July to mid-September.

MANAGEMENT

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 (100 KB, PDF).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.

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.

• Summary of Important Links

Common name		Amount/Acre**	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
A.	METHOXYFENOZIDE
	(Intrepid) 2F	8–16 fl oz	4	1
	MODE OF ACTION GROUP NUMBER1: 18A
	COMMENTS: Research information regarding the efficacy of this material for western yellowstriped armyworm is lacking; used primarily in the San Joaquin Valley for early season infestations. Less toxic to natural enemies than many other choices.
B.	METHOMYL*^
	(Lannate) 90SP	0.5–1 lb	48	1
	MODE OF ACTION GROUP NUMBER1: 1A
	COMMENTS: Will also control fuitworm, beet armyworm, and cabbage looper. 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.
C.	ESFENVALERATE*^
	(Asana XL) 0.66EC	5.8–9.6 fl oz	12	1
	MODE OF ACTION GROUP NUMBER1: 3
	COMMENTS: Some resistance has been documented. Do not use this product if leafminers are present because it is destructive of their parasites. Do not exceed 0.5 lb a.i./acre/season.
D.	SPINOSAD
	(Entrust)#	1.25–2.5 fl oz	4	1
	(Success)	4–8 fl oz	4	1
	MODE OF ACTION GROUP NUMBER1: 5
	COMMENTS: Research regarding efficacy of Entrust formulation for yellowstriped armyworm control is lacking, but efficacy information for the Success formulation suggests it has potential for controlling this species.
E.	BACILLUS THURINGIENSIS ssp. AIZAWAI#
	(various products)	Label rates	4	0
	MODE OF ACTION GROUP NUMBER1: 11.B1
	COMMENTS: This material is also somewhat effective on other worm pests.
	**  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. ^ Do not apply when bees are present. # 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 http://www.irac-online.org/.

IMPORTANT LINKS

• Armyworm monitoring and sampling form (100KB, PDF)
• Photos of natural enemies of armyworms

Back to management

PUBLICATION

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470
Insects and Mites
F. G. Zalom, Entomology, UC Davis
J. T. Trumble, Entomology, UC Riverside
C. F. Fouche, UC Cooperative Extension, San Joaquin County
C. G. Summers, Entomology, UC Davis/Kearney Agricultural Center, Parlier
Acknowledgments for contributions to the insects and mites section:
N. C. Toscano, Entomology, UC Riverside

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