How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Webworms overwinter as larvae or pupa and emerge in early spring. Adults are small buff to smoky brown colored moths that are active at night. They may be readily observed flying out of the foliage during the day as you walk through the field. Eggs are small, yellow or green in color, and laid in groups of 2 to 20 on the underside of leaves. Beet webworm eggs are laid end to end, while those of the alfalfa webworm are in overlapping groups. The beet webworm is dark green on hatching; mature larvae are about 1.5 inches long and olive green in color with a dark band running along the center of the back and lighter stripes on each side. The alfalfa webworm is yellowish to dark green with a broad light-colored stripe down the back and a darker stripe parallel to the light stripe. The garden webworm is also yellowish to green with a pale double stripe along the center of the back and a lighter line on each side of the body. The body also has numerous distinct black spots.

DAMAGE

Damage caused by the three species of webworms is nearly identical. They consume large amounts of foliage by skeletonizing leaves and can completely defoliate a field in a very short period of time. As they devour leaves, webworms spin a web drawing leaves together or folding individual leaves together to form a tube in which they hide when disturbed.

MANAGEMENT

Webworms are only occasional problems in sugarbeets. Plants can tolerate considerable defoliation and many biological control agents attack webworms. However, fields with populations of webworms should be closely monitored. If management is needed, choose materials such as Bacillus thuringiensis or spinosad that have low impact on natural enemies.

Biological Control
Numerous parasites and predators have been reported on webworms. However, it is doubtful that any of these agents are capable of regulating webworm populations.

Cultural Control
Injury and defoliation appear to be worse in weedy fields. Therefore, keep fields weed-free, particularly from pigweed and lambsquarter.

Monitoring and Treatment Decisions
No treatment thresholds are available for webworms. As with armyworms, the plants can tolerate considerable defoliation without yield loss. However, because of the rapidity with which webworm can defoliate plants, closely monitor fields in which webworms are active and apply a treatment if defoliation continues.

Common name		Amount/Acree	P.H.I.+
(trade name)			(days)
When choosing a pesticide, consider information relating to water quality and impact on natural enemies and bees.
A.	BACILLUS THURINGIENSIS ssp. KURSTAKI#
	(various products)	Label rates
	MODE OF ACTION: A microbial (Group 11.B2)1 insecticide.
B.	SPINOSAD
	(Entrust)#	1.5–3 oz	3
	(Success)	4.5–6 fl oz	3
	MODE OF ACTION: A microbial (Group 5)1 insecticide.
	COMMENTS: Do not apply more than 0.33 lb spinosad/acre/crop.
C.	METHOMYL*
	(Lannate) LV	0.75-3 pt	7-to harvest
	(Lannate) SP	0.25-1 lb	30-tops
	MODE OF ACTION: A carbamate (Group 1A)1 insecticide.
+	Preharvest interval. Do not apply within this many days of harvest.
#	Acceptable for use on organically grown produce.
1	Modes of action are important in preventing the development of resistance to pesticides. 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. 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 is assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.
*	Permit required from county agricultural commissioner for purchase or use.

PUBLICATION

UC IPM Pest Management Guidelines: Sugarbeet
UC ANR Publication 3469
Insects and Mites
C. G. Summers, Entomology, Kearney Agricultural Center, Parlier
D. R. Haviland, UC IPM Program, Kern Co.
L. D. Godfrey, Entomology, UC Davis

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