Agriculture: Plum Pest Management Guidelines

Tent Caterpillars

Description of the Pest

Tent caterpillars overwinter in the egg stage; eggs give rise to caterpillars in spring and early summer. The western tent caterpillar is hairy and dull yellow brown with a row of blue spots adjacent to orange spots on top of the body. The forest tent caterpillar is dusky gray, sparsely hairy, with fine yellow-brown stripes on the shoulder and side separated by a broad blue lateral stripe. Its most distinguishing feature is a series of white diamond or keyhole-shaped spots running along its back. Both caterpillars have one generation each year.


Damage caused by tent caterpillars may be serious on individual trees. From April to June western tent caterpillars build large silken tents over leaves on which they feed. Forest tent caterpillars build mats of webbing rather than tents. They forage in all directions from these mats but return to the colony when not feeding. Tent caterpillars do not eat leaf veins.


Populations of tent caterpillars tend to be concentrated in individual trees scattered throughout the orchard. Treatment is only occasionally required and can be limited to small areas of the orchard.

Organically Acceptable Methods

Bacillus thuringiensis sprays and pruning out infestations are organically acceptable management methods.

Treatment Decisions

On small trees, cut out and destroy infested twigs. Spray programs for other insects generally reduce populations. If insecticide treatments are required, localized treatments on individual trees and branches are generally all that is necessary. Treat when small caterpillars are first observed. The addition of a wetting agent to increase penetration of the webbing by the insecticide enhances control.

Common name Amount to use** REI‡ PHI‡
(Example trade name) (conc.) (dilute) (hours) (days)
Not all registered pesticides are listed. The following 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. Always read the label of the product being used.
  (various products) Label rates 4 0
  COMMENTS: Most effective on small caterpillars. Does not destroy natural enemies.
B. DIAZINON* 50 WP 3 lb 1 lb 24 21
  4EC 3 pt 1 pt 24 21
  COMMENTS: Avoid drift and runoff into surface waters. Where plums are grown near waterways, do not use diazinon.
* Permit required from county agricultural commissioner for purchase or use.
** For dilute applications, rate is per 100 gal water to be applied in 300–500 gal water/acre, according to label; for concentrate applications, use 80–100 gal water/acre, or lower if the label allows.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
# Acceptable for use on organically grown produce.
Not recommended or not on label.
1 Rotate insecticides 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; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers for insecticides and miticides (un=unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).
Text Updated: 04/09
Treatment Table Updated: 04/09