Description of the Pest
Larvae of the fall webworm are pale brown or gray caterpillars with long white hairs arising from black and orange spots. Fall webworms spend the winter as pupae on the tree trunk or ground litter. Moths emerge in late spring and lay eggs on undersides of leaves; eggs hatch in late summer. Larvae feed in webbed colonies on ends of branches. In the lower Sacramento Valley there are two complete generations per year, but in most other sections of the state only one generation occurs.
Damage
From July to September, fall webworm caterpillars are found eating leaves (not veins) and forming silken tents on host trees.
Management
Insecticide sprays applied for other pests often keep these leaf-eating caterpillars in check.
Cultural Control
On small trees, cut out and destroy infested twigs.
Organically Acceptable Methods
Cultural control and Bacillus thuringiensis (Bt) sprays are organically acceptable management methods.
Treatment Decisions
If insecticide treatments are required, localized treatments on individual trees applied when evidence of caterpillars is first observed are generally all that is necessary. 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. | |||||
| A. | BACILLUS THURINGIENSIS ssp. KURSTAKI# | ||||
| (various products) | Label rates | — | 4 | 0 | |
| MODE OF ACTION GROUP NUMBER1: 11.B2 | |||||
| COMMENTS: Most effective on small caterpillars. Does not destroy natural enemies. | |||||
| B. | DIAZINON* 50WP | 3 lb | 1 lb | 24 | 21 |
| 4EC | 3 pt | 1 pt | 24 | 21 | |
| MODE OF ACTION GROUP NUMBER1: 1B | |||||
| 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). |