Description of the Pest
Webworms overwinter as larvae or pupae 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 free from pigweed and lambsquarters.
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 per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (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. | METHOXYFENOZIDE | |||
| (Intrepid 2F) | 8–16 fl oz | 4 | 7 | |
| MODE-OF-ACTION GROUP NUMBER1: 18 | ||||
| B. | BACILLUS THURINGIENSIS ssp. KURSTAKI# | |||
| (various products) | Label rates | 4 | 0 | |
| MODE-OF-ACTION GROUP NUMBER1: 11A | ||||
| C. | SPINOSAD | |||
| (Entrust)# | 1.5–3 oz | 4 | 3 | |
| (Success) | 4.5–10 fl oz | 4 | 3 | |
| MODE-OF-ACTION GROUP NUMBER1: 5 | ||||
| COMMENTS: Do not apply more than 0.33 lb spinosad/acre per crop. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging. | ||||
| D. | METHOMYL* | |||
| (Lannate LV) | 0.75–3 pt | 48 | See comments | |
| (Lanate SP) | 0.25–1 lb | 48 | See comments | |
| MODE-OF-ACTION GROUP NUMBER1: 1A | ||||
| COMMENTS: Preharvest interval is 21 days for roots, 30 days for tops. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging. | ||||
| ‡ | 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. |
| 1 | Group numbers for insecticides and miticides are assigned by IRAC (Insecticide Resistance Action Committee). Rotate pesticides 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. |
| * | Permit required from county agricultural commissioner for purchase or use. |