Description of the Pest
Spider mites resemble tiny spiders, no larger than a pinhead. Nearly identical in appearance, the carmine mite occurs on sugar beets primarily in the Imperial Valley and the twospotted spider mite is more common in other beet-growing areas of California. When newly hatched, spider mites have three pairs of legs, but as they mature, they develop four pairs. They vary in color from nearly transparent to yellowish, greenish, or even red. They have two darkly pigmented spots, one on each side of the body. While visible to the unaided eye, they are best seen and identified with the use of a 10X hand lens. Both species cause similar damage and are managed in the same manner.
Damage
Mites feed on the undersurface of the leaf. They puncture cells on the leaf surface and feed on the sap from leaves, leaving them blotched with white or pale yellow spots, which range in size from mere specks to larger areas as the specks coalesce. The damage is visible on both the upper and lower surface of the leaf. Spider mites spin very fine webs over the surface that become prominent as the number of mites per leaf increases. Severe infestation can cause defoliation.
Management
Spider mites are usually controlled by predatory insects and mites. Outbreaks are often the result of plant stress or dusty conditions. Mature sugarbeets can tolerate dozens to hundreds of mites per leaf without significant reductions in yield or quality. Treatments may be necessary on early- and mid-season sugarbeets although treatment thresholds have not been established.
Biological Control
Spider mites have numerous predators, including lacewings, assassin bugs, damsel bugs, minute pirate bugs, bigeyed bugs, and sixspotted thrips. Predatory mites are often found attacking spider mites. Predatory mites can be distinguished from spider mites by their slightly larger size and a more flattened, pear-shaped appearance. Predatory mites are clear, lacking any spots or coloration. These natural enemies can keep spider mite populations in check and may bring spider mites under control. Always check for their presence when you find mites in your fields.
Cultural Control
Mites are more serious on stressed plants, particularly water-stressed or dust-covered plants. Outbreaks may occur as a result of foliar applications of organophosphates, carbamates, and pyrethroids used to control other pests such as armyworms. Avoid using these insecticides where possible and observe good cultural practices including adequate nutrition and irrigation.
Monitoring and Treatment Decisions
There are no economic thresholds for spider mites. If numbers continue to increase in spite of the presence of biological control agents, and injury becomes severe, treatments might be necessary.
| 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. | NALED | |||
| (Dibrom 8)* | 1 pt | 48 | 2 | |
| MODE-OF-ACTION GROUP NUMBER1: 1B | ||||
| COMMENTS: May give good initial control, but resurgence (primarily from hatching eggs) can be a serious problem. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging. | ||||
| B. | SULFUR DUST# | 35–40 lb | 24 | NA |
| MODE OF ACTION: Unknown. An inorganic insecticide. | ||||
| C. | MICRONIZED SULFUR# | |||
| (Microthiol Disperss) | 5–10 lb | 24 | NA | |
| MODE OF ACTION: Unknown. An inorganic insecticide. | ||||
| ‡ | 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. |
| 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. |
| # | Acceptable for use on organically grown produce. |
| NA | Not applicable. |