Description of the Pest
The most common spider mite encountered on beans is the twospotted spider mite, but the strawberry spider mite and Pacific spider mite can also be present. Infestations may include a mixture of spider mite species. Adult mites are nearly microscopic, have four pairs of legs, are greenish to pink or cream colored, and have different numbers and sizes of black spots on the body. Spider mites spin fine webbing over portions of the host plant. Under warm conditions, spider mites can move rapidly within the webbed area.
Spider mites have four stages of development:
- Egg: oval, somewhat translucent egg
- Larvae: a six-legged translucent larval stage
- Nymph: an eight-legged nymph stage
- Adult: an eight-legged adult stage
A resting or quiescent stage occurs at the end of the larval and nymph stages. A generation may complete development in as few as 5 to 7 days in summer and up to a month during cool periods.
Damage
All active stages of spider mites damage beans by piercing individual plant cells and sucking juices from infested leaves and pods. Damaged leaves have stippling on the upper surface and a grayish webbing on the undersurface where most feeding takes place.
Spider mites are most serious on lima beans and common dry beans, but can cause problems in blackeyes, especially on field edges near roads or after applications of broad-spectrum insecticides that reduce natural enemy numbers. In general, spider mite populations do not develop on blackeyes as rapidly as they do on limas, kidneys, and small white beans.
Management
Cultural Control
To prevent spider mite damage:
- Keep fields, field margins, and irrigation ditches clean of weed hosts such as field bindweed and common lambsquarters.
- Reduce dust, such as the dust caused by driving on dirt roads, as spider mite numbers may increase more rapidly in areas where dust deposits are heavy on bean plants.
- Consider sprinkler irrigation; spider mites are usually less severe in sprinkler-irrigated fields than in furrow-irrigated fields.
Biological Control
Spider mite numbers may be held at very low levels by various insect and mite predators, particularly early in the season. Western predatory mite, spider mite destroyers, predaceous midges, minute pirate bugs, bigeyed bugs, and six-spotted thrips are effective early-season predators. Plant habitat that attracts natural enemies and avoid the use of broad-spectrum pesticides known to harm natural enemies.
Organically Acceptable Methods
Monitoring and Treatment Decisions
There is no precise survey technique for evaluating spider mite infestations. Infestations usually begin on the lower portions of the plants and move upward as mite numbers increase. Start inspecting plants for spider mite damage during the vegetative growth stage. Continue looking for mites from the flower bud to bloom stages and during the pod fill stage.
Evaluating spider mite infestations is most efficient if older, lower leaves are randomly selected and inspected for stippling on the upper surface and webbing, mites, and feeding scars on the lower surface. If spider mites can be found easily on older leaves during the early bloom stage and before the first insecticide application for lygus bug control, it would be advisable to use an acaricide at the time of the first spray for lygus bug control.
| 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. | BIFENAZATE | |||
| (Acramite 4SC) | 16–24 fl oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NUMBER1: 20D | ||||
| B. | ABAMECTIN | |||
| (Agri-Mek SC) | 1.7–3.5 fl oz. | 12 | 7 | |
| MODE-OF-ACTION GROUP NUMBER1: 6 | ||||
| COMMENTS: Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging. For use on cowpeas that are grown only for dry seed. Do not allow livestock to graze cowpea forage and do not harvest cowpea forage or hay for use as livestock feed. | ||||
| C. | HEXYTHIAZOX | |||
| (Onager) | 10–24 fl oz | 12 | 14 | |
| MODE-OF-ACTION GROUP NUMBER1: 10A | ||||
| Onager will not control adult mites; it is an ovicide and larvicide. It needs to be applied prior to mite buildup. | ||||
| D. | PROPARGITE | |||
| (Comite) | 32–48 fl oz | 216 (9 days) | 14 | |
| MODE-OF-ACTION GROUP NUMBER1: 12C | ||||
| COMMENTS: Do not use on fresh market pod varieties. Do not feed or forage sprayed vines or trash after harvesting. | ||||
| ** | Mix with sufficient water to obtain full coverage. |
| ‡ | 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 | 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; pesticides with a 1B group number should be alternated with pesticides that have a group number other than 1B. Mode-of-action group numbers (un= unknown of uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee). |