Webspinning Spider Mites

Pacific and twospotted mites overwinter as reddish orange mature females in protected places on the tree, in soil, and in decomposing leaf litter on the ground. During warm weather in spring, overwintered females begin feeding on pistachio leaves and ground cover in the orchard. During periods of active feeding twospotted mites have a dark spot on each side of the body. Pacific mites have a second pair of dark spots near the posterior end. Often, however, the spots are barely visible or may coalesce to large dark areas, making it difficult to distinguish the two species.

Colonies develop on the underside of leaves and also on the upper sides when numbers build up. Eggs are spherical and translucent when first laid, becoming opaque soon before hatching. Immature mites molt three times before becoming adults. Spider mites are tiny, less than 1/20 inch (1.3 mm) long. The first stage mites have six legs; later stages and adults have eight legs. These mites reproduce rapidly in hot weather and may become numerous in June or July. They produce many generations a year. If temperature and food supply are favorable, a generation can be completed in 7 days.

Pacific and twospotted spider mite damage initially appears as a loss of green color on infested leaflets. Continued feeding causes the leaf to develop small, necrotic spots (stippling) with webbing. All mite life stages can be found feeding within this webbing. Low mite levels of 3 to 4 per leaflet are sufficient to cause defoliation in pistachio. Clusters of brown leaves are often the first sign of increasing mite numbers. Defoliation early in the season will greatly reduce nut yield and quality; defoliation late in the season will interfere with harvest.

Areas with somewhat alkaline soils are more likely to have spider mite problems. As in other crops, leaf loss relative to mite levels is enhanced by plant stress from lack of water or high salinity. Greater incidence of damage has been reported with increased pyrethroid use for plant bug control. Many cases involve severe defoliation and a shriveled crop on infested trees.

Spider mites are rarely pests in pistachio orchards, except where natural enemies are absent due to the use of broad-spectrum pesticides or where trees are highly stressed. Tree health and use of selective materials for other pests usually provides adequate spider mite control.

Sixspotted thrips, Scolothrips sexmaculatus, are the most important predators of spider mites in pistachios. Since webspinning spider mite populations develop very slowly on pistachios, thrips often provide effective biological control and insecticides are not needed. Other predators include the spider mite destroyer, Stethorus picipes, and minute pirate bugs.

Orchard management practices can reduce mite problems.

• Minimize dust by oiling orchard roads and maintaining a ground cover.
• Well-irrigated, vigorous trees sustain less damaged from mite infestations.
• Choose selective pesticides when controlling other pests and avoid pyrethroids, organophosphates, and carbamates until later in the season.

Biological control, cultural controls, and sprays of narrow range oil are acceptable for use in an organically certified crop.

Monitor from June through August by looking for the presence of spider mites and their predators on leaves. If spider mites are found, wait one week and resample to determine if populations are increasing, decreasing, or staying the same. Spider mite treatments in pistachios are rarely needed, but can be considered if populations and leaf stippling are increasing and defoliation occurs.

	Common name	Amount per acre	REI‡	PHI‡
	(Example trade name)		(hours)	(days)
Pesticide precautions Protect water Calculate VOCs Protect bees
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.	SPIRODICLOFEN
	(Envidor)	16–34 oz	12	7
	MODE-OF-ACTION GROUP NUMBER1: 23
B.	ETOXAZOLE
	(Zeal Miticide 1)	2–3 oz	12	28
	MODE-OF-ACTION GROUP NUMBER1: 10B
C.	FENPROXIMATE
	(FujiMite 5EC)	2–4 pt	12	14
	MODE-OF-ACTION GROUP NUMBER1: 21A
	COMMENTS: Apply by ground and use a minimum of 100 gallons of water per acre.
D.	NARROW RANGE OIL#
	(TriTek, etc.)	See label	4	0
	MODE OF ACTION: Contact including smothering and barrier effects.

‡

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 chemicals 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; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode-of-action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.