How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Twospotted spider mite eggs are laid on the undersides of leaves and are spherical, clear, and colorless when laid but become pearly white as hatch approaches. Nymphs, adult males, and reproductive adult females are oval shaped and generally yellow or greenish in color. There are one or more dark spots on each side of their bodies, and the top of the abdomen is free of spots. Adult female twospotted spider mites may cease to reproduce during the coldest winter months in production areas of colder inland valleys. Diapause is indicated by a change in color to bright orange. In coastal growing areas it is rare to have a significant proportion of the population undergo diapause. Mating and egglaying typically occur year round in all coastal strawberry-growing regions.

Carmine spider mite, a close relative of the twospotted spider mite, is bright red in color. It is commonly found at low densities in southern California, Central Coast, and San Joaquin Valley growing regions. Populations usually decline as temperatures warm in spring.

Take care to correctly identify these mites in the field, particularly in winter. Twospotted spider mites in diapause and carmine mite may be mistaken for the predaceous mite Phytoseiulus persimilis. However, the predaceous mite can be distinguished from these two mites by its much faster movement.

DAMAGE

Twospotted spider mite and carmine spider mite damage to strawberries appears as stippling, scarring, and bronzing of the leaves and calyx. Twospotted spider mite feeding is particularly damaging during the first 2 to 5 months following transplanting in late summer or fall, and yield loss is detectable at all mite infestation levels exceeding one mite per leaflet. Mite feeding during this critical period of plant growth substantially reduces berry number per plant and overall plantation yield.

Plants are less sensitive to mite feeding after initial berry set; substantial yield loss results from densities of 15 to 20 mites per mid-tier leaflet at this time. Plants that sustain infestations of greater than 75 mites per leaflet may become severely weakened and appear stunted, dry, and red in coloration. The highest twospotted mite populations are often observed following the peak spring fruit harvest, and this peak is typically followed by a rapid, natural decline in mite density when the plant enters a vegetative growth cycle. Twospotted mite densities may again increase later in summer as fruit production by day-neutral cultivars again increases.

MANAGEMENT

Cultural practices that favor vigorous plants are key to minimizing damage from spider mites. In addition, protect populations of natural enemies as much as possible by choosing insecticides and miticides that are least harmful to beneficials. If necessary, populations of natural enemies can be supplemented with the release of predatory mites. When treating for mites, choose the most selective miticide and alternate it with a miticide of a different chemistry or mode of action to avoid the development of resistance.

Biological Control
Predator mites such as Phytoseiulus persimilis, Galendromus occidentalis, Amblyseius californicus, and Neoseiulus fallacis are commercially available for release. Of the commercially available predatory mites, Phytoseiulus persimilis is most commonly used for suppressing spider mite populations. It is an aggressive feeder, and it multiplies and spreads rapidly. They will leave the field, however, if spider mite densities become too low to sustain the predator population.

Make predatory mite releases early in the season before spider mite populations begin to build or following winter spider mite treatments intended to reduce overwintering populations. On the central coast, spider mite populations are often first observed in January-February, while further south spider mite infestations may first develop in fall. Applying a short-residual miticide to reduce spider mite densities before a predator release may improve biological control under some conditions. Monitor fields on a regular basis to determine spider mite population densities.

Following releases of predator mites, it is important to monitor spider mite densities closely to evaluate the effectiveness of the predatory mites in maintaining the pest mites below economically injurious levels. Insecticides, miticides, and fungicides that are not selective will kill the predators. Make releases only after residues are below lethal levels following any pesticide application. Phytoseiulus persimilis has become established in most coastal strawberry-growing areas, and naturally occurring populations often move into spider mite-infested fields on their own. Amblyseius californicus has also been found to naturally infest strawberry plantations in some growing areas and can effectively maintain spider mite densities that are below threshold levels. Another predator mite, Phytoseiulus macropilus, occasionally occurs in strawberries early in spring.

Other natural enemies include minute pirate bug (Orius tristicolor), a small, black lady beetle (Stethorus spp.), a small, black rove beetle (Oligota oviformis), bigeyed bugs (Geocoris spp.), brown lacewings (Hemerobius spp.), green lacewings (Chrysopa spp.), sixspotted thrips (Scolothrips sexmaculatus), damsel bugs (Nabis spp.), a cecidomyiid fly maggot (Feltiella acarivora), and a predaceous midge.

Cultural Control
Strawberry cultivars vary in susceptibility to twospotted spider mite infestation and tolerance of twospotted spider mite feeding. When transplanted in fall, short-day cultivars are generally less tolerant of mite feeding than day-neutral cultivars, particularly later in the fruit-production season. When transplanted in summer, short-day cultivars are relatively tolerant of mite feeding.

Preplant chilling (vernalization) directly promotes plant vigor. Fall transplant, nursery location, preharvest chilling, nursery harvest date, and length of pretransplant supplemental cold storage can all affect a plant's vernalization. Plants with low amounts of chilling will have low vigor and will often develop intolerable mite infestations. Excessive chilling will promote increased vigor and reduce mite abundance, but other production factors are adversely affected (i.e., delayed flowering, large plant size, increased vegetative runner production). Be sure transplants have received adequate chilling and receive proper irrigation and fertilization.

Other controllable factors that can be used to promote plant vigor are soil preparation and fumigation, use of polyethylene plastic mulch, and proper irrigation to prevent water stress. Road dust control is also important in inhibiting mite infestations. Cultivars and cultural practices vary between production regions. Obtain information on cultivars and cultural practices pertinent to a particular growing region from your University of California County Cooperative Extension office or from cooperatives before making planting decisions.

Organically Acceptable Methods
Cultural and biological controls, including releases of predatory mites, and sprays of rosemary oil or organic stylet oil are acceptable for use on organically certified strawberries.

Miticide Resistance
Twospotted spider mites have a history of rapidly developing resistance to miticides when a miticide is repeatedly applied to the same population. Alternating miticides that have different modes of action may reduce development of resistance to a specific miticide. Avoid unnecessary spraying and treat only infested portions of the plantation. Organophosphate, carbamate, and pyrethroid insecticide applications can induce twospotted spider mite outbreaks. If possible, avoid early season insecticide applications or apply insecticides that are less disruptive to beneficial arthropods. Careful selection and use of insecticides early in the season can potentially reduce the number of miticide applications.

Monitoring and Treatment Decisions
Vigorous plant growth during the first 4 months following fall transplant is a key factor in strawberry production. Monitor mid-tier leaves during this critical period when mite feeding is extremely damaging. Mid-tier leaflets can be monitored by examining the undersurface with a hand lens to count the number of mites or by using a mite-brushing machine. Randomly select 10 leaflets/acre in small fields and 5 leaflets/acre in larger fields. When using a mite-brushing machine, the leaves from each acre can be brushed as one sample. The established economic threshold for this period is an average of five mites per mid-tier leaflet. Summer transplants have a higher threshold of an average of 10 mites per mid-tier leaflet during this same period. Record your observations on a sampling form (109 KB, PDF).

Once harvest begins, strawberries become more tolerant of mite feeding and treatment thresholds increase to an average of 15 to 20 mites per mid-tier leaflet. Treatment thresholds may vary somewhat depending on location, time of season, cultivar, overall plant vigor, yield potential, and the availability of an effective miticide.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
A.	BIFENAZATE
	(Acramite) 50WS	0.75–1 lb	12	1
	MODE OF ACTION GROUP NUMBER1: un
	COMMENTS: Do not make more than 1 application per harvested crop. Two sprays may be made per year if more than 1 crop is harvested each year; minimum period between application is 21 days. A good resistance management strategy is to use bifenazate as the winter spray (if needed) and as a rotational pesticide with abamectin and hexythiazox during the season. It has low toxicity to predatory mites and predatory insects. Bifenazate can be used once per year in strawberry nurseries.
B.	ACEQUINOCYL
	(Kanemite) 15 SC	21–31 fl oz	12	1
	MODE OF ACTION GROUP NUMBER1: 20B
	COMMENTS: Do not use less than 100 gal water/acre and do not apply more than twice/year. Allow a minimum of 21 days between treatments. Crops other than strawberries may not be rotated for at least 1 year following treatment.
C.	SPIROMESIFEN
	(Oberon) 2SC	12–16 fl oz	12	3
	MODE OF ACTION GROUP NUMBER1: 23
	COMMENTS: Do not make more than 2 applications per crop season.
D.	ETOXAZOLE
	(Zeal)	2–3 oz	12	1
	MODE OF ACTION GROUP NUMBER1: 10B
	COMMENTS: A mite growth regulator that is most effective against eggs and immatures. Most effective when applied before populations develop large numbers, but it will eventually control even a large population. Effective against both twospotted and carmine spider mites but not against cyclamen mite. Do not apply more than 3 oz/acre/season.
E.	HEXYTHIAZOX
	(Savey) 50DF	6 oz	12	3
	MODE OF ACTION GROUP NUMBER1: 10A
	COMMENTS: Limited to 1 application/season. Follow label directions for last date material can be applied because this varies by region. Most effective against eggs and nymphs, so best used when mites begin to actively reproduce. Not registered for nurseries.
F.	ABAMECTIN*
	(Agri-mek) 0.15EC	16 fl oz	12	3
	MODE OF ACTION GROUP NUMBER1: 6
	COMMENTS: Abamectin is less effective under cold weather conditions than in warm weather because movement into the leaf does not readily occur. Abamectin is most effective when used in paired applications 7–10 days apart when mites reach detectable levels under warmer temperatures in late winter/spring. Repeat the paired applications if necessary to maintain twospotted spider mite control. Do not exceed 16 fluid oz/acre/application or 64 fl oz/acre (4 applications) in a growing season. Do not apply in less than 100 gal water/acre (200 gal/acre is optimal). Do not repeat treatment within 21 days of 2nd application. Abamectin is not registered for strawberry nurseries.
G.	DICOFOL 4E	2 pt	12	2
	MODE OF ACTION: un
	COMMENTS: Resistance to dicofol has been widely reported in many twospotted spider mite populations. Recent observations indicate that dicofol can be effective at controlling twospotted spider mites for 1 or 2 applications following an extended period of no use. Because this material is one of the few remaining miticides registered for use on and effective against cyclamen mite, it is best to not apply dicofol for twospotted spider mites and to use it sparingly for cyclamen mite control. Toxic to predaceous mites but relatively nontoxic to general predators and parasites. Do not apply after fruit formation or when temperatures exceed 90°F. Do not make more than 2 applications/season,
H.	NARROW RANGE OIL#
	(Omni Oil) 6-E	1–2%	12	0
	MODE OF ACTION: Contact including smothering and barrier effects.
	COMMENTS: The potential for phytotoxicity has not been fully evaluated. Growers are encouraged to test product or product mixes for phytotoxicity before field applications to determine safety margins. Acceptable for use on organically grown crop only when fruit are not present. Apply in 60 gal water/acre with air-assist, low-volume ground equipment or 200 gal water/acre with standard ground spray equipment. Use this material for low-to-moderate population levels; higher levels of mite infestation require treatment with more effective miticides. Make applications only during winter months when plants are semi-dormant to reduce the risk of phytotoxicity. Do not use oil from peak bloom through fruiting period or when air temperatures are expected to exceed 75°F within several days following application. Do not apply from Jan 16 to May 30 in Orange and San Diego counties or the Oxnard Plains; do not apply from Feb 1 to Jun 15 in the Santa Maria Valley; and do not apply from Mar 1 to Jun 30 in Monterey and Santa Cruz counties.
I.	ROSEMARY OIL#
	(Ecotrol)	Label rates	0	0
	MODE OF ACTION: Contact including smothering and barrier effects.
	COMMENTS: The potential for phytotoxicity has not been fully evaluated. Growers are encouraged to test product or product mixes for phytotoxicity before field applications to determine safety margins. Danger of phytotoxicity when used at higher rates and when temperatures are warm. Do not apply in less than 50 gal water/acre. No residual activity, so repeat applications at 10-day intervals while mite populations are increasing.
J.	STYLET OIL
	(Organic JMS Stylet Oil)#
	(JMS Stylet Oil)	72 fl oz in 75 gal	4	0
	MODE OF ACTION: Contact including smothering and barrier effects.
	COMMENTS: The potential for phytotoxicity has not been fully evaluated. Growers are encouraged to test product or product mixes for phytotoxicity before field applications to determine safety margins. There is a danger of phytotoxicity when oils are applied incorrectly, especially under conditions of high temperature and low humidity; not recommended for use in southern California. Use of ceramic spray nozzles is recommended by the manufacturer. Make applications at a minimum pressure of 400 PSI. Lower pressures lead to larger droplet sizes, increasing the potential for phytotoxicity. Only organic JMS Stylet oil is acceptable for use on organically certified produce.
K.	FENBUTATIN-OXIDE*
	(Vendex) 50WP	1.5–2 lb	48	1
	MODE OF ACTION GROUP NUMBER1: 12B
	COMMENTS: Pest resistance to fenbutatin-oxide has been widely reported and persists within a population. Two applications of 1–2 lb/acre can be effective at suppressing twospotted spider mites following an extended period of no use in controlling a given population, but resistance will again become prevalent in the surviving twospotted spider mite population. Fenbutatin-oxide is more effective in warm weather conditions and appears to work in some areas of the Central Valley. Do not apply more than 3 applications per season or more than 9 lb/acre/season.
L.	PHYTOSEIULUS PERSIMILIS#
	COMMENTS: If low spider mite populations are present in localized areas, make spot releases. Although research is lacking, experience suggests release rates of an average of 2–3 predators/plant when pest populations are low and an average of 5 predators/plant when the pest mite population is at threshold level. For more widespread infestations early in the season when spider mite populations are low, releases can be made of about 30,000/acre (about 1.5 predatory mite/plant) either as a single, large release or as three smaller releases of 10,000/acre, depending on severity of weather conditions and spider mite population density in the field. Once mite densities increase to threshold levels, inundative releases may reduce twospotted spider mite infestations, but these must be made at release rates exceeding 100,000/acre because once spider mite populations begin to increase, it is difficult for predators to contain their densities below economic thresholds. Follow all releases of predatory mites with close monitoring of the spider mite population.
—	Information not available.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.
*	Permit required from county agricultural commissioner for purchase or use.
#	Acceptable for use on organically grown produce.
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/.

IMPORTANT LINKS

• Lygus survey form (109KB, PDF)

PUBLICATION

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468
Insects and Mites
F. G. Zalom, Entomology, UC Davis
M. P. Bolda, UC Cooperative Extension Santa Cruz County
P. A. Phillips, UC IPM Program/UC Cooperative Extension Ventura County
Acknowledgment for contributions to the Insects and Mites:
N. C. Toscano, Entomology, UC Riverside

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