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 (111 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.

• Summary of Important Links

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 Co.
P. A. Phillips, UC IPM Program/UC Cooperative Extension Ventura Co.
Acknowledgment for contributions to the Insects and Mites:
N. C. Toscano, Entomology, UC Riverside

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