Spider Mites

Twospotted spider mite

Twospotted spider mite eggs are about 0.006 inch (0.14 mm) in diameter and are laid on the undersides of leaves. They are spherical, clear, and colorless when laid but become pearly white as hatch approaches.

Nymphs, adult males, and reproductive adult females are oval and generally yellow or greenish. 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 stop reproduction 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 egg laying typically occur year-round in all coastal strawberry-growing regions.

Carmine spider mite

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

Strawberry spider mite

Both strawberry and twospotted spider mites look similar; they can only be distinguished by the morphological characters of male genitalia. Twospotted spider mite is the predominant species in strawberries grown on the Central Coast. Strawberry spider mite occurs in some of these areas, with mixed populations of both twospotted and strawberry spider mites seen particularly during the warmer parts of the production season.

Twospotted spider mite and carmine spider mite damage 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. Mite feeding during this critical period of plant growth substantially reduces berry number per plant and overall plantation yield. Yield loss is detectable at all mite infestation levels exceeding one mite per leaflet.

Plants are less sensitive to mite feeding after initial berry set.

• Substantial yield loss results from 15 to 20 mites per mid-tier leaflet after berry set.
• Plants that sustain infestations greater than 75 mites per leaflet may become severely weakened and appear stunted, dry, and turn red.

The highest numbers of twospotted mites are often observed following the peak spring fruit harvest. This peak is typically followed by a rapid, natural decline in mite numbers when the plant enters a vegetative growth cycle. Twospotted mite numbers may again increase later in summer as fruit production by day-neutral cultivars again increases.

• Cultural practices such as vernalization of transplants that favor vigorous plants are key to minimizing damage from spider mites.
• Protect natural enemies as much as possible by choosing insecticides and miticides that are least harmful to natural enemies.
• If necessary, supplement natural enemies by releasing 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.

Predator mites commercially available for release:

• Phytoseiulus persimilis
• Neoseiulus (Amblyseius) californicus
• Neoseiulus fallacis
• Galendromus occidentalis

Of the commercially available predatory mites, only the first three are commonly used for spider mite suppression, most frequently Phytoseiulus persimilis. P. persimilis is an aggressive feeder, multiplies and spreads rapidly, and is a specialist predator that feeds exclusively on pest mites. However, in the absence of these pest mites, P. persimilis feeds on its own species, causing the population to gradually disappear.

Predatory Mite Releases

1. Monitor fields on a regular basis to determine spider mite numbers.
2. Applying a short-residual miticide to reduce spider mite numbers before a predator release may improve biological control under some conditions.
3. Make predatory mite releases early in the season before spider mite numbers begin to increase or following winter spider mite treatments intended to reduce overwintering mites. On the Central Coast, spider mites are first observed typically in January or February, while further south spider mite infestations may first develop in fall.
4. Following predator mite release, monitor spider mite numbers closely to evaluate the effectiveness of the predatory mites in maintaining the pest mites below economically injurious levels.

If low spider mite numbers are present in localized areas, make spot releases. Although research is lacking, experience suggests the following release rates:

• When pest numbers are low:
  • Release of an average of two to three predators per plant.
  • For more widespread infestations early in the season when spider mite numbers are low: make releases of about 30,000 per acre (about 1.5 predatory mite per plant) either as a single, large release or as three smaller releases of 10,000 per acre, depending on severity of weather conditions and spider mite numbers in the field.
• When the pest mite numbers are at threshold level:
  • Release an average of five predators per plant.
  • Once mite numbers increase to threshold levels, inundative releases may reduce twospotted spider mite infestations, but these must be made at release rates exceeding 100,000 per acre because once spider mite numbers begin to increase, it is difficult for predators to reduce their numbers below economic thresholds.

Insecticides, miticides, and certain 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. Neoseiulus (Amblyseius) californicus has also been found to naturally infest strawberry plantations in some growing areas and can effectively maintain spider mite populations that are below threshold levels. Another predator mite, Phytoseiulus macropilus, occasionally occurs in strawberries early in spring.

Other natural enemies of mites include minute pirate bug (Orius tristicolor), a small, black lady beetle (spider mite destroyer, 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.), and a predaceous midge larvae (Feltiella sp.).

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 have low vigor and can develop intolerable mite infestations. Excessive chilling promotes increased vigor and reduces mite abundance, but other production factors are adversely affected (i.e., delayed flowering, large plant size, and increased vegetative runner production at the expense of flowering and fruiting). Be sure transplants have received adequate chilling for the variety and area 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 transplant nurseries before making planting decisions.

Use cultural and biological controls, including releases of predatory mites, sprays containing plant-derived oils or organic stylet oil, and bioinsecticides such as Beauveria bassiana, Burkholderia sp. strain A396, or Chromobacterium subtsugae strain PRAA4-1 on organically certified strawberries.

Twospotted spider mites have a history of rapidly developing resistance to miticides when a miticide is repeatedly applied to the same population.

• Alternate miticides that have different modes of action to reduce development of resistance to a specific miticide.
• Spray based on sampling, to avoid unnecessary pesticide applications. Spray only infested portions of the field, if applicable.
• 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.

Vigorous plant growth during the first 4 months following fall transplant is a key factor in successful strawberry production. Monitor mid-tier leaves during this critical period when mite feeding is extremely damaging.

1. Randomly select 10 leaflets per acre in small fields and 5 leaflets per acre in larger fields. When using a mite-brushing machine, the leaves from each acre can be brushed as one sample.
2. Examine the undersurface of mid-tier leaflets with a hand lens to count the number of mites or use a mite-brushing machine.
3. Record your observations on a sampling form (PDF).

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. 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.