Two Spotted Spider Mite Control in Strawberries

Two-spotted spider mites are sporadic pests of strawberries, damaging outbreaks do not occur every year, or on every farm. Scouts should watch closely for these pests however, because outbreaks are not predictable.

Factors that affect mite populations include:

• Cultivar: Some varieties are more susceptible to mites than others. Sparkle and Jewel are reported to be very susceptible. Examples of varieties where we have seen mite problem in the past include Annapolis and Mesabi. and the day neutral variety Seascape.
• Crop nutrition: Plant tissue high in nitrogen favours mite development
• Crop management: Irrigation can affect mite populations. Drought stressed plants are more susceptible to injury, and favour increased mite populations. Management practices such as renovation have an effect. Mowing leaves after harvest helps to reduce mite numbers.
• Pesticide use: Certain pesticides, for example pyrethroid insecticides, are harmful to beneficial insects and beneficial mites.
• Surrounding vegetation: Severe outbreaks on adjacent fields of beans or apple orchards, can spread to strawberry fields.
• Weather: Hot dry weather is favourable for mite development.

Damage

Two spotted spider mite live mostly on the underside of the leaves, they cause damage by sucking plant juices. Damage appears as flecking or stippling on the upper leaf surface. Heavily damaged plants lose vigour, leaves turn dry and off-colour, which is known as bronzing. Fruit production and yield can be reduced if mites cause this type of damage before harvest.

Identification and Life Cycle

Adults mites are greenish yellow colour with two black spots on the side of their backs (Figure 1). They are small (< .5 mm), but can be easily seen with a hand lens. Nymphs are similar to adults, but smaller and the earliest instars do not have dark spots. Eggs are clear and round.

Figure 1

TSSM adult females overwinter in plant debris, and on old strawberry leaves. The overwintering mites are orange in colour. As the weather warms up and mites begin to feed they develop their normal appearance. There are multiple generations per year, and all stages may be present at one time. Webbing may be present when populations are high. Mites can move into strawberry fields from adjacent vegetation, especially if populations are high on these alternate hosts (beans, orchards, broad-leaved weeds).

Monitoring

First, determine where mites are present in the field. Watch for signs of flecking and stippling on the older, lower leaves (Figure 2). Walk the field in a zig-zag pattern, periodically stopping to pick a middle-aged leaf (not the oldest, not the youngest). Scan the underside of the leaf with a hand lens for mites.

Figure 2

Once you have determined where mites are the biggest problem, collect a sample of 60 leaflets from 40-60 random plants the variety in question. A leaflet is one of the trifoliates that make up a strawberry leaf. Take the leaflet from fully expanded leaves, avoiding the lower, oldest leaves next to the ground.

Use the presence absence technique to estimate the population of mites in the field.
If mites are present on 25% of the leaflets, there are approx 5 mites per leaflet.
If mites are present on 55% of the leaflets, there are approx 20 mites per leaflet.

Thresholds

Thresholds have not been validated in Ontario. Thresholds from other regions range from 5-20 mites per leaflet. Consider using the low threshold before harvest and the higher threshold after harvest or in non-bearing plants.

Timing

Timing depends on the miticide to be applied and the crop stage. See Table 1, Miticides for use on strawberries for two-spotted spider mite control, for best timing of specific products. Miticides work best when applied in warm weather, because mites are more active and eggs are hatching more quickly. In cool weather, mites do not feed or move as much and are less likely to pick up miticide residue.

Early spring applications may not be effective because strawberry plants are rapidly growing in the spring and leaves that expand after the miticide is applied do not have adequate residue for control. Miticides should not be applied until mites are moving from older overwintering leaves to newer growth.

Table 1: Miticides for use on strawberries for two-spotted spider mite control

Precautions

The window for mite control closes a week to 10 days before harvest. Scouts should be sure to provide growers with detailed information 2-3 weeks before harvest so controls can be applied if necessary.

Successful treatment requires thorough spray coverage, particularly of the leaf undersurface. Most currently registered miticides are not systemic: mites must come in contact with residues. Excellent spray coverage, with lots of air to move the leaves around, is needed for good control. Remember, mites are on the lower leaf surface . Drop nozzles between the rows can be used to direct spray up into the beds.

Mites thrive on succulent plant tissue high in nitrogen. Avoid excessive use of nitrogen on strawberries, especially before harvest.

Biological control

Beneficial mites are available for purchase from a number of suppliers. Amblyseius fallacis is more likely to survive in field conditions. Phytoseiulus persimillus is better in greenhouse conditions. Beneficial mites must be introduced before large populations of mites develop, but after insecticides for tarnished plant bug have been applied. Pyrethroids (Decis, Matador, Cymbush, Ripcord) and Thiodan are toxic to beneficial mites. A list of suppliers of beneficial insects and mites can be found on the OMAF webpage.

Related Links

• List of suppliers of beneficial insects and mites

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