The two-spotted spider mite, Tetranychus urticae, is a very common pest in soft fruit crops such as strawberry and raspberry. Existing acaricides have become increasingly less effective against this pest due to resistance, hence growers largely rely on biological control of spider mites in their crops.
Control of spider mite control is achieved with the use of predatory mites, the most effective being Phytoseiulus persimilis (Spidex). This product is supplied in bottle sizes of 2,000 and 10,000. The mite is a specialist predator of spider mites and does not feed on anything else. As a result it is not usually possible to use this predator in a preventative fashion; instead it is normally introduced as soon as the first spider mites are found in the crop. Adult Phytoseiulus will feed on all stages of spider mites, and reproduce faster than the pest at temperatures between 15-25°C, and a relative humidity above 70%.
Phytosieulus attacking a spidermite.
Due to the wide variety of growing systems in UK soft fruit crops, the introduction rates for Phytoseiulus differ according to the particular crop, e.g. being expressed per plant in strawberries, though per m2 in cane fruit such as raspberries. The table below is a guideline for application rates in different crops, and different pest pressures.
Application rates for Spidex:
| Preventative | Low pest pressure | Medium pest pressure | High pest pressure |
Strawberries | NA | 1 per plant | 2-3 per plant | 4-5 per plant |
Raspberries | NA | 10-15 per m2 | 20 per m2 | 25-50 per m2 |
Blueberries | NA | 8 per bush | 20 per bush | 40 per bush |
Applications of Spidex should continue on a weekly basis until a good balance between predator and prey is achieved. This can be seen by the new growth of the plant being free of damage, and the presence of predatory mites in spider mite colonies.