Hot weather leads to increase in cases of two-spotted spider mite

ADAS has warned growers to be alert after increased outbreaks of two-spotted spider mite (TSSM) caused by the prolonged period of hot weather in some parts of the country.

According to ADAS nursery stock consultant David Talbot, the pest has been a particular problem on crops of Ceanothus, Choisya, Cordyline, Crocosmia, Heuchera, Lobelia, Passiflora, Verbascum and Weigelai in recent weeks.

Talbot said: "Two-spotted spider mite thrives in low humidity and high temperatures. (Biological control) Phytoseiulus persimilis doesn't perform brilliantly under hot conditions because it tends to move down the canopy of the crop, whereas the TSSM tends to move up it.

"It's been more of a problem this year because it breeds so much more quickly in these temperatures. If you don't keep up your crop monitoring, it can really explode in hot conditions."

Consultant John Atwood added: "It wasn't such a problem early on, but when we had very high temperatures numbers increased substantially." In temperatures of 30 degsC the pest can complete its life cycle up to five times faster than at 15 degsC.

Phytoseiulus persimilis slows consumption of prey at these elevated temperatures and is most comfortable at 20-25 degsC, so the desired control in poorly-vented tunnels is not always achieved.

Talbot said a ratio of more than 10:1 prey to predators is unlikely to give satisfactory control and more than one spray application may be necessary for non-integrated pest management users.

Where Phytoseiulus persimilis has not achieved control, Talbot advised increasing ventilation and air movement using fans or releasing Amblyseius californicus. He warned that the withdrawal of Bifenthrin on 5 November could further compound the problem should conditions be repeated next year.

The weather has also increased the incidence of other pests, according to ADAS. "Whitefly and western flower thrips are also causing problems," Talbot added. "Both can increase quite dramatically under the hot conditions."

ADAS has reported incidents of whitefly on Ceanothus, Cistus and Heucheralla, while thrips have been affecting Chaenomeles, Clematis, Solanum and Rosa banksiae. Atwood added that the warmer conditions had also helped reduce downy and powdery mildew.


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