Careful use of predators key to controlling thrips

Research points to best strategy for growers to deal with western flower thrips to prevent bronzing on stawberry crops.

Thrips: cause of costly visual damage across a wide range of crops - image: Gary Chang
Thrips: cause of costly visual damage across a wide range of crops - image: Gary Chang

Western flower thrips (WFT) in strawberries can only be effectively controlled by careful use of predators, last month's East Malling Research (EMR)/Horticultural Development Company (HDC) soft-fruit day heard.

The insect, which first appeared in the UK in 1986, causes costly visual damage to a wide range of edible and ornamental crops, including "bronzing" of strawberries, on which both larvae and adults feed, leading to fruit being downgraded.

EMR research leader Dr Jean Fitzgerald said: "It rapidly becomes resistant to insecticides, whatever you spray on it." Instead, the main control is now the predatory mite Neoseiulus cucumeris, but this feeds only on the thrips' larvae. "You need to get the mites in early," Fitzgerald explained. "Also, they are not resistant to pesticides, so only use those with low persistence beforehand."

When these conditions were met, Defra-funded trials showed that two releases of the mites yielded a 92 per cent rate of marketable fruit, with the remaining eight per cent rejected due to bronzing, while only 69 per cent was marketable where no such release was made.

"WFT overwinters in strawberry fields and in growbags, while weeds also act as overwintering refuges," Fitzgerald added.

Further trials on the everbearer Camarillo by Clare Sampson of Keele University found a correlation between WFT numbers on flowers and subsequent levels of fruit bronzing. Meanwhile, a potential supplement to N. cucumeris, Orius laevigatus, is "a voracious predator of both WFT larvae and adults, but viewed as expensive and slow to establish", said ADAS senior research entomologist Jude Bennison.

The two together "gave good control of thrips in all treatments", although the companion planting of ornamental alyssum "banker plants" had little effect on numbers, she added.

However, roller traps combined with pheromone lures at two trial sites showed reductions in thrips numbers of 48 and 60 per cent, with "corresponding reduction in fruit damage", although the reductions were less marked at other sites where effective biocontrol measures were already in place, she said.

"You couldn't rely on them on their own - they would need to be part of integrated pest management (IPM)," said Bennison. "We still need a biopesticide, such as an entomopathogenic fungus."

More guidance on implementing mites as part of an IPM strategy emerged from Sampson's trials of six "good" and "bad" commercial sites. These confirmed that applications of conventional pesticides against other pests such as capsids and aphids impacted on the biological control of WFT, with the number of such applications found to be twice as high on sites with "poor" control.

In those with "good" WFT control, no long-residual insecticides were used during cropping, while fungicide use was also lower. N. cucumeris was released fortnightly from flowering or even before. "You need to keep topping it up," said Sampson. Given the interplay of factors particular to each site "there is no blueprint programme", making monitoring essential, she concluded.

Thrips Useful tips

- Start applying Neoseiulus cucumeris early and top up through the season.

- Minimise spray treatments.

- Supplement with mass trapping, such as blue sticky traps.

- Clean up thoroughly at the end of the season to reduce overwintering of other pests and consider IPM-compatible controls for these.

- Keep weeds under control to minimise overwintering.

- Consider one-year crops where WFT is out of control.

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