Diamond-back moths, the scourge of brassicas, seem to like the milder UK climate, with last year’s summer influx surviving colder weather, and pesticides, to emerge again

  • 24
  • NOV
  • 2017

It was a Monday in August when Steve Foster took a call from a worried grower in Somerset. The grower was struggling to control an invasion of what looked like diamond-back moths that had laid eggs through the nets designed to protect his swede crop.

“Diamond-backs are quite an exotic moth, originally from Asia,” says Foster, an entomologist at Rothamsted Research who works on insecticide resistance in crop pests, including aphids, beetles, weevils, thrips and moths. “For our studies, I rear them at around 25 degrees [Celsius].”

But the moths are far from unknown in the UK; last year, there was an unprecedented invasion of the pests. Though poor flyers as adults, the wind can carry them long distances from the warmer climate of continental Europe; they then lay eggs and, as caterpillar larvae, can wipe out the foliage of cabbage, cauliflower, swede and other brassicas.

Plutella xylostella. Diamond-back moth. Credit: Rothamsted Research

From samples sent by the Somerset grower, Foster confirmed that the pests were indeed diamond-back moths and set about assessing their resistance to several insecticides, as he’d done last year with three samples from Lincolnshire, Suffolk and Scotland.

Foster raised a colony through two generations, and then fed sets of the caterpillar larvae on Chinese cabbage leaves soaked in three types of pesticides: “Two compounds worked but the pyrethroid insecticide, which can work well against Lepidoptera, was again no good,” he says.

“The sample showed the same resistance profile as those of 2016, and the moths were also lighter coloured, as last year’s,” says Foster. “The indications are that last year’s moths reproduced and survived over our UK winter. And under the [swede] nets, they probably benefited from the closeted environment.”

Foster sent the findings to the Agriculture and Horticulture Development Board, a levy board funded by farmers and growers, which released them this week.

The moths’ resistance to pyrethroids and their ability to overwinter present an increasing problem. Growers need to remain alert for the presence of this pest in their crops, says Foster, particularly if the coming winter is mild.

Growers and agronomists can contact Steve Foster at Rothamsted for advice. The institute funded the diamond-back moth work in 2016; this year, a large consortium of organisations, including AHDB, is funding the operation.

About Rothamsted Research
Rothamsted Research is the longest-running agricultural research institute in the world. We work from gene to field with a proud history of ground-breaking discoveries, from crop treatment to crop protection, from statistical interpretation to soils management. Our founders, in 1843, were the pioneers of modern agriculture, and we are known for our imaginative science and our collaborative influence on fresh thinking and farming practices.
Through independent science and innovation, we make significant contributions to improving agri-food systems in the UK and internationally. In terms of the institute’s economic contribution, the cumulative impact of our work in the UK was calculated to exceed £3000 million a year in 20151. Our strength lies in our systems approach, which combines science and strategic research, interdisciplinary teams and partnerships.
Rothamsted is also home to three unique resources. These National Capabilities are open to researchers from all over the world: The Long-Term Experiments, Rothamsted Insect Survey and the North Wyke Farm Platform.
We are strategically funded by the Biotechnology and Biological Sciences Research Council (BBSRC), with additional support from other national and international funding streams, and from industry. We are also supported by the Lawes Agricultural Trust (LAT).
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1Rothamsted Research and the Value of Excellence: A synthesis of the available evidence, by Séan Rickard (Oct 2015)

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About LAT
The Lawes Agricultural Trust, established in 1889 by Sir John Bennet Lawes, supports Rothamsted Research’s national and international agricultural science through the provision of land, facilities and funding. LAT, a charitable trust, owns the estates at Harpenden and Broom's Barn, including many of the buildings used by Rothamsted Research. LAT provides an annual research grant to the Director, accommodation for nearly 200 people, and support for fellowships for young scientists from developing countries. LAT also makes capital grants to help modernise facilities at Rothamsted, or invests in new buildings.