How parasitic wasps could help farmers kill a problematic pest
PUBLISHED: 14:35 17 January 2020 | UPDATED: 15:00 17 January 2020
Anna Jordan / John Innes Centre
Parasitic wasp larvae are among the natural predators which could help farmers kill a marauding crop pest in the absence of banned pesticides, said a leading scientist.
East Anglia's oilseed rape fields have become increasingly vulnerable to attack by cabbage stem flea beetles following the EU ban on neonicotinoid seed treatments which used to protect the crop.
Dr Rachel Wells, senior scientist at the John Innes Centre (JIC), outlined alternative strategies to combat the beetle while speaking to farmers at Frontier Agriculture's 3D Thinking seminar at the Norwich Research Park.
She said researchers continue to search for genetic resistance in brassica plants which could be incorporated into commercial varieties, and they have identified differences in the palatability of some lines to adult flea beetles.
But they have also been researching the potential of natural predators in the field, and how farmers could tailor their management regimes to help them thrive.
Dr Wells said the JIC team, working with the Natural History Museum in London and the Swedish Museum of Natural History, had identified a tiny parasitoid wasp, named microctonus brassicae, in 2017.
The female lays its eggs inside the flea beetle, affecting its ability to reproduce and eventually killing it when the larvae emerges to become an adult wasp.
"We found this parasitoid was quite effective at destroying our captive research colonies of flea beetles - something that really hampered the research, which was very annoying, but obviously very fortunate," she said.
"We showed that within captivity in the trials we were doing we had a parasitism rate of up to about 53pc. So this shows good potential for biocontrol within the field.
"I think we can really look at exploiting and promoting the use of this wasp. As such, management regimes can be adjusted to promote these natural controls."
To that end, Dr Wells suggested farmers should avoid unnecessary use of pesticides which the beetles are developing resistance to, but which could harm beneficial insects.
And as these "natural enemies" can be damaged by soil disturbance such as ploughing, she also suggested providing a habitat outside the crop so they can thrive in the margins, hedgerows or fallow land, as well as a nectar source so the adults can feed.
The JIC is also exploring how entomopathogenic fungi can infect and kill pest insects, as well as agronomic solutions including defoliating the plants during winter to reduce the larvae numbers in December, January and March.
"We all know there has been a massive increase in cabbage stem flea beetle numbers since neonicotinoid withdrawal," said Dr Wells.
"But not just one approach is going to solve this. We can't just throw chemicals on this and expect to have control.
"It is going to take approaches of agronomy, it is going to take novel biopesticides and novel solutions to combat the flea beetle. By working together is where we are going to see the most benefit."
Another speaker at the seminar was Andrew Hill, Frontier's national grain origination manager, who said despite the decreasing area of oilseed rape being grown due to the cabbage stem flea beetle problem, there was still a strong domestic market for the crop.
"In 2019 the demand for oilseed rape in the UK was two million tonnes, and that is not supplied entirely by the UK farmer because of the problems we have been hearing about," he said.
"I always talk to farmers about growing for the market and while there are other crops out there that might be better for you agronomically, from a marketing perspective you should grow for your market, and the rapseseed market in the UK has fantastic demand. That 2m tonnes demand for rapeseed crush is not going anywhere.
"So it is good to know the scientists are working really hard and coming up with some fantastic solutions which we will hopefully see in the market place before too long."