Scientists have learned that cabbage and cauliflower crops could potentially be "devastated" by a species of moth arriving from continental Europe.
BBC News understands that tens of millions of diamondback moths are thought to have come to the UK in the past week.
This is 100 times the number that arrive in the entire year.
Researchers describe the species as a "super pest" because it is thought to be resistant to several insecticides.
An alert has been issued by researchers at the Rothamsted Research in Harpenden in Hertfordshire.
The Twitter feed @migrantmothUK reported a two mile cloud of moths on Saturday night near Leominster. One subscriber to the feed reported that it was like "driving through rain".
Steve Nash, who administers the feed, said much worse was yet to come.
"Once the progeny of this influx arrives in mid-July, numbers could be biblical," he said.
Dr Steve Foster, who works at Rothamsted Research described how they devastate crops.
"There are swarms of them, a bit like plagues of locusts - there are so many of them that they seem like a brown cloud."
Dr Foster and his colleagues learned of the infestation on Friday. They will study the moths to see if they can identify an insecticide that can be used against them as a matter of urgency.
This process could take a few weeks. If they find an effective spray in that time the infestation will be an inconvenience to growers. If not, the moths could "devastate" cabbages, cauliflower, broccoli and oil seed rape across the country, according to Steve Foster.
"Hopefully there will be an Achilles heel and one of the insecticides available to growers will work out," he explained.
"But if you haven't got anything available what you can do apart from sit down and pray, because they are just going to destroy your crops?"
His advice to growers is to try to consult with their authorised advisers about spraying their crops.
If the moths are already resistant to the insecticide, it could result in killing friendly insects such as wasps and ladybirds which prey on other pests - which could make their problems with the moths even worse.
Rothamsted Research learned of the infestation because it produces a survey of moths at the end of each year for conservation purposes. But the survey's administrator, Chris Shortall, noticed chatter on Twitter a week ago about an exceptionally high number of diamondback moths and also noticed high numbers in traps at the research centre.
He rang around his network of "monitors" across the country and asked them if they had noticed an upsurge. They reported that hundreds of the moths were caught in traps each night last week when normally there would be just one or two.
"I'm concerned for cabbage and cauliflower growers which is why I wanted to get this information out as quickly as possible. What I've seen so far is the moth community saying 'we are seeing all these moths isn't it great," he told BBC News.
A trap in Oxfordshire collected 173 moths in one night, another in Guernsey collected 310, in Bedfordshire it was 260, in North Yorkshire it was 71, in County Durham it was 61 and in Berkshire more than 1,000 were trapped over three nights.
The Diamondback moth has also been found in large numbers in in Kirton, Lincolnshire and Wellesbourne, Warwickshire.