People are being urged to report any sightings of Asian hornets to help stop them from spreading.
The non-native invasive species, recently spotted in Somerset for the first time, preys on other insects including honeybees, butterflies and beetles.
That sighting has raised concerns the hornets could be heading towards Devon and Cornwall. South West Water has asked people to report any sightings.
Officials in the Channel Islands reported a record number of Asian hornet queens and nests last year, and said this year would be a critical indicator since queens had been caught earlier than ever before.
According to the Devon Hornet Action Team, Asian hornet queens are being blown across the Channel from France towards Kent, and "every year, they're getting closer to us as an established group".
The group's Simon O'Sullivan said: "In Somerset, just over the border from Tiverton, we have seen Asian hornets, so they're slowly coming towards us and we're trying to get organised."
A single nest of the apex predators can eat around 11kg (24lbs) of native insects within a single season.
As of 3 September, the National Bee Unit had recorded sightings of 167 yellow-legged hornets and 53 nests across Britain.
According to O'Sullivan, if not controlled, "they'll spread very rapidly".
"We've had three nests in Devon in the last few years; we had the second nest in the UK in Woolacombe in 2016 and then there was a couple in Plymouth," he said. "But now the concern is, it's this established population which is slowly working its way towards us."
The time it could take for the insects to become established in the South West is not clear, but experts say the weather is a factor.
The number of Asian hornets trapped in Alderney this year almost halved compared to last year, which experts attributed in part to favourable wind direction and strength from France.
O'Sullivan said weather was key. He said: "These long, hot summers will certainly increase the population, but again, very long, wet winters will have an adverse effect on overwintering queens. If we were to have a very long hot summer, like we've had, but then a very dry winter, the population would probably increase quite dramatically. I would imagine within the next two or three years, we're going to have a lot more reports of hornets within Devon, and I would imagine similar in Cornwall."
Michael Grindley, an Asian hornet coordinator in Cornwall, said the insects were "very adept at stowing away under vehicles in cargo" and added they could also arrive "anywhere yachts can come in" such as Falmouth, Newlyn and Fowey.
He said half of their food intake is from bees and the remainder flies, wasps and butterflies, which pollinate fruit crops. "So we'll have lots of wheat and lots of corn, but no tomatoes, no fresh fruit, no soft fruit," he added.
Concern Mounts Over Asian Hornets Spreading