Three adult Asian hornets and an abandoned embryo nest, 6cm by 4cm, were found on Thursday at a property near Torteval Church.
The hornets were collecting aphids from a tree, which indicates that they are seeking protein to feed their young.
The embryo nest, which has been sent to the National Bee Unit for confirmation, has sparked fears among beekeepers that worker bees have moved away to build a larger nest.
Chris Tomlins, president of the Guernsey Beekeepers' Association, said these nests are usually built high up in the tree canopy.
'Guernsey's bees are now in a very serious situation as any undiscovered hornet nests will produce up to 1,800 workers, which will feed initially on the island's fauna.
'As these decline in numbers they will turn their attention to the island's bee population,' he said.
n A spokesman for Agriculture, Countryside and Land Management Services said people should get in touch with them if they see a nest or hornet.
Sightings should be reported by calling ACLMS on 234567 or emailing aclms@gov.gg during working hours. The States of Guernsey will arrange for inspection and any necessary eradication.
