The cicada killer wasp (picture: University of Tennessee)

An internal memo to executive directors has warned that the giant wasps make their home at the department entrances every summer.

But management has been quick to reassure diplomats and officials that the insects are only lethal if you are a cicada - a noisy cricket-like insect common in the humid Foggy Bottom district of Washington DC.

The memo, released last month said: "The insects are cicada killer wasps and are not aggressive, do not pose a hazard to people and are considered to be beneficial in most circumstances."

But anyone walking barefoot in the offices or being careless about where they sit can expect a nasty sting if they accidentally crush a female wasp.

If someone accidentally presses a female wasp, with a bare foot, for example, she will sting in self-defence, this scenario is unlikely in our workplace environment US State Department memo

The memo warns: "The female wasps do not bother passers-by and do not use their stings to defend nests - just to paralyse cicadas.

"Although it is true that if someone accidentally presses a female wasp, with a bare foot, for example, she will sting in self-defence, this scenario is unlikely in our workplace environment."

Male cicada killer wasps "do not sting at all and are completely harmless".

The memo includes a full-colour photograph of a typical cicada killer wasp - courtesy of the University of Florida entomology department - so employees can identify the insect.

Cicada killers, also known as giant ground hornets, can reach up to 4 cm (1.5 inches) in length.

The BBC's David Bamford said a state department official confirmed that pest control experts had been called in to deal with the invasion.