Cineraria

The Cineraria were a species that invaded by stealth instead of force.

Biology
The Cineraria had long heads with small eyes on the top and a slit-like mouth on the bottom. (NSA: The Story of Martha: Breathing Space)

Technology
The Cineraria relied on biotechnology. The Cineraria ship looked like conical mound of bone, held together by a grey resin. Though capable of cloaking themselves, the ship had little defence and would be unable to survive a strike from a nuclear weapon.

When claiming to help Earth, the Cineraria sent in what were called "whales", mainly for their size and communication like whalesong. The "whale" were large, larger than any human ship, but were mostly featureless except for the gill-like openings along their sides. They would suck up all the toxic gases in the atmospheres, ostensibly to extrude it in space but actually storing it to produce a deadly gas explosion. (NSA: The Story of Martha: Breathing Space)

History
The Cineraria evolved on a gas planet, making fire-based weaponry a dangerous choice. Instead, they relied on stealth to take over a planet, strip it of resources and then leave.

In 2088, the Cineraria came to Earth, hiding their ship and calling themselves The Benefactors. They claimed that the "whales" had been sent to clean the atmosphere. In reality, the "whales" would collect the explosive gases and then crash into the cities, killing the inhabitants. Though the humans were initially accepting of the deal, the Tenth Doctor was able to disable one of the "whales" and uncloak the hidden ship. When the Cineraria relized their cover was blown, they fled the planet, taking the "whales" with them. (NSA: The Story of Martha: Breathing Space)

Behind the scenes
Cineraria is also the name of a genus of flowers.