Mandrels are natives of the plant Eden. They are aggressive, but slow moving. They are strong, but they are not immune from weapons.
The Doctor and Romana encountered a group of Mandrels on board the space liner the Empress after some escaped from a sample of Eden that had been taken by Professor Trsyt. Tryst was using the Mandrels to smuggle the hugely addictive, and deadly, drug Vraxoin.
After a Mandrel was electrocuted after attacking the Doctor the body quickly crumbled into a power - Vraxoin. By transporting live Mandrels Tryst was able to smuggle the drug without it being detected.