Firefighters fought for 12 hours to tackle a blaze after yobs set fire to hay bales on a farm in Northolt.
Crews were forced to make repeated two mile round trips to collect water to douse the blaze.
The mindless youths torched 80 bales on the back of a transporter in Sharvel Lane and to more waiting to be loaded on board as they made their way through the fields.
Four engines from Ealing and Hillingdon to fight were called in - one to douse the hay with water while the others to shuttle water from a hydrant in West End Road, Ruislip.
Crew manager Mark Reddy was with the first shift at the scene at about 6pm on Sunday (28) said: "It may seem like fun to these kids but it's a drain on our resources and the farmer lost about £2,000 worth of hay. He was really upset."
"These youngsters have gone across the fields, set fire to the pile in three places then set fire to single bales on their way back."
The flames were under control by about 11.45pm.
Police are appealing for information about the culprits on 020 8810 1212.