Crimestoppers is offering a reward up of to £5,000 for information on a series of deliberate grass fires across south Wales.
More than 100 blazes across Rhondda Cynon Taf and Merthyr Tydfil have been reported to police since the beginning of March.
Porth, in the heart of the Rhondda valley, has suffered more grass fires than any other area – with one coming close to destroying a farm building.
South Wales Police has arrested a number of adults and children in connection with the incidents and enquiries are ongoing.
Officers are also urging people to watch out for the signs of the intent to commit arson - including clothes smelling of smoke and charred skin as well as missing matches, lighters and accelerants.