Yes, PEST is a Scrabble word!

Letter values in Scrabble US: P 3, E 1, S 1, T 1.

Is it a valid word?

PEST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a serious (sometimes fatal) infection of rodents caused by Yersinia pestis and accidentally transmitted to humans by the bite of a flea that has bitten an infected animal
any epidemic disease with a high death rate
a persistently annoying person
any unwanted and destructive insect or other animal that attacks food or crops or livestock etc.

Not playable in Scrabble US: PESTIS.