Adweek claims the company is engaged in a market by market appraisal of its media arrangements. It is currently conducting a review of its media in China.
Wrigley's US media spend, which totalled $250m last year, is managed by WPP's MindShare.
The $530m Mars account is handled by Publicis Groupe's MediaVest.
Sources said both agencies were defending although they declined to comment. Mars did not respond to invitations to comment.