The Marathon wiki

OVERVIEW: The carnivorous ticks infesting some corners of NEW CASCADIA are a unique type of mega-Cetoniinae assumed to be native to the TAU CETI system.

Their nests are oddly fleshy growths usually found in clusters and suggest a communal nature to their existence. Use caution on approach as they tend to charge anything that invades their space, "self-destructing" into a super-heated bio-mess as a means of nest defense.

TICKS' aggressive nature toward RUNNERS is likely due to this territorialism, so don't take it personally, though there is a theory that they are drawn to both the heat emitted by RUNNER SHELLS and any scent they may recognize as bio-matter [read: food]. Another theory is that their aggression—should they recognize the human form—has less to do with identifying a food source and more to do with cellular memory triggering survival instincts in response to encounters with something they view as a natural predator due to severe harm species during colony-era research endeavors.

REMARK: A TICK's internal juices are caustic; that exterminating them before they get too close seems inflicted on their part is no rumor, so wise .