Lease language. Most multifamily leases give the resident a duty to report and the community a right to enter and treat after notice. Some assign treatment cost to the resident when the introduction is tied to their furniture or a guest. Others keep treatment as an owner cost because insects move through shared construction. Read the lease in front of you.
Notice. Residents usually must report promptly. The community usually must give access notice before entry. If the file does not show when the ticket opened, when access was requested, and when the unit was treated, later arguments rest on memory.
Origin. Origin is often unclear on day one. Used furniture, visitors, turnover, and the unit next door all get named. An origin argument does not kill the population. Record what was reported and found. Do not wait for a courtroom-grade source before the first visit.
Vacant versus occupied. Vacant and make-ready units are usually treated as an operations cost before the next lease starts. Occupied units are where charge-back language gets used. Even then, many communities treat first, because an occupied unit that stays live is a neighboring-unit risk.
When the owner or community treats versus charging back. Typical practice: the community treats when more than one unit is involved, when origin cannot be isolated, when the complaint is already a habitability issue, or when the owner would rather close the file than win a small charge-back and lose three units. Charge-back is more common when a single occupied unit has a documented, resident-caused introduction and the lease assigns that cost. Even then, schedule the work so the adjacent stack is not left waiting.
Rule of thumb: treat the risk as community-scale until a written file says otherwise.