Mice Active Despite Bait Take, Resistance vs Reinfest Decision Tree

Why this matters

Bait is disappearing from the stations, the customer is still hearing and seeing mice, and the easy story is "the bait is not working, the mice are resistant." Genuine anticoagulant resistance in house mice is documented but far rarer than the four mundane explanations a technician should rule out first: bait take by a non-target (another rodent, insects, or a pet), continuous reinfestation from an unsealed structure that refills faster than the bait kills, a sub-lethal dose from wrong formulation or placement, or normal lag because second-generation anticoagulants take four to ten days to kill after a lethal feed. Misdiagnosing reinfestation as resistance sends the tech rotating to a more hazardous active when the real fix is exclusion. This tree forces the cheap, high-probability causes to the front and reserves the resistance call for last.

Symptom presentation

Quantify the bait take precisely. Heavy, even take across many stations versus take at one or two stations near a single entry tells different stories. Note the species: confirm Mus musculus (small droppings 3 to 6 mm, pointed both ends, musky odor) versus a rat (Rattus, droppings 12 to 18 mm) versus non-rodent take by insects or a pet that defeated the station. Establish the timeline: days since first bait placement, and whether the customer saw a population drop at all before the rebound. Capture structural facts: has the building been sealed, or is bait the only intervention? Bait-only programs against an open structure are the single most common reason mice "never stop."

Quick checks

Inspect the bait stations themselves. Take with no carcasses, no sick mice, and no decline in activity over ten-plus days suggests the active is not reaching a lethal dose: wrong block formulation, station tampered, or a non-target consuming it. Look for insect infestation of the block, gnaw patterns inconsistent with mouse incisors, or evidence a pet or wildlife reached the bait.

Verify the bait is fresh and labeled for mice. Rancid, moldy, or moisture-degraded blocks lose palatability; mice nibble and move on without a lethal feed. Check the placement: mice travel short distances along walls, so stations more than about ten feet apart along the runway leave gaps where the population never encounters bait.

Walk the structure for open ingress with the exclusion lens. A dime-sized gap (about one-quarter inch) admits a mouse. Utility penetrations, garage door corner seals, weep gaps, foundation cracks, and door sweeps are the usual suspects. A structure that is actively refilling will show heavy, sustained take that never tapers because the bait is killing residents while migrants replace them one to one.

Isolation tree

Branch A: Heavy sustained take, no decline, open ingress points found, fresh sign at the openings. Reinfestation, not resistance. The bait is working but the structure is a sieve. The remediation is exclusion-first: seal every quarter-inch-plus gap with rodent-proof material, then let the bait clean up the residents. Adding a stronger active without sealing repeats the cycle and raises secondary-poisoning risk for no gain.

Branch B: Take present but no carcasses, no sick mice, take pattern or gnaw marks inconsistent with mouse, or pet/wildlife access to station. Non-target take. The "mice" eating the bait may be a different animal, or insects are destroying the block. Switch to tamper-resistant stations secured against the actual culprit, relocate to true mouse runways, and re-verify with a non-toxic monitoring block plus tracking patches to confirm the species before re-baiting.

Branch C: Light or inconsistent take, degraded or wrong-formulation bait, wide station spacing. Sub-lethal dosing. Mice are not getting a full lethal feed. Replace with fresh, labeled bait in the correct formulation, tighten station spacing along the runways (closer along walls), and add snap or multiple-catch traps as a fast knockdown alongside the bait.

Branch D: Good take, correct fresh bait, sealed structure, tight placement, full lethal feeds occurring, yet the population does not decline after 10-plus days across multiple feed cycles. Only now consider anticoagulant resistance. Confirm by ruling out every branch above, then rotate to a chemically distinct mode of action (a non-anticoagulant such as bromethalin or cholecalciferol where labeled and appropriate) and document the suspected resistant population for the account history.

Confirming diagnosis

Branch A is confirmed by sustained heavy take that does not taper plus documented open ingress and fresh sign at those openings; sealing and re-monitoring shows take fall off as the structure stops refilling. Branch B is confirmed by tracking-patch prints, a non-toxic monitor that reveals the true feeder, or direct evidence of pet/wildlife access. Branch C is confirmed when replacing degraded bait and tightening placement produces carcasses and a population drop within a week. Branch D is the diagnosis of exclusion: every other branch negative, full lethal feeds documented, no mortality across multiple cycles, and a rotation to a different mode of action that finally produces decline.

Remediation

Branch A: seal-first exclusion to NPMA structural standards, then maintain bait until activity ceases, then reduce to monitoring. Branch B: secure stations, confirm species, re-bait only after the real feeder is identified. Branch C: fresh correct bait, tight placement, traps for knockdown, recount at 7 days. Branch D: rotate mode of action, document resistance, and still verify exclusion is complete because a "resistant" call against an open structure is almost always misdiagnosed reinfestation. Across all branches, never escalate to a more hazardous active before exclusion is verified.

References

• US EPA Office of Pesticide Programs, Risk Mitigation Decision for Ten Rodenticides (May 2008), tamper-resistant station and second-generation anticoagulant restrictions.
• FIFRA, 7 U.S.C. 136 and 40 CFR 156.10, label Directions for Use, placement, and quantity limits as enforceable terms.
• EPA-registered labels for bromadiolone, difethialone (second-generation anticoagulants) and bromethalin, cholecalciferol (non-anticoagulants), current editions, for mode-of-action rotation.
• National Pest Management Association, Best Management Practices for Rodent Exclusion, structural sealing material and method standards.
• University of Nebraska-Lincoln, Internet Center for Wildlife Damage Management, house mouse biology and anticoagulant resistance literature.
• CDC and HUD Healthy Homes guidance on integrated rodent management and exclusion-first strategy.