Depending upon climate and exposure, the composition of trap fluids may be adjusted to meet needs. At least in my area, pitfall trap in most habitats need only very dilute formalin. In cool mountane habitats traps can be left out easily for two months, otherwise one month is better. Of course, the larger the interval the less information on phenology. Foramlin will not necessarily completely solve the mammal problem, but in general they will not destroy the entire trapline, as I have seen them do with glycol.
Black bears can be especially troublesome.
I have also experimented with vinegar. In fact I did this last summer in northeast Washington at ca. What I did was mix one pint of methanol with one gallon of vinegar, and a little dish soap. This avoided the poison problem of formalin, which made the whole process more enjoyable. The is quite a literature on this with regard to pitfalls and carabids.