Thank you for all the good advice above.
After inspection it turns out one of the tenants brought German cockroaches with them when they moved in. They didn't say anything, but an adjacent tenant did when they decided to move into her unit. It will cost me $600 to treat the building (4 units).
I like the advice above of deciding whether I want a small problem now vs big problem later and factoring the cost of annual service into the rent. I am going to treat all the units and eat the cost.
One additional question:
I have received a recommendation to just have all the leases written to say that the tenant is responsible for all pest control costs. That seems viable in a SFH, but in a multi-unit property could lead to big problems for everyone, except for the guy who doesn't care if he lives in a roach infested home.
Has anyone written leases with tenants responsible for pest control in a multi-unit building and managed to get it to work?