Following observations by a service engineer (mid-2007), 12 of the hundreds of installed packaged plant (PP) sewage systems were producing relatively large numbers of Culex quinquefasciatus (Say) and a variety of filth flies in George Town, Grand Cayman. Due to the vector potential of C. quinquefasciatus in the Caribbean, we explored the efficacy of industry-standard insecticides against C. quinquefasciatus. Since PP systems contain an organic biofilm on the water surface, we devised a modified technique for collecting immatures and for the application of insecticides. We treated functioning and non-functioning PP systems with Bacillus sphaericus (VectoLex® WDG), Bacillus thuringiensis var. israelensis (Bti; VectoBac® CG), temephos (Abate® 4-E), monomolecular film or ethyoxylated alcohol surfactant (Agnique® MMF) and (S)-methoprene (Altosid® Pro-G). B. thuringiensis var. israelensis gave control for 35 and 48 days for functioning and non-functioning systems, respectively, with less control from B. sphaericus (high label rate) (c. 25 and 35 days) and temephos (suggested label rate) (c. 27 and 33 days). B. sphaericus (low label rate), monomolecular film and (S)-methoprene gave limited control of immatures (i.e. no mortality detected after 24 h). Treatment options for PP systems are discussed with regard to economics, operational feasibility, and within the context of integrated mosquito management. Functioning systems were recolonized at a faster rate than those that were not functioning, but the level of infestation in the non-functioning systems was considerably greater.