The quantity of mosquito larval habitat in a specified area was assessed by placing a known number of ovitraps in the same area. The ovitraps competed for oviposition with the unknown quantity of larval habitat, and that quantity was deduced by comparing the number of eggs laid in the ovitraps when the number of ovitraps was changed from one time to another. This method can be used to assess the effectiveness of source reduction. It also provides key information for using ovitraps to eradicate local mosquito populations by competing with existing breeding sites for oviposition. The same ovitraps that are used for control can provide information on how many ovitraps are necessary to ensure eradication.