Yes—if the drainage saucer under an urban planter box holds water long enough, it can attract mosquitoes. The planter box itself is not usually the problem. The risk comes from standing water in the saucer, which is exactly the kind of small, sheltered water source that container-breeding mosquitoes such as Aedes species often use. A female mosquito looking for a place to lay eggs may treat a water-filled saucer like a miniature pond.
Mosquitoes do not need deep water. Even a shallow layer in a drainage saucer can be enough. If the water remains for about a week or more, especially in warm weather, mosquito eggs can hatch, larvae can develop, and adults can emerge. Urban planter boxes can therefore contribute to mosquito problems when saucers stay wet after rain, overwatering, or poor drainage. The effect is stronger when saucers are shaded, filled with leaves or debris, or overlooked behind dense plants.
The good news is that the risk is easy to reduce. Empty the drainage saucer after watering and after rain. If the saucer fills quickly, adjust watering, improve drainage, or elevate the planter slightly so excess water can escape. For saucers that are difficult to empty, add a thin layer of sand or gravel to reduce standing water, or use a mosquito-control product labeled for container water, such as Bti dunks or granules, according to the instructions. Avoid letting water sit for more than a few days, and check hidden saucers under large urban planter boxes regularly.
In short, urban planter boxes do not attract mosquitoes simply because they are planter boxes. They become a mosquito concern when drainage saucers collect and hold water. Regular emptying, better drainage, and simple container-water management will greatly reduce the chance that your planter boxes become mosquito breeding sites.