Not necessarily. A reservoir at the bottom of an urban planter box does not automatically cause mosquitoes to breed. The real issue is whether mosquitoes can reach standing water and whether that water stays stagnant long enough for eggs to hatch and larvae to develop. Many self-watering planter boxes are designed with enclosed reservoirs, so they are often less risky than open saucers or exposed trays. However, poorly designed or neglected planters can become mosquito breeding sites.
Mosquitoes need standing water, and in warm weather some species can complete their life cycle in about a week. A bottom reservoir can become accessible through loose lids, open fill tubes, overflow holes, cracks, seams, or blocked drainage that lets water seep into a saucer or tray. Overfilling the reservoir, heavy rain, and clogged overflow ports can also create small pools of exposed water. Even a shallow layer of stagnant water is enough for mosquitoes if it remains undisturbed.
So the practical answer is: reservoir planter boxes can cause mosquitoes if the water is exposed or poorly maintained, but well-sealed, properly draining, and regularly checked planters generally do not. To reduce the risk, check the planter weekly, drain excess water, keep lids and fill ports closed, cover overflow openings with fine mesh where possible, fix leaks, and empty any saucers or trays. If water must remain in an accessible reservoir, use a mosquito larvicide such as Bti according to the label. Avoid overwatering, refresh water regularly, and make sure the reservoir is not allowed to sit stagnant for more than a few days. With good design and simple maintenance, bottom-reservoir planter boxes can be a low-risk addition to an urban garden.