Planters can become a nest location for fruit flies but in most cases, they'd prefer some type of food on which to lay eggs. Unless the soil is very moist, it's more likely they've found a location by the garbage pail or sink-erator. To be sure, the best thing to do is to remove the planter for two weeks to see what happens. If the fruit flies disappear once the planter is removed, then it's obviously the culprit and you can treat it confidently knowing your effort should solve the problem. If the fruit flies still persist even after the planter is removed, then you know you need to find the real source of the problem.

Once found, treat with the Gentrol for long term control. The PT-565 can be used to kill off any adults that are still active or you can opt to use one of the Fruit Fly Traps listed in our post on Fruitfly Control.

Here are direct links to the information and products listed above:

Gentrol Aerosol:  http://www.bugspray.com/item/gentrol_aerosol.html

PT-565:  http://www.bugspray.com/catalog/products/page79.html

Fruit Fly Traps:  http://www.bugspray.com/item/fruit_fly_glass_trap.html