Until you have to empty it. 
Permethrin is an arichmicide and will take care of the problem. Understand use it to kill ticks. It can be bought in a spray can format. It can also be bought in concentrated form.
Sporting goods stores like Cabelas, BassPro, or Dicks Sporting Goods Cary it. You can also get it at Home Depot and maybe Lowes.
Bob