HARRISBURG, Pa. (AP) - Millions of little pests are about to encounter a big problem.

The Pennsylvania Department of Environmental Protection said Monday aerial spraying for black flies has begun in a program that targets more than 1,600 miles of waterway in 37 counties.Spraying began last week for the flies, also known as gnats, along the Juniata and Susquehanna rivers and their tributaries.Officials say weather and other factors will determine where and when spraying will occur.They're suggesting anyone concerned about sprayers with backpacks or helicopters check to see if the black fly suppression program is active in their area.