you need to boil or bake (or both) the eco earth to kill eggs that are inherant in it. otherwise yeah, you'll get bugs. bring your water to a boil and pour it over the ecoearth in a bucket and allow it to hydrate like normal, after it cools break up any clumps and spread it out on a cookie sheet, then bake at 250 degrees, stirring and turning it over every 15 minutes or so, until it's dry. that should kill any pests that have hitched along for the ride.
you could also go to a mushroom grow website, like mushbox.com, and get substrate starter, it contains a nematode that feeds on fly eggs, just mix a few tablespoons into your ecoearth and then use it. I found it online and have been using it to keep my organic planted tank from having fruit flys or other flys in my tank. I have earthworms, roaches and pillbugs in my tank and haven't noticed any adverse effects upon their population.
1 man dreamed of becoming, another awoke and became
22.214.171.124 crested geckos
0.1 gargoyle gecko (fry)
1.0 leopard gecko (lucky)
0.1 (I think) ocelated skink (slinky)