Different nematodes live at different levels in the soil and feast upon different prey. So, you want to be certain to get the "right" nematodes for termites. (Heterorhabditis bacteriophora
and Steinernema carpocapse
are the nematodes to use when targeting termites.)
When to apply depends on soil temperature (you want to apply when soil is moderately warm, above 50 degrees) and the life stage of the intended prey - when prey are apt to be producing larvae, that's the best time to strike. In more southern states, that could be all year long. But, where you are, there are times when termites are more active than others.
Also, after laying down the nematodes, you are going to want to put down diatomaceous earth (which is not "earth", but the exoskeletons of diatoms) around your foundation.
There's a good worm farmer over in New Jersey, when I come back to provide links to more specific information, I'll include one to his site too. (Here's the NY worm farmer's site: http://www.wormman.com/pd_nematodes.cfm
.) His site also has information about the application of nematodes, but you are going to want to do a google search for more detail.
You're not going to like this - nematodes are effective within a narrower temperature range than chemicals, and are more impacted by less-than-perfect soil, thatch depth, and watering frequency. It sounds like you're not treating your house or your foundation now - just baits around the lawn. So, I'm going to assume that the house is "clean" and what is being done now is to prevent re-infestation? Know that nematodes aren't going to do much of anything about termites within your walls. They are aimed at termites in the soil and around your foundation (you can insert tubes leading to the foundation to pour some of the nematode solution close the foundation). Most of your H. bacteriophora juveniles are going to die within a few days of application and this is the nematode that goes to the deeper soil to attack termites. So, it's important to apply the right amount for the area you need to cover.I found a short blurb in a recent edition of an online environmental magazine that referred to a product called BioBlast, said to be an effective fungal parasite for both drywood and subterranean termites. It might be worth looking into this as a product to apply if you do get a reinfestation.
Hope this info helps. And, I would suggest you contact the entomology department of the University of Pennsylvania. They should have someone with specific information for the application of nematodes in your area.