It depends on your covering. If it's plastic or polycarbonate some insecticides can react with it.
Closing up the greenhouse tightly for a few days should kill anything in there.

Thanks for response cactusfreak. The GH is a Greenline GH, made of fiberglass. Do you know if there is such a reaction with fiberglass?

well,i've often thought about setting off a bug bomb-with the greenhouse cleaned out for the summer-but polycarbonate can be effected by chemicals.i bought my greenhouse used-when i disassembled it last year-i found at least 25 live,healthy blackwidows inside.it did have a shade cloth on it-but the automatic ventilation was not hooked up.so-despite extreme summer heat it went through-the black widows thrived.i wonder about the aphids,and spider mites that i battled last winter....

Well, I went ahead and set one off. Haven't seen any kind of reaction with fiberglass yet. I guess time will tell. I just can't bear the thought of going through another winter battling those dang whiteflies. Although they may get in again at least it won't be leftovers from the year before. I will definately be more attentive of prevention this year than last. I guess this will be one of those learning experiences to see if I'll do it again next summer.

Bengal actually has a bomb that is a dry powder and doesn't leave a residue. It's kind to poly carbonate as well.

That said, I'm not an advocate of insecticides. Close it up for a couple of weeks and superheat the insects out if you must. Note that once the conditions become favorable in the GH again for whiteflys and aphids, they'll be back regardless of what you do.