Use the right tool for the job.
Small plant, small pot, transplant as necessary to prevent rootbound issues. If you are putting a seedling in a large pot, the tendency is to overwater to keep the soil hydrated. But it takes too long for the plant to uptake the excess moisture, and large pots of soil retain water too long for seedlings and young clones. This eventually causes a toxic enviornment for the roots. (root rot)
At the very least, overwatering can cause nutrient lockout issues.

So do you have bugs or not?

When posting, best to list all pertinent info in the post, (no links or post-hopping) and do not confuse the issue with photo's lifted from the web. Use a cell phone or borrow a camera.

If you have a few minutes, fill-out the troubleshooting form, and we can possibly see where you went wrong on this last grow. The link is in my signature. Copy-n-paste it here, and fill it out as best you can. Might be able to prevent this from happening again. :thumbsup:

A preventative spray of poison...? :wtf: Poison the plants ONLY if need be, and rinse the residue off when treatments are done. The chemicals may break-down fairly quickly, but the residue can clog respritory pores. (the stomata) If the soil is bringing-in bugs and egg cases, ditch it and get some fresh, or get it from somewhere else.
If bugs are coming-in from outside, get a screen door.