Eradicating bedbugs requires repeat chemical treatments applied at the correct time. It is essential that the egg to nymph to adult lifecycle be broken, or else they come right back. A first chemical spray will kill everything but the eggs. The eggs then hatch and become nymphs. Due to variability in the time the eggs hatch, and for the nymphs to become adults (and start laying eggs), there is only a very small window to apply the second round of treatment.