Common German cockroach biology
The life cycle of German cockroach starts with an egg capsule oötheca visibly located at the posterior end of pregnant female's abdomen. The female drops her egg capsule after she carries it for most of the incubation period. Incubation takes 28 days. On the average 37 nymphs emerge from the oötheca, pass through six or seven instars, and molt into winged adults. The average development period from nymph to adult is 103 days. Females can live more than 200 days during which period they can produce on the average six egg capsules each.
German cockroaches are thigmotactic, meaning that they prefer to squeeze into small cracks where their back and underside make contact with other surfaces. They evolved an excellent ability for intercepting chemical stimuli through their antennae rather than depending on vision or sound. Cockroaches are most active 20 minutes to two hours before dark, and increase their activity to a peak which ends before daybreak.
Studies have shown that without food and water
female German cockroach survive only 13 days at 40% humidity. The survival
time with food alone is about the same as the survival without food and
water. However, with water but no food the survival time increases dramatically
up to 42 days. The survival rate increases with the relative humidity of
the environment as well. From this, one can conclude that water availability
and relative humidity are crucial for survival of this insect.
Why some exterminations work and many don't?
As scavengers, cockroaches will be happy with anything they can find. However as we saw above, water is the most crucial controlling factor in their distribution. This is why they are freequently found in kitchens near the sink and in bathrooms. They also thrive in areas and times of high humidity. The second crucial element in their survival and distribution is their habitat. In their biology overview above it was mentioned that cockroaches are thigmotactic, they absolutely require cracks and crevices within their habitat (your kitchen) to hide in.
It follows that as for cockroaches:
Elimination of cockroach access to water and humidity in cities like Montreal or Toronto is impractical or even impossible, especially in summer.
Elimination of cracks and crevices within cockroach habitat is the only effective approach. The importance can not be overemphasized. This is the reason why substantial cockroach infestations are predominantly found in older structures. New constructions and renovated units initially will not have this problem. Any cockroach infestations in renovated units are generally short-lived and easily treated as the insect can not get established. Even in these units however, with time the wood of kitchen furniture eventually dries up, cracks and crevices develop. Eventually every human habitation becomes vulnerable to insect colonization.
Finally, often times we witness exterminators using potent neurotoxins being sprayed within apartment buidings. One major drawback of use of these heavy knock down agents is that while they will kill a substantial portion of the population they will disperse the rest of the insect population to areas within the walls as well as to the other neigbouring apartments. Once the active agent is neutralized with time through photo- and chemical degradation processes, the insects, and often the more resistant individuals, will return.