You saw one. It was gone before you found the light switch. With German cockroaches, that single insect is the visible edge of a population already breeding within a metre or two of your sink.
The short answer
German cockroaches are not a hygiene problem you can scrub away. They are a reproductive problem. A gel bait, rotated between two active ingredients and placed in the cracks the insects already use, collapses a colony in two to four weeks. A spray along the baseboard does not.
How fast they multiply
A female carries her egg case, an ootheca, for about three weeks and drops it hours before 30 to 40 nymphs hatch. She can produce four to eight cases in a lifetime. In a heated kitchen the full cycle, egg to egg-laying adult, runs in roughly 60 to 90 days, allowing six or more overlapping generations a year.
The arithmetic is the point. One mated female carried home in a grocery bag can become a thousand insects inside twelve months. Nymphs mature through six or seven molts, and each molt leaves a shed skin that the others eat.
Where they actually live
German cockroaches need warmth, moisture and a gap about 1.5 mm wide. That is why they cluster on the hinge side of cabinet doors, behind the dishwasher panel, inside the motor housing of a refrigerator and under the lip of a sink.
They forage one to two metres from harbourage, following chemical trails laid in their own droppings. A bait placed across the room is ignored; the same bait at the cabinet hinge is found within hours.
What works
- Gel bait, rotated. Pea-sized dabs every 30–40 cm along hinges, corners and pipe penetrations. Change active ingredient — indoxacarb to fipronil to imidacloprid — about every eight weeks, roughly one generation.
- Insect growth regulator. An IGR does not kill adults; it stops nymphs from becoming reproductive. Use it beside bait, never instead of it.
- Sticky monitors. Four traps, one per corner, tell you whether the population is falling. Count nymphs, not adults.
- Steam and vacuum. A steam jet into hinge gaps kills egg cases that bait cannot reach.
What makes it worse
Total-release foggers. The mist coats exposed surfaces, not the cracks, and it flushes roaches deeper into the wall cavities. Repellent sprays do the same. Both produce a floor full of dead insects that reads like success while the colony moves out of reach.
The seven-day check
After baiting, leave the gel alone. It stays palatable for weeks, even when it looks dry and untidy. Check the monitors on day seven. If nymphs are still appearing in the same corner, the harbourage was missed, not the product.