The story is almost always the same. A few cockroaches spotted near the sink at night, an insecticide can bought in a hurry, a few quiet days. Then they come back in greater numbers, and sometimes the upstairs neighbour starts seeing them too. It is not bad luck: it is the predictable result of spraying German cockroaches.
Know your opponent
The German cockroach (Blattella germanica) is the species that invades kitchens and bathrooms in homes, restaurants and shops. Adults are about 1 to 1.5 cm long, light brown, with two clearly visible dark parallel stripes on the thorax, just behind the head.
Its strengths:
- it loves warmth and humidity: fridge motors, the back of the dishwasher, cupboard hinges, worktop joints;
- it gets through cracks just a few millimetres wide;
- the female carries her ootheca, the capsule holding her eggs (usually around thirty), until they hatch or nearly so, which shields the eggs from most treatments;
- at room temperature, a generation turns over in just a few months.
Three reasons sprays make things worse
1. They scatter the population
Most consumer aerosol insecticides contain pyrethroids, which are irritant and repellent. Cockroaches caught in the cloud die, but the hidden ones smell the product and flee. They settle further away: another room, service ducts, the neighbours' flat. One hotspot becomes several.
2. They select the most resistant
Pyrethroid-resistant German cockroach populations are documented in many countries. Every spray kills the susceptible individuals and leaves the rest to breed. After a few months, the product barely does anything.
3. They spoil the bait
This is the least known consequence. Insecticide sprayed on or near gel bait contaminates it: cockroaches detect the repellent and refuse to eat the bait. A common mistake is to lay gel and then "back it up" with a spray, which cancels out the treatment that works.
Why gel bait works
A gel bait combines a highly attractive food matrix with a slow-acting active ingredient. The delay is deliberate.
- The cockroach eats the bait, then goes back to hide in its shelter with the others.
- It dies in the shelter, not in the middle of the kitchen.
- The other cockroaches, especially young nymphs that do not yet come out, are poisoned in turn by eating its droppings or its body. That is the domino effect.
This reaches individuals that never went near the bait point, which is exactly what no spray can do.
| Criterion | Aerosol can | Gel bait |
|---|---|---|
| Hidden cockroaches | Not reached, they flee | Reached through the domino effect |
| Spread to neighbours | Common | Low |
| Pyrethroid resistance | Major problem | Other families of active ingredients |
| Occupant exposure | Cloud in the room | Small dots placed in cracks |
A word on aversion
Cockroaches can also learn to avoid a bait. A well-known case published in 1993 showed that some populations had developed an aversion to glucose, a sugar used in baits at the time. Manufacturers adapted their formulas, and professionals rotate gels. That is one reason why a treatment followed up by a technician works better than buying the same product over and over.
Applying gel properly
Gel is easy to use, but placement makes all the difference:
- small dots the size of a lentil, plenty of them, rather than one big bead;
- in cracks and right next to shelters: hinges, corners of base units, the back of appliances, pipe openings under the sink;
- never wipe bleach or degreaser over the dots, and never spray insecticide next to them;
- leave no competing food: crumbs, pet bowls, bins left open overnight;
- check and replace dots that have been eaten or have dried out.
Sticky monitoring traps under the sink and behind the fridge let you follow progress week by week.
When you need a professional
If cockroaches are visible during the day, are present in several rooms or the building is affected, the infestation is established. Our technicians follow the Lock-Down Method: detect shelters with a torch and monitoring traps, neutralise with selected professional gels used in rotation, lock down the gaps (seals, pipe openings, skirting boards) and monitor with a second visit a few weeks later to reach the nymphs that hatched in the meantime.
Find out more about our cockroach treatment, part of our insect treatment service, and read our cockroach guide. Our prices are public and you can book online.
FAQ
Your questions
How long does gel bait take to get rid of cockroaches?
The first cockroaches die within a few days, but the goal is the whole colony, including nymphs from the oothecae. In practice, allow several weeks, with checks and fresh dots. That is why professionals usually plan a second visit.
Can I use gel and an insecticide spray at the same time?
No. A spray applied near the gel makes it repellent, and cockroaches stop eating it. It also scatters the population into other rooms. If you have already used sprays, tell the technician so they can adjust where the bait goes.
Why do I see more cockroaches right after treatment starts?
It is common in the first few days: the active ingredient disrupts the insects, which leave their shelters, sometimes in broad daylight. It is not a sign of failure. Do not wipe away the gel dots and do not spray anything: catches on the monitoring traps should then drop clearly.
