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Why Do I See More Roaches After Bombing or Treatment?

Bombing roaches is one of the management methods considered in the case of an invasion. Fumigation techniques are always subject to correctly using the product of your choice to kill all roaches.

Improper fumigation use as well as having more roaches in the house than you initially estimated are among the first reasons why there are more roaches around after using a roach bomb.

The reasons for spotting more roaches are numerous and they can be categorized as follows.

A human error in fumigation – this includes not properly using the fogger and not preparing the house correctly for fumigation, according to the instructions on the label.

Multiple roach nests – a higher number of roaches means there’s a higher chance of some of them escaping alive.

Higher roach activity looking to get out – roaches might simply come out looking for a way out.

Not deeply cleaning the area after fumigation– some roach eggs may survive insecticide and hatch within weeks.

What is roach bombing?

Roach bug bombs are a type of pesticide spray used to kill a roach infestation indoors. They spray the pesticide in the air which then coats all of the surfaces it falls on. By direct contact, it kills cockroaches on the spot or within a few hours.

Key facts

  • Improper home preparation may allow some roaches to survive a bombing
  • Having multiple hidden roach nests indoors means more roaches can escape fumigation
  • Hiding roaches begin to come out after you begin fumigation
  • Not opening up the doors of cabinets or cupboards for fumigation allows ‘safety pockets’ roaches can escape in
  • Some roach bug bombs take 2-24 hours to kill roaches

Common reasons you see more roaches after a bombing

Fumigation is one of the recommended pest control techniques for killing roaches at home. It may be done by professionals but it is also a process you can do yourself.

While it is reasonable to expect cockroaches to be dead immediately after fumigation, they might move around more or it might even like there are more roaches now, after fumigation. Here’s why this is either an impression or a fact.

Roaches become agitated trying to find a way out

Fumigation releases insecticides that kill roaches, such as cypermethrin. These types of ingredients may kill roaches on the spot or within hours.

Cockroaches that don’t diet immediately after bombing may be looking for a quick way out. They are seeking gaps in walls, crevices, and other safe places not affected by fumigation.

Roaches hide during fumigation in hard-to-reach areas

More cockroaches can be seen after fumigation as a result of having hiding spots to stay clear of insecticides.

They might go back to their nest, which is typically in a hidden space. They can also crawl under furniture or appliances and escape fumigation.

Multiple cockroaches might even be hiding when you begin fumigation which means they might not be in direct contact with the insecticide which would kill them.

Ensure you spray pesticide and fog into all hard to reach areas in your home

You haven’t opened all cabinet doors, cupboards, and drawers

Not following the clear instructions of your roach bomb is another common reason why there are more roaches running around the house once you begin the process or after fumigation.

You might have not followed all the recommendations such as opening cabinet doors and cupboards. Even drawers need to be pulled out as roaches can crawl inside and stay there for a few days, essentially surviving fumigation attempts.

You’ve Only used foggers in small/enclosed areas

Foggers should be used in rooms or open spaces of the house. One common mistake which limits fumigation results is only adding a roach bomb in cupboards, drawers, or a wardrobe.

This mistake is typical in homes where roaches are only found in these places. Homeowners might think this is the only place roaches are in if they find them nesting inside.

Ensure all rooms and areas are fogged, not just places where you commonly see cockroaches

You Left windows open

In reality, roaches might have already invaded the houses. This means the roach bomb only acts locally, allowing cockroaches in other rooms and areas to survive.

Open windows and fresh air limits or even cancels the effect of roach bombs. You might not be able to effectively kill all roaches when the sprayed insecticide escapes with air currents.

Make sure you close all windows, doors, air vents, and cracks in walls where air currents can draw out the pesticide.

Gaps under doors are among the places where air currents can draw sprayed pesticides out or where roaches can escape the area only to return later for more food in areas they are already familiar with.

Roaches made a quick escape in cracks and crevices

Most roach bombs advise closing doors and windows but some don’t offer information about tiny crevices where roaches can escape through.

Even a 1-inch opening can be sufficient for roaches to crawl through and make their way out of the bombed areas.

Many can escape and make it look like you have more roaches after fumigation as these start moving around.

Sealing all escape routes, particularly around pipes, is crucial. Make sure you seal all of these tiny gaps before another fumigation attempt.

Your fogger is too small for your home

A typical fogger may cover an area of up to 2.000 cubic feet. This means large multi-floor areas such as hallways or entryways may need extra foggers to kill all roaches in an area.

Larger homes, multi-car garages, as well as offices or warehouses need more bombs to effectively clear all roaches.

A 2-level apartment requires fumigation on both floors to effectively kill all cockroaches.

A single fogger is calculated to cover a room with a standard ceiling of around 8 feet. Make sure to increase the number of foggers you use even in smaller rooms with higher ceilings to effectively kill roaches.

There are too many roaches in the house

A roach nest numbers anywhere between tens and a few hundred roaches. Multiple nests are often spotted around large homes or in different areas of the house.

A nest might be present in the basement while another nest may be found in the house.

Fumigation may activate all of the roaches in the house, even those that you aren’t aware of. Most people decide to cover the area they see most roaches in but additional roach nests homeowners are unaware of might also be one of the reasons more cockroaches are spotted after fumigation.

You haven’t waited 2 hours for the insecticide to work

Most fumigators are made to cover a single room and kill roaches on the spot or within a couple of hours.

Instructions often recommend opening up the doors and windows after 2 hours. You should not enter the fumigation area sooner, allowing all of the roaches to die.

Opening up the windows and doors sooner may effectively allow the stronger roaches to survive fumigation.

Not being able to vacate the property for a couple of hours is one of the reasons residents often open up doors and windows sooner, limiting the effect of the roach bomb.

Roaches out hiding are now coming out driven by fumigation

Roaches that are hiding also come out during fumigation. This is one of the reasons why a larger number of cockroaches is visible once you’ve used a smoke bomb.

Both the visible roaches and those crawling out of their hiding places are killed by a roach bomb. It might not be easy to drive all of them out before you start fumigation. I

It’s always tempting to start squishing roaches once they appear, but beware of the risks of stepping on roaches as you are likely to introduce more problems.

How to get rid of live roaches after a bombing

Surviving cockroaches after a failed or partially-successful fumigation attempt requires another bombing. This time, you should ensure all of the directions on the canister are properly followed. Here’s how to get rid of cockroaches for good.

  • Try bombing again

    You can apply a new canister in a new fumigation attempt. You should not use more than 1 canister per room as you can spread out canisters. Most fumigators cover areas of a minimum of 5 X 5 feet.

  • Close all doors, and windows, and brings appliances to the center of the room

    It might also be a good time to move the location of your cabinets and appliances such as refrigerators to allow fumigation in all corners. Bringing furniture to the middle of the room is one of the tips to consider in a second fumigation.

  • Close out all hiding spots and crawling spaces

    Small cracks in walls as well as opening around windows and door frames need to be sealed before fumigation. These are areas roaches escape through and which they might use to come back inside.

  • Apply additional insecticide in key hiding areas after bombing

    Some fumigators have a difficult time reaching small areas such as behind furniture. Make sure you use an insecticide spray in all areas that are hard to reach.

    You should still use an insecticide canister in the area you see roaches as well as in other hard-to-reach areas where there are no roaches.

  • Clean the house to eliminate dead roaches and their eggs

    Insecticide doesn’t always kill roach eggs. This means new roaches can emerge even 50 days after fumigation.

    Even more, roach eggs that are in the ootheca aren’t killed by the insecticide.

    You need to properly clean the entire house and the fumigated area of dead roaches and eggs so that new and even more roaches don’t emerge again within a few weeks.

    These eggs can be identified by their 1/3 to ½-inch size and their capsule-like appearance.


You may spot more cockroaches after the bombing as a result of drawing them out of their hiding spots. Improper use of fumigators may also allow some roaches to survive. Finally, failing to clean the area of roach eggs might mean a new problem is going to arise with hatching eggs within a few weeks.

How long after bombing do roaches come?

Roaches are resilient creatures, and after a fog bombing, they frequently look for food and shelter. Although it is impossible to provide a precise time frame, the appearance of roaches following a bombing can happen fairly quickly. In some instances, roaches might show up days or even hours after the bombing. This is due to roaches’ high adaptability, which allows them to easily find new food and shelter sources.

It is also crucial to keep in mind that roach populations that were present before the bombing are likely to reappear more quickly because they already have access to resources for food and shelter.

What are the signs that roaches have gone?

While there’s no surefire way to tell if roaches are gone, there are a few signs that can suggest they are gone.  A decline in bug sightings is one indicator. It may be a sign that they have decided to move on if you used to see them frequently but now only occasionally or never. 

A reduction in odors and droppings is another signal. It’s possible that the cockroaches have left if the odors and droppings they leave behind have vanished. 

Another indication that the roaches have permanently left your home is the absence of egg cases and shed skins. If all of these indications appear to be present, then the cockroaches probably exist.