I specialize in inner city multi family bed bugs and have treated thousands of suites to date. Almost from the very beginning we chose to treat only suites that had actual bed bugs and simply monitored surrounding suites with climbup interceptors or modified glue boards. We found that dispersal to neighbouring suites was actually quite rare with perhaps 80% of all cases we treat involving a single infested suite. The key to this method is aggressive monitoring of suites surrounding the infestation, regular building wide monitoring so the infestations are not so severe, and educating tenants before they get bed bugs so panic induced harm is reduced.
When there is dispersal to other suites there usually is a discernible reason. The biggest factor in dispersal is the size of the infestation. Small infestations do not disperse nearly as often as larger ones. Of course if there are very few places for bed bugs to hide in a suite (encased beds, sparse furniture, sealed baseboards, no books, etc) the bugs will disperse faster even with smaller infestations. The key here is the bugs disperse when available harbourage is either filled with bugs (larger infestation) or there are very few acceptable hiding spots in a smaller infestation.
The construction of the building also plays a part in dispersal. Wood frame structures are absolute sieves and are very prone to dispersal. Sometimes I remove a baseboard in one suite and can see daylight in the next suite! Modern precast concrete buildings are also subject to dispersal through the floor/ceiling cracks. My own apartment block has concrete block walls with pan poured concrete floors which is the best construction method. I rarely have issues of dispersal in my building.
Tenant behaviour also contributes to dispersal. If a tenant self treats his/her suite with aerosol pyrethrin (RAID, KONK, etc) there is a high likelihood that dispersal will occur regardless of the size of infestation. Raid sprayers typically push bugs to the suite underneath and across the hall.
If a tenant drags an infested mattress down the hall the likelihood of dispersal is much higher. In one recent case I noted a gentleman discovered a beautiful (but infested) mattress in the back lane and dragged it through the apartment block to his suite. 5 other suites got bed bugs in the process. Interestingly enough the suite most at risk of mattress dragging is the main floor suite nearest the elevator. This suite has the enviable position of having all the upper floor examples of mattress dragging go right past his door.
Suites beside the laundry room are also more likely to get bed bugs because everyone with bed bugs will bring their infested laundry there. Many people are not very careful and succeed in infesting the area.
The suite beside the garbage dumpster is also more prone to bed bugs. People will dump their infested furniture within a few feet of a window and the bugs crawl back in the building.
Where a tenant sleeps also influences dispersal. If a tenant sleeps on the couch and sits on it during the day a percentage of the bugs will attempt to feed during the day. These diurnal bugs hitch a ride on the tenants clothing and dispersal occurs. In one case, when I arrived to treat a suite, I insisted that a couch sleeper with a severe infestation remove his clothing before leaving the suite. That clothing went directly in the dryer and when the cycle finished and the clothing came out of the dryer at least 10 dead bugs fell on the laundry room floor. Impressive. Interestingly enough it was discovered that this couch sleeper regularly sat at the same dining table in the building. All the people who regularly shared that table also had bed bugs despite their suites being hundreds of feet apart.
Dispersal also occurs when tenants keep items near sleeping areas that are continually removed from the suite. For example when clothing is left on the floor right beside the bed those clothes are at high risk of infestation. Or perhaps a Bible is left on the head board and brought elsewhere later. That Bible is at tremendous risk even in smaller infestations. When that tenant goes to a Bible study somewhere in the building dispersal is likely to occur.
The exterminator also plays a role in dispersal. Due to increasing pesticide resistance the technicians get frustrated and over apply chemicals. This causes dispersal. Also the residual effect is often so poor the tech might try injecting every hole or crevice with aerosol pyrethrin to hit the bugs directly. This also causes massive dispersal.
So we see that dispersal has many causes but an experienced exterminator can often figure out why it happened. A landlord can mitigate these risks by educating tenants before they get bed bugs, monitoring all suites regularly as many people do not report bed bugs, requiring all tenants to have a bed with box spring and legs which increases harbourage and reduces dispersal potential, and hiring an exterminator that won’t abuse chemicals.