Your house is full of creepy-crawlies, and that’s probably good

Humans have lived under the same roof with bugs since we first began building shelters, 20,000 years ago. Next time you reach for a can of insecticide, however, consider the results of a new study published recently in the journal Scientific Reports: “Rooms with more kinds of arthropods may well be healthier rooms.”

Included in the findings by researchers from the California Academy of Sciences, North Carolina State University and the Natural History Museum of Denmark are that arthropods — insects, spiders, ants and their relatives — prefer rooms with high traffic, on the ground floor, with carpet, with many windows and doors, and they like large common areas such as living rooms rather than kitchens and bathrooms.

“Even though we like to think of our homes as shielded from the outdoors, wild ecological dramas may be unfolding right beside us as we go about our daily lives,” says lead author of the report, Misha Leong. 

“We’re learning more and more about these sometimes-invisible relationships and how the homes we choose for ourselves also foster indoor ecosystems all their own.”

The researchers investigated 50 houses around Raleigh, North Carolina, in the south-eastern United States, gathering data from 554 rooms. The rooms were categorised as attics, basements, bathrooms, bedrooms, common rooms, and kitchens. Rooms not conforming to one of the categories were classified as “other” and excluded, leaving an analysed total of 531.

Living and dead arthropod specimens on all visible surfaces were sampled in every room through hand-collecting by trained entomologists. Specimens were identified to the family level in most cases, with provisional identifications of genera and species where possible. 

The dataset eventually included more than 10,000 specimens that comprised 304 unique arthropod families. Each home had from 24 to 128 distinct families, and from 32 to 211 “conservatively estimated” morphospecies (species identified by shape alone). 

“We are just beginning to realise, and study, how the home we create for ourselves also builds a complex indoor habitat for bugs and other life,” Leong says. 

“We’re hoping to better understand this age-old coexistence, and how it may impact our physical and mental well-being.”

The idea of a home teeming with bugs may sound unappealing, but having them in your house may contribute to health, in a roundabout way, says senior author and California Academy curator of entomology Michelle Trautwein. 

“A growing body of evidence suggests some modern ailments are connected with our lack of exposure to wider biological diversity, particularly microorganisms, and insects may play a role in hosting and spreading that microbial diversity indoors.”

The report says that apart from bacteria and fungi, arthropods are the most diverse group of organisms found indoors. Because of their prevalence and mobility, arthropods can serve as important agents of movement for microbiota, and they also make up a diverse indoor community themselves, with many species having functional consequences for homes, humans, and pets.

“The idea that open rooms on ground floors come with increased uninvited biodiversity may not seem appealing,” the report says, “ yet a growing body of evidence suggests that many of our chronic, modern diseases are associated with our failure to be exposed to biological diversity, particularly that of microbes, some that may be vectored by insects. In this light, rooms with more kinds of arthropods may well be healthier rooms.”

The researchers looked into factors such as tidiness, and their findings indicate that having a neat house does not play a significant role in insect diversity. More cluttered areas hosted a greater number of arthropods but on the whole human behaviour plays a minimal role in determining the composition of bug communities in the houses.

The presence of cats or dogs, house plants, pesticides, and accumulated dust also had no significant impact, suggesting that our indoor communities are more strongly influenced by the outside environment.

Most of the arthropod species found in houses have no known impact on humans, and most have simply wandered into the home and become trapped. 

On the other hand, the report says, the existence of a multibillion-dollar pest-control industry suggests they do have “direct impacts on humans”, in structures, stored food products, “or simply nuisance pests”. 

“Thus,” the researchers write, “the indoor arthropod community reflects a spectrum of association with humans, from outdoor vagrants trapped indoors, to arthropods that thrive inside human dwellings as both non-pests and pests. Such has been the case for no less than 4000 years, and likely much longer.”

Not surprisingly, the researchers found that more diverse arthropod populations outside the house corresponded to greater variety indoors. This pattern also influences the number of microbes likely to arrive on arthropods, as well as allergens to which the people in the house are exposed. 

The study narrowed its dataset by focusing on arthropod families that were frequently found between and within houses, resulting in 47 families. The groups in the core indoor community represent a range of characteristics. Some live out their entire lives indoors – for example, silverfish (Zygentoma) and book lice (Liposcelididae) – while others live both in houses and outdoors: for example, fruit and vinegar flies (Drosophilidae) and ladybirds (Coccinellidae). 

Many indoor pest groups, such as house centipedes (Scutigeridae), subterranean termites (Rhinotermitidae), fleas (Pulicidae), and bed bugs (Cimicidae) were not included in the core indoor community, based on filtering methods. Although they are closely associated with human houses, they were relatively rare in the surveys. Some can be abundant and conspicuous when present (house centipedes, for example) but occupy a relatively small proportion of houses at any given time.

Several groups of arthropods commonly considered to be room-specific were instead represented across all room types. Ants (Formicidae) and cockroaches (Blattidae) are often thought of as kitchen pests, but were found fairly evenly across room types. Pantry moths (Pyralidae) were the only family of known kitchen pests found to be overrepresented in kitchens.

In a separate and somewhat contradictory study, dust samples from 1462 houses across the United States revealed greater arthropod diversity in houses with pets. 

The disparity could be due to sample size, researchers in the latest study say. Although they did not find significant difference in arthropod diversity when pets were present, houses with dogs did tend towards a greater mix. 

Intriguingly, however, they found reduced diversity in houses with cats. “While our data does not differentiate between outdoor and indoor house cats, reduced arthropod diversity could be due to cats actively hunting arthropods indoors,” the researchers wrote.

The new study is part of a global effort in which scientists visited each of the seven continents on earth and examined “the great indoors”, to get a better understanding of arthropods and how they’ve evolved with us, and in response to us, over the course of hundreds of thousands of years. 

In May researchers examined four suburban houses in Canberra and two rural properties on Queensland’s Magnetic Island to collect samples of different arthropod species. Their findings were not included in this latest report but Trautwein says the team “immediately noticed differences between the suburban and more rural home environments we visited”.