Roaches aren’t the worst critters in our homes, though. Yes, they can carry pathogens, but your neighbors or children carry more. Also, experts haven’t yet documented any cases in which someone has actually gotten sick from a pathogen that a cockroach spread, whereas people get sick every day from pathogens spread by other humans. The most serious problem the bugs pose is that they are, in great densities, a source of allergens. In response to this real problem, and the many perceived problems, we have spent enormous resources trying to kill them.

The German cockroach appears to have made its way through Europe during the Seven Years War (1756-1763), a time when people were traversing Europe with containers large enough to hold quite a few cockroaches. Just who transported them is unknown. Carl Linnaeus, the father of modern taxonomy, asserted it was the Germans, but he might have been biased. Linnaeus was Swedish, and during the Seven Years War, the Swedes fought several battles with the Germanic Prussians. Linnaeus thought that “German cockroach” was a fitting moniker for a species he didn’t like.

By 1854, the insect was in New York City. It now lives from Alaska to Antarctica, having moved with peoples of nearly every nation in their boats, cars and planes. It is surprising the roaches aren’t yet on the International Space Station.

If It Doesn’t Kill You …

Certain conditions are needed for animals to adapt quickly to pesticides: the targeted species must be genetically diverse (or borrow new genes from other species), the chemical kill almost all of the targeted species, the chemical exposure happens repeatedly, and the targeted species’ competitors (parasites and pathogens) be missing.

Cockroaches meet these conditions, but so do nearly all the other pests we target. Besides roaches, bed bugs, head lice, houseflies, mosquitoes and other common insects in our houses have developed resistance to our pesticides.

If a species becomes resistant to a pesticide or other means of population control, that species can readily move to another home that uses the same control measure. In rural environments, the spread of resistant species might be slow. But in cities, it can happen rapidly because apartments and houses are closer together. Even though human social networks often fall apart in cities, with people feeling lonely and isolated, the resistant pests stay connected. Their network is a kind of river of our own making, and it flows through our windows and under our doors.

Although resistance is quick to evolve among the pests, it is less likely to evolve in species we don’t target. This is doubly problematic. The first problem is the simple loss of the biodiversity around us, on which wild ecosystems depend. A recent study found that over the last 30 years, the biomass of insects in Germany had declined in wild forests by 70 percent. The jury is still out on the cause, but many scientists believe a likely one is pesticides. The second problem is that among the species killed by pesticides are those that benefit us, including the natural enemies of the pests we are trying to control.

Along Came a Spider

Whether you like it or not, spiders are great at pest control. If you kill them in your home — and this is precisely what we do with many kinds of pesticide applications — you do so at your own expense.

As children, we learned about the old woman who swallowed a spider after swallowing a fly. That case didn’t turn out well. (Spoiler alert: She died.) Others have turned out better.