The indiscriminate spraying of pesticides has probably caused as many problems as it has solved, but here's one that was not expected: some bacteria have decided that one insecticide is a very tasty meal. Unfortunately for us, one of the strains of bacteria that has evolved the ability to digest the toxin happens to be able to find a home in an insect's gut. When it does so, it provides the insect with resistance.

Several factors had to come together for this to take place, but one was the heavy use of fenitrothion, which is described as "one of the most popular organophosphorus insecticides used worldwide" by the authors of a study of these insects. It has apparently been so widely deployed that a variety of bacteria have evolved the ability to use it as a food source. Most of these simply inhabit the soil in the fields where it is used and, at worst, cut down on the level of insecticide present and thereby make life a bit easier for the insects.

One of the bacterial species, Burkholderia has a rather unusual ability. Not only can the strain make a living in the soil, but it can also take up residence inside the gut of insects, acting as what's called a "endosymbiont." In most cases, insects transfer endosymbionts as contaminants on the egg, which take up residence in the offspring as they hatch—these bacteria never have to spend any time living outside an insect. Burkholderia, then, is rather unusual, in that the bugs seem to pick up an infection from the environment. It's still a symbiont, though, as animals that carry these bacteria tend to live longer and grow larger than their peers that don't have any in their guts.

The researchers describe a series of fairly straightforward experiments that make their case. When they took a pot of soil and treated it weekly with fenitrothion, they found that it actually boosted the bacterial population, and that 80 percent of the bacteria that grew out were able to digest the toxin. (Conveniently, an intermediate in the digestion process is yellow, while the original compound is completely colorless.) This confirmed that the pesticide could be a useful food source.

They then grew plants in both sets of soil, and added eggs from an agricultural pest called the bean bug (Riportus pedestris in formal situations). Those bean bugs that were grown on insecticide-treated soil ended up with bacteria in their midgut that could digest the pesticide. Surprisingly, this not only provided protection when the insects ingested fenitrothion, but it protected them from surface exposure as well.

Is this a problem out in the fields? Their first scan of insects from agricultural areas came up empty. But the authors noted that the Japanese agricultural system (they were based in Japan) doesn't tend to use heavy doses or repeated spraying of these insecticides, suggesting there wasn't sufficient selective pressure for either the bacteria or the insects to partner with them. So, they specifically looked for areas where fenitrothion has been used extensively, finding it on a remote subtropical island far from the main islands of Japan. The bacteria isolated from the sugarcane fields there could digest the pesticide, and the insects that carried them showed high levels of resistance to it.

We've already known from countless examples that repeated, heavy use of a single pesticide (or drug) can help select for the evolution of resistant organisms. But generally, this selection is thought to occur over multiple generations which, for insects, tends to take a fair bit of time. Bacteria, however, can go through several generations in a day, and a single source of Burkholderia can presumably be transferred to many insects within a population.

The good news here is that, where fenitrothion is used sparingly, this resistance doesn't seem to be a problem. If the bacteria can't guarantee it will be there to provide a meal, they won't evolve or maintain the genes needed to digest it.

PNAS, 2012. DOI: 10.1073/pnas.1200231109 (About DOIs).