Much of our common knowledge of cockroach behavior is gained in that brief interval between flicking on the kitchen light and racing to stomp on the little buggers. But the chaotic movements we observe while roaches scurry for their lives belies a far more interesting truth: behind our cupboards, roaches live highly organized social lives. Indeed, social contact among roaches is so crucial for their development and well-being that roaches kept on their own exhibit ‘isolation syndromes’: they develop more slowly, are reluctant foragers and show diverse deficiencies in mating and reproduction. If social contact is so important, how do young roaches ensure they stay together? New research published in Proceeding of the National Academy of Sciences by Ayako Wada-Katsumata and her colleagues from North Carolina State University in the USA provides compelling evidence that the key to aggregation lies in the alluring bouquet of roach poop.
It has long been known that faeces are potent attractants for developing roaches. But the source of the chemicals that coordinate aggregation had remained unknown. While most researchers argued that aggregation pheromones are produced by the cockroaches themselves, the North Carolina team considered a novel alternative: that the pheromones were instead derived from gut bacteria excreted into the roach faeces.
To test this idea, the team reared germ-free roaches and then measured the attractiveness of their germ-free faeces to other roaches. In short, they didn't much like it. Irrespective of developmental stage, roaches significantly preferred extracts of ‘normal’ faeces to those isolated from sterile insects. But when uncontaminated roaches were recolonized with their endogenous microbial community, their faeces regained their appeal.
So what makes normal roach faeces so attractive? When the team compared the chemical profiles of normal with germ-free poop, they identified a series of volatile compounds that were only found in the former. Furthermore, they could pin the production of a majority of these compounds to a set of six bacterial species that were resident in the cockroach guts. If these microbes were absent, or present with reduced diversity, test roaches lost their interest.
Cockroaches are some of our most pernicious pests. They carry disease, violate our food and (often) induce a general sense of queasy revulsion; we just don't like them, and we certainly don't want them in our homes. By understanding the factors that bring them together, Wada-Katsumata and colleagues hope to design a synthetic pheromone cocktail to bait roaches to their doom.
But this may be harder than it seems. Other researchers have also identified attractor compounds in roach faeces, but these only partially overlap with the volatiles discovered in this study. Moreover, the roaches in the present study didn't much care for the other chemical cocktail. At the moment it remains unclear whether this is due to the different methods used by different researchers, or to differences in the gut flora carried by different roach populations. If the latter, are these differences manifest at the level of individual colonies, or of individual families? Can a good whiff of poop identify a parent, a sibling or a suitable mating partner? The approaches and insights developed in this work provide a clear path towards answering these questions.