Beetles and bugs

THE coffee-berry borer is a pesky beetle. It is thought to destroy $500m-worth of unpicked coffee beans a year, thus diminishing the incomes of some 20m farmers. The borer spends most of its life as a larva, buried inside a coffee berry, feeding on the beans within. To do so, it has to defy the toxic effects of caffeine. This is a substance which, though pleasing to people, is fatal to insects—except, for reasons hitherto unknown, to the coffee-berry borer. But those reasons are unknown no longer. A team of researchers led by Eoin Brodie of Lawrence Berkeley National Laboratory and Fernando Vega of the United States Department of Agriculture had a suspicion the answer lay not with the beetle itself, but with the bacteria in its gut. As they outline in Nature Communications, that suspicion has proved correct.

The team’s hypothesis was that the borer’s gut bacteria are shielding it by eating any caffeine it has ingested before the poison can be absorbed through the insect’s gut wall. Experiments on a laboratory-reared strain of the borer suggested this hypothesis was probably true. Initially, the larvae’s droppings were caffeine-free. When the lab-reared insects were dosed with antibiotics, this changed. Caffeine started appearing in their droppings, and the animals themselves began, as it were, dropping off the perch. Over the course of an experiment lasting 44 days after their guts had been sterilised (a period that let the insects complete an entire life cycle of egg, larva, pupa and adult), the population of the experimental colonies fell by 95%—and even those larvae that did not die had trouble pupating. Clearly, immunity to caffeine was being conferred by bacteria. The question was, which ones?

To answer that, Dr Brodie and Dr Vega turned to wild beetles. They collected samples from seven coffee-growing countries and combed through the insects’ gut floras, looking for features in common. By constructing what was, in effect, a Venn diagram of microbes from these populations, and also those from their lab-bred strain, they were able to focus on the bacterial species found in all of them.

They tried growing each of these on a medium whose only source of carbon and nitrogen for metabolism was caffeine. Some of the bugs were able to survive on this diet, others were not. Of the survivors, the most abundant in beetle guts was Pseudomonas fulva. This species, a genetic analysis showed, is blessed with an enzyme called caffeine demethylase, which converts caffeine into something that can be dealt with by normal metabolic enzymes.

Kill P. fulva, then, and you would probably kill the borer. But that is easier said than done. Even if spraying coffee plantations with antibiotics were feasible and would do the job (by no means certain, for the larvae would have to ingest sufficient antibiotic for the purpose), it would be undesirable. The profligate use of antibiotics encourages resistance, thus making them less effective for saving human lives.

There might, though, be another way of getting at P. fulva. This would be to craft a type of virus, known as a bacteriophage, specific to the bug—an approach already being investigated for the treatment of human illness caused by a different species of Pseudomonas.

In practice, more than one type of phage would probably be needed, for if P. fulva were knocked out, another caffeine-consuming bacterium in the beetle’s gut might end up replacing it. But, regardless of the details, this study has introduced a novel way of thinking about pest control. Many plants use poisons to protect themselves from insects. Sometimes, such plants are crops. Being able to circumvent these natural insecticides is an important part of becoming abundant enough to constitute a pest. It is possible other agronomists who have been seeking to understand how critters do this have been looking in the wrong place—ie, at the critters themselves, rather than among the bacteria in their guts.