For parasites to thrive, they must first locate and then derive resources from a host. Some parasites can use chemical cues – such as CO₂ released through respiration – to find hosts, and when multiple hosts live in the same area, parasites often preferentially infest certain individuals (or certain hosts are more resistant to parasite infestation). There are several characteristics that can make hosts more preferable for (or, alternatively, more vulnerable to) infestation by parasites, including specific behaviours, poor overall health and being injured. However, whether parasites might be able to select hosts based on their metabolic rates – and, if so, whether they might select hosts with high metabolic rates (which would produce resources for the parasites at high rates) – remained unknown. Because many parasites can detect CO₂ produced by hosts and because CO₂ production increases with metabolic rate, Collin Horn and two colleagues from the University of Alberta in Canada wondered whether parasitic mites might be able choose specific fruit fly hosts based on their relative CO₂ production. If so, they also wondered whether higher metabolic rates in injured flies might underlie the mites’ documented preference for hurt hosts.

To answer their first question, the researchers first determined the resting metabolic rate of individual flies by measuring each one's CO₂ production in a tiny respirometer. Then, they placed a parasitic mite at the base of a Y-connector and immobilized two fruit flies with different metabolic rates at the other end of the connector – one fly in each arm – creating a two-choice tunnel for the mite. After an hour, the group inspected the flies under a microscope to see which fly the mite had chosen. Horn and his colleagues found that the mites did indeed tend to infest the fly with the higher metabolic rate, suggesting that these parasites can detect not only the presence of CO₂, but also small differences in CO₂ concentration.

To determine whether mites might be attracted to injured flies because of increases in the flies’ metabolic rates during healing, Horn and his colleagues measured the metabolic rates of flies that had been punctured with a pin. Unexpectedly, they found that metabolic rate did not change in injured flies, indicating that mites must use another cue to detect injured hosts.

To further determine what might attract mites to wounded flies, the scientists placed either haemolymph (equivalent to fruit fly blood) or water on the bodies of uninjured flies. They then allowed mites to choose between haemolymph- and water-treated flies in the same Y-connector setup used in the first experiment. The researchers found that the mites preferentially infested flies that had received a drop of haemolymph, suggesting that these parasites rely on chemical cues from the injury itself to find vulnerable hosts.

Overall, Horn and the other researchers discovered that this common parasite of fruit flies is able to use both exhaled and ‘leaked’ chemicals to pick the perfect host. Their results also suggested that the mites can discriminate tiny differences in odours produced by hosts and select the victims that will produce the resources that they require most rapidly via higher metabolic rates. If fruit flies think they're going to escape these bugs, they shouldn't hold their breath – or maybe they should!

• © 2018. Published by The Company of Biologists Ltd