In many species of fly, females are attracted to lay eggs where other females have previously laid. The activity of large numbers of larvae makes the food more edible, often by increasing the rate of substrate decay, thereby aiding larval growth and survival. In the housefly, Musca domestica,however, the carnivorous habits of the larvae means that newly hatched larvae may turn into food for older maggots. Kevin Lam and co-workers from Simon Fraser University investigated their hypothesis that to avoid this danger,female flies should be attracted to lay near to freshly deposited eggs and avoid older egg masses.
They first tested whether the presence of eggs of different ages affected survival rate. They found that significantly fewer flies hatched from a mixture of fresh and 24-h-old eggs compared with a single-age control. This supported the idea that mixed-age larval masses represent a significant cost for houseflies. The authors then measured female egg-laying preferences,finding that female flies preferred to lay on food containing freshly laid eggs over egg-free food, but preferred egg-free food over food containing 24-h-old eggs. This showed that the females were using a signal to discriminate between the two egg ages.
To test the hypothesis that this signal was chemical, the authors washed eggs of various ages, treated food with the resultant solution and observed the egg-laying responses of female flies. The results were identical to the real-life situation: the females preferred the fresh egg solution and avoided the 24 h egg solution. When presented with clean food and food containing a solution from 8 h or 16 h eggs, the females showed no significant choice,suggesting that the chemical cue was gradually accumulating or changing over time.
The authors hypothesized that the source of this cue might be microorganisms on the eggs. They therefore filtered the solution from washed 24 h eggs to remove any potential microorganisms and found that females did not avoid laying eggs on food treated with this solution. Having identified microorganisms as the source of the signal that alters oviposition behaviour,the authors next demonstrated that the flies' attraction to microorganisms associated with fresh eggs shows a dose–response curve and that no significant effect was induced by microorganisms isolated from food that had no eggs on it.
The final part of the team's study involved a shift from chemical ecology to microbiology. They isolated 19 different strains of bacteria from fresh egg-washes and discovered that a single Gram-negative oxidase-positive strain, Klebsiella oxytoca, strongly inhibited oviposition on its own. This led them to hypothesize that changes in the amount of K. oxytoca on eggs might form the basis of the signal used by females. To test this idea,they first showed that the number of K. oxytoca found on eggs increased by 1.46× 108 between 0 and 24 h. To see whether this increase could alter behaviour, they added this amount of K. oxytoca and of 14 other bacterial strains to fresh eggs. Only the addition of K. oxytoca increased female avoidance behaviour,demonstrating that female flies use the level of this bacterium as a measure of egg age.
Chemical communication in insects often involves direct measures of age,for example through changes in cuticular hydrocarbons. This investigation shows that insects can also take advantage of the presence of other organisms– in this case, bacteria – to detect evolutionarily significant age changes. This intriguing and rigorous study provides researchers studying chemical communication with a stimulating question: how many other examples of inter-kingdom cues are there?