Mated moths smell different plants

Confronted with a mixture of scents, some coming from plants you can feed from or lay eggs on, and others from plants that are just a distraction, how do insects like moths track down and tell apart the sources of the useful from unhelpful odours? It turns out that they can discriminate very precisely between them, and their sensitivity also changes depending upon whether they've mated or not. Speaking with Chris Smith, Sonja Bisch-Knaden is at the Max-Planck-Institute for Chemical Ecology in Jena, Germany…

Sonja - I wanted to know how moths can, just by using their sense of smell, find important plants that, for example, provide nectar, for feeding, and plants that are suitable as host plants for their offspring. This is especially interesting in the case of moths as most moths are specialised on specific plants. And if these plants are rare in the habitat, this means they have to find these resources against a huge variety of odours that are emitted from other plants that are not relevant for the moths.

Chris - But moths do have an exquisitely sensitive sense of smell, don't they? I mean, I remember one friend of mine pointing out to me that moths can pick up the smell of another moth at less than parts per billion concentrations. So why should it be a challenge for them to find the plants they're after?

Sonja - The challenge is that these plants that they want to find release a complex mixture of odours, but plants that are not important release also mixes of odours. And the components in these mixtures might be very similar, just differ in the ratio or the concentration. So they have to somehow differentiate or distinguish important plants from the unimportant ones. We know that they find the plant, but we don't know exactly how!

Chris - How did you then pursue it to work out how they were doing it?

Sonja - So we went to the natural habitat of one of these moths and there we were at the field station of the University of Tucson, and there is a lot known already about the moth species that live there and also about the plants. So that means we could go there and we were sure that this is the correct habitat, and there are all the plants there that the moth needs. And then we did collect the orders that are released by the plants in the field. And we did this with the plants that are important for our moths, but also we sampled from other species that were around trees or from grass flowering plants in the vicinity of the important plants.

Chris - So you end up basically with a smell snapshot of what the environment that these moths are operating in is like. How do you then work out how they respond to those different odorants that are in those samples?

Sonja - Yeah. We used these samples as stimuli in physiological experiments, and we wanted to know what the antennas - or the nose - of the moths can detect. And then, in a second step, we wanted to know how these odour mixtures from each of the plants is processed in the brain of the moth.

Chris - How on earth do you do that?

Sonja - For what the moth can detect? We have a GC - that's a gas chromatograph - that separates a complex mixture into single components. A stimulus is then going to the antenna and each time the antenna can detect one of these stimuli, we see this as an electrical amplitude. So then we know this chemical is detected by the antenna.

Chris - And what about the response in the insect's brain? How does it then interpret that constellation of signals coming in?

Sonja - We focused on the first processing centre, the antennal lobe. This is equivalent to the human olfactory bulb. It consists of 60 or 70 subunits. And each of these is targeted by one specific receptor type. And as each of the subunits has a different range of odours that it detects, in the end if our moth, for example, has 70 of these subunits, the moth can detect much more than 70 odours by this kind of combinatorial coding.

Chris - And do you see then that the insect has a unique brain response for the plants that are high value for the insect and lower for the plants that are just contributing to the, the sort of "smell scape" that's going on in the environment?

Sonja - Yeah. In we did another kind of experiment. We did also test virgin and mated females, as we thought, looking for a host plant, where the eggs can be laid and the larvae develop, is more important for the mated female. And there we thought, maybe that we find a difference. And what we saw in the virgin female, especially the flower odours were very much represented, but plants that are host plants were activating only one of these subunits and very weakly.

Chris - So you've got quite an interesting set of findings there then, which is sort of multi-dimensional, isn't it in the sense that a) they can detect the plants that are high value that are gonna be good hosts that, or that are gonna be a good food source, but that depends on whether they are looking for a host plant on which to lay eggs, or not. And so it sort of switches in response to, to their mated state as well?

Sonja - Yes, in the mated female, we expected that the host plant, so the important plant for egg-laying will be represented in a more, in a increased way in somehow maybe more subunits or higher activation, but this was not the case. It was exactly the same, but the activation of the background plants almost vanished. So the moths didn't smell this background plants anymore. Only the floral odours that are important for feeding, and also the host plants for their own larvae. But at the moment, we don't know exactly how the mating influences the olfactory system. So we don't know the mechanism.