Insects amazing sense of smell

Bente Haarstad
Photo: Jens Søraa and Zoologisk Institutt, NTNU
Illustrations: the Department of Zoology, NTNU

For many animal species the detection of odours eliciting attraction or avoidance reactions can be important for life or death. We, however, spray ourselves with synthetic perfumes, unaware of how delicate and crucial the sense of smell is for other animals.

Insects may have about 100,000 hairs on each antenna. Each hair is one olfactory organ.

Olfaction is of major importance for the survuval of insects, the real experts on the sense of smell.

''It is quite an event to 'watch' and to 'hear' instruments pick up nerve impulses which are elicited by the insect olfactory receptor cells when they have been stimulated with odorants of biological importance'', says Hanna Mustaparta, professor of zoology at NTNU.

Mustaparta has established a research group at NTNU, the only one in Norway studying insect olfaction / neurobiology and behaviour.

The researchers at NTNU has employed a variety of methods to present the odours to the insects; odours that humans either cannot smell or at amounts below what we can detect. But the insects are very responsive.

Professor Hanna Mustaparta wants to find out which of the substances we produce that are detected by the sensory cells of the malaria mosquito.

The response can be seen and heard

''The sensory cells of insects are able to fire more than 200 impulses per second. In the research setting, these impulses are made visible and transformed into sound by amplifier and loudspeaker, producing a very nice rhythmic pattern which create a jolly atmosphere in the laboratory'', says Mustaparta.

The group at NTNU has, among other things, shown that the pine weevil has at least 30 different types of olfactory receptor cells, each type specially designed to detect one or a few of the numerous substances produced by the conifer trees.

All plants produce complex mixtures of hundreds of volatile substances, so that the pine weevil manage to work its way through the jungle of odours by using their highly specialized noses. One particular blend of substances might send out the message 'good food', whereas a slightly different blend may mediate information about unsuitable food. This means that the various odour messages are mediated in the pine weevil by nerve impulse activity in certain sets of the more than 30 sensory cell types. Thus the olfactory system differs from the visual system, where only three types of sensory cells (blue, green and red) are able to produce all the different spectral nuances.

Both in the pine weevil and in humans there are a large number of sensory cells mediating information about different odour blends, such as the odour of pine, spruce or roses.

''The perfume industry is always searching for mixtures of odours which produce a'round', pleasant experience of smell. In the same way, insects rely upon a mixture of substances which, in specific combinations, trigger a mechanism which attracts the insects to a host-plant or to a partner. After having identified some of the substances which influence the sensory cells in the pine weevil, we are now able to test how these biologically relevant substances influence the insect behaviour, and what mixtures are the best'', explains Hanna Mustaparta.

Insect pests

Mustaparta expresses that the work is primarily basic research, but it is possible to see aspects of application of these studies. Knowledge about odours that modify the behaviour of pest insects can be used in the developement of methods for biological control of the insects.

The pine weevil, for example, is one of the insect pests which cause the greatest ecomonical loss in forestry in Norway and elsewhere in Europe. Adult weevils eat the bark of newly planted spruce and pine trees, and are able to destroy large, cultivated areas. Consequently, it has been interesting to find out which odors might influence the behaviour of the pine weevil. Such results may be used in the ongoing search for new methods in biological control of insect pests.

The spruce bark beetle is another dreaded pest species in forestry, which was at the centre of attention in the late seventies, when insect traps containing the bark beetle's own scents were tried out. These pheromones, which such olfactory substances are called, led millions of beetles into the traps which also contained insecticides. A crucial problem in such strategies is that almost all beetles must be killed, so that they wouldn't breed even more the following years. It is in general difficult to use mass trapping for controlling pest insects, particularely insect populations of high density and in landscape that are not uniform.

A male insect is pursuing the desirable trace of scent from a female of the same species.

Females of two related species with one pheromone substance in common. But one of these species also has a substance which makes the male run away, because he is certainly not going to mate with the neighbouring species female.

A scheam, illustrating odour molecules diffusing through pores in the wall of the olfactory hair. From the sensory cells the nerve impulses are conducted by the nerve fibres to the brain.

Pheromones are still important

The use of pheromones in pest control is particularly interesting for many agricultural insect species. The researchers in Trondheim are also studying mechanisms of pheromone communication in a group of insects which includes some of the most economically significant pest species in agriculture. In this group of moths, it is the larvae that cause the damage by feeding on buds of many economically important plants, like cotton, corn, paprika etc

In one strategy used for controlling these insects, the idea is not to kill them, but to confuse the males when they are searching for the females.

''The pheromones emitted by the female is taken by the wind, forming a pheromone plume which the males are following until they find the partner. By placing capsules with relatively large amounts of pheromones on the plants in the field, the air becomes saturated with pheromones. As a result the male is not able to find the plume with pheromones emitted by the female. In fact, the male insect becomes totally passified by such an overwhelming amount of the odour'', says Hanna Mustaparta.

Along with her team, she is working in an international network of researchers within this field. The pine weevil project was carried out by the researcher Atle Wibe, who has just completed his PhD. In his study, he used a technique of combining chemical analyses and the measurement of nerve impulses from single olfactory cells. Each cell was exposed to plant substances, one by one, after they had been separated in a gas chromatograph. In this way each cell could tell which of the numerous plant compounds could be detected by the insect.

''It is amazing to see how often substances present in large quantities do not seem to have any effect, whereas substances which have left only tiny traces may trigger off a strong, long-lasting train of nerve impulses. Further chemical identification of such substances would not be possible without close collaboration with chemists'', says Mustaparta.

At the moment, she is working with a group of researchers at the Royal Institute of Technology in Stockholm. They test and analyse voilatile substances from various plants which insects depend on for survival, as well as from plants which are not used by the insect species in question. This is because insects, just like human beings, are probably able not only to sense what is good for them, but even more what might be dangerous for them or what they don't like. The researchers believe that insects use odours from certain plants to tell them where they should not go, and what they should not eat.

The pine weevil, for example, eats on spruce and pine trees, but it does not like juniper. Even so, the substances to which this insect reacts, can be found in spruce and pine as well as in juniper trees.

''This means that there may not be one particular substance in juniper which turns the insect off, but that particular combination of the odorants in juniper makes them less attractive, or even repulsive, to the weevil'', Mustaparta reasons.

Hunting for the malaria mosquito

Recently, researchers at NTNU have been encouraged to start a project to examine the mosquito which is responsible for the highest number of human fatalities caused by insects: the malaria mosquito.

The research here focus on substances emitted by humans in order to identify those odorants that cause the mosquito's sensory cells to react.

The NTNU malaria project is financed by the World Health Organisation (WHO). Colleagues in other countries have shown in fiels experiments, carried out in African, that air drawn from one particular cottage with a person inside attracted a much larger number of malaria mosquitoes, than air drawn from a cottage without a person inside.

''We want to test and analyse volatiles from different persons and animals in order to find out which ones might be detected by the mosquito's olfactory cells and whether these substances are present only in some species and not in others. One interesting aspect of such research projects is to identify new mosquito repellents. However, we are aware of that the project is very challenging. It is a long way to go before we can expect to reache the goal'', says Hanna Mustaparta about the work on this tiny, delicate insect which has even fewer sensory cells than many other insects.

Sharing interest with perfume producers

''Perfumers have special interests in people's body odour. When human beings are used as experimental individuals, the advantage is that they can press a button when they want to tell that they are sensing an interesting odour. Insects, however, cannot do this, so we have to find other methods, like recording from their sensory cells'', says Mustaparta.

In addition, the researchers in Trondheim are particularly interested in finding out how each organism receives and discriminate between different odours, i.e. which substances each olfactory cell reacts to, where in the brain the cell conduct the information, and how, in turn, the brain deals with it. This kind of basic research is carried out on many organisms in the international community of researchers.

''For many reasons, insects are suitable model organisms for such studies. Even if people and insects look very different, our olfactory system shows many similarities with that of the insects'', says Hanna Mustaparta.

The research on olfaction in Trondheim has mainly been financed by the Norwegian Research Council, but also other sources, like WHO has been involved.

International collaboration

''WHO has placed insect olfaction on its list of priority within molecular entomology of vectors, i.e. insects that transmit deseases'', says Mustaparta. For many years she has worked closely with researchers in many countries in Europe and the USA. ''Thanks to many organizations, promoting international collaboration, like the NATO Scientific Affaires, Deutsche Akademische Austauschdienst, European Science Foundation, Fulbright Foundation etc, it has been possible for me to work in several leading groups in this area and enjoy fruitful collaborations with good colleagues and friends in this fascinating area of research'', says Mustaparta. She has during several years had fruitful collaborative projects together with researchers at the University of Arizona.

Replaces spraying

The Norwegian researchers on insect olfaciton take part in a scientific jigsaw-puzzle, in which the main research aim is to discover nature's own solutions. One goal is to find natural «perfumes» which can replace poisonous insecticides.

Such odours will work only against the species you really want to destroy, whereas all other organisms will go free. In addition, the amounts of such compounds involved are small, and with apparently no negative effect on the environment.

''If we could identify the important substances and determine how they affect insect behaviour in nature, we might be able to manipulate the insects in their most crucial phases of reproduction'', says Mustaparta about the very extensive work going on internationally.

Each insect species has its own, particular blend of substances affecting their senses of smell and taste, as well as its special biology. Therefore they need to be studied separately.

Natural protection

Mustaparta reports that almost all plants produce a large number of substances, including poisons, in order to protect themselves against attacks of different organisms. Some animals consume only a little of each plant, which means that they do not digest too much of each poison. Other animals are well adjusted to their diet and may feed only on one plant species. However, cultivated agricultural plants have lost some of their natural ability to defend themselves against insects.

This means that insect pests can have the most glorious lives within monocultures, with plenty of food and no poison. These pest insects are our worst enemies at the 'global dinner table', the reason why insecticides are being used so extensively in such monocultures. The insects, however, develop resistance if the poison is used over and over again, which means that higher quantities of poison is needed.

''Insects are bound to win this race. We must try to find other ways of outwitting our competitors. Although it seems to be difficult to eliminate the insecticides, the use can be reduced. It is important to encourage studies on insect-plant interactions, an area where different disiplinces of reserch meet, zoology, botany and chemistry'', says Hanna Mustaparta.

Olfactory researchers at NTNU are working with one of the world's most dreaded insect pests. The larvae of this moth can destroy whole crops of cotton, corn and tobacco.

Exotic guests

Exotic insects which create enormous agricultural damage all over the world often 'visit' the laboratories in Trondheim.

''These insects cannot survive our climate. Even so we take a lot of precautions so that none of them will ever be able to escape. Males and females are introduced into the country separately. It is important to take extreme care, and we are really in full control'' Mustaparta assures.

Gemini fakta

SOMETHING ABOUT SCENT
The success of the perfume industry clearly demonstrates the importance of the sense of smell in human beings. Five thousand years ago incense and myrrh were used for religious ceremonies. Both the ancient Greeks and Romans used scents made of roses, lilies and violets, as perfumes.
The biological importance of olfaction is closely linked to eating, drinking, sex and memory.
The research on olfaction has enjoyed several breakthroughs, not least because of developments of new methods in neurophysiology, bio-chemistry and molecular biology. It has for instance been shown that olfactory cells have receptor proteins in the membrane which have similarities with the photo pigment in retina. When odorants are transported by other proteins in the mucus (surrounding medium) they elicite a cascade of intracellular reactions which trigger the nerve impulses.
In general, all such reactions are similar in mammals, fish and insects. The next question to answer is how this olfactory system enables us to distinguish between the different odorants.