Martin H. Villet
Does even reading about scents invoke a vivid impression of them for you? Consider the irresistible aroma of freshly brewed coffee and newly-baked bread, the power of smells to evoke memories of people and places, and the pleasing way that perfumes can focus our attention, even if they are only in our detergents and cleaning agents. Smells can be compelling, and many animals use them quite deliberately as a means of communicating. When conflict arises between these animals and humans, we have developed ingenious ways to weaponise these signals and turn them against the offending animal.
Smells are chemicals carried through the environment. When they are produced by one organism to cause a response in another organism, they serve as signals that are called semiochemicals. Many species depend on these chemical signals to run their lives. When the two organisms exchanging the signal belong to the same species, the chemical is termed a pheromone.
As a channel of communication, pheromones have advantages over signals that must be seen or heard. They can travel slowly, move around objects, diffuse over long distances, and tend to persist. Because specific receptors are often required to detect them, pheromones can be very private messages that do not attract the attention of predators and parasites. Pheromones therefore often used to attract mates and mark territories, and this is where humans have hijacked them.
Attract and Kill
Bolas spiders synthesize and secrete chemicals that are the same as the mate-attracting pheromones of particular moths, and use this to lure lusty moths close enough to become the spider’s meal. Humans have adopted this tactic too, usually using synthetic mate-attracting pheromones and aggregating pheromones to lure pests to a trap that may contain poison. In this role, these synthetic pheromones are called attracticides and they have been developed and refined for legal population control.
The trick is to put a pheromone-impregnated plastic or rubber lure inside a trap chamber with a one-way entrance or walls coated in non-drying glue. A well-designed lure emits the pheromone slowly, allowing traps to be effective for weeks at a stretch, and cutting down on maintenance needs.
Pheromones have many appealing characteristics for pest control. The first is that they work well even at low populations densities. When pest populations are at low densities, one has to spray insecticides in many places to ensure that they find their target, but insects are finely tuned to detecting pheromones and following them to their source. Conversely, when pest populations are large, pheromone traps are at a disadvantage because they have to compete with too many other sources of pheromones, and then well-placed insecticide sprays become an appealing weapon.
Second, for pheromones to work as signals, there must be co-adaptation between the signaler and the receiver. If either one changes its preferred signal, the pheromone’s message will not get through, so this signaling system has a built-in conservatism. On the other hand, the massive die-offs caused by insecticides soon lead pests to evolve insecticide resistance, often within a decade of the introduction of a new pesticide. By luring pests into a trap, pheromones will lead to their death eventually, even if they are resistant to the pesticide.
In addition, the pheromones are non-toxic and biodegradable chemicals, so they pose no environmental problem, and the insecticides that kill the pests are confined to the inside of the trap where few insects go unless they are responding to a pheromonal ‘invitation’. Using pheromones protects us from agricultural poisons.
Third, the fact that pheromones are highly private, species-specific messages has the special advantage that other species do not need to get caught in the cross-fire. These non-target species may in fact be beneficial to humans because they are predators or parasites of the pest species, and killing them too would be unwanted collateral damage. When insecticides kill the non-target beneficial predators and parasitoids, it leads to outbreaks of secondary pests like spider mites, which are difficult to control with pesticides. Using pheromones protects beneficial insects from agricultural poisons.
Of course, pheromone traps are not perfect solutions. They can attract pests from neighboring fields and orchards, and can be sensitive to bad weather. Mate-attracting pheromones are generally limited to one sex and do not attract the sexually immature stages that may be the main cause of damage, but aggregation pheromones can sidestep this problem. Technical challenges include a lack of knowledge of the pheromone for some pest species, a need for specialized application techniques or equipment, high research and production costs, and the need to use pesticide sprays when populations get very large.
Pheromone-based tactics can be particularly successful in enclosed areas like storage facilities and homes, but it is generally impractical to try to eradicate a field pest using the attract-and-kill approach. Fortunately, some of the advantages of pheromones can be used in a more subtle technology – mating disruption.
Sowing chaos amongst the enemy
If insects do not mate, they generally do not reproduce. One way to reduce the rate of mating is to fill the environment with so much mate-attracting pheromone that the insects cannot locate potential mates’ signals. This works well at low population densities, where the insects are unlikely to meet at random.
When populations grow larger, a more devious strategy can be used. The trick is to use a mate-attracting pheromone to draw pests to a dispenser where they can become coated in the pheromone. The dispenser is not a trap, so the pests can escape. When the insects fly off, their pheromone coating disguises them as being of their opposite sex and makes them attractive to other members of their own sex, which then waste time and energy in fruitless courtship. If enough cases of mistaken identity occur, mating and egg-laying will be severely delayed, and this has been shown to result in decreased egg-laying and increased numbers of sterile eggs in several species.
A sustained mating disruption programme can erode a pest population in a way that attract-and-kill programmes don’t.
Pheromones can become expensive to deploy as a curative measure on the scale of a whole crop, especially if the target pest is not actually sufficiently common to be a threat. The answer is to be vigilant for pests, and to take action only when they are active. Pheromones have a role in this monitoring process too.
Because insects are so sensitive to pheromone signals, they will almost certainly be attracted to monitoring traps, especially at low population densities, which is when they are hardest to detect by other means. In this context, pheromone traps are often used to detect the first appearance of a pest during a season or to keep tabs on the arrival of an exotic pest at a new location, perhaps in imported goods. This monitoring function is the keystone of what is called integrated pest management.
A prime challenge of the future is to make more use of the knowledge that we already have about pheromones. The Pherobase (http://www.pherobase.com/) is a free, publically accessible database of pheromones and semiochemicals of more than 7000 species and 3500 semiochemical compounds.
A second challenge is to improve the effectiveness of our pheromone delivery formulations and to bring down their cost. Current technologies include minute polymer capsules of pheromone that can be sprayed just like insecticides, hollow plastic fibers from which pheromones slowly evaporate, and high-emission dispensers that blanket bigger areas and bring down labor costs.
There are also new classes of pheromones to deploy because attracting mates and forming aggregations are not the only roles that pheromones play. Honey bees use them to regulate the reproduction in the hive, and ants mark their foraging trails with trail pheromones. Some insect use them for marking territories, or for marking flowers that they have already visited, or for marking plants that they have laid eggs on so that other insects know that that resource has already been used. This last type of pheromone is termed an epideictic pheromone, and it has great potential for protecting crops from pests.
Beyond this are the signals that the plants themselves use in their battle against insects. It has been discovered that when many plants start to be eaten by insects, the plant will release a volatile semiochemical that signals the location of the pest to its predators and parasites, rather like sending out an S.O.S. message or calling the police. These chemical signals are called kairomones to distinguish them from pheromones because they carry messages between species rather than within them. Perhaps by harnessing the right kairomones, we can attract these beneficial insects into our crops to do guard duty.
One thing is for certain: bringing together the fields of chemical ecology and biotechnology may be another step in a long arms race, but it will certainly prove interesting.
*Martin Villet is Professor of Entomology at Rhodes University.