|Pheromones in plants|
Pheromones are semiochemicals that carry information between members of a single species. To do this, the pheromone must be released into the atmosphere or placed on some structure in the organisms’ environment.
It is thus made available to other members of the species for interpretation and response. It is also available to members of other species, however, so it is a potential allelochemic.
Types of Pheromones
In complex interactions, a pheromone may also be acting as a kairomone, passing messages between species to the benefit of the recipient; an allomone, passing messages between species to the benefit of the sender; or a hormone, passing messages within a single organism.
One possible example of a hormone as a pheromone is the plant hormone ethylene, which is produced by an individual plant to stimulate ripening of fruit, loss of leaves, and other physiological changes.
Some evidence suggests that ethylene, produced in response to damage caused by insects feeding on the plant, stimulates production of chemicals that are detrimental to the insects, thus acting as a hormone.
It also passes through the atmosphere to surrounding plants and stimulates their production of defensive chemicals, thus acting as a pheromone. Not everyone is convinced by the evidence that has been presented for this phenomenon, but the possibility is intriguing.
There are two general types of pheromone: those that elicit an immediate and predictable behavioral response, called releaser or signal pheromones, and those that bring about a less obvious physiological response, called primer pheromones because they prime the system for a possible behavioral response.
Pheromones are also categorized according to the messages they carry. There are trail, marker, aggregation, attractant, repellant, arrestant, stimulant, alarm, and other pheromones. Their functions are suggested by the terms used to name them.
The chemical compounds that act as pheromones are numerous and diverse. Most are lipids or chemical relatives of the lipids, including many steroids. Even a single pheromonal message may require a number of different compounds, each present in the proper proportion, so that the active pheromone is actually a mixture of chemical compounds.
Functions and Sources
Different physical and chemical characteristics are required for pheromones with different functions. Attractant pheromones must generally be volatile to permit atmospheric dispersal to their targets.
On the other hand, many marking pheromones need not be especially volatile because they are placed at stations which are checked periodically by the target individuals. Some pheromones are exchanged by direct contact, and these need not have any appreciable volatile component.
Pheromones are widespread in nature, occurring in most, if not all, species. Most are poorly understood. The best-known are those found in insects, partly because of their potential use in the control of pest populations and partly because the relative simplicity of insect behavior allowed for rapid progress in the identification of pheromones and their actions. Despite these advantages, much remains to be learned even about insect pheromones.
Pheromones and Pest Control
Pheromones and other semiochemicals are of interest from the stand point of understanding communication among living things. In addition, they have the potential to provide effective, safe agents for pest control.
The possibilities include sex-attractant pheromones to draw insects of a particular species to a trap (or to confuse the males and keep them from finding females) and repellant pheromones to drive a species of insect away from a valuable crop.
One reason for the enthusiasm generated by pheromones in this role is their specificity. Where as insecticides generally kill valuable insects as well as pests, pheromones may target one or a few species.
These chemicals were presented as a panacea for insect and other pest problems in the 1970’s, but most actual attempts to control pest populations failed.
Lack of understanding of the particular pest and its ecological context was called the most common cause of failure. Some maintain that pest-control applications must be made with extensive knowledge and careful consideration of pest characteristics and the ecological system.
In this context, pheromones have become a part of integrated pest management (IPM) strategies, in which they are used along with the pest’s parasites and predators, resistant crop varieties, insecticides, and other weapons to control pests. In this role, pheromones have shown great promise.