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Chemical communication
Living beings communicate with each other by transmitting
information and messages via the five senses. Hearing and touch are
essentially physical senses, whereas taste and smell are chemical--that
is, based on interactions between molecules and receptors in the
appropriate organs. Sight combines the initial physical processes with
chemical ones. The two chemical senses have scarcely been investigated,
and limited information is available regarding the mechanisms involved.
Sustained interest in the subject has developed only in recent years.
Chemical communication among animals occurs by way of many groups of
chemicals, classified according to the agent of dissemination, the
receiving individual, and the benefit to the sender or to the receiver.
These chemicals, known collectively as semiochemicals or infochemicals,
include pheromones and allelochemicals (see sidebar, The
infochemicals: Classification and history) (3).
The communication chemicals shown in Figure 1 are among the
pest-control agents currently on the market and substances with
pesticidal potential. In the rest of this article, I focus specifically
on pheromone action among insects, the largest class in the animal
kingdom. My emphasis is moths, the larvae of which form the largest
group of agricultural pests; however, beetles, flies, and other insects
also inflict severe crop damage.
Sex pheromone
The role of this key pheromone is to summon the opposite sex
of the species to engage in reproductive activity. Like most
pheromones, it is made up of a mixture of substances that
individually are not pheromones; only the complete set, with the
relative concentrations of the components, can determine the
unique character of the pheromone. The exclusive composition of each
species' sex pheromone ensures the reproductive isolation of related
species by preventing undesirable interspecies hybridization.
Field measurements have demonstrated the impressive navigational
skills of insects. In experiments in which tagged males were
distributed in a field, most of the males were recovered in traps
containing virgin females secreting the sex pheromone. Most recoveries
were in traps placed along the flight path upwind from the source, as
expected. The pheromone quantities secreted and scattered into the air
are minute, on the order of 1 ng (10-9 g). A few thousand
molecules are sufficient to set the chemobiological process in motion.
Laboratory tests have shown that concentrations of as little as
10-18 g/cm3 of air are enough to sexually
arouse the male silkworm and cause it to search for the source of the
smell (6).
Basic research has been under way during the past few years with
the aim of understanding the sensing mechanism completely: from first
penetration of the pheromone molecules in the antenna hair, to the
transport of the molecules in the fluid medium by special proteins, the
interaction between the complex (or single pheromone molecules alone)
and appropriate receptors, the triggering of an electric current, and
transmission of the nerve impulse to a central or local nerve center in
the insect, leading to a change in behavior or other processes. Also
under way are studies of the biochemical processes responsible for
expelling the pheromone molecules from the receptor, to enable renewal
of nerve activity and ensure its continuity, and the biosynthesis of
sex pheromones and hormonal control of the process (7).
Application of pheromones for pest control
The principal potential applications of pheromones--especially
sex pheromones--in a sophisticated and environmentally friendly IPM
system are monitoring, mass trapping, attract-and-kill, and disruption
of communication (also known as confusion) (8,
9).
Monitoring. Because pheromones are unique--specifically,
because every species has an exclusive sex pheromone--they can be
exploited to determine the size of a pest population. Traps containing
the appropriate sex pheromone are placed in the field, duly protected
from environmental influences such as light and oxygen, as a bait to
attract the opposite sex of the species. A farmer can obtain precise
and reliable information concerning the size of the pest population,
where it is located, when it first appeared, and so on and subsequently
decide when to spray pesticides, which section of the field to spray,
and which chemical and concentration will be effective.
This approach does not eliminate the use of standard pesticides, but it
does reduce the quantity applied to the field and the amount of the
field that must be sprayed. Efficient monitoring makes it possible to
time treatments to coincide with the start of the pest's active season
or with massive onslaughts. The pheromone serves as a sensitive
detector that may be exploited to make intelligent decisions about
pesticides. This approach is used worldwide for a wide variety of
crops.
Mass trapping. The underlying assumption of this
approach is that scattering traps in the field ensures that many
individuals will be trapped, significantly hindering fertilization and
the production of a new generation of pests. As a result, fewer
chemical treatments are required; in some cases, the need for chemical
treatment is eliminated. Some pests that have been controlled using
this method include the Egyptian cotton leafworm in Israel, which
blights many crops (especially cotton, alfalfa, and vegetables); the
processionary pine moth in Spain, a severe pest of pine; and the bark
beetle in Scandinavia, which affects forest trees.
Attract-and-kill. Pesticides are placed inside centrally
located containers or other point sources that are baited with sex or
aggregation pheromones, eliminating the need to scatter the insecticide
in the field. The insect is attracted to the dispenser loaded with the
pheromone and comes into contact with the poisonous chemical. The
efficiency of chemical pest control is increased, and environmental
pollution is minimized. This method has been successful in cotton field
trials.
Disruption of communication or "confusion".
Individuals are drawn to a pheromone source against the wind, from a
zone of low concentration to a zone of high concentration. If an
artificial "zone" is created in which pheromone concentration is
constant, uniform, and higher than that generated naturally by males or
females, then the individual's capacity to detect and locate the
pheromone source will be significantly impaired. In other words, an
individual will be unable to find its way to the source of the
pheromone and accomplish fertilization; the species will not be
perpetuated.
The mechanism or chemobiological process responsible for
confusion is not fully understood. Still, the evidence indicates
several possibilities. One is a false trail: The individual flies
toward the artificial source of pheromone, which diffuses a
larger dose of active chemical(s) than the natural source (e.g., a
calling female) and thus conceals it. Another possibility is
habituation or adaptation. Marked by a desensitization of the sense of
smell, it is as if the sensors are switched off and lose the capacity
to react. Perhaps all the receptors in the pheromone-sensing cells
become saturated by an uninterrupted stream of molecules, making them
unable to produce the required nerve pulse. Yet another explanation is
the recipient's inability to detect concentration differentials that
point toward the natural source of the pheromones.
Whatever the mechanism involved, the results are inhibited mating and
the laying of unfertilized eggs, leading to near-complete
eradication of the next generation of the pest. Confusion has been
applied in cotton fields to control the pink bollworm,
Pectinophora gossypiella (United States and elsewhere;
initial trials were made in Egypt, Israel, and Pakistan), and also has
been used to eradicate pests that attack deciduous fruit trees (Europe,
New Zealand, United States, and Australia) and vineyard grapes
(Europe).
Of all the approaches described, the latter (disrupting communication)
presumably will become the preferred pest control method because it is
environmentally friendly, free of toxins, economically attractive, and
effective. This method already competes, with varying degrees of
success, with currently used chemical pesticides, and the day when the
heavy pesticides industry also adopts this new approach may not be far
away.
Other approaches. Monitoring, mass trapping,
attract-and-kill, and confusion are pest control methods based
mainly on the sex pheromones. Insects secrete other pheromones that
also can be exploited for pest control. For example, alarm pheromone
sprayed on plants keeps certain leaf aphids away. Compounds that
resemble the pheromone in structure can inhibit and disrupt chemical
communication. Some compounds act as repellants, scattering the pests
and keeping them away from the affected crop. Recent preliminary trials
used egg-laying-deterrent pheromones, which affect fertilized females
by interfering with the egg-laying process.
Global pheromone use
In the early 1990s, I conducted a survey to determine the
scope of pheromone use worldwide (10). Questionnaires were
sent to 142 researchers, scientists, and people from industry and
marketing associated with pheromones--half of whom were from
universities and research institutions and half from industry.
Twenty-five countries were represented. From the responses (~60% of
the individuals surveyed), I determined the percentage of use for each
of the following methods:
| Monitoring |
32.1 |
| Mass
trapping |
23.3 |
| Attract-and-kill |
2.2 |
| Confusion |
42.4 |
From the survey data, I determined that 1,313,000
hectares (ha) were treated with pheromones--1% of the cultivated land
in the world (see sidebar, Pheromone use worldwide). More areas
are being treated now than when the survey was conducted, and the use
of pheromones in pest control is increasing.
The pheromone use data indicate that the principal target pests
are moths. This majority results partly because research has largely
focused on this particular group, which includes numerous harmful
pests, but also because the components of moth pheromones belong to
very limited classes of organic compounds: alcohols, acetates, and
aldehydes of straight-chain C10-C18 molecules
with one or two double bonds or, in a few instances, three double
bonds. This feature has facilitated the rapid characterization of the
components. The starting materials vary only slightly, and the methods
used to produce the various pheromones are very similar.
Use of the confusion technique is expected to increase in the
future. The technique is clean and easy, farmers like it, and it meets
the requirements of the growing number of consumers who demand
pesticide-free agricultural products. These consumers are prepared to
pay double the price of conventional produce (or more) for the
"organic" designation, particularly in the United States, Japan,
and Europe.
The confusion method is preferred mainly in countries that have already
experimented with pheromones: the developed countries of western Europe
and North America as well as Japan, Australia, and New Zealand. In line
with the extensive research on ways to protect valuable cotton crops
from pests, pheromone use is already widespread in that industry. With
the development of efficient confusion techniques over wide areas,
fruit and other produce (e.g., rice, corn, and vegetables) soon will
benefit from pheromone-based pest control methods.
In the early 1990s, annual sales of pheromones were valued at
some tens of millions of dollars. The annual sales forecast for the
next decade is hundreds of millions and even billions of dollars,
equivalent to about 20% of the insecticide market, even though the
cost for pheromones has dropped rapidly (from $10-20/g down to a few
dollars per gram).
What about resistance to pheromones?
An important question we must consider is whether
insects can become pheromone-resistant. The answer is, unequivocally,
no. Resistance implies nullification of the harmful effect of the given
active substance. Pheromones are essential to insects for
communication, information transfer, fertilization, and reproduction;
therefore, nullification is not an option. Insects do respond to
pheromone treatment, but in a way that I call "evasion" rather than
"resistance".
With the confusion method, some insects subjected to continuous,
prolonged, and acute exposure to a large dose of sex pheromone may
react, impeding the success of the method. In regions where insects
tend to migrate or wander from field to field or area to area, the
resulting pressures will tend to be counterbalanced to some extent;
however, in isolated areas or over vast tracts, the stressed population
will react in some way.
For example, in a given population, the composition and titer of
pheromones are neither fixed nor uniform, so it is reasonable to expect
that although confusion will affect most of the pest population and
prevent them from mating, some of the population will nonetheless
succeed in reproducing. If the characteristic that helps these
individuals overcome the high concentrations of pheromone and reproduce
successfully is genetic and is passed on to the next generation, this
characteristic will be intensified in the surviving population.
Populations may eventually develop that can no longer communicate
chemically because their pheromones differ significantly, that is, the
pheromone composition has changed (Figure 2). This process probably
will lead to the emergence of new species. I propose to name this
process "chemospeciation".
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Figure 1. Proposed classification...
Figure 2. Schematic representation...
