With the introduction of insect pheromones in plant protection, an old dream has come true: "Toxic" insectides are being replaced with "harmless" natural products. This is most apparent in the technique of mating disruption which directly uses pheromone chemicals to control specific pests. But even when used to attract insects to a monitoring trap, as in hundreds of insect species worldwide, pheromones have made a significant contribution towards a reduction of pesticide use.
In spite of these achievements, progress in the application of pheromones has been rather slow, and technology is still in its infancy. While producing material for applications on a large scale, industry is still in great need to make formulations more effective and suitable for a wide range of pests and climatic conditions. It is not only the scientific knowledge that drives this process, but to a great extent also trial and error. Users thus still find it difficult to obtain material of predictable quality for field use.
Dispensers for mating disruption
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Mating disruption can be effective as long as the proper chemical, usually a synthetic pheromone or its derivative, is present in the atmosphere in sufficient concentration to prevent male insects from locating calling females. This requires a device releasing the active ingredient over an extendend period of time, usually several months.
Measuring the release rate of dispensers is quite a difficult task. Weight loss
provides reliable data only under laboratory conditions where degradation can
be controlled. In the field, dispensers can even gain weight, e.g. with
accumulation of debris, change of humidity or oxidation of the remaining
product. These variations become important towards the end of the growing
season when the supply of active ingredient is low and insect pressure possibly
at its highest. Analysis of the remaining chemical is a more reliable, yet
destructive technique, and provides no valid information about future release
of the chemical. The only valid procedure is the measurement of pheromone
concentration in the dispenser effluent. The techniques to accomplish this can
be quite involved (Leonhardt et al. 1988, McDonough et al. 1989,
Van der Kraan & Ebbers 1990).
The release of pheromone chemicals from an unused RAK 1+2 ampoule (BASF AG) at
various temperatures is shown in Fig. 2. The release rate increases by about a
factor of 10 from 15 to 35°C. Similar temperature gradients were observed
for dispensers for the codling moth, such as Isomate C (Shin-Etsu) or Ecopom
(Isagro). Since these orchard and vineyard insects are sexually active in the
evening or during the night when temperatures are low - as is the case for most
moths using sex pheromones - this temperature effect represents a serious waste
of active material during the hot summer days.
To the user it is important to know how a dispenser will release the active
material over the entire growing season. Fig. 3 shows the results obtained
with two dispensers of different design and lure content that were exposed
facing the sun outside the laboratory. For each measurement, the dispensers
were taken inside for one day and placed back outdoors again. Release rates
were determined at 30°C to avoid the need for excessive refrigeration during
sampling.
Accelerated exposure tests
The described tests were quite useful to back up an actual field trial. They
could have given some clues in case of failure to control the specific pest.
However, dispenser performance should actually be determined before their use
in the field. Where the manufacturing process still evolving, a procedure is
needed in which exposure to the elements, possibly under extreme conditions, is
simulated in the laboratory. The Collaborative Pesticide Advisory Council
(CIPAC) has adopted protocols for accelerated storage tests for pesticides.
These are carried out at elevated temperatures, typically 54°C (Dobrat &
Martijn 1995). Procedures to be developed for pheromone dispensers should
provide for additional tortures such as UV radiation and wind.
Dispensers used in monitoring traps
The chemistry of insect sex attractants is well documented. In
Lepidoptera alone, pheromones and sex attractants of over 1600 species have
been described (Arn et al. 1992, 1996). Numerous companies serve the
needs of plant protection services and growers by manufacturing lures and
traps. However, users of pheromone traps frequently complain about the
variability of data obtained with certain commercial lures. In a letter sent to
commercial suppliers in the early 90's, Günter Schruft of the Institute of
Viticulture in Freiburg im Breisgau bitterly complained that half of the lures
sold to grape growers caught either nothing or the wrong species. Similar
reports have been heard from various orchard and forest entomologists. This
situation has led to insecurity among growers and plant protection advisors.
Purity Requirements
It is well known that chemical purity can be very critical to biological
activity of pheromones. Insects are very sensitive to trace components present
in attractant blends. Any synthetic product, crude or purified, contains some
impurities. Even when close to or beyond the detection limit, these byproducts
can have positive or negative effects on trap catch.
Every chemical ecologist should read the chapter by Turk and Turk (1975) on
chemical purity of odorants. It begins as follows: "The idea of `ultimate
purity', usually considered to be the condition of a substance composed
entirely of like molecules, has no operational meaning in the laboratory. We
cannot examine each molecule in a sample, and even if we could, we might not be
able to describe the purity of the sample because we would not necessarily
know which differences between molecules were sufficiently permanent to persist
after separation. All the chemist can do is to look for evidence of impurity. A
`pure substance' is then taken to be one for which no evidence of impurity is
found. The criterion of purity is thus always conditional and temporary."
The importance of isomeric purity has been established for many species. In
many cases, however, the reasons for a better performance of one batch of
chemical over another have remained obscure. The detection of a positional
isomer as an inhibitor of the Anarsia lineatella pheromone (Millar et
al., this volume) is an example of the high quality of research needed to
guarantee the efficacy of a lure. Ironically, some chemical batches are more
active than others due to the presence of a synergistic impurity. This seems to
be the case in the grapevine moth, Lobesia botrana, where highly pure
(E)-7,(Z)-9-dodecadienyl acetate is less attractive than many
crude synthetic products,
Importance of field tests
Following these considerations it is not possible by chemical analysis alone to
predict whether a given batch of chemical will give a good attractant. Once
chemical analysis has demonstrated the presence of essential and the absence of
detrimental constituents in a synthetic product, the only demonstration of
biological activity can be obtained in the field. The procedure of providing of
lures suitable for distribution thus always consist of 3 steps: 1) Synthesis of
ingredients, 2) chemical analysis to assure the presence of essential and
absence of known antagonistic components and, if necessary and feasible,
purification, and 3) field testing and comparison with standards in various
habitats.
Batch certification
In order to assure a continuous supply of lures of comparable quality and to
allow comparisons between lures of different origin, we propose to adopt the
procedure of batch certification. It consists of two principles:
1) Any chemical or blend prepared for insect monitoring is given a batch number
which is carried over to all dispensers made from it.
2) Each batch is field-tested by experts and the results made publicly
available.
Since lures for monitoring and detection are not normally subject to
registration by government authorities, certification can be accomplished on a
nearly informal basis within the international scientific community. Batch
specifications and test results could be published on the Internet and readily
updated. Some of the procedures will need to be discussed in more detail. Where
the dispenser material has been shown to be important, it should be included
in the procedure. A consensus should also be reached on experimental design of
field tests and analysis of data.
A dilemma can be anticipated concerning the chemical information to be revealed
for certification. To the educated user it is important to know the basic
ingredients of each lure. Insect attractants are constantly being refined, and
the user should have access to the most effective and reliable mixture. For
example, racemic disparlure, which has been used for decades to monitor the nun
moth, will soon be replaced by the far more attractive blend containing
monachalure which was recently identified (Gries et al., 1996). One
could even argue that analytical samples of certified batches should be made
available to the scientific community and that the results of the chemical
analysis be published. This could lead to an improvement of our knowledge of
effects of secondary components and lure specificity. In this context it would
even be desirable to know the synthetic pathway. On the other hand, we should
also respect the position that for commercial reasons the composition of a
particular lure may need to remain a well-garded secret.
Batch certification will lead to a gain of confidence in the trapping results
obtained with pheromones. Attempts to relate trap catch with population density
can begin to be fruitful as soon as a the batch of chemical used in different
tests is the same. Certification is of critical importance for quarantine pests
in which it is often impossible to confirm biological activity without going
to another continent.
Batch testing has long been established for pesticides and pharmaceuticals in
cases where it was necessary to track down unwanted side effects. In insect
monitoring and detection, the procedure will require a closer cooperation of
suppliers and scientists. This small effort is by far outweighed by the gain of
confidence.
Pheromone technology started in the late sixties when researchers began testing
their first synthetic samples in the field. Some of the home-made slow-release
devices of those days, such as rubber tubes and polyethylene caps (Glass et
al.) are still in use today. Field experiments in which an attempt is made
to control all the chemical and physical elements needed for biological
activity are still quite scarce (Witzgall et al, 1995, Färbert
et al. 1996). Such efforts are essential to guarantee the survival of
pheromones as tools in pest management.
Acknowledgement
The investigations were supported by the German Ministry of Science and
Technology (BMFT) and BASF AG, Germany.
References
Arn H, Tóth M, Priesner E (1992) List of Sex Pheromones of Lepidoptera
and Related Attractants, 2nd edition. International Organization of Biological
Control, Avignon.
Arn H, Tóth M, Priesner E (1996) List of Sex Pheromones of Female
Lepidoptera and Related Male Attractants, Internet Edition.
Dobrat W, Martijn A, eds. (1995) CIPAC Handbook, Volume F. Collaborative
International Pesticides Analytical Council Ltd., pp. 149-153
Färbert P, Koch UT, Färbert A, Staten RT, Cardé RT (1996)
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Specificity of nun and gyspy moth sexual communication through
multiple-component pheromone blends. Naturwiss. 83:382-385
Leonhardt BA, Dickerson WA, Ridgway RL, Devilbiss ED (1988). Laboratory and
field evaluation of controlled release dispensers containing grandlure, the
pheromone of the boll weevil (Coleoptera: Curculionidae). J. Econ. Entomol.
81:937-943
McDonough LM, Brown DF, Aller WC (1989) Effect of temperature on evaporation
rates of acetates from rubber septa. J. Chem. Ecol. 15:779-790
Millar J, McElfresh S, Rice RE (1997) Technological problems associated with
use of insect pheromones in insect management (This volume)
Pop L, Arn, H, Buser HR (1993) Determination of release rates of pheromone
dispensers by air sampling with C-18 bonded silica. J. Chem. Ecol.
19:2513-2519
Turk A & Turk J (1975) The purity of odorant substances, pp 63-73 in
DG Moulton, A. Turk, JW Johnston Jr (eds.) Methods in Olfactory Research,
Academic Press, London
Van der Kraan C, Ebbers A (1990) Release rates of tetradecen-1-ol acetates from
polymeric formulations in relation to temperature and air velocity. J. Chem.
Ecol. 16:1041-1058
Witzgall P, Bengtsson M, Karg G, Bäckman AC, Streinz L, Kirsch PA, Blum Z,
Löfqvist J (1996) Behavioral observations and measurements of aerial pheromone
in a mating disruption trial against pea moth Cydia nigricana F. (Lepidoptera, Tortricidae).
J. Chem. Ecol. 22:191-206
adapted from Pop et al. (1993)
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http://nysaes.cornell.edu/pheronet/,
http://quasimodo.versailles.inra.fr/pherolist/pherolist.html,
http://mpi.seewiesen.mpg.de/pheronet/pherolist.html
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