The primary uses of pheromones world-wide at present are:

These points can be implemented by using some of the sixty different pheromone traps that are commercially available along with the wide variety of synthetically manufactured pheromones. The two most commonly used pheromone traps are the delta and wing traps seen in the image below. The type of data supplied by insect catches from pheromone traps is termed a relative estimate (Dent 1991). This means that the insects within the trap do not originate from a known volume of area, unlike those insects that could be found in a quadrat which gives a absolute estimate. The main advantages of relative estimates is that it is less costly as implementation of traps is not as labour intensive as using quadrats. It also allows for those insects that are airborne to be included. Traps used for relative monitoring can be either mechanical, such as suction traps and pitfall traps, or attractant traps such as light or pheromone traps. The area around a pheromone trap which has a high enough pheromone concentration to attract an insect is termed the active area. This active area falls inside the capture area, this is the spatial area around the trap to which an insect is able to orientate and then move into the active area and thus the trap (Nakamura & Kawasaki, 1977). The advantage of a pheromone trap is that both the active and the capture areas are large compared to mechanical traps. The actual area of a pheromone trap will vary according to many factors such as the target species, the height of the trap, the prevailing weather conditions, and the time of day (Mc Neil, 1991) These variables are disadvantages of this method as they make interpretation of catch data difficult.

Before moving on to look at trap design and monitoring techniques a short explanation of what a pheromone is and how it affects an insects behaviour is worthwhile. Pheromones are chemicals which when released result in either attracting or dispersing insects of the same species, that is to say they are intraspecific . This separates pheromones from other chemicals released by insects which influence the behaviour of other species, these chemicals are called allelochemicals and act interspecifically. Pheromones are sub-divided into three categories, sex pheromones which generally originate from females and attract males for mating. Aggregation pheromones which may or may not be produced by either sex to congregate the species for feeding or reproduction, and alarm pheromones that serve to rapidly disperse a group of insects usually as a response to predation (Nordlund 1981).

At present the majority of pheromones used are those involving control and monitoring of Lepidoptera such as pink bollworm Pectinophora gossypiella, the oriental fruit moth Grapholita molesta and the tomato pinworm Keiferia lycopersicella. There are others, such as the alarm pheromone (E)--farnesene, of aphids, or the aggregation pheromone of Dendroctonus spp, but at present the majority of pheromone traps are used to monitor and control Lepidoptera. Although this may seem a very limited range of use the Lepidoptera are a very important economic pest order and so pheromones can be very effective for their monitoring and control. The exact mechanism of how the synthetic pheromone disrupts the orientation of males is not precisely known, although habituation, competition between source points of formulation and females plus a camouflage of the female pheromone are all thought to be involved (Carde & Minks 1995). The shape and design of the trap itself is also a crucial factor in the effectiveness of the pheromone as the plume structure will differ according to trap design (Hoyt et al.. 1983; McNally & Barnes 1981; Saario et al..1970) In an experiment using a sex pheromone to trap the pea moth Cydia nigricana it was found that traps placed just above crop level caught most moths in moderate winds. Ground level traps caught the most at higher wind speeds however due to the dispersal of the pheromone at high wind speeds being to great. The best location for the trap to be reasonably effective in all conditions was midway between the top of the crop and halfway down the crop height (Lewis & Macaulay 1976) Different shape traps will also result in differing pheromone plumes and so the best shape trap as well as the optimum position also needs to be determined for the most effective monitoring and control. This procedure is extremely difficult as pheromone plumes are not visible and so a large amount of trial and error is needed before the best trap can be selected.

Having now established that pheromones are capable of enabling a monitoring and control program providing certain criteria are met the following review will discuss recent experimental work that highlights the advantages of pheromone use for monitoring and control and the limitations. As already mentioned many pheromone monitoring and control programs involve the order Lepidoptera. The advantage of a sex pheromone used against these species is that often there is quite an extensive range over which the pheromone can be effective. However on the downside is the fact that only males are trapped. This means that it is possible for some males to have located a female before being captured as male Lepidoptera have the capacity to mate several times (Drummond 1984). This was shown in the gypsy moth by mark and recapture experiments using males that had mated, releasing them and then finding them in pheromone traps at the same rate as unmated males. The usefulness of monitoring becomes apparent here as this allows a picture of peak mating times to be established and so a high density of traps can be used to reduce the amount of males which will find a female before a trap. A recent study in India with the diamondback moth Plutella xylostella , the larvae of which feed on Brassica, revealed that the released sex pheromone from the female was most attractive to males during her first day of emergence. The pheromone of the female moth has been broken down into its component parts which are, (Z)-11-hexadecenal and (Z)-11-hexadecenyl acetate. These two compounds have the greatest attracting powers when in a ratio of 7:3 respectively (Tamaki et al..1977). These pheromones can be synthesised in the laboratory and this is a common factor with many pheromones. The study on Plutella xylostella revealed that the synthetic pheromone was attractive to males for up to 84 days although over this period capture rates fell from 200 in the first 14 days to 10 in the last 14 days. The highest capture rates were always between 16.00hrs and 18.00hrs for this species. Field trials are know planned to ascertain if pheromone traps would be more cost effective than spraying the cabbage and cauliflower crops in India (Reddy & Urs 1996). Even if the pheromone trapping method of P. xylostella is not as economically viable as spraying it will allow more precise spraying at a time that will be the most effective so reducing wasteful spraying and environmental damage.

A successful case of using pheromones to control a lepidopteran pest can been seen with the pink bollworm P. gossypiella. Due to its ecology the pink bollworm is particularly difficult to control with conventional insecticides as the larvae feed in the actual flowers or bolls on the cotton plant. This feature affords them considerable protection from spraying and intensive spraying was targeted at the imago stages instead, this however can lead to resistance, loss of beneficial insects and heavy production costs. The pheromone of P. gossypsiella consists of two components, (Z-Z)- and (Z-E)-7,11- hexadecadienyl acetates at a 1:1 ratio. This pheromone was the first synthetically manufactured pheromone and was registered in 1978 for use as a direct control of an agricultural pest (Brooks et al.. 1979). The pheromone was dispersed in a 1981 control campaign by impregnating hollow fibres which were then applied to the cotton crop aerially, these had an effective emission period of approximately 14 days. The main aim of the campaign was to dispense of the need for early spraying by suppressing the early season population. The damage to the crop in the 1981 control program fell from 30% in 1980 using conventional spraying to 5% in 1981 using the mate disruption technique (Doane et al.. 1983). The campaign against P.gossypsiella in the United States was on a large scale with a high amount of labour input. A recent study in India has looked at how pheromone traps can enhance the yield and lower the control costs for smallholdings that grow their own rice. The target species was the yellow stem borer Scirpophaga incertulas which is the most economically important lepidopteran pest of rice in Asia. The pheromones used originates from Chilo suppressalis another Lepidoptera species and the pheromone is impregnated into resin which is coated onto PVC. This prolongs the active life of the pheromone by protecting it from environmental degradation. The formulation is manufactured in the UK by Agrisense-BCS and is used against other pests such as Earias insulana and E.vitella. The pheromone traps were placed as smallholders transplanted rice and it took four men less than half a day to treat one hectare. The PVC strips were attached to bamboo sticks which were available locally. A measure of the damage in a pheromone plot was compared to a traditionally managed plot and a significantly lower measurement of "white head" and "dead heart" was found. Dead heart is the result of the larvae boring into the central leaves of the tillers causing them to dry up. White head is caused by the second generation larvae and results in no grain being produced within the panicle. This field trial showed that it was a very practical method of control for smallholders as trap placement could be integrated with crop planting. The trial also showed that larval S. incertulas in the traditional plot was reduced from 88% to 65% in the pheromone plot.

The use of pheromones in both monitoring and control of pests will continue to increase as more research and data becomes available. The forecasting systems at present allow farmers to phone various institutions and apply the information they receive to planting times and spraying regimes. A good example of the use of pheromone and light trap data is that used in Wormbase. This is a management database system for African armyworm forecaster. The service began in 1969 at Nairobi, Kenya and published a weekly report to the East African countries to allow preparation for outbreaks as the population of armyworm travelled across the continent. The data is now a accumulation of information from light and pheromone traps, knowledge of population trends and seasonal outbreaks combined with weather maps from meteorological offices. The recent advances in computer hardware and software now allows a current information source to be on-line for extension workers, information that can then be used to advise growers. The use of pheromones in trap catch data is an important integral part of these forecasting systems and combined with control methods they contribute substantially to insect pest management programs and population monitoring.