The discovery, isolation and chemical identification of sex pheromone (bombycol) of the silk worm, Bombyx mori (Linnaeus) in 1959, provided impetus for the exploitation of pheromones in pest management. Upto 1970s, pheromones and pheromone mimics for more than 200 insects were known and by the end of 1980s the number rose to over 2000.
Some of the important sex pheromones with a potential in pest management are disparlure (gypsy moth), gossyplure (pink bollworm), grandlure (cotton grey weevil) and frontalin brevicomin (pine beetles). Besides pheromones, chemical attractants have been utilized for management of insect pests, viz., methyl eugenol (Oriental fruit fly), cue-lure (melon fly) and amlure (European chafer).
These chemicals have been employed for disrupting the activities of insect pests in three ways:
Strategy # 1. Monitoring:
Pheromone baited traps for monitoring pest populations provide a highly sensitive means of detecting the insect pests with many advantages over conventional methods such as light traps and scouting programmes. Pheromone traps can be used to detect both the presence as well as the density of pest species.
Insect population can thus, be estimated and new areas of infestation detected at a very early stage. The trap catches may be used to forewarn regarding outbreaks of important pests. Monitoring of quarantine pests such as gypsy moth, Lymantria dispar (Linnaeus) and Mediterranean fruit fly, Ceratitis capitata (Wiedemann), is being successfully accomplished by use of pheromone traps.
Detection of Pest:
Pheromone-baited traps provide a relatively simple and reliable means of detecting the presence of insects. They are one of the primary tools employed in quarantine surveys, where the aim is to determine the presence of a species and prevent its establishment and spread.
Traps are routinely deployed around airports and harbours to detect potential introductions of exotic pests at these high-risk sites. Similarly, large number of traps is used in regional survey programmes to determine the distribution of specific pests and provide the required information for preventing spread to new areas.
The use of attractant-baited traps to demonstrate pest-free production and storage zones is assuming significance. The process often requires implementing a defined monitoring programme to allow for the export of various agricultural commodities to specific countries.
Measurement of Pest Density:
Traps baited with sex pheromones are commonly used to monitor the population density of pests. Pheromone traps have also been used in area-wide studies of pest disruption and dispersal. Stored product pests have been successfully monitored from food processing plants and warehouses, using pheromone or food traps.
Sex pheromones are used for moths, and beetles usually use aggregation pheromones and/or food baits. Quantitative relationships between adult captures and counts of larval stages or signs of larval feeding, such as feces or damage, have been found for pests of tree fruits, annual crops and forests.
Assessment of Density of Natural Enemies:
Sex pheromones have been employed to trap the insect biological control agents. Pheromone traps have also been used for monitoring the establishment of a biological control agent, Cydia succedana Denis & Schiffermuller (Tortricidae), introduced for the control of a weed, gorse, Ulex europeaus Linnaeus, in New Zealand.
This new tool would permit monitoring of presence, phenology and relative abundance of the biocontrol agents and could give an indication to the growers whether the population in future might be high enough for successful control of the particular pest.
Assessment of Pest Phenology:
The combination of pheromone traps and predictive phenology models can provide a reliable method for predicting the timing of flight activity or life stages. The first moth catch (referred to as ‘biofix’) has been used as a predictor of the beginning of adult emergence. The precision of this method has been well documented for codling moth, a key pest of pome fruits throughout the world.
The phenology model for this pest is based on accumulating degree-days (base 10°C) beginning on the day the first moth is captured in a pheromone trap, provided moths are captured on two successive trapping dates. In 6 of the 10 years in the state of Washington, USA, this model predicted the start of egg hatch on the same day that it was observed in the field and there was never a discrepancy of more than two days between the predicted and observed event.
Traps have generally been less effective in predicting peak emergence and the emergence of later generations probably because of trap saturation, pesticide use, female moth-trap competition, and the variability associated with the degree of trap and lure maintenance. This approach is likely to be particularly useful for biorationals such as IGRs, which require precise timing as they are primarily active against specific instars or life stages.
Assessment of Effectiveness of Mating Disruption:
An increasingly important use of attractant- baited traps is to measure the efficacy of mating disruption formulations. Capture of zero (complete shutdown) or very few moths in a pheromone-baited trap has been used to indicate successful disruption of the target pest. However, it is not uncommon to record low moth catches in traps and still have less than adequate pest control in pheromone-treated plots.
In some cases, it is also possible to greatly inhibit catches in pheromone traps but still detect substantial numbers of females mating. Two approaches have been suggested to improve the utility of pheromone-baited traps as monitoring tools in pheromone-treated plots.
Firstly, a lure with an emission rate closer to the natural rate would seem to be the most suitable for measuring the efficacy of a disruption treatment. The second approach is to use lures with very high release rates as a means following changes in adult population densities in spite of air permeation with pheromone.
Monitoring of Insecticide Resistance:
Pheromone-baited traps are one of the more widely adopted methods for monitoring insecticide resistance in several lepidopteran pests. The technique involves collecting large number of males in traps and testing for expression of resistance by topical application of insecticides or through incorporation of the insecticide through glue.
The major advantage of pheromone-trap bioassays is the rapid collection and determination of resistance for large samples of the pest without incurring the costs in time and money associated with rearing of large numbers of insects.
However, the technique may overestimate the impact of resistance in the field because only males are captured and assayed. Sex-related differences in tolerance to insecticides in insects are known, with females more susceptible than males. Moreover, this approach cannot be used to monitor for resistance to materials whose primary mode of action requires ingestion.
Decision Support:
Pheromone-based monitoring systems can be used to assess population trends and determine the need to treat. Thresholds of catch have been developed for a large number of insects and used as the basis for conventional pest management interventions. Basing management decisions on adult catches rather than taking a preventive or calendar-based approach is a key step in many efforts to reduce the number of insecticide applications.
The approach works on the principle that an intervention with a spray is required only if certain defined sampling threshold is exceeded. The decision may be based on a single weekly catch, consecutive catches or cumulative catches over an extended period, such as a generation. A threshold of apple leafroller, Epiphyas postvittana (Walker), pheromone trap catch has been determined from a correlation of catch with fruit damage at harvest.
Thus catches greater than the threshold led to recommendation for application of a selective insecticide. Treatment thresholds for codling moth, Cydia pomonella (Linnaeus), based on moth captures in pheromone traps, have been developed for most pome fruit producing regions of the world. A threshold of 1-5 moths per trap per week has been established as the point at which pesticides need to be applied depending upon location. This need-based application has led to 50- 75 per cent reduction in pesticide use.
Strategy # 2. Mass Trapping:
Mass trapping aims at catching substantial proportion of a pest population before mating, opposition or feeding and thus preventing damage to the crop. Success with this technique requires the combination of a very attractive lure and a highly efficient trap. The lure should be very attractive, eventually out-competing the naturally occurring attractant.
For Lepidoptera, it is essential that males are trapped before mating, and it is most likely to succeed with insects that mate only once. In case of Coleoptera, trapping based on aggregation pheromones aims to reduce the number of both sexes before eggs are laid or damage is done by feeding adults. It is most important that there is minimal influx of the pest from outside the protected areas.
In addition, mass trapping is rather cost-and labour-intensive because-of trap maintenance. As with other traps, there may also be problems with the blend, change of release rate, or trap efficiency over time. The ability to attract and retain very high numbers will be affected by trap design, placement and maintenance.
Mass trapping is considered most effective for pests, which are geographically isolated and/ or at low densities. Four year of mass trapping with a sex and floral lure reduced a small pocket of Japanese beetles in a city park by 97 per cent. Male removal using sex pheromone traps was shown to be an effective means of controlling Chinese tortrix, Cydia trasias (Meyrick), on street-planted Chinese scholar trees.
Success of these efforts in urban or park settings was, in part, due to the isolation of the sites and relatively low population densities. Similarly, food warehouses and other enclosed situations provide a high level of isolation, which should enhance the prospects for mass trapping.
Perhaps the most successful use of mass trapping has been for the control of several species of beetles on forest trees. One of the most effective uses has been for the control of ambrosia beetles in timber-processing facilities in British Columbia. In this case, the programme probably benefited from the trapping being somewhat isolated from beetle populations in the forest.
Controls of some forest beetles may be enhanced by use of deterrents to push the target beetles away from a host, combined with attractant baited traps or trap trees to ‘pull’ them away. Recently, the potential of using lures containing the aggregation pheromone components in combination with ethyl acetate, cut sugarcane and insecticide (permethrin), was demonstrated for mass trapping of New Guinea sugarcane weevil, Rhabdoscleus obscurus (Boisduval), in Guam.
Lure and Kill:
The lure and kill approach is a modification of mass trapping, where instead of being trapped, the responding insects come in contact with a toxicant and get killed. In many ways, this method also suffers the same constraints as for mass trapping, e.g., population density, attractiveness of the lure and efficiency of the method of killing.
However, the problem of trap- saturation may be eliminated and this may improve the effectiveness of control in high-density situations. The problems of trap maintenance and high cost of the control programme may also be mitigated to some extent, especially if the system relies on attracting the insects to a plant surface treated with an insecticide rather than to some kind of target device.
The lure and kill formulations have been developed for the control of various beetles, moths and flies. Some of the earliest applications of attractants in combination with insecticides have been for the control of tephritid fruit flies. The most successful example is the control of olive fly, Bactrocera oleae (Gmelin) in Greece.
Protein/insecticide-bait sprays have been used to control this pest in most Mediterranean olive-growing areas for a number of years. However, due to toxicity to natural enemies, a system was developed based on the use of target traps baited with either a food- attractant or a sex pheromone dispenser. This target-device method of controlling B. oleae was effective in reducing fruit infestation, especially when applied on area-wide basis.
The most recent development for fruit fly control has been microencapsulated sprayable formulation, comprising of the sex pheromone of this species, 1, 7-diozaspiro, and an insecticide (either dimethoate or Malathion). Recently, biodegradable or wooden spheres laced with a low dose of imidacloprid have shown promise for the control of Rhagoletis pomonella (Walsh) in apple and R. mendax Curran in blueberry.
Lure and Infect:
This innovative and promising approach combines an attractive lure with an entomopathogen. This technique is also called ‘autodissemination’. In this case, the insects that arrive at the source are not killed, but are inoculated with the pathogen with the idea to magnify the treatment by spreading the disease to other individuals. Different pathogens could be used with slightly different pathways including viruses, bacteria, fungi and nematodes.
This approach can generate disease outbreaks that can multiply in the area and affect the pest populations. Fewer insects may need to be directly attracted to the pathogen stations, which could reduce the cost and labour required. However, the critical requirements for success with pathogens may be difficult to achieve and include biological factors as well as operational factors such as formulation and delivery systems.
The lure and infect approach has been explored with nucleopolyhedrosis virus against tobacco budworm; a granulosis virus against codling moth; a protozoan against stored-product insect, Trogoderma glabrum (Herbst); and fungi against diamondback moth, Japanese beetle and termites. Fungi seem to be the best candidates as they are transferred between adults and larvae, and do not require consumption or copulation to be pathogenic.
Once an appropriate pathogen is selected, a formulation must be developed that protects the organism from environmental degradation. A major constraint with these systems, as with mass trapping, is likely to be the ability to make them cost-effective as many bait stations may need to be developed for the approach to be effective.
Strategy # 3. Mating Disruption:
Control of insect pests by mating disruption technique is achieved by widespread application of synthetic pheromone over the treated crop. Various slow-release pheromone formulations have been developed which either permeate the air with relatively high levels of pheromone so as to achieve sensory adaption or provide numerous discrete point sources so as to mask trail following or to create false trail.
The development of hand applied, slow release dispensers for season long control have undoubtedly contributed to make the technique effective, reliable and economical. Pink bollworm, Pectinophora gossypiella (Saunders), has been successfully managed in a number of countries including USA, Egypt and Pakistan by using controlled release formulations of its female sex pheromone gossyplure.
In Egypt, 150,000 ha of cotton were treated with pheromones against P. gossypiella. In Pakistan, a single twist-tie formulation containing pheromones of both pink and spotted bollworms has been successfully used to obtain season-long control of bollworm complex. Other successful examples include control of Oriental fruit moth, Cydia molesta (Busck), over 2000 ha stone fruit area in South Africa and Chilo suppressalis (Walker) in 2500 ha rice area in Spain.
Another area which has received considerable attention during the last decade is the utilization of a variety of semiochemicals for aphid management. (E)-β-farnesene (EBF) has been identified as alarm pheromone of aphids. The application of alarm pheromone along with host plant volatiles caused the aphids to move away from feeding sites and may be utilized to prevent colonization of the host plants by the aphids.
It also increased the mobility of the aphids and this is useful for increasing the pick up of contact insecticides or pathogens by the pest. EBF is now commercially available for use against a number of aphid species including Aphis gossypii Glover and Lipaphis erysimi (Kaltenbach).
Proteinaceous artificial honeydews sprayed onto foliage have been found to attract aphid natural enemies like chrysopid and syrphid adults and arrest the movement of a number of coccinellids.
Among the volatiles in artificial honeydew attractive to Chiysoperla carnea (Stephens) adults was a tryptophan degradation product, indole acetaldehyde. Parasitism by several aphid parasitoids including Aphidius spp., Dacus rapae and Praon volcure (Haliday) is increased by spraying nepetalactone which is a component of the male sex pheromone of several aphid species.