A new approach to pest control in food crops  

The European consumer is increasingly demanding that food is produced with minimal inputs of synthetic pesticides. A simple reduction in pesticides use must be accompanied by the introduction of alternative safer methods of pest control. There have been promising developments in pest control strategies utilising pheromones and other biological control agents reported here by Roslyn McKendry.

Biological pest control methods have been used for many years, mainly for control of insects, from the first introductions of exotic predators through to more recent developments in microbial bio-pesticides such as Bacillus thuringiensis (Bt), baculoviruses, nematodes and entomopathogenic fungi. Alongside developments in biological control agents, isolation and characterisation of insect chemical messengers (semiochemicals) has opened up new possibilities for use reduction of undesirable synthetic insecticides. Biological control agents and semiochemicals are both considered mainstays of the Integrated Pest Management (IPM) approach.
    Semiochemicals are chemicals that evoke a behavioural or physiological response in individuals of the same or other species. Their range of effects can be exploited for insect monitoring and control. For example, pheromones are compounds that enable signaling between organisms of the same species, and sex pheromones are most well known for insect lures and mating disruption. Another group of semiochemicals are the allelochemicals. These are chemicals produced by one species that modify the behaviour of a different species and are important for parasite-host and plant-pest interactions. Allelochemicals are further classified according to the beneficiary of the interaction: 

• allomones where the emitter benefits from the message (carnivorous plants that release a substance which attracts insects, clearly benefitting the plant and not the insect)
• kairomones where the receiver gains (for example when pest insects are attracted to plant volatiles) 
• synomones where both sides benefit.

The first semiochemical to be isolated and characterised was the sex pheromone for the silkworm moth (in 1959). Since then several thousand species-specific pheromones have been discovered and used in experimental and commercial pest monitoring and control systems. These chemicals are now mainly synthesised rather than being purified from insects, however, new possibilities for production are now being investigated.
    In contrast to conventional insecticides, insect sex pheromones have a very low toxicity to mammals, birds and insects. The main environmental benefit is that the pheromones are species-specific, with a strong effect on the targeted pest, and no direct effect on local populations of natural predators or pollinators. Considering the possible effect of pheromone residues on food quality, no health problems due to human consumption are known or expected, and the quantities of pheromone residue will always be several hundred times less than typical synthetic insecticides – in fact research in the USA has shown zero detectable residues when pheromones have been used at several hundred grams of active ingredient per hectare1. However, since the current EU law judges that pheromones should be regulated to the same degree as synthetic insecticides, and this is a barrier to exploitation2,3, it is important to demonstrate that pheromone residues are negligible, present no risk to human health and do not affect taste or other sensory qualities of food.
    Insect pheromones tend to be volatile and also to photo-oxidize, so they dissipate and react rapidly once released into the air. The formulations include a UV screen and antioxidants to increase shelf life. In order to maintain the optimum concentration of pheromone in the air, continuous release is required and this can be expensive, despite the small quantities, owing to the high cost of production. Some plants absorb pheromones in their leaves and release gradually, and this can mean that pheromone concentrations in the air are maintained even after a dispenser has run out.
    Traps baited with insect sex pheromones are used for monitoring of insect populations, mainly to assist with spraying decisions against defined thresholds. This in itself can rationalize the use of conventional insecticides with reductions of up to 50%. The most popular designs are delta traps that are suitable for most cases except for serious infestation. The pests can be caught by sticky liners, or killed by a soap solution or strips impregnated with insecticide.
    For actual control of Lepidoptera, the simple bait trap is limited, especially where high populations must be dealt with. It is likely that reducing a part of the male population does not reduce the frequency of mating, as those left mate more frequently with the females. Mating disruption is a more effective approach that has had some success for control of lepidopteran pests, and several techniques are used. The most widespread method is to dispense relatively large quantities of female pheromone into the air so that individual female pheromone trails are indistinguishable by the males. Further confusion is achieved through habituation of the male pheromone receptors: a continuously high signal of pheromone overloads the receptor and it becomes unresponsive, effectively blinding the male to directional pheromone signals. Steady release of pheromone is achieved by spray dispensers, impregnated ropes hung throughout the site, or microencapsulated formulations. Some types of pheromones require higher doses. 
    Considering the high cost of pheromones, a strong driver in the development of alternative techniques is to reduce the quantity required. In one approach, dispensers can provide false trails that confuse the males and reduce the frequency of mating. This requires less pheromone. Other systems have sophisticated metering of pheromones according to the time of day when mating occurs or when an increase in pest numbers are detected. Much research has been carried out on the optimum placing of dispensers, with analysis of plume structures arising from each dispenser in order to avoid major overlapping, minimising the number of required dispensers and hence the cost. Overall the current approaches for mating disruption have not been shown to be sufficient on their own for control of lepidopteran pests, and the cost is not always competitive with using conventional insecticides. 
    A new system has now been developed as an alternative for mating disruption of lepidopteran pests. This new system uses an electrostatically chargeable natural wax powder formulated with pheromone and placed in a delta trap dispenser. The particles adhere to the male insect when it enters the trap, and this works to achieve mating disruption in several ways: 

• the antennae of the treated male are coated with pheromone, thus rendering it blind to actual female pheromone trails. Partial coating will still interfere with the sensing of females
• the treated male will be rejected by females after their detection of the female pheromone
• the treated male will become a mobile dispenser of female pheromone, providing an additional false trail for other males
• where other males attempt to mate with the treated male, the powder is transferred to additional males thus spreading the effect throughout the pest population.

The system is currently on trial in 18 countries both for the control of agricultural and amenity lepidopteran insect pests and regulatory approvals are being sought – a very expensive and time consuming process since under current regulations it is necessary to make an application on a ‘by pest’, ‘by crop’ basis, with extensive field data over 2-3 years and submit volumes of toxicological and residue data in the same way as for a chemical insecticide4. (see PN61).
    More innovations are at the research stage using microbial biopesticides. A company project will test the system with new formulations of Bt and baculoviruses against Lepidoptera, mixed with electrostatic powder. It is thought that the ability of these biopesticides to target pests will increase if they physically adhere to the insects. This is particularly interesting considering that resistance to Bt is a growing problem. However, this method of using biopesticides may have impacts on environment and food safety that should be investigated. It is anticipated that environmental risks will be much reduced by more targeted delivery.
    One drawback of using insect sex pheromones is that only the males are affected. It is desirable to target females as well, and one approach currently gaining favour is to exploit plant volatiles that attract male and female pests. Plant volatiles of interest are a range of compounds including food odours that draw the insect pests in to spoil the produce – therefore classed as kairomones. To date, the main application of kairomones for insect control has been in mosquito traps. For agriculture, no applications are yet mature, but new research is identifying kairomones among plant volatiles that can be used as lures for a wide range of insect pests – extending potential targets to sucking pests like aphids and jassids as well as fruit flies.
    Several companies working together plan to investigate plant volatiles emitted by the test crops as kairomones as lures for Lepidoptera. Female Lepidoptera use plant volatiles to identify suitable sites for oviposition, with damage to the crop resulting from the burrowing of emerging larvae. Using kairomones to attract females to the biopesticides system will provide the possibility of directly targeting insect larvae or eggs with biopesticides, resulting in the reduction in the target insect population.

References
1. Guidance for Registration Requirements for Pheromones and Other Semiochemicals Used for Arthropod Pest Control. OECD series on pesticides, 12, OECD Environment, Health and Safety Publications, 2002.
2. Chandler, JA, Pheromones fall foul of EU pesticide directives, Pesticide News 59. March 2003, 10-11.
3. Chandler, JA, A call for common sense: EU regulations hold back eco-friendly pest control solutions, Organic farming, Spring 2003, Issue 77, 24-25.
4. Chandler, Op. Cit. 2 and 3.

For further information please visit the ExoSect website www.exosect.com

[This article first appeared in Pesticides News No. 62, December 2003, pages 10-11]