16.9. Pheromones and other insect attractants
Insects use a variety of chemical odors called semiochemicals to communicate within and between species (Section 4.3.2). Pheromones are particularly important chemicals used for signaling between members of the same species — these are often mixtures of two, three, or more components, which, when released by one individual, elicit a specific response in another individual. Other members of the species, for example prospective mates, arrive at the source. Naturally derived or synthetic pheromones, especially sex pheromones, can be used in pest management to misdirect the behavior and prevent reproduction of pest insects. The pheromone is released from point-source dispensers, often in association with traps that are placed in the crop. The strength of the insect response depends upon dispenser design, placement, and density. The rate and duration of pheromone emission from each dispenser depends upon the method of release (e.g. from impregnated rubber, microcapsules, capillaries, or wicks), strength of formulation, original volume, surface area from which it is volatilized, and longevity and/or stability of the formulation. Male lures, such as cuelure, trimedlure, and methyl eugenol (sometimes called parapheromones), which are strongly attractive to many male tephritid fruit flies, can be dispensed in a manner similar to pheromones. Methyl eugenol is thought to attract males of the oriental fruit fly Bactrocera dorsalis because of the benefit its consumption confers on their mating success (see here). Sometimes other attractants, such as food baits or oviposition site lures, can be incorporated into a pest management scheme to function in a manner analogous to pheromones (and parapheromones), as discussed below.
There are three main uses for insect pheromones (and sometimes other attractants) in horticultural, agricultural, and forest management. The first use is in monitoring, initially to detect the presence of a particular pest and then to give some measure of its abundance. A trap containing the appropriate pheromone (or other lure) is placed in the susceptible crop and checked at regular intervals for the presence of any individuals of the pest lured to the trap. In most pest species, females emit sex pheromone to which males respond and thus the presence of males of the pest (and by inference, females) can be detected even at very low population densities, allowing early recognition of an impending outbreak. Knowledge of the relationship between trap-catch size and actual pest density allows a decision about when the ET for the crop will be reached and thus facilitates the efficient use of control meas- ures, such as insecticide application. Monitoring is an essential part of IPM.
Pheromone mass trapping is another method of using pheromones in pest management and has been used primarily against forest pests. It is one form of attraction—annihilation — a more general method in which individuals of the targeted pest species are lured and killed. Lures may be light (e.g. ultraviolet), color (e.g. yellow is a common attractant), or semiochemicals such as pheromones or odors produced by the mating or oviposition site (e.g. dung), host plant, host animal, or empirical attractants (e.g. fruit fly chemical lures). Sometimes the lure, as with methyl eugenol for tephritid fruit flies, is more attractive than any other substance used by the insect. The insects may be attracted into container or sticky traps, onto an electrocutor grid, or onto surfaces treated with toxic chemicals or pathogens. The effectiveness of the attraction—annihilation technique appears to be inversely related to the population density of the pest and the size of the infested area. Thus, this method is likely to be most effective for control of non-resident insect pests that become abundant through annual or seasonal immigration, or pests that are geographically restricted or always present at low density. Pheromone mass trapping systems have been undertaken mostly for certain moths, such as the gypsy moth (Lymantriidae: Lymantria dispar) (see Plate 6.7), using their female sex pheromones, and for bark and ambrosia beetles (Curculionidae: Scolytinae) using their aggregation pheromones (section 4.3.2). An advantage of this technique for scolytines is that both sexes are caught. Success has been difficult to demonstrate because of the difficulties of designing controlled, large-scale experiments. Nevertheless, mass trapping appears effective in isolated gypsy moth populations and at low scolytine beetle densities. If beetle populations are high, even removal of part of the pest population may be beneficial, because in tree-killing beetles there is a positive feedback between population density and damage.
The third method of practical pheromone use involves sex pheromones and is called mating disruption (previously sometimes called “male confusion”, which as we shall see is an inappropriate term). It has been applied very successfully in the field to a number of moth species, such as the pink bollworm (Gelechiidae: Pectinophora gossypiella) in cotton, the oriental fruit moth (Tortricidae: Grapholita molesta) in stone-fruit orchards, and the tomato pinworm (Gelechiidae: Keiferia lycopersicella) in tomato fields. Basically, numerous synthetic pheromone dispensers are placed within the crop so that the level of female sex pheromone in the orchard or field becomes higher than the background level. A reduction in the number of males locating female moths means fewer matings and a lowered population in subsequent generations. The exact behavioral or physiological mechanism(s) responsible for mating disruption are far from resolved but relate to altered behavior in males and/or females. Disruption of male behavior may be through habituation — temporary modifications within the central nervous system — rather than adaptation of the receptors on the antennae or confusion resulting in the following of false plumes. The high background levels promoted by use of synthetic pheromones also may mask the natural pheromone plumes of the females so that males can no longer differentiate them. Understanding the mechanism(s) of disruption is important for production of the appropriate type of formulation and quantities of synthetic pheromone needed to cause disruption, and thus control.
All of the above three pheromone methods have been used most successfully for certain moth, beetle, and fruit fly pests. Pest control using pheromones appears most effective for species that: (i) are highly dependent on chemical (rather than visual) cues for locating dispersed mates or food sources; (ii) have a limited host range; and (iii) are resident and relatively sedentary so that locally controlled populations are not constantly supplemented by immigration. Advantages of using pheromone mass trapping or mating disruption include:
- non-toxicity, leaving fruit and other products free of toxic chemicals (insecticides);
- application may be required only once or a few times per season;
- confinement of suppression to the target pest, unless predators or parasitoids use the pest’s own pheromone for host location;
- enhancement of biological control (except for the circumstance mentioned in the previous point).
The limitations of pheromone use include the following:
- high selectivity and therefore no effect on other primary or secondary pests;
- cost-effective only if the target pest is the main pest for which insecticide schedules are designed;
- requirement that the treated area be isolated or large to avoid mated females flying in from untreated crops;
- requirement for detailed knowledge of pest biology in the field (especially of flight and mating activity), as timing of application is critical to successful control if continuous costly use is to be avoided;
- the possibility that artificial use will select for a shift in natural pheromone preference and production, as has been demonstrated for some moth species.
The latter three limitations apply also to pest management using chemical or microbial insecticides; for example, appropriate timing of insecticide applications is particularly important to target vulnerable stages of the pest, to reduce unnecessary and costly spraying, and to minimize detrimental environmental effects.