Pheromones and trapping in pest management
How insects communicate by scent, how pheromones are used for monitoring, mass trapping, mating disruption and attract-and-kill, and where each works best.
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Insects live in a world of smells. A female moth releases a few billionths of a gram of a chemical blend, and males several hundred metres downwind can follow it to her. These chemical signals between members of the same species are pheromones, and because they are so specific, they have become precise tools for watching and controlling pests without poisoning anything else.
Pheromone traps and mating disruption are now part of routine pest management in orchards, vineyards, cotton, vegetables and stored grain worldwide, and a standard topic in entomology courses and exams.
What pheromones are
The term pheromone was coined in 1959, the same year the German chemist Adolf Butenandt and colleagues identified the first insect pheromone, bombykol, the sex attractant of the silkworm moth, after extracting it from half a million female moths. Pheromones belong to a wider class of signalling chemicals, semiochemicals:
- Pheromones act between individuals of the same species.
- Allelochemicals act between different species: for example kairomones that benefit the receiver (a host plant odour that guides a pest, or a pest odour that guides its parasitoid) and allomones that benefit the sender (repellents).
The main kinds of pheromone used in agriculture:
| Type | Function | Examples in pest management |
|---|---|---|
| Sex pheromones | usually released by females to attract males for mating | most moth pests: codling moth, pink bollworm, tomato leaf miner, fall armyworm, stem borers |
| Aggregation pheromones | released by one or both sexes, attract both sexes to a food source or mates | many beetles and weevils: red palm weevil, stored-product beetles, bark beetles |
| Alarm pheromones | warn others of danger | aphids; studied for repelling them |
| Trail and other pheromones | mark trails, regulate colonies | ants, termites |
Most pheromones are blends of a few compounds in precise ratios; the blend, not a single chemical, makes them species-specific. Synthetic versions, made identical to the natural compounds, are released from lures and dispensers such as rubber septa, plastic tubes, membranes, microcapsules and aerosol emitters.
Monitoring
The most widespread use is in monitoring traps, which catch the males of one pest species:
- Detecting a pest early, including new invasive species at ports and borders.
- Timing: the first consistent catch (the biofix) starts a degree-day model that predicts egg-laying and hatching, so sprays, biological releases or other actions can be timed to the vulnerable stage.
- Estimating abundance relative to past seasons, or against a trap threshold.
Timing action from a pheromone trap and degree-days
Insect development depends on temperature. A degree-day model adds up, each day, how far the mean temperature exceeds the pest's lower developmental threshold.
Suppose a moth's lower threshold is 10 °C, and a local model says egg hatch begins about 150 degree-days after the biofix (the first sustained trap catch). After the biofix, the daily mean temperatures are about 20 °C.
- Degree-days per day = 20 − 10 = 10.
- Days to egg hatch = 150 ÷ 10 = 15 days after the biofix.
If the weather warms to a mean of 25 °C, each day adds 15 degree-days and hatch comes after 10 days instead. Thresholds and degree-day totals differ for every pest and region, so use the locally validated model; the method is the same.
Trap catches must be read with care: they measure male activity, are affected by weather, trap type, lure age and placement, and do not always correspond closely to larval damage. Thresholds based on catches are usually crop- and region-specific.
Mass trapping
Placing many traps to remove a large share of the population can work where the pheromone attracts both sexes, as aggregation pheromones do, so that females are removed too. It has been used against the red palm weevil in date and coconut palms, against some stored-product beetles, and against bark beetles in forests. For moths, where traps catch only males, enough males usually escape to mate unless trap densities are very high.
Mating disruption
In mating disruption, dispensers release so much synthetic sex pheromone over a field or orchard that males cannot find females: the plumes are masked, males follow false trails, or their responses become exhausted. Fewer females mate, fewer eggs are laid, and the next generation is smaller.
- Proven at large scale against the codling moth in apples and pears, grape berry moths in vineyards, the oriental fruit moth in stone fruit, the pink bollworm in cotton, and the tomato leaf miner in tomatoes, among others.
- Works best when pest densities are low to moderate, over large, uniform areas (ideally area-wide, including neighbours), so that mated females do not fly in from untreated fields.
- Needs monitoring to check that it is working (traps in disrupted blocks should catch almost nothing) and is often combined with other controls when populations are high.
Attract and kill, and lure and infect
A lure draws insects to a small point source treated with an insecticide or a pathogen, so only a tiny amount is needed and it does not reach the crop. Male annihilation against fruit flies uses powerful male lures (parapheromones) in bait stations; protein baits with small amounts of insecticide attract both sexes. Lure-and-infect devices contaminate insects with an entomopathogenic fungus that they spread to others.
Pheromone methods are often combined with the sterile insect technique, in which mass-reared males sterilised by radiation are released to mate with wild females, used area-wide against the Mediterranean fruit fly and the New World screwworm.
Strengths and limits
Strengths
- Species-specific: no effect on natural enemies, pollinators or other wildlife.
- Non-toxic at the doses used, with no residues on the harvest; generally registered with simple data requirements.
- Compatible with biological control and other IPM tools.
- Low resistance risk, although behavioural shifts are possible.
Limits
- Each product controls one or a few species; other pests still need management.
- Disruption and mass trapping need large areas and low starting populations.
- Lures and dispensers cost money and must be replaced as they are used up.
- Results take a generation to show, which demands patience and monitoring.
Key points
- Pheromones are chemical signals within a species; with allelochemicals (kairomones, allomones) they make up semiochemicals.
- The first insect pheromone, bombykol, was identified in 1959.
- Sex pheromones (mostly moths) and aggregation pheromones (mostly beetles) are the main ones used in agriculture.
- Uses: monitoring and timing (with degree-day models), mass trapping, mating disruption, attract-and-kill and lure-and-infect.
- Mating disruption works best area-wide, at low to moderate pest densities, with monitoring.
- Pheromones are specific, non-toxic and compatible with natural enemies, but control only target species.
What the exams ask
- School-leaving and entrance papers: what pheromones are, examples of sex pheromone traps, and their use in pest monitoring.
- Degree and research-fellowship entrances: types of semiochemicals, bombykol, the mechanisms and conditions of mating disruption, mass trapping, attract-and-kill, and degree-day timing.
- Plant protection examinations: designing a monitoring programme, interpreting trap catches, area-wide pheromone programmes and their integration with the sterile insect technique and biological control.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Do pheromone traps control the pest?
Monitoring traps mostly do not; there are too few of them. They tell you when to act. Control comes from mass trapping, mating disruption or attract-and-kill, which use many more lures or dispensers.
Why did my traps catch nothing in a field with damage?
Possible reasons: the lure was for a different species or was old, the trap was placed badly, the damage came from a different pest, or the field was under mating disruption, which suppresses catches.
Are pheromones safe for people and bees?
At the doses used, they have very low toxicity and act only on the target species' behaviour. They are among the safest crop protection tools available.
Sources
- Karlson, P. and Lüscher, M. "'Pheromones': a new term for a class of biologically active substances." Nature 183, 55–56 (1959).
- Witzgall, P., Kirsch, P. and Cork, A. "Sex pheromones and their impact on pest management." Journal of Chemical Ecology 36, 80–100 (2010).
- Cardé, R. T. and Minks, A. K. "Control of moth pests by mating disruption: successes and constraints." Annual Review of Entomology 40, 559–585 (1995).
- El-Sayed, A. M. The Pherobase: Database of Pheromones and Semiochemicals. Online.
- Dyck, V. A., Hendrichs, J. and Robinson, A. S. (eds) Sterile Insect Technique: Principles and Practice in Area-Wide Integrated Pest Management, 2nd edition. CRC Press, 2021.
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