Semiochemicals in Hydroponics
From Research to Practice
This article is the third part of a series on integrated pest management in hydroponic systems.
1. Fundamentals of Chemical Ecology
Definition of Semiochemicals
Semiochemicals are messenger substances that serve for information transfer between organisms. They include both intraspecific (within a species) and interspecific (between species) chemical signals (Nordlund & Lewis, 1976).
Classification of Semiochemicals
Intraspecific Signals (Pheromones)
- Sexual Pheromones: Mate finding and recognition
- Aggregation Pheromones: Accumulation of individuals
- Alarm Pheromones: Warning of dangers
- Trail Pheromones: Marking of paths
Interspecific Signals
- Allomones: Advantage for the emitter
- Kairomones: Advantage for the recipient
- Synomones: Advantage for both species
Physico-chemical Properties
| Property | Significance | Practical Consequence |
|---|---|---|
| Volatility | Range of signal effect | Dispenser design, application interval |
| Stability | Durability under environmental conditions | Storage, formulation |
| Specificity | Species-specificity of effect | Selectivity of control |
| Threshold | Minimum effective concentration | Dosage optimization |
2. Pheromones: Species-Specific Communication
Sexual Pheromones in Pest Control
| Target Organism | Pheromone Components | Formulation | Application Rate | Duration of Action |
|---|---|---|---|---|
| Tomato Leafminer (Liriomyza bryoniae) |
(Z)-9-Tricosene + Acetate ester |
Microencapsulated 200-400 μg/dispenser |
50 dispensers/ha or 1/10m² |
4-6 weeks |
| Whitefly (Trialeurodes vaporariorum) |
Neutralite® (Propyl-(E)-3,7,11-trimethyl-2,4-dodecadienoate) |
Septa dispenser 10 mg/dispenser |
500-1000/ha or 1-2/100m² |
8-12 weeks |
| Thrips (Frankliniella occidentalis) |
n-(R)-Lavandulyl acetate + Isomer |
Gel formulation in trap systems |
1 mg/day release rate | 6-8 weeks |
| Spider Mites (Tetranychus urticae) |
(E)-β-Ocimene + (E)-β-Farnesene |
Emulsion for foliar application |
0.1-0.5 g/ha/day | 2-3 weeks |
Mating Disruption Technique
Principle: High pheromone concentrations in the air disorient male insects, preventing them from finding females.
Application Requirements:
- Enclosed spaces (greenhouses)
- Low initial population
- Species-specific pheromone formulation
- Continuous application
Mass Trapping
Principle: Pheromone-based traps attract and eliminate pests.
Effectiveness Factors:
- Trap density (optimized per pest)
- Placement (height, exposure)
- Combination with visual stimuli
- Regular maintenance
3. Kairomones: Interspecific Signals
Plant-based Kairomones
Green Leaf Volatiles (GLVs)
- (Z)-3-Hexenol: Freshly damaged plant tissue
- (Z)-3-Hexenyl acetate: Herbivore attraction
- Hexanal: General stress indicator
Application: Artificial application to divert from main crops
Terpenoids and other Secondary Metabolites
- β-Caryophyllene: Root signal during herbivore infestation
- Methyl Salicylate: Systemically acquired resistance
- Jasmonates: Direct defense induction
Application: Push component in combination with attractants
Kairomones for Beneficial Insect Promotion
| Beneficial Insect | Target Pest | Effective Kairomones | Application Method | Increase in Efficiency |
|---|---|---|---|---|
| Phytoseiulus persimilis (Predatory mite) |
Spider mites (Tetranychus spp.) |
Spider mite pheromones + Plant scents |
Dispenser near infestation sites |
40-60% higher predation rate |
| Encarsia formosa (Parasitoid wasp) |
Whitefly (Trialeurodes) |
Whitefly pheromones + Honeydew scent |
Foliar application as a formulation |
35-50% more parasitism |
| Amblyseius swirskii (Predatory mite) |
Thrips, Whitefly | Thrips alarm pheromones + Plant stress scent |
Slow-release formulations |
50-70% better establishment |
4. Application Techniques and Formulations
Microencapsulation
Principle: Encapsulation of active ingredients with polymeric materials for controlled release.
Advantages for Hydroponics:
- Protected active ingredients from degradation
- Controlled release kinetics
- Reduced application frequency
- Better compatibility with nutrient solutions
Typical Carrier Materials: Polyurethanes, Chitosan, Alginates
Dispenser Systems
Passive Dispensers
- Septa Dispensers: Rubber or polymer matrices
- Membrane Dispensers: Controlled diffusion
- Matrix Systems: Porous carrier materials
Active Dispensers
- Electronically Controlled: Precise timing
- Environmentally Responsive: Temperature/humidity controlled
- PWM Systems: Pulse width modulation
Liquid Formulations
Emulsions and Microemulsions
- Oil-in-water Emulsions: For foliar application
- Microemulsions: Increased stability
- Nanoemulsions: Improved penetration
Hydrogels and Gels
- Temperature-sensitive Gels: Sol-gel transitions
- pH-responsive Systems: Controlled release
- Biodegradable Gels: Sustainable formulations
Dosage Calculation for Closed Systems
Space Volume-based Calculation
Formula: V = L × W × H (Room volume in m³)
Example: Greenhouse 10m × 5m × 3m = 150m³
Pheromone Requirement: 150m³ × 0.1g/m³ = 15g active ingredient
Plant Count-based Calculation
Formula: n = Number of Plants × Application Rate/Plant
Example: 100 Tomato plants × 0.05g/plant = 5g
Correction Factor: × 1.2 for air circulation = 6g total
5. Practical Implementation in Hydroponic Systems
Airspace Management
In NFT systems with vertical plant growth, the vertical distribution of semiochemicals is critical.
Optimal Dispenser Placement
- Lower Level: 30-50cm above ground
- Middle Level: Plant center
- Upper Level: 20-30cm below ceiling
- Horizontal Distance: 2-3m between dispensers
Combination with Climate Control
- Fan use for distribution
- Temperature control (15-25°C optimal)
- Relative humidity 60-80%
- Avoidance of dead zones
Optimize Water-Air Transfer
The higher humidity in DFT systems affects the evaporation and distribution of semiochemicals.
| Parameter | Influence on Semiochemicals | Adjustment Measure |
|---|---|---|
| Humidity >80% | Reduced evaporation | Set higher release rate |
| Water Temperature | Affects air convection | Optimize cooling/heating |
| Ventilation Intensity | Distribution speed | Increase circulation |
6. Case Studies and Success Measurement
Case Study: Thrips Control in NFT Tomatoes
Initial Situation
- Crop: Tomatoes (Solanum lycopersicum)
- System: NFT, 200m² greenhouse
- Problem: Frankliniella occidentalis
- Infestation Level: 15-20 Thrips/yellow sticky trap/day
Implemented Measures
- Push: Methyl jasmonate foliar application
- Pull: Thrips pheromone + blue sticky traps
- Dispensers: 25 pieces, evenly distributed
- Application: Continuous over 8 weeks
Results after 8 Weeks
- Infestation Reduction: 87%
- Damage Index: From 3.2 to 0.4
- Yield Increase: 22%
- Cost-Benefit: 1:4.3
References
- Nordlund, D. A., & Lewis, W. J. (1976). Terminology of chemical releasing stimuli in intraspecific and interspecific interactions. Journal of Chemical Ecology, 2(2), 211-220.
- Pickett, J. A., et al. (2014). Aspects of insect chemical ecology: exploitation of reception and detection. Trends in Plant Science, 19(5), 272-281.
- Bruce, T. J., & Pickett, J. A. (2011). Perception of plant volatile blends by herbivorous insects–finding the right mix. Phytochemistry, 72(13), 1605-1611.
- Kaplan, I. (2012). Trophic complexity and the adaptive value of damage-induced plant volatiles. PLoS Biology, 10(10), e1001437.
- Dicke, M., & Baldwin, I. T. (2010). The evolutionary context for herbivore-induced plant volatiles: beyond the 'cry for help'. Trends in Plant Science, 15(3), 167-175.
Next Article in the Series: Biological Control in Hydroponic Systems: Beneficial Insect Application and Management
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