Greenhouse operations applying contact-only acaricides under relative humidity above 85% operate in a deposition regime where the water carrier does not dry at the same rate as in field spray programmes. Contact miticides such as bifenazate and fenpyroximate act by direct contact with motile stages; they do not translocate acropetally or basipetally in the vascular system, and therefore the deposit must remain on the abaxial leaf surface where Tetranychus urticae feeds. Relative humidity must be measured in the crop zone, not at the climate computer return air point, because dense canopies can maintain a leaf boundary layer near saturation even when the central sensor reads 78%. Ventilated capacitive sensors calibrated at two high-humidity points by ASTM E104-02 saturated salt solutions reduce measurement error to ±2% between 80% and 95%; unventilated sensors at low air speed frequently over-read by 5–10 percentage points during condensation. A dielectric leaf wetness sensor placed in the lower canopy gives a direct record of liquid film duration, which is more relevant to residue retention than ambient humidity alone.
The thermodynamic driving force for droplet evaporation is vapour pressure deficit. At 20 °C, saturation vapour pressure is 2.34 kPa; at 85% relative humidity the air contains 1.99 kPa of water vapour, leaving a VPD of only 0.35 kPa. By contrast, at 60% relative humidity at the same temperature the VPD is 0.94 kPa. A water droplet of 120 µm initial diameter released from a nozzle 0.5 m above the canopy therefore undergoes negligible diameter reduction at 0.35 kPa VPD before impact. The droplet arrives almost fully hydrated and coalesces with pre-existing condensation on the leaf surface. This mechanistic point explains why spray quality selection that is acceptable in a dry greenhouse is not valid when leaf wetness is continuous, and why contact miticide failures at high humidity are frequently coverage failures rather than biological resistance.
Field and protected-crop deposition work distinguishes between initial spray retention and residual deposit after drying. At high relative humidity the initial retention can appear visually acceptable on water-sensitive paper while the residual deposit later migrates or is removed by condensation drip. Nozzle selection must therefore prioritize droplet size and carrier volume that reduce runoff rather than those that maximize visible wetting. Standard flat fan nozzles at 3 bar produce a medium spray with Dv0.5 of 236–340 µm under ASABE S572.1; medium droplets remain on a dry glasshouse tomato leaf well, but on a pre-wet leaf they tend to coalesce into a continuous film that drains to leaf margins. Air-induction nozzles generating very coarse droplets at 404–502 µm resist evaporation and can reduce drift, but they may bounce from younger cucumber leaves with high contact angle. A practical compromise is a coarse air-inclusion nozzle operated at 4–6 bar with an air-assisted boom. Carrier volume for contact acaricides in a high-wire tomato canopy should be lowered when condensation is present because the leaf’s maximum retained volume is already reduced; published data for this exact configuration is limited, so water-sensitive paper should be inspected for run lines before adding more water.
| Spray class | Approximate Dv0.5 | High-RH retention behaviour | Application consequence for contact acaricides |
|---|---|---|---|
| Fine | 145–235 µm | Droplets remain wet, coalesce on pre-wet leaves, and form runoff films | Not suitable; drift and lower-canopy coverage failure occur under condensation |
| Medium | 236–340 µm | Coalesces on existing leaf wetness; rapid runoff from tomato leaf margins | Adequate only with air-assist and dry leaf surfaces; not preferred above 85% |
| Coarse | 341–403 µm | Reduced drift; moderate splash on wet surfaces; better individual droplet persistence | Acceptable with 4–6 bar air-inclusion nozzle and lower carrier volume |
| Very Coarse | 404–502 µm | Resists evaporation; may bounce or roll from waxy cucumber leaves | Use in dense lower canopies with air velocity 15–25 m/s at the leaf surface |
Application timing relative to condensation is more important than nozzle change alone. The safest window in a Venlo glasshouse is after the climate computer reduces the vent position and after horizontal circulation fans have broken the leaf boundary layer for at least 20–30 min; this usually occurs mid-morning when the VPD rises above 0.50 kPa. Do not begin a miticide application while condensation drops are visible on fruit trusses or when the lower canopy leaf wetness sensor reads above 30% full scale. If night condensation is severe, overhead gutters should be inspected; anti-drip films and properly sloped gutters reduce condensation drip but do not eliminate it. Applying a contact miticide at 1–2 h before sunset is frequently discouraged because the deposit may not dry before condensation forms, extending solvent contact and allowing redistribution into the leaf margin.
Contact miticide deposits are not necessarily removed as intact particles during condensation; the dried formulation matrix rewets, the water-soluble dispersants and surfactants dissolve, and the hydrophobic active ingredient may be carried with the liquid front toward the leaf edge. The result is a variable exposure: some zones retain more than the intended dose, while the abaxial lower leaf surface that held the original spray deposit can become depleted. In a high-wire tomato crop with leaf area index above 3, the upper canopy intercepts most condensation drip and can protect the lower leaf surface from direct water flow; but the lower canopy remains humid and wet, and any water moving down the stem can still remobilize deposits on the petiole and leaf base where mite colonies are dense. Water-sensitive papers placed on abaxial leaf surfaces before application should be retrieved after the first condensation cycle and inspected for percent coverage, droplet number per cm², and run-off streaks. Where coverage falls below 50% of the original reading, the subsequent reapplication interval should be shortened only after validating mite counts by leaf brushing; otherwise, repeated sublethal exposure can select for resistant mite populations within a few generations.
Formulation type interacts with condensation. Emulsifiable concentrate formulations containing aromatic hydrocarbon solvents can produce phytotoxicity when high humidity prolongs the drying period, because the solvent remains in contact with the leaf cuticle for longer than under a 50% relative humidity condition. Water-based suspension concentrates and water-dispersible granules are less likely to produce this solvent burn, but they still contain dispersants that rewet and can move active ingredient during condensation. A contact acaricide with low water solubility and log Kow above 4 will generally remain associated with the cuticle wax; the water-soluble coformulants migrate, leaving the active ingredient as a non-uniform deposit. Without a wetting agent, the remaining deposit may be less available to the mite tarsi; adding a nonionic wetter at label rates can improve initial coverage, but at high humidity it also lowers surface tension and increases runoff from hydrophobic leaf surfaces. Therefore the label restriction for surfactant addition should be followed exactly, and a jar test or spray tank wetting test should be performed before large-scale use under condensation-prone conditions.
The leaf surface of greenhouse tomato and cucumber responds to high humidity through cuticular hydration, which changes the apparent contact angle of water droplets. Tomato leaves with abundant trichomes retain spray by impalement and surface tension, but the abaxial surface has a less dense cuticle and is the preferred habitat of two-spotted spider mite; cucumber leaves are more hydrophilic but their margins are prone to runoff. When the cuticle swells at high relative humidity, the contact angle of a water droplet on a waxy adaxial surface can decrease, but this does not automatically improve residual retention. If the leaf already carries a liquid film from condensation, the applied droplets merge with that film and move according to gravity and capillary forces. For contact acaricides, this means that the lower leaf surface cannot be compensated by translaminar movement; the pesticide must be delivered to the abaxial side by air turbulence and boom angle. Spray equipment should be configured so that a vertical boom places nozzles under the canopy at an angle of 30–45° from horizontal to expose the abaxial leaf side; air velocity at the leaf surface should be 15–25 m/s to induce leaf flutter and turnover of the boundary layer. A stagnant lower canopy with air velocity below 0.3 m/s will retain a high-humidity boundary layer and effectively trap condensation on mite-infested leaves, while also preventing the contact deposit from reaching the sites where adult females feed and oviposit.
Some operations use a night temperature pulse of 40–70 °C in the heating pipes or an air heater to lower the greenhouse relative humidity below 85% before midnight. This tactic can increase the air VPD and reduce condensation on glass surfaces, but it does not necessarily dry the lower leaf canopy. Warm air rises from heating pipes placed below the crop, moving through the canopy and increasing leaf temperature; if leaf temperature remains equal to or lower than the dewpoint of the surrounding air for part of the night, water still condenses. Horizontal circulation fans must maintain air speed of 0.3–0.5 m/s through the crop zone to replace the saturated boundary layer. Tracer smoke tests and hand-held hot-wire anemometers can identify stagnant zones behind columns and under gable ends where air exchange time exceeds 10 min. If a night heating period is used to dry the crop, the growers must confirm that the lower leaf surfaces actually dry before a contact miticide is applied the following morning; otherwise the application coincides with residual leaf wetness even though the roof glass is clear. The physical measurement is leaf wetness duration, not ambient humidity, because the two can diverge in a heated, poorly mixed greenhouse.
Operational decisions under RH above 85% are best made with a small set of in-house validation measurements. The following matrix does not replace label requirements but provides threshold values that distinguish a deposition-limited application from a rainfall-like wash-off environment.
| Measurement | Threshold at RH above 85% | Instrument or method | Corrective action |
|---|---|---|---|
| Ambient relative humidity | >85% | Ventilated capacitive sensor or aspirated psychrometer calibrated per ASTM E104-02 | Delay contact acaricide release until VPD exceeds 0.35 kPa or ventilation lowers canopy RH |
| Leaf wetness duration | >30 min continuous | Dielectric leaf wetness sensor in lower canopy | Run horizontal fans and delay spraying until reading falls below 30% full scale |
| Spray Dv0.5 | <300 µm at 3 bar | Laser diffraction or ASABE S572.1 reference classification | Install coarse air-inclusion nozzle and adjust pressure to 4–6 bar |
| Water-sensitive paper coverage | >60% wetted with run lines | Image analysis of collectors placed in upper and lower canopy per ISO 22522:2007 | Reduce carrier volume, change boom angle, or increase air-assist until run-off streaks disappear |
| Deposit recovery after first condensation | <50% of original visual coverage | Water-sensitive paper or fluorescent tracer on abaxial leaf discs | Shorten reapplication interval only after leaf-brushing mite counts confirm survival; otherwise correct placement first |
Resistance management must account for compromised application quality. Contact-only acaricides are most often degraded by coverage failure before metabolic resistance is confirmed in a population. If a grower observes mite survival in patches after an application at RH above 85%, the first diagnostic step is to quantify coverage on water-sensitive paper and leaf wash-off, not to switch active ingredient. When coverage is confirmed adequate but mites survive, an IRAC mode-of-action rotation is indicated. Repeated contact miticide applications under condensation conditions without deposition verification create sublethal exposure gradients that accelerate selection of polygenic resistance. Therefore the application record should include: RH and VPD at start and end, leaf wetness sensor state, nozzle type and pressure, carrier volume, boom speed, air-assist setting, water-sensitive paper readings from upper and lower canopy, and the reapplication interval. This record allows a production-scale greenhouse to distinguish environmental deposition failure from biological resistance, avoiding an unnecessary active ingredient change and preserving the limited chemical rotation for twospotted spider mite control.
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