3.2 Dry Formulations: Dusts, Granules, Pellets & Wettable Powders

3.2 Dry Formulations: Dusts, Granules, Pellets & Wettable Powders

Core Principle: Dry formulations fall into two distinct operational categories: dry concentrates engineered to be diluted in water and sprayed through hydraulic nozzles (such as wettable powders and water-dispersible granules), and ready-to-use dry formulations applied directly to target sites without liquid carriers (such as granules, pellets, dusts, and baits). Understanding their physical mechanics prevents fatal wildlife poisonings, nozzle scour, and applicator inhalation injuries.

Dry pesticide formulations offer chemical stability, extended shelf life, and resistance to sub-freezing winter storage temperatures that would freeze and crack liquid pesticide jugs. However, dry products introduce unique operational challenges—ranging from severe inhalation hazards during tank loading to the physical abrasion of sprayer components and selective poisoning of non-target wildlife.

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Wettable Powders (WP or W)

A Wettable Powder (WP) is a dry, finely ground formulation consisting of an insoluble active ingredient (typically 25% to 80% by weight) blended with an inert mineral carrier (such as talc, kaolin clay, or bentonite) and synthetic wetting and dispersing agents.

Physical Behavior in the Spray Tank

Because the active ingredient and the mineral carrier are insoluble in water, a wettable powder does not dissolve. Instead, the wetting agents allow water to penetrate and surround each dry particle, enabling the powder to disperse throughout the water carrier. This produces a liquid-solid mechanical suspension that resembles cloudy, muddy water.

Wettable Powder (WP) + Water Carrier
                 │
                 ▼ Wetting & Dispersing Agents Activate
Mechanical Suspension Formed (Insoluble Solid Particles Suspended in Water)
                 │
      ┌──────────┴──────────┐
      ▼                     ▼
Continuous Agitation    Agitation Interrupted
(Particles Suspended)   (Particles Settle into Dense Sludge at Tank Bottom)

Operational Demands & Maintenance Requirements

1. Continuous, Vigorous Agitation: Suspended WP particles have a higher specific gravity than water. If spray tank agitation stops—even for a 15-minute transit or lunch break—the mineral particles rapidly settle by gravity, consolidating into a dense, caked layer across the tank floor. This sludge blocks suction lines, chokes strainer screens, and causes uneven chemical concentration. Applicators must maintain continuous mechanical or hydraulic jet agitation from the moment of mixing until the tank is completely empty.
2. Severe Equipment Abrasiveness: The mineral clay particles in wettable powders act as an abrasive slurry under pressure. As the spray mixture flows through the system, it rapidly scours and enlarges nozzle orifices. Soft metal nozzles—particularly brass and aluminum—wear out quickly, causing increased flow rates (gallons per acre), distorted spray patterns, and costly over-application. Applicators applying WP formulations must equip sprayers with wear-resistant nozzles fabricated from hardened stainless steel or ceramic, and perform frequent nozzle flow-rate calibrations.
3. Screen Clogging: WP particles will not pass through extremely fine filter meshes. Nozzle screens must be sized appropriately (typically 50-mesh or coarser) to prevent immediate clogging.

Applicator Inhalation Hazard

Wettable powders present one of the highest acute inhalation hazards among all pesticide formulations. When opening bags, scooping powder, and pouring the product into the spray tank hatch, fine chemical dust clouds billow into the air. Inhaling these airborne particles delivers toxic active ingredient directly across sensitive pulmonary membranes into the bloodstream. Applicators mixing and loading wettable powders must wear a NIOSH-approved particulate respirator (such as an N95, R95, or P100 particulate respirator), chemical-resistant gloves, and protective eyewear or a face shield.

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Water-Dispersible Granules (WDG) / Dry Flowables (DF)

Water-Dispersible Granules (WDG), also termed Dry Flowables (DF), represent a modern engineering innovation designed to preserve the strengths of wettable powders while eliminating their severe inhalation dust hazards.

Formulation Chemistry & Handling

To manufacture a WDG or DF, the manufacturer blends the active ingredient, mineral clay carrier, and dispersing agents, but agglomerates the fine powder into uniform, micro-sized granules or beads (resembling dry instant yeast or granulated sugar). When poured into a spray tank filled with water, the granules dissolve their outer water-soluble binders within seconds, breaking apart into microscopic particles that form a suspension identical to a wettable powder.

Operational Advantages Over WP

• Drastic Reduction in Inhalation Hazard: Because the product is granulated, it pours cleanly without generating billowing airborne dust clouds during measuring and tank loading.
• Easy Measurement: Granules pour freely from containers and can be measured rapidly using graduated volumetric measuring cylinders or digital scales.
• Cleaner Packaging: Very little chemical dust adheres to the interior walls of the packaging, simplifying empty bag disposal.
• Operational Parity: Once dispersed in the tank water, WDGs require the same continuous agitation and wear-resistant nozzles as wettable powders.

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Soluble Powders (SP or WSP)

A Soluble Powder (SP) is a dry, finely ground chemical formulation consisting of an active ingredient (often 50% to 90% by weight) that is completely soluble in water.

Physical Behavior & Operational Advantages

Unlike wettable powders, when a soluble powder is added to the spray tank, it dissolves completely at the molecular level, forming a true, transparent solution.

• No Continuous Agitation Required: Once the powder is thoroughly dissolved in the water carrier, it remains in solution permanently and will never settle out or stratify.
• Completely Non-Abrasive: Because there are no suspended mineral particles, SP solutions cause no abrasive wear on pump impellers, pressure regulators, or nozzle tips.
• No Screen Clogging: Does not clog fine line strainers or nozzle screens.

Water-Soluble Packaging (WSP)

To protect applicators from dust exposure, both soluble powders and wettable powders are frequently packaged in Water-Soluble Packets (WSP). These pre-measured packets are fabricated from a clear polyvinyl alcohol (PVA) film. The applicator drops the entire sealed packet directly into the water-filled spray tank without opening it. Within minutes, the PVA film dissolves, releasing the chemical into the tank. This eliminates all direct applicator contact and inhalation exposure during loading. Applicators must handle WSP packets with completely dry gloves and store them in moisture-proof outer containers.

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Granules (G) and Pellets (P)

Granules (G) and Pellets (P) are ready-to-use dry formulations applied directly to target sites without water carriers. They are formulated by impregnating, binding, or coating a low concentration of active ingredient (typically 1% to 15%) onto coarse, solid carrier particles—such as clay, crushed corn cobs, walnut shells, or vermiculite. Pellets are extruded into uniform cylindrical shapes and weights, whereas granules have irregular granular shapes.

Application Methods & Practical Strengths

Granules and pellets are applied using rotary (centrifugal) spreaders, drop spreaders, or specialized aerial application equipment. They offer distinct advantages:

• Minimal Drift Hazard: Because the individual particles are relatively large and dense, they fall rapidly by gravity, presenting virtually zero airborne drift under light wind conditions.
• Dense Canopy Penetration: Granules pass cleanly through dense turf thatch, groundcovers, or agricultural crop canopies to reach the soil surface or root zone.
• Simple Ready-to-Use Delivery: Eliminates the need for water hauling, tank mixing, agitation, and high-pressure pumps.

Severe Avian Toxicity & Environmental Risks

Despite their low drift risk, granular formulations present one of the most acute, catastrophic hazards in pesticide management: severe poisoning of birds and waterfowl.

Pesticide Granule (Size, Shape, Texture)  ◄─── Closely Resembles ───►  Grit / Weed Seeds
                               │
                               ▼ Ingested by Bird
        Muscular Gizzard Grinds Granule to Digest Food
                               │
                               ▼ Highly Concentrated Toxin Absorbed
        Rapid Systemic Poisoning / Fatal Songbird Mortality

Birds possess no teeth and must grind tough seeds and food in their muscular gizzards. To accomplish this, birds instinctively seek out and ingest small pebbles, hard sand grains, and seeds—a biological necessity termed digestive grit. Pesticide granules match the exact physical dimensions, hardness, and appearance of natural grit and weed seeds.

When birds forage across turfgrass or field borders treated with granular insecticides (especially organophosphates or carbamates), they consume the chemical granules. In the gizzard, the abrasive action breaks down the carrier, rapidly releasing a lethal dose of active ingredient into the bird's bloodstream. Ingestion of a single organophosphate granule can kill an adult songbird.

Mandatory Granular Stewardship Rules

1. Soil Incorporation and Watering-In: Applicators must incorporate granules into the soil mechanically or apply sufficient irrigation/rainfall immediately following application as specified by the product label.
2. Impervious Surface Cleanup: Granules landing on sidewalks, driveways, patios, or road shoulders must be immediately swept or blown back onto the target turf to prevent bird foraging and storm sewer contamination.

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Dusts (D)

A Dust (D) is a ready-to-use dry formulation containing a very low concentration of active ingredient (typically 0.5% to 10%) blended with an ultra-lightweight, finely ground inert mineral diluent such as talc, chalk, or pyrophyllite.

High Drift Risk & Restricted Deployment

Because dust particles are microscopic and exceptionally lightweight, dusts exhibit the highest drift hazard of any pesticide formulation. Applied outdoors, even a gentle 2 mph breeze can transport dust plumes hundreds of yards off-target. Consequently, outdoor broadcast dust applications are largely obsolete and strictly restricted.

Specialized Indoor & Structural Applications

Dusts remain highly valuable in structural and perimeter pest management when applied using specialized bulb dusters, bellows dusters, or hand dusters. Applicators inject dusts directly into enclosed, undisturbed harborage zones—including:

• Hollow wall voids and plumbing chases
• Electrical conduit junction boxes (where liquid sprays would cause electrical shorts)
• Attics, crawlspaces, and narrow structural cracks

In these dry, dark voids, dust formulations do not degrade rapidly and provide long-lasting residual control of crawling pests like cockroaches, termites, ants, and bed bugs.

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Baits (B)

A Bait (B) is an edible mixture combining a low concentration of active ingredient (typically 0.1% to 5%) with an attractive food carrier (such as grains, sugars, vegetable oils, proteins, or peanut butter) and attractant pheromones.

Targeted Delivery & Non-Target Hazards

Baits are designed to be ingested directly by the target pest or carried back to social nests (e.g., ants, yellowjackets, termites). While highly targeted, baits carry significant risks:

• Attractiveness to Non-Target Animals: Children, domestic pets (dogs and cats), livestock, and non-target wildlife are naturally attracted to the edible food base.
• Secondary Poisoning: Anticoagulant rodenticide baits pose severe secondary poisoning hazards to predatory raptors (owls, hawks, eagles) and mammalian carnivores (foxes, bobcats, coyotes) that hunt and consume poisoned rodents. Applicators must secure rodenticide baits inside EPA-approved, tamper-resistant bait stations.

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Other Formulations: Fumigants, Impregnates, Animal Systemics & Fertilizer Combinations

The core manual groups several products that are neither ordinary liquids nor ordinary dry formulations:

• Fumigants deliver the active ingredient as a gas. Some are liquids under pressure that become gases when released, some are volatile liquids, and some are solids that release gas in humid conditions or on contact with water or water vapor. They are used in structures, food and grain storage, soil, greenhouses, and regulatory quarantine treatments.
  • Advantages: toxic to a wide range of pests; penetrate cracks, crevices, wood, soil, and packed commodities; a single treatment usually kills most pests in the treated space.
  • Disadvantages: the site must be enclosed or covered to hold the gas, and most fumigants require specialized PPE, often supplied-air respiratory protection. Connecticut certifies fumigation separately: categories 7Ci (structural), 7Cii (soil), and 7Ciii (pipe) for supervisors, and fumigation endorsements for private applicators.
• Impregnates incorporate the active ingredient into a solid, usually plastic, that releases it over time — livestock ear tags, pest strips, adhesive tapes, and pet collars. Fertilizers may also be impregnated with pesticides.
• Animal systemics are absorbed into and move within the treated animal. They are applied externally as pour-ons, sprays, or dusts, or orally as feed additives, capsules, pastes, or liquids, and they control fleas, other blood-feeding insects, and internal parasites.
• Pesticide-fertilizer combinations (usually granules or pellets) control pests and supply nutrients in one pass, as in "weed-and-feed" lawn products. Connecticut's school, day care, and playground statutes still treat these as lawn care pesticides when the pesticide component is labeled for lawn use (Section 1.6).

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Dry Formulations Summary Matrix