How Pesticides Mix with Water: Role of Adjuvants and Synergists

Nov 19

Adjuvants: What They Are and How They Help

Definition: Adjuvants are non-active ingredients added either by the pesticide manufacturer or in the spray tank to improve the performance of the pesticide. Purdue Extension+2National Pesticide Information Center+2
Types & Functions:
- Wetting agents / spreaders: lower the surface tension of the spray, allowing droplets to spread out more, which improves coverage. Purdue Extension+1
- Emulsifiers: allow oil-based pesticide formulations (like EC = emulsifiable concentrate) to mix properly with water. Natural Resources Conservation Service
- Stickers / deposit builders: help the active ingredient adhere better to plant surfaces or structures, reducing wash-off and increasing persistence. Purdue Extension+1
- Penetrants: increase the ability of the pesticide to penetrate into waxy or tough surfaces. Purdue Extension
- Buffers: stabilize the pH of the spray solution, which can affect how soluble and stable the active ingredient is in water. Natural Resources Conservation Service
- Thickeners / drift control: increase droplet size, reduce drift, or reduce foaming in the tank. Purdue Extension
Why they’re important: Without proper adjuvants, some active ingredients may not mix well in water, may not stick to the target surface, or may drift off-target, reducing effectiveness. Purdue Agriculture+1

Definition: Synergists are chemicals that don’t kill pests on their own, but when combined with a pesticide, they increase its lethality. Purdue Extension
How they work:
- A common mode is inhibiting insect detoxification enzymes, especially cytochrome P450s. For example, piperonyl butoxide (PBO) is widely used with pyrethrins or pyrethroids; it slows down the insect’s ability to break down the insecticide, making the treatment more potent. National Academies Press
Why they matter: By using a synergist, you can often use less of the active ingredient while achieving better control because more of it stays effective in the insect’s system. Purdue Extension

The half-life of a pesticide is the time required for half of the applied dose to degrade or dissipate in a particular environment. National Pesticide Information Center+1
Degradation depends strongly on the environment (soil, water, plant surfaces, indoor surfaces), so a single pesticide may have multiple half-lives under different conditions. National Pesticide Information Center

Chemical degradation: Hydrolysis, photolysis (breakdown by light), and pH-dependent reactions. Adjuvants that buffer pH can affect how quickly these reactions happen. Purdue Agriculture
Biological degradation: Microbial action in soil or on surfaces; microbes can ingest or break down the molecules over time. National Pesticide Information Center
Physical processes: Volatilization (evaporation), wash-off, and binding to surfaces (adsorption) all contribute.

Adsorption: Many pesticides, especially hydrophobic ones (those that don’t mix well with water), bind strongly to surface materials. This depends on:
- The chemical’s polarity: non-polar (hydrophobic) molecules will more readily adsorb to waxy, organic or earthy surfaces.
- The surface’s chemistry: rough or porous surfaces (like soil, wood, fabric) provide more binding sites.
Use of stickers / spreaders: As noted above, adjuvants like stickers increase how well the pesticide lingers on a surface, by forming a semi-persistent film. Purdue Extension
pH effects: pH also influences how well some pesticides bind. For example, if a pesticide is charged, it may bind more strongly to surfaces of opposite charge. Purdue Agriculture

Here are some examples, based on typical environmental behavior:

Bifenthrin (pyrethroid): Very hydrophobic, very low water solubility, and can have a very long half-life in soil (from days up to months) depending on soil type. Wikipedia
DDT (organochlorine, though largely banned): Extremely persistent; its half-life in some soils can be decades; its breakdown products also persist. Wikipedia

Permethrin (pyrethroid):
- In soil: ~40 days (can be as low as 11 days to as high as 113 days depending on conditions). National Pesticide Information Center
- In water: half-life of ~19–27 hours in the water column; but if it binds to sediment, it may last much longer. National Pesticide Information Center
Dimethoate (organophosphate): Fairly water-soluble, degrades relatively rapidly depending on pH and microbial activity — half-life in some conditions ranges from 2.5 to 31 days in soil. Wikipedia

Long persistence = extended control, but also higher environmental risk: Chemicals like bifenthrin or DDT (if still used) can provide long-term residual effects, but they also pose more risk of buildup, runoff, or non-target exposure.
Short persistence = safer but may require reapplication: Pesticides with short half-lives (like permethrin in water) degrade quickly, which reduces long-term residue risk but may require more frequent re-treatment to maintain pest control.

Adjuvants improve how well pesticides mix with water, cover surfaces, penetrate targets, and stick to surfaces — but they don’t kill pests themselves.
Synergists boost the potency of the pesticide by interfering with the pest’s ability to detoxify the chemical.
Half-life is a critical measure of how long a pesticide remains active or present; it varies dramatically depending on environment (soil, water, leaves, indoor surfaces).
Persistence and adhesion depend on chemical properties (hydrophobic vs hydrophilic), the surface, and how the pesticide is formulated.
Which pesticide lasts longest vs shortest depends on chemistry: pyrethroids like bifenthrin are very persistent; other classes like some organophosphates break down much faster.