4.4 Pesticide properties and modes of dose transfer
There are hundreds of pesticides that work in various ways, and the different types of control action affect the amount, efficiency, speed and mode of dose transfer to the target pest.
Farmers (and researchers) may not always appreciate that, except in certain circumstances, direct contact with spray is a relatively unimportant dose transfer mechanism.
Many insecticides rely on pests picking up a lethal dose after crawling over deposits (secondary contact) or by ingestion. Fungicides such as copper, which only have protectant action, must similarly be well distributed on the surface of the plant, in order to prevent infection by fungal diseases.
In practice, contact insecticides and protectant fungicides must be applied with a good coverage of spray droplets in order to make contact with the target (although copper deposits may redistribute over the surface of the plant by rainwater). Fumigant action is especially important for control of storage pests.
Certain older insecticides (e.g. lindane, endosulfan: see Insecticides below) were especially effective, since fumigant action often helped to compensate for inadequate application in the field (difficult at the best of times with cocoa). Repellency may not always be beneficial - especially if deposits are short lived or if pests consequently pick up sub-lethal doses.
However, the concept of lure and kill (where an insecticide is mixed with an attractant) has been used very successfully for control of pests such as fruit-flies.
Ingestion of insecticides may occur via various routes: either from a residual deposit (as illustrated) or by translocation - where pesticides have an ability to be absorbed into the plant and are redistributed, including to the site of attack.
Depending on their physical-chemical properties (section 4.4.1), some pesticides may be translaminar (travelling short distances through the surface of leaves into the tissues) or systemic (where the insecticide, fungicide or herbicide is translocated over greater distances).
Systemic action is an important feature of many modern fungicides and herbicides, besides being often effective for control of sucking insects (aphids, capsids, mealybugs, etc.) and ‘cryptic’ pests (e.g. insects that are unlikely to come in contact with a pesticide spray by burrowing into the plant). Systemic translocation is usually acropetal, moving up the plant from the point of application, or towards the edges of leaves if these are sprayed.
Only herbicides (and rare examples of phosphonate fungicides and one recently introduced insecticide) move down the plant (basipetal translocation) towards the roots.
4.4.1 Physical and chemical properties (and where to obtain information)
Readers wanting to know more about pesticides can consult the Pesticide Manual [1], which is available either as a book or electronically (the latter is updated annually) [2].
Again, the importance of accuracy cannot be over- emphasised, and a reference work such as this is an essential tool for policymakers, senior crop protection scientists, etc.
The Pesticide Manual includes information on:
- Names: both international nomenclature and common product brand names
- Physical chemistry and methods of analysis
- Commercialisation and toxicological reviews (including Chemical Abstracts Service Registry Number [CAS RN] and status in EU regulations)
- Mode of action, common uses and formulation types
- Mammalian toxicology
- Ecotoxicology and environmental fate
Although much of this information is specialist in nature, anyone advising on pesticides should be familiar with the function of certain crucial entries.
Information on properties such as vapour pressure, solubility and partition coefficient (log P) can give important clues on the behaviour of a compound in the plant or environment.
- Solubility: Unless stated otherwise, units for solubility in water are in mg per litre (mg L-1). Measurements are influenced by the temperature, the pH and the method used.
- Partition Coefficient: Kow (expressed as Log P) is a measure for the lipophilicity/hydrophilicity of a substance. With most pesticides and other organic substances, Kow provides a useful predictor of their properties, provided the molecular weight is not too high. It is a dimensionless parameter and is the measured ratio (at equilibrium) of dissolved mass of the substance, between equal layers of n-octanol and water. Kow is often expressed as Log P (which is log to the base 10 of the Kow) and is considered to be a good indicator of:
- systemic action, with low values (generally <=2) indicating likely systemic translocation of pesticides or pesticidal breakdown products; very low (or negative) values often indicate basipetal translocation, as with many systemic herbicides.
- accumulation in organisms and food chains (bio-accumulation: with a positive correlation with log P).
Vapour pressure (vp): is a measure of how readily it will volatilise and for pesticides can be considered advantageous or in a negative light:
- a pesticide with fumigant action can have useful penetrative powers, but …
- a high vp can cause vapour drift and environmental pollution; first noted with some of the early synthetic auxin herbicides.
The usually used SI unit for vapour pressure is the milliPascal (mPa = g·m-1·s-2 or 0.001 N·m-2).
- Henry’s constant: or air-water partition coefficient (sometimes Kaw) describes the concentration ratio of a substance in equilibrium between air and water - thus the tendency of a material to volatilise from aqueous solution to air. Sometimes measured, but more usually calculated, as the ratio of vapour pressure (in Pascals) × molecular weight / solubility (mg L-1).
- Adsorption Coefficient: Koc is the ratio (at equilibrium) of the mass of a substance, adsorbed onto a unit mass of soil, relative to the mass remaining in water solution. It is heavily influenced by the organic carbon content (OC) of the soil and the value is also dependent on the type and pH of the soil; it must therefore be used carefully and a range of given values is commonplace.
