Pesticide information box

Biological activity (pest control, environmental impact, etc.) will be affected substantially by:

• the mode of action (MoA) of the active ingredient(s)
• spectrum of action (influenced by MoA)
• formulation and method of application
• mode of dose transfer (related to the above, combined with other properties of the pesticide).

The contents of the product

The active ingredient (AI: also called the active substance) and its concentration that is of most interest from the point of view of efficacy, safety and residue tolerances. These properties will also be influenced by the formulation and application technique.

From a legal point of view, one of the main methods of communication between an agrochemical company and the user is the product label. Labels should give the chemical name – which follows rules of nomenclature set by the International Union of Pure and Applied Chemistry (IUPAC) as adapted for indexing in Chemical Abstracts. In practice, the common names (for which there are ISO standards) are generally used for describing AIs.

However, the most noticeable words on the label will usually be the trade name (or brand), and it is of course in the chemical company’s interest to promote its particular brand of pesticide. Routine use of brand names can cause confusion because:

• Often (and increasingly) the brand name represents a product containing a mixture of active ingredients
• Different brand names may be used for the same product in different countries and languages
• Active ingredients – especially of successful products – may be changed over time
• The formulation names (and numbers used in the name) may not conform to international standards.

An example of a commonly used pyrethroid insecticide is:

• • Common Name (ISO) – lambda-cyhalothrin – which is easier to remember than the …
  • Chemical Name – of two stereo-isomers: (S)-?-cyano-3-phenoxybenzyl (Z)-(1R,3R)-3-(2-chloro-3,3,3-trifluoroprop-1-enyl)-2,2-dimethylcyclopropanecarboxylate and (R)-?-cyano-3-phenoxy-benzyl (Z)-(1S,3S)-3-(2-chloro-3,3,3-trifluoroprop-1-enyl)-2,2-dimethylcyclopropanecarboxylate
  • Trade names are numerous (especially now that the patent for the compound has expired) but they include: ‘Karate’, ‘Kung Fu’ and ‘Matador’ (as used by the same Company in different countries)
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Measurement of activity (efficacy, toxicity, etc.)

Biological activity can be expressed quantitatively as in the concentration of a substance required to give a certain biological response – often initially in laboratory experiments called bioassays. Biological effect is often described by scientists as a response and it is dose dependent – which usually means that the higher the dose, the more individuals in a population of organisms will be affected (and ultimately killed). The population in question could be the target pests, but also unintentionally exposed human beings or other non-target organisms (beneficial or harmless animals and plants). This is assessed in laboratory bioassays, where response is measured over a range of doses (different quantities of pesticide [AI] delivered individually to target organisms).

Described on a graph, the response is non-linear (i.e. not in a straight line), but usually in the form of a sigmoid (‘S’ shaped) curve – (see below). This sigmoid curve has been derived from the normal distribution – the bell shaped curve that describes natural variability which is widespread in living organisms (e.g. the height of people, the weight of fruits, the ability of animals to withstand drought). By analysis of this dose response line, an estimate can be made of the median lethal dose or LD50 of a pesticide to a group of organisms (i.e. the exact dose which would kill 50% of a test population of pests).

Median lethal dose (LD50)

The LD50 is derived from the dose-response curve and represents the dose at which 50% of test organisms (such as pests) are killed. In practical experiments, there is often considerable variability in measured mortality at different dose rates and statistical methods (called logit or probit analyses) are used to determine LD50s.

Other levels of response can be used such as LD10 and LD90 (i.e. the 10% and 90% level of control respectively), but LD50 is most commonly used since it represents the point at which the dose can be estimated most accurately. In some bioassays, the pesticide is not administered directly to the target, so the true dose applied to a given individual is not known. The term dosage is used both here and in field applications, where an amount has been applied to an area or volume (e.g. a field, a length of river, etc.). In bioassays, different dosages may have been applied using different rates of surface deposit from various concentrations of pesticide mixtures; the result is expressed as the median lethal concentration or LC50. In efficacy experiments, the effective dose or concentration (ED50, EC50 ) may be quoted.

These measures of toxicity apply to both pests and non-target organisms including mammals.

Mode of dose transfer (residual, systemic, vapour acting, etc.)

There are many pesticides that work in various ways, and the different types of control action affect the amount, efficiency and speed of dose transfer to the target pest. The cartoon below shows some of the insecticide dose transfer mechanisms.

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 useful for control of insects of storage and cryptic pests. Certain older insecticides (e.g. lindane, endosulfan) 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 some pesticides may be trans-laminar (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 chemicals typically have a high solubility and a LogP<2.

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 the rare example of phosphonate fungicides) move down the plant (basipetal translocation) towards the roots.

All pesticide applications involve a number of complex interactions, influenced by operating, atmospheric and plant surface conditions. Young [1] has illustrated the complexity of processes (below), and noted that changes in atomisation technology have taken place, with consequences to deposition efficacy that were “not adequately anticipated.”

Spectrum of action

The mode of action (MoA) will often determine the degree to which a pesticide discriminates between target and non-target organisms. A selective pesticide affects a very narrow range of species other than the target pest. The chemical itself may be selective in that it does not affect non-target species or it may be used selectively in such a way that non-target species do not come into contact with it. Non-selective or broad-spectrum pesticides kill a very wide range of weeds, insects, plant disease organisms, etc.

MoA describes the way a pesticide attacks some biological process (often a certain biochemical pathway in a particular kind of living cells) within the pest. For example:

• • • Selective herbicides attack specific photosynthetic pathways (photosystems) in the chloroplasts of susceptible plant cells (i.e. weeds not crops).
    • Pyrethroid and neonicotinoid insecticides attack nerve cells of a wide range of insects.
    • Phenylamides that attack specific nucleic acid synthesis pathways in Oomycetes such as Phytophthora.

The impact of a pesticide is dependent on other factors though, especially its application.

Environmental impact

Agrochemical companies are now obliged to allocate substantial resources to assess the environmental fate of compounds (and their metabolites). The environmental impact of a given pesticide treatment is a function of its properties and the way that contamination takes place.

Screening of new compounds includes risk assessment of both ground and surface water contamination, involving computer modelling. A number of standard tests take place on non-target organisms including birds (such as mallard ducks), fish (including rainbow trout), algae, water fleas (Daphnia spp.), bees and other beneficial species.

Inappropriate application can lead to off-target contamination due to spray drift, and “run-off” from plants causing contamination of the soil. Several studies have concluded that point source contamination (entry of pesticides to water courses/groundwater following spillage of concentrate or after washing equipment) often causes the greatest harm – especially to waterways. During training sessions, time should be allocated to considering crop protection activities relative to the positions of water courses and wells. For example, in order to protect water sources, it is especially important that farmers consider waste flows when washing out sprayers in order to avoid point source contamination.

[1] Young, B.W. (1986) The need for greater understanding in the application of pesticides. Outlook on Agriculture 15 (2), 80-87.

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: Inclusion of compounds or products are for illustration only and does not imply recommendation or otherwise.