5.4 Pesticide breakdown
After application, pesticides are degraded by chemical and physical processes in the environment such as sunlight, soil and water (called abiotic degradation) or metabolised within living organisms (both target and non-target animals and plants, soil bacteria, etc.).
Breakdown of a pesticide (and many other substances) in the environment can be thought of as following a decay curve. This is a function of the chemical’s half-life, which is the time (most usually expressed in days) required for half of the applied pesticide to become converted into degradation products (which may in turn be biologically active and have substantial half-lives).
The rate of breakdown depends on many factors, not least the chemical stability of the pesticide in question, but factors such as temperature and pH are extremely important, so the half-life may be expressed as a range (e.g. 3-10 days).
Probably the most important mode of pesticide degradation is oxidation: especially by activated oxygen (e.g. ozone and hydroxyl radicals generated by sunlight, hydrogen peroxide generated in plants, etc.) rather than O2 in the atmosphere.
Allowing sufficient time to elapse between application and harvest enables any residue to degrade to acceptable levels (i.e. the MRL) and the Pre-Harvest Interval (PHI) has a built-in safety factor.
Reducing the dosage reduces the time to which acceptable levels are reached, but pest control may be impaired. Excessive residues occur with short harvest intervals, overdosing, or worst of all both.
5.4.1 Implications for application and environmental impact
Improved (as opposed to just competent) application techniques are an especially promising way of mitigating residues and lowering environmental impact, but unfortunately research in this field has been very limited. Targeted dose-transfer [1] can increase pest mortality for a given level of application to the crop, while maintaining equivalent pest control [2].
