4.6 Technical problems with pesticides (the ‘three Rs’)
Besides residues, which will be discussed further in Chapter 5, two other phenomena can be described as ‘technical issues’, in that they relate to the effectiveness of pest control rather than the toxicological and environmental risks associated with pesticide use.
However, in both cases one of the practical consequences is that some farmers, by not understanding these phenomena, may be encouraged to apply more pesticides in the short-term, thus increasing the risk of high crop residues.
1. Development of resistance
Where pests adapt over time after exposure to control agents, which become ineffective (e.g. loss of effectiveness of certain fungicides for the control of Phytophthora spp.). Among the first cases of insecticide resistance detected was resistance against organochlorines by cocoa mirids [1].
Resistance is an evolutionary process that has been defined as: “a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species” (source: IRAC).
Furthermore, the problem may be compounded by cross-resistance: where resistance to one pesticide confers resistance to another active substance, even if the pest has not been exposed to products containing the latter.
Because insect and fungal populations are usually numerous and reproduce quickly, the rate at which resistance evolves is greatest when fungicides and insecticide are over-used.
2. Pesticide induced resurgence
Especially following the use of broad-spectrum insecticides that cause a ‘flare up’ of pests that were previously of minor importance; this is sometimes called the “pesticide treadmill”.
An example of resurgence in cocoa was the dramatic increase in populations of the trunk borers Eulophonotus myrmeleon (Cossidae) and Tragocephala castinia theobromae (Cerambicidae), which were previously considered to be minor pests, following destruction of their natural enemies with applications of BHC and dieldrin - applied to control insects such as mirids [2].
