Pesticides can be generally separated into three main categories (indications) for use: insecticide, fungicide or herbicide. In order to be effective against the three target organisms, each of these categories of pesticides require different physical and chemical characteristics. When examining the .chemical space’ occupied by these compounds, it is therefore sensible to review the indications separately.
From a review of pesticides listed in the 12th Edition of the Pesticide Manual (Delaney et al, in Tomlin, 2000), the following statistical distributions for each indication can be presented (Figure 17).
The fact that ionisable compounds are more likely to be within the herbicide indication is not an accident. Weak acids (within a certain log KOW range) tend to have the most favourable properties for leaf uptake, concentration in the phloem of the target plant and translocation to the roots. Therefore many foliar applied, systemic herbicides are weak acids.