Introduction

Occurrence and spread of resistance to triazine herbicides

The first incidences of resistance in weed species to triazine herbicides occurred in 1968 in the State of Washington USA. During the 1970s and 1980s, notably in North America and mainly western Europe, and to a lesser extent in Israel in the 1980s there has been an irregular but relatively steady addition to the occurrences of new species becoming resistant mainly to triazine herbicides (but also to some others). Figure 1 gives an indication of the total number of species in different countries which are occurrences of resistance not previously recorded. Worldwide 49 species of 33 genera have become resistant to triazine herbicides (Le Baron pers. comm. 1987) and a further 11 species have evolved resistance to other herbicides.

A record of the occurrence of a herbicide-resistant biotype will often conceal the fact that numerous populations have independently evolved in many different locations over a period of just a few years. For example, in Hungary, Amaranthus retroflexus resistant to-s-triazines occurred in scores of locations and 75% of the maize growing area has become infested by the resistant biotype (Hartmann, 1979). In the USA, evolution of resistance to s-triazines in A. hybridus first occurred in Maryland in 1972 but between 1976 and 1982 there were numerous reports of resistant populations occurring throughout Virginia, New York, Delaware, Pennsylvania, Massachusetts, West Virginia and Illinois (Le Baron & Gressel, 1982; Le Baron pers. comm., 1987).