and Integrated Pest Management - part - usaid

PESTICIDE RESISTANCE: THE REALITIES
D. M. Evans
(Vice Chairman, GIFAP Agriculture and Environment Committee)
Manager of Environmental Operations and Registrations
Monsanto Agricultural Company Europe-Africa
270-272 Avenue De Tervuren, 1150 Brussels, Belgium
Natural ecosystems are often represented as stable and balanced, and in a
sense, they are. The balance, however, involves life or death competition
amongst the species present. Evolutionary advances by one species that alter the
balance in its favor thus are niadc at the expense of others. The forces of
evolutionary change are no less at work in agriculture. Agricultural practices
reduce the competition offered to crops and animals so they will be more
productive. Most crops and domestic animals would not survive long in the
absence of that care. It is thus essential that crops and animals be protected and
pes'licidcs have a vital role to play in this care.
Resistance has been a problem since the beginning of agriculture.
Resistance also does not result exclusively from the use of pesticides against
pests. Pests may also evolve resistance to introduced biological controls,
including pathogenic parasites and predators; to control measures based upon
physical factors; and to mechanical action (CAST 1983). For example, in
Englapd and Australia, strains of rabbits resistant to the disease Myxamatosis
have appeared. The ploughing of land for the control of weeds has contributed
over the centuries to weeds that not only survive such action, but prosper tinder
these conditions. In fact, our most serious weeds tnrive only when the soil is
disturbed frequently. The development of resistance is part of the endless process
of evolutionary adaptation. This phenomenon of resistance is also exploited by
plant breeders in the development of new varieties.
THE BACKGROUND ON PESTICIDE RESISTANCi-
The discovery of resistant strains of insects, fungi, and weeds is not a recent
event. The resistance of insects to certain specific agrochcmicals was first
reported 79 years ago. Similar observations involving plant pathogenic fungi
were made 28 years ago and a resistant weed species was confirmed in 1964.
A survey in 1981 by Georghiou (Georghiou & Mellon 1983) showed that
428 species of arthropods out of an estimated 20,000 insect species had
developed strains that were resistant to one or more insecticides that were once
effective against them. However, only about 20 out of the 428 are economically
important agricultural pests. In 1983, Ogawa et al. (1933) reported 90 or more
species of plant pathogens with resistance, and in 1982 LeBaron and Gressel
(1982) reported 36 species of weeds with resistance.