Pest management - Ecological Applications: Toward a Sustainable World - page 146

Biological control
Biological control involves introducing or augmenting an enemy of the target pest. This may be a
predator (e.g. a ladybird that eats aphids), a parasitoid (an insect that lays its eggs in a pest insect,
which is consumed from the inside out) or a disease organism. The control agent is often imported
from the area where the pest originated. Figure 6.2d, which is really a mirror image of Figure 6.2c
with the addition rather than subtraction of an enemy, illustrates how the pest declines from an
equilibrium population around the EIL to a new, much lower equilibrium, and one that may be
maintained because of the persistence, at low densities, of both target and control agent. When
successful, biological control has the particular advantage that it only needs to happen once,
whereas pesticides need to be used time and again.

Evolution of resistance
At best, chemical pesticides have to be used repeatedly and forever. However, most pesticides have
a limited life because pests evolve resistance to them. This is simply natural selection in action. It
is almost certain to occur when vast numbers of individuals in a genetically variable population are
killed in a systematic way by the pesticide. One or a few individuals may be unusually resistant
(perhaps because they possess an enzyme that can detoxify the pesticide). If the pesticide is
applied repeatedly, each successive generation of the pest will contain a larger proportion of resist-
ant individuals. Pests typically have a high intrinsic rate of reproduction, and so a few individuals
in one generation may give rise to hundreds or thousands in the next, and resistance spreads very
rapidly in a population (Figure 6.3).
It has been extremely common for resistance to pesticides to evolve. The evolution of resistance
to biological control agents seems less common, but is not unheard of.

Genetically resistant
Not resistant

Spray 1

Spray 2

Spray 3

Fig. 6.3 Evolution in a pest population of resistance to a control agent. When the pesticide is
fi rst applied (Spray 1) the majority of susceptible individuals are killed while a few are
naturally resistant. The latter contribute more offspring to the next generation, passing on
the genes that confer resistance, and resistant individuals are better represented when Spray
2 is applied. Again, most of the susceptible individuals are killed, but now there are even
more resistant individuals to contribute resistant offspring to the next generation. Within a
few generations, most individuals are resistant and the pest controller has to look for another
pesticide.

6.2 Chemical
pesticides

Chemical pesticides are a key part of the armory of pest controllers but they have
to be used cautiously because population theory (Box 6.1) predicts undesirable
responses to careless attacks. You have to know your enemy's weaknesses, develop
chemical weapons to exploit them, and be ready for the pests to fi ght back (by
evolving resistance). It's an arms race.

6.2.1 Natural arms
factories

Plants have been in the defence game for millennia. Relentless attack from herbi-
vores has ensured that individual plants with features that reduce the impact of
grazing pass on more than their fair share of genes to future generations (essentially