Pest management - Ecological Applications: Toward a Sustainable World - page 143

campaign in the eastern USA aimed at a root parasite, African witchweed ( Striga
asiatica ), is based on strict quarantine of the infested area, the use of herbicide
against the plants and soil fumigation to destroy the long-lived seed-bank (Eplee,
2001). But a much more notable success involved the screwworm fl y ( Cochliomyia
hominovorax ) in a large area of the USA and Central America, and later when it
invaded North Africa - it was eradicated by a bit of biological trickery. The fl y
cements its eggs near a wound on an animal such as a sheep or cow, and occasion-
ally on people ( hominovorax actually means man eater). The maggots enter and feed
in the wound, producing a foul-smelling, pus-discharging sore. The surprising trick
was to breed more screwworm fl ies, but only males - a large factory was established
to breed millions of them. By swamping the population with sterilized males (pupae
exposed to gamma radiation), the females breed but produce no eggs - the birth
rate drops to zero and the population disappears. It really works.
Most often, however, once invaders have established and spread through a new
area and are classifi ed as pests, they become just one more species at which the pest
manager's armory must be directed. Eradication is not usually the objective. Rather,
the aim is to reduce the pest population to a level at which it does not pay to achieve
yet more control (the Economic Injury Level). The underlying theory is outlined in
Box 6.1.
We can attempt to combat pests by physical means (e.g. simply keeping invaders
from arriving, keeping pests away from target crops by planting preferred 'trap'
crops in their path or picking them off by hand when they arrive), by 'cultural
control' (e.g. rotating crops planted in a fi eld so pests cannot build up their numbers
over several years), or using chemical or biological agents.
My emphasis will be on the latter two classes. You w ill learn how chemical pes-
ticides actually work and, by exploring a range of examples, understand how to
avoid unwanted outcomes such as pest resurgence and secondary pest outbreaks
(Section 6.2). The basic theory behind these topics is presented in Box 6.1. An alter-
native to pesticide application is biological control - the introduction or augmenta-
tion of natural enemies of the pest (Section 6.3). When t hi s works, the benefi ts can
be huge. But, as with pesticides, there may also be a downside when nontarget
species are adversely affected. One of the biggest problems facing pest managers is
the tendency of pests to evolve resistance to control agents, whether chemical, bio-
logical or cultural - I discuss the implications of evolution of resistance in Section
6.4. The optimal pest control strategy, taking into account the pros and cons of each
approach, turns out to be a judicious selection from the complete pest control
armory - this is termed integrated pest management (Section 6.5).

Box 6.1 Population
dynamics theory 2

Economic Injury Level (EIL)
Consider a pest of a farm crop. Since every control measure has an associated cost (e.g. payment
for pesticide and the equipment and labor to administer it), pest control is only worthwhile if the
economic gain from increased crop productivity outweighs the cost of the control measure. The
EIL then is the pest population size at which it does not pay to achieve yet more control. Above the
EIL, pesticide application is worthwhile because there is a net benefi t to the farmer; below the EIL
it does not pay.
Every herbivorous insect that feeds on a crop might conceivably achieve pest proportions, but
whether it will do so depends on the EIL for that particular species, the densities its population
typically achieves and the degree to which its density fl uctuates. You saw earlier how a variety of