Pest management - Ecological Applications: Toward a Sustainable World - page 147

the same story as illustrated in Figure 6.3). The physical defences that have evolved
are obvious to every one - just think of stinging nettles and acacia thorns. The
chemical defences, on the other hand, are invisible but ubiquitous and amazingly
diverse. And people have known for centuries that certain insects can be fought
using naturally occurring substances, collectively known as botanicals , such as nico-
tine from tobacco plants ( Nicotiana rustica ) and pyrethrum from fl owers in the
Chrysanthemum genus. Both are nerve agents, disrupting nerve impulses and killing
insects. Many natural insecticides are unstable on exposure to light and air, but
chemists have overcome this by producing their own versions, such as the synthetic
neonicotinoids and pyrethroids that are now among the most widely used of
pesticides.

6.2.2 Take no
prisoners

The use of the so-called inorganics goes back to the dawn of pest control and, along
with the botanicals (Section 6.2.1), they were the chemical weapons of the expand-
ing army of insect pest managers of the nineteenth and early twentieth century. The
inorganic insecticides, mostly metallic compounds or salts of copper, sulfur, arsenic
or lead, are primarily stomach poisons effective only against invertebrates with
chewing mouthparts. A range of inorganic herbicides was also commonly used
against weeds. However, the persistence in the environment of toxic residues has
led pest controllers to abandon inorganics, with rare exceptions. Borates (absorbed
by plant roots and translocated to above-ground parts) are still sometimes used
where no vegetation of any sort is wanted. And you heard in Section 6.1.2 how an
invasion of the Caribbean black-striped mussel was greeted by the application of a
massive amount of copper sulfate. The fearful inorganics still have some uses but
only when the decision is to take no prisoners.

6.2.3 From
blunderbuss to
surgical strike

With advancing knowledge of cellular physiology, a chemical warfare catalog has
gradually been put together. Like pyrethrum, chlorinated hydrocarbons disrupt
nerve-impulse transmission. They are contact poisons (no need to eat them) that
are insoluble in water but show a high affi nity for fats, thus tending to become con-
centrated in animal fatty tissue, and to pass along food chains. The most notorious
is DDT: a Nobel Prize was awarded for the discovery of its pest control properties
in 1948, but it was suspended from all but emergency uses in the USA in 1973. It is
still used in South America, Asia and Africa in an attempt to control the mosquitoes
that spread malaria. Arguably, in these areas the human costs of malaria outweigh
the environmental damage caused by the use of DDT.
One of the biggest problems of DDT and related compounds, such as toxaphene,
dieldrin and chlordane, is their toxicity to just about everything, coupled with a
very long life in the environment. They can cause particularly severe problems
because of biomagnifi cation - this happens when a pesticide is present in an organ-
ism that becomes the prey of another and the predator fails to excrete the pesticide.
It then accumulates in the body of the predator, and so on up the food chain. Top
predators (including people) in aquatic and terrestrial food chains, which of course
were never intended as targets, can accumulate extraordinarily high doses (Figure
6.4). And this was the cause of a famous decline of peregrine falcons (Section 5.6.2)
when calcium metabolism was upset and egg shells became dangerously thin.
The application of chlorinated hydrocarbons might be characterized as a blun-
derbuss approach, because it is not just the target that gets hurt. But a better analogy
would be with the dreadful land mines left behind after human confl ict: the weapon