Pest management - Ecological Applications: Toward a Sustainable World - page 150

is taken out, collateral damage is avoided, and the control agent does not persist for
long, travel far or become concentrated up food chains.

6.2.4 Cut off the
enemy's
reinforcements

In the previous section I considered pesticides that disrupt cellular functioning.
Another group of pesticides upsets growth and development, thereby reducing the
chance of juvenile stages surviving to become reproducing adults.
Insect growth regulators mimic natural insect hormones and enzymes, and hence
interfere with normal insect growth and development. As such they are generally
harmless to vertebrates and plants, but they may be as effective against a pest's
natural insect enemies as against the pest itself. The two main types are the chitin-
synthesis inhibitors, such as difl ubenzuron, which prevent the formation of a proper
exoskeleton when the insect molts, and juvenile hormone analogues, such as metho-
prene, which prevent insects from moulting into their adult stage, and hence reduce
population size in the next generation.
The equivalent herbicides are the phenoxy or hormone weed killers. These are also
relatively selective. For instance, 2,4,5-dichlorophenoxyacetic acid (2,4-D) is highly
selective against broad-leaved weeds, whilst 2,4,5-trichlorophenoxyethanoic acid
(2,4,5-T) is used mainly to control woody perennials. Put these together and you
have the notorious Agent Orange, used to defoliate vast areas in the Vietnam War
and with signifi cant health consequences for the combatants. The hormone weed
killers appear to act by inhibiting the production of enzymes needed for coordinated
plant growth. Substituted amides (e.g. diphenamid) and nitroanilines (e.g. t r i fl uralin)
are largely effective against seedlings rather than adult plants, and are applied to
the soil around established plants as a 'pre-emergence' herbicide, preventing the
subsequent appearance of weeds and reducing future problems by cutting off
reinforcements.
While pest wars have been primarily waged against invertebrate pests and weeds,
there are plenty of vertebrate pests too. Sea lampreys ( Petromyzon marinus ) invaded
the Great Lakes of North America from the Atlantic Ocean through shipping canals
in the early 1900s, contributing signifi cantly to declines in several highly valued
fi sh species. The lampricide TFM (3-trifl uoromethyl-4-nitrophenol), when carefully
applied for a few days to produce just suffi ciently high concentrations in the small
streams where larval lampreys develop, kills them without detriment to other fi sh
(although invertebrates and amphibians may be locally affected). The populations
of adult lampreys in the Great Lakes, starved of reinforcements, have been very
substantially reduced.

6.2.5 Changing pest
behavior - a
propaganda war

In human warfare, propaganda pamphlets dropped behind enemy lines are intended
to infl uence behavior and disrupt the enemy. There is an equivalent here too, devel-
oped more recently than the inorganic and organic toxins and thus sometimes called
'third-generation insecticides' (Forrester, 1993). Semiochemicals (literally 'chemical
signs') are not toxins but chemicals that elicit a change in the behavior of the pest.
They are all based on naturally occurring substances, but in a number of cases it
has been possible to synthesize them in the laboratory. Sex-attractant pheromones
have been used, for example, to control pest moth populations by interfering with
mating or to attract moths to cages holding a virus that is deadly to the moth's
caterpillars. In a different context, an aphid alarm pheromone has been used to
increase the effectiveness of a fungal pathogen against pest aphids in glasshouses