Pest management - Ecological Applications: Toward a Sustainable World - page 151

by increasing the mobility of the aphids, and hence, their rate of contact with fungal
spores (Hockland et al. , 1986).
One of the oldest of pest control tools also aims to reduce pests, this time verte-
brates, by modifying their behavior. Thus scarecrows are intended to reduce feeding
by birds on valued seeds and crops. In similar vein, model (or real) hawks may be
placed around airports to reduce bird densities and the risk of bird strikes.

6.2.6 When
pesticides go wrong
- target pest
resurgence and
secondary pests

Mexicans in the mountain village of Atascaderos noticed a build up in the rat popu-
lation when these pests appeared in numbers in their food stores in 2003. Their
response was to apply poison, but this backfi red when cats and other rat-eaters suc-
cumbed to the pesticide. Target pest resurgence (discussed in Box 6.1) was dramatic:
as a result of the unintended loss of their enemies, each household in the village
became infested by as many as 200 rats! (The villagers' response was equally dra-
matic - they placed advertisements in Chihuahuan newspapers to recruit a replace-
ment army of 700 cats.)
Similar, if less repulsive, examples of target pest resurgence include surges in
density of damaging cyclamen mites ( Steneotarsonemus pallidus ) on strawberries
after the application of parathion dust, and much higher densities of pacifi c mites
( Te t ra nychu s pacifi cus ) attacking vineyards after spraying with carbaryl as compared
to unsprayed areas (Debach & Rosen, 1991). In both cases, the natural enemies of
the pests were more strongly affected by pesticide application than the pests
themselves.
A classic example of pesticides gone wrong concerns the insect pests of cotton in
the southern part of the USA. When mass use of organic insecticides began in 1950
there were two primary pests of cotton, the Alabama leafworm ( Alabama argillacea )
and the boll weevil ( Anthonomus grandis ), the latter an invader from Mexico. Smith
(1998) estimates that the boll weevil alone has cost cotton producers more than $15
billion since it arrived a century ago. Chlorinated hydrocarbons and organophos-
phate insecticides were applied fewer than fi ve times a year and initially had spec-
tacular results - cotton yields soared. By 1955, however, three secondary pests had
emerged as a result of reductions in natural enemy populations: the cotton bollworm
( Heliothis zea ), the cotton aphid ( Aphis gossypii ) and the false pink bollworm ( Heli-
coverpa armigera ). The pesticide applications rose to 8-10 per year. This reduced the
problem of the aphid and the false pink bollworm, but led to the emergence of fi ve
further secondary pests. By the 1960s, the original two pest species had become
eight and there were, on average, an unsustainable 28 applications of insecticide
per year.
A study in the San Joaquin Valley, California, provides examples of both target
pest resurgence and secondary pest outbreaks. Figure 6.5a shows how cotton boll-
worm, the target pest, resurged after application of azodrin, as a result of pesticide-
generated reductions in its natural enemies. Figure 6.5b and c, on the other hand,
show how cabbage loopers and beet army worms became secondary pests after
natural enemies declined when target pests - lygus bugs in this case - were sprayed
with bidrin or a mixture of DDT and toxaphene.
A surge in pest numbers happens because the pest population recovers more
quickly than its enemies. Predators will often have a lower intrinsic rate of popula-
tion increase than their prey and this partly explains the pattern. But the predators
may also be slower than the pests to recolonize after the pesticide loses its potency.