Pest management - Ecological Applications: Toward a Sustainable World - page 164

and virus. They found that rabbit mortality declined from more than 90% to less
than 30% within 8 years. Individuals that happened to be least susceptible to the
virus were more likely to pass on their genes to future generations - the population
did indeed evolve resistance (Figure 6.3 applies to this case just as well as to pesti-
cide resistance). Less expected was the fact that the virus itself evolved to become
less virulent. This was because when virulence was very high, rabbits died before
the virus could be passed to other rabbits (by mosquitoes feeding on live rabbits).
Viruses with an intermediate level of virulence proved to be fi ttest, contributing
most to subsequent generations of the virus.
Another kind of biocontrol agent is the endotoxin-producing bacterium Bacillus
thuringiensis (Bt - mentioned in Section 6.3.4) but, for example, mosquitoes have
evolved resistance to Bt in areas where it has been used intensively. Two strains of
the bacterium effective against mosquitoes are Bt subspecies israelensis and sphaeri-
cus ; the fi rst produces two classes of endotoxin called 'Bin' and 'Cry' while the
second produces another called 'Cyt'. A fundamental principle in the management
of the evolution of resistance is that the more complex and potent a toxin mixture,
the slower will be the development of resistance. Thus, Park et al. (2005) have
genetically engineered forms of the bacterium to produce novel combinations of
endotoxins. Those engineered to produce only Cry and Cyt were between 9 and 15
times less potent at killing mosquitoes ( Culex quinquefasciatus ) than others that
produced Cry, Cyt and Bin.
Some forms of pest control do not involve pesticides or biological control agents,
but resistance can still evolve. Take, for example, the northern corn rootworm ( Dia-
brotica barberi ), a beetle pest of corn crops in North America. The most effective
method of control (a case of so-called 'cultural' control) was simply to rotate the
corn crop with another crop every second year; this worked because rootworm eggs
could not survive through more than one winter. Then corn producers in some areas
began to notice rootworm damage even in rotated crops. This occurred because
some individuals now lay eggs that can survive for more than one winter (Levine
et al., 1992), an apparent case of evolved resistance to a cultural control method.
Judicious insecticide application will be needed in future to supplement the effect
of crop rotation.

6.5 Integrated pest
management (IPM)

Having considered chemical and biological approaches to pest control separately,
the pros and cons of the different approaches should now be clear. However, it is
vital to turn to a broader perspective and consider how all the different tools at the
pest controller's disposal can be deployed most effectively, both to maximize the
economic benefi t of reduced pest density and to minimize adverse health and envi-
ronmental consequences (including the evolution of resistance).
This is what integrated pest management (IPM) is intended to achieve, by combin-
ing physical, cultural, chemical and biological control, and the use of resistant crop
varieties. IPM accepts that crops, forests and any other managed biological resources
are part of a functioning ecosystem, and that management decisions must take into
account the way that control actions will affect not only the pest but also other
species in the web of community interactions. IPM also involves accepting that the
mere presence of a pest is not necessarily a problem; before a potentially disruptive
control action is taken, we need to know that the benefi t in reducing pest numbers
will be justifi ed by the cost of control (Box 6.1). IPM was, in part, a reaction to the