Pest management - Ecological Applications: Toward a Sustainable World - page 144

infl uential environmental factors determine a species' equilibrium density (or carrying capacity,
Box 5.1). This is shown as EP (Equilibrium Population) in Figure 6.1a. In this case, the population
stays at a low and relatively stable density, far below its EIL. It is not a pest, but could become one
if circumstances changed and its birth or immigration rate increased, or its death or emigration
rate decreased.

(a) (b) (c)

EIL

EIL

EIL

ET
EP

ET

EP

EP

Fig. 6.1 Population fl uctuations of hypothetical pests about their equilibrium population
(EP) density - set by physicochemical environmental conditions and the pest's interactions
with its food and enemies. (a) The EP of this species is well below the economic injury level
(EIL) at which it would be worthwhile to apply control measures. (b) Population fl uctuations
of this pest break through the EIL in some years. Control measures should be applied
(arrows) when density reaches the economic threshold (ET), so that the measures take effect
before the EIL is reached. (c) This population fl uctuates through the ET in most years.

The Economic Threshold (ET)
The hypothetical species shown in Figure 6.1b, on the other hand, has a density that fl uctuates
markedly and occasionally breaks through the EIL. When a pest population has reached a density
at which it is causing economic injury, however, it is generally too late to start controlling it. So
let's add another idea into the mix. ET is the density at which action should be taken against a pest
to prevent it reaching the EIL. The EP of the pest illustrated in Figure 6.1c is closer to its EIL (than
those in Figure 6.1a or b), and its wide population fl uctuations take it above the ET most years.
EILs and ETs are predictions based on detailed studies of pest populations and past outbreaks,
and usually require the pest controller to monitor the pest, weather conditions and even densities
of the pest's natural enemies, because these may help keep pest density down. For example, the
prescription for the spotted alfalfa aphid ( Therioaphis trifolii ) on hay alfalfa in California is to apply
pesticide in spring or winter when aphids have reached densities of 40 or 50-70 per stem, respec-
tively. In summer and autumn, however, the prescription is more complex: apply when the popula-
tion reaches 20 aphids per stem, but do not treat the fi rst three cuttings of hay if the ratio of
ladybirds (beetle predators of the aphids) to aphids is at least one adult per 5-10 aphids or three
ladybird larvae per 40 aphids (Flint & van den Bosch, 1981). The most useful estimates of EIL and
ET involve detailed cost-benefi t economic analyses (Ramirez & Saunders, 1999) that take into
account the current value of the crop - because when the value of the crop at the farm gate is high,
it pays to control pests at a lower EIL.

Target pest resurgence
In the history of pesticide use there have been many examples where pest density increases rapidly
again soon after application. There are two explanations.
Note fi rst that if the pest population was originally subject to density-dependent processes (lower
rates of birth or immigration, or higher rates of death or emigration, at high than low density),
reduction of its density by pesticide application will cause density-dependent effects to be relaxed.
Thus, the population may quickly return to its original equilibrium level (Figure 6.2a). The speed
at which this happens will depend on the pest's intrinsic rate of natural increase ( r ) (Box 5.1). Pests
usually have very high values for r (that's one reason they become pests), so numbers can build
up very rapidly and, as a result, several pesticide applications may be needed each year. Recovery
of the pest population will be delayed if the pesticide persists in active form in the environment for
months or years.
There have also been many records of a pest population recovering after pesticide application
to densities far above the original level. This is known as 'target pest resurgence'. A pesticide gets
a bad name if, as is often the case, it kills more species than just the one at which it is aimed. In
the context of agricultural productivity, the bad name is especially justifi ed if the pesticide kills the