The use of natural enemies to control pest populations below their
economic damage level, or EDL, is called biological
control. Two factors are important in the biological control
of a pest; (1) pest suppression, the degree of reduction
of the pest population below EDL, and (2) pest fluctuations,
or the stability of the community equilibrium created by interaction
with the predator(s) and/or parasitoid(s), which determines the
probability of the pest population exceeding EDL. Let us illustrate
these concepts by reference to the model of the black-headed budworm
that is regulated at low population density by effective insect
parasitoids (click here to see
the model). In other words, the parasitoids are effective at pest
suppression but there may be considerable pest fluctuations
around the community equilibrium. Let us assume that the EDL occurs
at 6000 budworm larvae per 100 m2 of host foliage.
In a variable environment, the budworm and parasitoid populations
will tend to cycle around the community equilibrium and may occasionally
exceed the economic damage level (Fig. 1). The greater the degree
of environmental variability, the greater the likelihood of the
pest population exceeding EDL. The job of the pest manager, of
course, is to minimize this probability.
Figure 1. Stochastic simulation with the budworm-parasitoid
model in which outbreaks arise when the pest population exceeds
EDL; Phase portrait shown on the left with gray line the time
series trajectory, and time series plot on the right.
The first thing to notice is that the further the system is from the community equilibrium, the wider are the cyclical orbits, and the greater the likelihood of the pest population exceeding EDL as it goes through its natural predator-prey cycle. Hence, the pest manager should try to keep the system as close to the equilibrium point as possible, say within the circular target in figure 2. To do this requires tactics that change with the particular location of the system coordinate in predator-prey phase space. For example, if the populations are at location a (moderate pest and low parasitoid density), then the only tactic that will bring the system into the target area is to increase the parasitoid population in some way. This is usually called natural enemy augmentation. At location b, on the other hand, we need to reduce the pest population as well as increase parasitoids. This could be done with an insecticide that only kills the pest, a selective pesticide, plus enemy augmentation. At location c a selective pesticide alone would be the best tactic while at d we need to kill both pest and parasitoid, so the use of a non-selective pesticide is justified, or even essential. Finally, if the system is at positions e or f an augmentative release of the pest is required. In addition, a selective pesticide may be used against the parasitoid at location e. These seemingly counterintuitive tactics may be difficult for the manager to swallow, but the effectiveness of pest augmentation in suppressing pest outbreaks has been verified empirically (Maksimovic et al. 1970). The purpose of these treatments, of course, is to prevent the parasitoid population from overexploiting its food supply and dying out from starvation, or even going extinct locally. The set of tactics that maintain the pest population in the target area is the optimal pest control strategy.
Maksimovic, M., Bjegovic, P. L. and Vasiljevic, L. (1970) Maintaining the density of the gypsy moth enemies as a method of biological control. Zastita Bilja 1107: 3-15 (see also on page 162 of Berryman, A. A. 1986. Forest insects: principles and practice of population management. Plemun, New York)
©1998 Alan A. Berryman