  In this paper, the effects of periodic partial harvesting of a continuously
grown crop on augmentative biological control are analyzed. Partial harvesting
can remove a proportion of both pests and biological control agents, so its
influence on the control efficiency cannot be a priori neglected. An impulsive
model consisting of a general predator-prey model in ODE, augmented by a
discrete component to depict releases of biological control agents and the
periodic partial harvesting is used. The periods are taken as integer multiples
of each other. A stability condition for pest eradication is expressed as the
minimal value of the budget per unit time to spend on predators. We consider
the partial harvesting period to be fixed by both the plant's physiology and
market forces so that the only manipulated variable is the release period. It
is shown that varying the release period with respect to the harvest period
influences the minimal budget value when the former is carried out more often
than the latter and has no effect when releases take place as often as or less
frequently than the partial harvests.
