The importation of parasites from the native home of introduced insect pests now constitutes a recognised part of entomological practice in most countries under civilised control. However, since the method is still in the experimental stage, and since long periods of time often elapse before the parasites colonised become sufficiently abundant to exert any appreciable influence on the host, the entomologist often finds it necessary, in order to avert disaster, to utilise remedies which, though temporary, are more immediately effective. To this end he employs some one or other of the various methods of mechanical control, by which the population of the insect pest can be at once greatly reduced.

But in many cases, these methods of attack affect the parasites as well as the hosts. It is therefore important to consider what influence they will have upon the progress of the natural enemies and whether, in order to obtain the temporary relief afforded by mechanical methods, we are not sacrificing the hope of permanent control.

We have at present no data permitting us to attack this problem from the experimental angle ; but it can be studied in a broad general way when reduced to mathematical terms.