THE PRINCIPLES OF BIOLOGICAL CONTROL
Authors: THOMPSON W. R. 
Published: February 2008

Summary.

In view of the rapid development of practical experiments in biological control, a general survey of the subject seems desirable. The foregoing paper designed to meet tins need contains a discussion of the nature of biological control, the character and practical value of biotic controlling factors of various types, the pests to which the method can be applied, the situations in which it can be utilised and the results that may be expected from it.
By control we mean a check on the increase of an organism brought about by any cause whatever. By biological control we mean the reduction of the rate of multiplication of an organism effected through the agency of other, organisms as distinct from non‐living factors.
There is no necessary connection between control as a biological phenomenon and control in the economic sense.
The complex of factors responsible for the control of an organism differs in composition both qualitatively and quantitatively in the various districts inhabited by it; the measures necessary to re‐establish control, in the case of an outbreak, will also vary in different times and places. It would, therefore, be unwise to rely exclusively upon the methods of biological control for the solution of entomological problems.
The main factors of the biological control of insect pests are: (1) Pathogenic organisms; (2) Invertebrate parasites and predators; (3) Vertebrate predators.
Pathogenic organisms are sometimes very effective but practically impossible to manipulate successfully. Among the parasitic and predaceous Invertebrates, the entomophagous insects are by far the most important and the most useful. Vertebrate predators apparently play only a minor part in the control of insect pests, and there is little indication that they can be utilised in practice.
The control of insect pests by the use of entomophagous insects seems, on the whole, much easier to bring about than the control of plant pests by phytophagous insects, in the first place because the insect is much more susceptible to injury than the plant, and in the second place because the entomophagous insect, being of about the same size as its host, is capable of inflicting greater injury than the phytophagous insect can inflict upon the host.
The attempt to control plant pests by phytophagous insects is more dangerous than the attempt to control insect pests by entomophagous insects, because if the phytophagous insect changes its host after introduction it may attack a useful plant, whereas the entomophagous insect, even if it does attack hosts other than that against which it was introduced, is not likely to attack any beneficial insect.
The object of work in biological control may be either:(i) the intensification in the action of natural enemies already present in the area; or(ii) the introduction of natural enemies (a) temporarily absent from the area; or (b) never before present in it.In the latter case the species introduced may be either (i) species of which a known host already exists in the area; or (ii) species of which no known host exists in the area.In general, the most promising type of work concerns introduced 3pecies from other areas, though valuable or, at least, promising results have been obtained in the work with certain established speciec.
The results which may be expected from experiments in biological control depend primarily (other things being equal) on whether the host is increasing in numbers or not. If it is increasing, the increase of the parasite at its expense will not cause a reduction in damage or a decrease in numbers, which will not occur until the parasite has overtaken the host. If the host population has become stabilised, then the increase of the beneficial species at its expense will give immediate relief, though this may be slight at first. The reduction of damage will continue until the rarefaction of the host population begins to cause a diminution in the effective reproductive rate of the parasite, after which the host population will again increase for a time.
Generally speaking, no one species of parasite or predator is likely to bring the host under control over the whole of the infested area. To produce this result, the introduction of additional species will usually be necessary, while in many cases, their efforts must be aided by the methods of agricultural, chemical or mechanical control.