74 
Applied Entomology 
(1902, p. 221) has pointed out that a colony of scale insects may be 
extensively parasitised and yet betray no obvious manifestation of the 
fact except to the trained and expei’ienced eye. In such case it is, as 
a rule, highly injurious and superfluous to apply insecticides as they 
also destroy the beneficial insects which are already keeping down the 
abundance of the injurious species. These two instances may serve to 
illustrate the importance of a scientific knowledge of the insects we 
deal with. A too prevalent belief is that the methods of applied 
entomology only concern the destruction of insects. An equally im¬ 
portant side is the preservation and encouragement of the vast numbers 
of species which are directly beneficial to the agriculturalist. Measures 
for dealing with an insect pest may therefore be divided into 
1. The utilization of natural methods of control. 
2. The adoption of artificial methods. 
(ft) Natural Methods of Control of Injurious Insects. 
These for the most part consist in the preservation and increase of 
those organisms which are definitely beneficial in that they destroy 
injurious species of insects. They mainly comprise carnivorous insects 
and insectivorous birdsk Carnivorous insects may be divided into 
predaceous species and parasitic species. (1) Predaceous insects are those 
which attack other insects directly and devour them. Immediate benefit 
is thus derived from their action. They comprise a very large number 
of species including Coleoptera belonging to the families of the Cicin- 
delidae, Carabidae, Staphylinidae, Coccinellidae together with the 
Cleridae, Cucujidae and .several minor groups. The larvae of the 
Dipterous families of the Syrphidae and Leptidae, and the Neuro- 
pterous family of the Hemerobiidae are likewise actively predaceous. 
Similarly many Rhynchota, also Mantidae, Empidae, Asilidae and 
Vespidae are likewise beneficial. All such insects, some more so than 
others, directly aid the agriculturalist. (2) Parasitic insects which 
deposit their eggs in the bodies of other insects or in their immediate 
neighbourhood. They pass the greater part of their life within their 
hosts whose death they sooner or later bring about through devouring 
their internal tissues. Parasites amount to tens of thousands of 
species and constitute Nature’s most effective method of control over 
the excessive multiplication of insect life. They are almost exclusively 
confined to the orders of the Hymenoptera and Diptera. Among the 
* For the role birds play in connection with agricultural entomology, vide Newstead 
(1908). Also Collinge (1913) and F. V. Theobald (1907). 
