BENEFICIAL VERSUS INJURIOUS INSECTS. 
degenerated, or has suffered disuse atrophy, as its wings were no longer 
used, and consequently were less and less developed. Most of the para- 
- sites that are used in controlling insect pests show very little degeneration, 
being parasites for part of their life; and for the rest are extremely 
active independent insects. Parasites may be restricted in their 
parasitism to one stage in the development of their host, 7.e., they may 
parasitize the egg only, or the larva only, or may confine their attention 
to the pupa. A complete sequence of parasites is of greater efficiency 
than multiple parasitism. The introduction of predators and parasites 
which have counterparts in other places is of little value, while ‘the 
introduction of new species which form a new element in the chain of 
sequence has always great possibilities. In the first case, the introduced 
insect merely replaces a portion of the individuals of the local species; 
while in the second case there is no replacement, and the effectiveness 
of the introduced species is added to the effectiveness of the species which 
_ already occur in the region, but are of different habits. 
Spring Grain Aphis, Lysiphlebus tritici. Parasite on Green 
Bug, Toxoptera graminum. (After Hunter.) i 
The economic use of entomophagous insects may show three 
phases :— ; 
(1) The introduction from other ‘countries of species which do not 
occur in the country infested with the insect it is desired to control. 
(2) The transportation of the parasites from one distriet to another, 
(3) The utilization of native species already parasitic or predatory — 
in the district, increasing by artificial means the number of the 
individuals already present in such a way as to bring about a higher 
relative mortality in their host than would have occurred if left to act 
under normal conditions. This involves an attempt to make the 
entomophagous insects continually dominate their hosts, a condition 
which will not exist if natural conditions are allowed to prevail. Control — 
by this method has to take account of the fluctuations of hosts and 
parasites. The balance of nature is like a pendulum swinging to and 
é 477 
