NOTE ON BENEFICIAL INSECTS. l8l 



imported from Australia to California, and then many more 

 were bred. These ladybirds were set free here and there in the 

 orange groves; they started to feed on the Scale; soon they 

 mastered the Scale, and now successfully keep it in control. 

 There have been further successful importations to New 

 Zealand, the Hawaiian Islands and Portugal. Certain favour- 

 ing reasons made the importation of this ladybird successful 

 above the average. 



Parasitic insects belong chiefly to two Orders — the true flies or 

 Diptera and the membrane-winged insects or Hymenoptera. 

 In Diptera, flies of Tachinid species are very useful, a large 

 number of caterpillars — Moth Caterpillars and Sawfly Caterpillars 

 — being destroyed by the Tachinid larvse. For example, if one 

 collects a number of the pupae of the Magpie Moth a certain 

 percentage of Tachinid flies will come away instead of Magpie 

 Moths ; or if one collects cocoons of the Large Larch Sawfly, 

 some of the cocoons will yield not sawflies but Tachinids. 



It is to Hymenoptera, however, that most parasitic insects 

 belong, and chiefly to three nearly-related Families of the Order — 

 the Ichneumonidae, Braconidae and Chalcididae. The adults of 

 these Families (there are some exceptions among the Chalcids, e.g. 

 Megastigmus and Isosomd) lay their eggs on or in other insects, 

 in the %gg, or on or in the larva or pupa, and the larvae on 

 hatching feed at the expense of the insect parasitised, destroy- 

 ing the ^g^ or so interfering with the growth or development of 

 the host as to bring about its death. 



It is often difficult to estimate the part a native parasite plays 

 in checking increase of the species it parasitises. In ordinary 

 years, and without man's interference, the numbers of a species 

 remain about the same. Then conditions appear which so favour 

 a destructive species that its numbers come to be far above the 

 normal. The parasites of this species, favoured by the extra 

 numbers of the host insect, increase in their turn until the 

 numbers of host and parasite come again to be about the normal. 



Practically every insect has its parasitic enemies, and 

 entomologists expect to find some insects, in every brood they 

 rear, parasitised. During the last year or two I have had 

 occasion for class purposes to rear the Pine Sawfly, the Large 

 Larch Sawfly, the Large Cabbage White Butterfly, the Carrot Fly, 

 the Frit Fly, the Bluebottle, the Pine Beetle, and in every 

 case parasites were got, several of the insects having more 



