270 The N.Z. Journal of Science and Technology. [July 
is of very rare occurrence over a 'considerable period before favourable 
conditions present themselves for its abnormal development. This is illus¬ 
trated by the steel-blue saw-fly (Sirex juvencus), which, although occurring 
in New Zealand for approximately the last fifteen years and at times 
attacking conifers, is as yet comparatively rare. Of considerable interest to 
fruitgrowers is the initial outbreak this summer of the pear-midge ( Perrisia 
pyri) in the Auckland district, but precisely when this fly was first intro¬ 
duced is not at present known. On the other hand, the Australian gum- 
tree scale ( Eriococcus coriaceous) spread with marvellous rapidity practically 
from its introduction, so that within five or six years the Eucalyptus 
plantations of Canterbury and Otago were seriously infested. It is interest¬ 
ing to note that this scale insect does no damage to Eucalyptus trees in 
Australia, where it is preyed upon and held in check by a ladybird beetle 
(Rhizobius ventralis). Consignments of this latter insect, therefore, were 
brought from Australia and established in the infested areas, with the result 
that the scale was practically eliminated within a few years. 
If we review the noxious insect fauna of New Zealand it will be found 
that, of the approximate total of species already recorded, the indi¬ 
genous element equals only 18 per cent. The countries of origin of the 
exotic 82 per cent, are Europe, Australia, 
North America, Pacific islands, and South 
Africa. The European element is by far the 
most prominent, making up about 68 per 
cent.; of the others, the Australian accounts 
for approximately 9 per cent., the North 
American 2 per cent., and the Pacific 
islands, South African, and cosmopolitan 
1 per cent. each. The most pronounced 
elements, therefore, of our destructive in¬ 
sect fauna are the European and the 
indigenous, but in proportion the latter 
is hardly one-fifth of the former, although 
there is every likelihood of additional native 
forms becoming injurious. 
Considering our noxious insects from 
the viewpoint of their habitats, it will be 
found that approximately 28 per cent, are 
orchard pests, 22 per cent, affect live-stock 
stored products and household goods, and 19 per cent, damage field and 
vegetable crops. Although the orchard pests comprise the largest group, 
the damage caused by them varies inversely as their numbers, owing 
to the extensive use of contact and poison sprays. However, if the 
fight against these pests were at all lessened the damage due to them 
would correspondingly increase, with disastrous results to the fruit 
industry. In the early days of the Dominion, before most of these 
destructive insects had become established, exotic fruits could be grown 
without fear of attack, but later on the fruit industry was so dan- I 
gerously threatened by insect depredations that it would have come to a 
standstill had rigorous measures not been adopted. It is of interest to 
note that approximately two-thirds of these orchard pests are exotic and I 
one-third indigenous. 
The members of the group injurious to live-stock and man are, as far 
as we know, all exotic, and do not as yet cause such extensive damage 
to their hosts as they do in their native habitats. 
Proportion of number of insect 
species (black) to species of all 
other animals (white). 
and man, 21 per cent, attack 
