1919.] New Zealand Institute Science Congress. 271 
Of the stored-products and household pests approximately seven-eighths 
are exotic and one-eighth indigenous. 
Finally, the insects injurious to field and vegetable crops, although 
numerically inferior to any of the others already discussed, are responsible, 
perhaps, for a greater amount of damage in proportion to their numbers. 
The reason for this is probably due to the fact that the methods for 
controlling the other groups are better understood and more thorough, 
particularly relative to the orchard and live-stock pests, where legislation 
necessitates the adoption of suitable methods for their suppression ; also, 
the information regarding the field and vegetable foes is extremely meagre, 
so that there is very little in this respect upon which the agriculturist can 
base the disposition of his crops for the purpose of guarding as much as 
possible against the depredation of these insects. Unlike the other groups, 
where the exotic element is so well marked, the foreign and native species 
amongst our field-crop insects are more or less equally balanced. 
One of the great secrets in the reduction of noxious insects is the 
practice of clean farming. If it were fully realized that crop refuse and 
cuttings and rank growths fringing fields, ditches, and roadways are 
excellent harbours for various noxious insects to tide over the periods 
when crops are not available for them, these breeding-grounds would 
not be such a familiar feature throughout our agricultural districts. For 
example, the moths of cutworms and army-worms congregate amongst 
neglected growths of weeds and grass to deposit their eggs ; the larvae 
hatch therefrom and live upon these plants, awaiting the crop which 
is eventually placed at their disposal, and which they immediately 
attack. 
Again, in crop-rotation we have not only an invaluable factor in con¬ 
serving the soil-fertility, but also a method of incalculable value in the 
reduction of noxious insects. Before arranging a scheme of rotation the 
insect fauna should be considered, and a selection made in such a sequence 
that no two successive crops be closely related, thus breaking the con¬ 
tinuity of the food-supply of any noxious insect attacking one crop more 
than, or to the exclusion of, others. 
The advance of cultivation in a new country such as ours is responsible 
for upsetting the normal equilibrium and establishing sudden and unnatural 
changes in the environment affecting directly the various forms of indi¬ 
genous life, usually at the expense of these forms themselves, but frequently 
to the detriment of man’s economy. When the natural vegetation is 
cleared, for example, to make way for the plough, a great variety of plant- 
life, the home and food of innumerable insect forms, is destroyed. These 
insect inhabitants, then, must either perish or find new pastures ; some 
doubtless give up the struggle where they are, or are of necessity driven 
back into already fully stocked areas as yet untouched by cultivation, 
where the normal factors controlling the equilibrium will eventually reduce 
them ; but others, unfortunately, owing to lack of their natural food- 
supply, or because they are forced by the intensified struggle for existence 
due to the upsetting influence of cultivation, readily advance upon the 
farmers’ crops, which consist of a few selected plant varieties rendered 
more succulent than their feral relatives by long periods of intense domesti¬ 
cation. Illustrative of such phenomena are the New Zealand grass-grubs 
(Odontria spp.), the subterranean grass-caterpillars ( Porina spp.), the New 
Zealand army-worm ( Melanchra composita), the fig-tree borer ( Xylotoles 
grisens), and the apple-skin moth ( Cacoecia excessana ). 
