535 



An illustration of this is Xanthorhoe praefedata (New Zealand flax 

 moth), the larvae of which confine their attention normally to flax 

 [Phonnimn tenax] growing in comparatively dry conditions. As the 

 flax industry in New Zealand extended, swamp areas were drained 

 comparatively dry and covered with a growth of flax, with the result 

 that the moth, being provided with abundance of food under suitable 

 conditions, increased with alarming rapidity. The larvae of X. prae- 

 Jectata are heavily parasitised by an Ichneumonid, which is itself 

 limited by a hyperparasite, and it is probably the factors controlling 

 the hyperparasite which influence the abundance of the larvae of 

 X. praefedata. A chart shows the relation of X. praefedata to primary 

 and secondary parasites. Similar instances of indigenous phytophagous 

 insects developing injurious tastes are those of Melanchra composita 

 (New Zealand army worm) and Odontria spp. (New Zealand grass-grub 

 beetles). The natural food of M. composita consists of native grasses, 

 though it occasionally attacks oat crops in large numbers. Very little 

 is known of the Odontria, but their life-histories are being studied. 



While a certain amount of damage is caused to cultivated areas 

 through the depredations of native insects, far greater loss is caused 

 by injurious species accidentally imported from other countries. 

 These insects rapidly increase under new conditions in the absence 

 of their natural enemies. A notable example of this is Eriococcus 

 coriaceus (gum-tree scale), a native of Australia, where it does little 

 damage owing to the presence of natural enemies. The first outbreak 

 of this scale in New Zealand was noticed in 1900. when great destruction 

 was caused, and by 1905 the insect had spread north and south for 

 a distance of 180 miles, killing many trees and causing great damage. 

 Coccinellids that were known to control the scale in other territories 

 were then introduced from North Auckland and from Australia, 

 Rhizobi'us ventralis being the most effective in Canterbury, where it 

 became so well established that within three years E. coriaceus had 

 been reduced to a negligible quantity. It is found that when a 

 predaceous insect begins to reduce an outbreak of its host, although 

 the latter is superior in numbers and fertility, it will in a few generations 

 be in excess of its host. In the case of a parasite confined to one 

 host, however, the excess of the latter will of necessity disappear 

 owing to lack of sufficient food, until only enough remain to retain 

 the normal equilibrium. 



Oligotropus alopecuri (meadow foxtail midge) by its depredatious 

 upon the developing seed of the meadow foxtail grass has produced 

 a serious situation [see this Revieiv, Ser. A, v, p. 165]. Before the 

 midge was considered an important pest, there was every likelihood 

 that sufficient quantities of the seed would be produced in New Zealand 

 for local and export purposes. Owing to this pest, which was probably 

 imported in seed from Europe, there is now no seed grown in one 

 district where formerly the output was 18 tons annually. Infestation 

 was first noticed in 1910 ; by 1915, 70 per cent, of the seed was 

 rendered useless, and in the following year this percentage was even 

 higher. It is known that early settlers in the Dominion were able 

 to grow various exotic fruits without loss from insect pests. In 

 later years, however, these became so well established that fruit- 

 growing threatened to come to an end. Organisation and the 



