THE HEMP INDUSTRY.

Manawatu Herald, Volume XXXIX, Issue 1769, 27 December 1917, Page 3

THE HEMP INDUSTRY.

CONTROL OF FLAX GRUBS; The following article on the above subject, by Mr D. Miller, entomologist, appears in the current issue of the Journal of Agriculture, the previous ones having been reproducedrin these columns some months ago:— . In ah article on the investigation into the control of the New Zealand flax grubs (Xanthorhoe praefectata and Melanchra steropastis) published in the June, 1917, Journal, mention was made of a natural enemy infesting the former species in the Riverton district. On looking carefully into this important matter it has been found that a similar parasite infests the larvie of X. praefeetata in the greater part of those flax (Phormium tenax) areas extending south of the Manawatu River between Shannon and Foxton to Waikanae. This parasite is a small ichneumon Avasp, which apparently deposits its eggs in the young flax-grubs. Up to the present no larvae of X. praefeetata from the badly attacked flax of the Makerua swamp (i.e,, north of the Shannon-Foxton line) have been found parasitized by this species, but fully 30 per cent, of the larvae captured in the swamps south of the Shannon-Foxton line have been found to be infested, and here the flax is comparatively free from the attacks of larvae. At the present stage of the investigation the data,.show that the mature parasites emerge simultaneously with the hatching of the first spring brood of larvae of X. praefeetata, so that the latter are open to the attack of the parasite. None of the grubs parasitized by this small species of ichneumon wasp and kept under observation reached beyond medium grow lb, since, at this stage the parasite pupates within its victim, the cuticle of which is utilized as a covering, so that the parasite pupa assumes the shape and occupies the whole length of the Jlax grub except for the. head and first thoracic and anal segments. On the formation of the parasite pupa within the flax grub a glutinous substance is exuded from the anterior end of the latter, gluing the dead grub to the'support, which is usually the inside of a dead flax-loaf. When emerging, the mature ichneumon cuts a circular aperture on the upper side of the old grub-skin, near the posterior end, and gradually works its way out. Owing to the comparative scarcity of flax-grubs in the parasitized areas, and in order to obtain a more definite idea concerning the actual percentages of grubs parasitized, as many young larvae as possible are being collected from the badly attacked swamps and placed in special cages giving access to the parasite in the parasitized a robs. It is expected by this means to procure sufficient numbers of the parasite to liberate in those swamps suffering from larval attack. Why this parasite should exist in one area and not in another, although practically adjoining, is an important feature which is being 1 gone into. Another larger ichneumon wasp, found commonly in all the Mariawatn and Waikanae flax areas, occasionally attacks the larvae of X. praefeetata, hut, unlike the foregoing species of ichneumon, pupates in the pupae of X. praefeetata, and not in the grubs. This species, so far as can be seen at present, does not confine its attentiton sufficiently to X. praefeetata to be of much use in controlling the grubs. In the article already referred to it was stated that the larvae of a hover-fly—Syrphus ortas —-attacked the larvoe of X, praefeetata. This 1 fly* is also very common in those flax areas comparatively free from laval attack, and doubtless plays a certain part in the control of these grubs. ( , The other flax grub (M. steropastis), which attacks the edge of the leaf, and not the lower surface, as does the larvae of X. praefeetata, is attacked by another species of parasitic fly—Phorocera marginata. However, both this parasite and S. ortas attack several other species of larvae besides the flaxgrubs. Therefore, as the matter stands at present,' our most valuable parasitic ally in "the control of the grub of Xanthorhoe praefeetata is the small ichneumon found in the areas south of the Shannon-Foxton line and in the Riverton district. ; Detailed experiments are being earned out regarding the effect of moisture upon the pupae of the flax--grubs, both of which usually pupate in the ground, A full account of the results will be published later on when the work is more complete. The informataion collected to date shows that moisture is apparently an important factor of control, r