CONTROL OF THE NEW ZEALAND FLAX GRUB.

New Zealand Journal of Agriculture, Volume XII, Issue 6, 20 June 1916, Page 446

CONTROL OF THE NEW ZEALAND FLAX GRUB.

PRELIMINARY OBSERVATIONS.

DAVID MILLER.

Judging from observations made throughout the Dominion by the writer, the larval attacks- of Xanthorhoe praefectata upon New Zealand, flax (Phormium tenax) are most serious in the Makerua Swamp, on the left bank of the Manawatu River. It has been asserted that the regularity or irregularity of floods causes respectively a decrease or . increase of the grub; but it is found that Xanthorhoe ■ is just as liable, to seriously attack flax swamps which are very wet or subject to regular floods as areas that are perfectly dry. This is illustrated by the swamp at Otaitai Bush, in Southland, where four years ago, when the swamp was undrained and very- wet, the larval attacks were very serious; while now, though the swamp, is dry and well drained, the larvae have practically disappeared. Again, in the Waikato, the flax growing upon the river-bank and subjected to floods is : more seriously attacked than the flax on the higher levels. The best example is the extensive swamp area in the vicinity of Te Puke (near Tauranga), where the flax grows not only at sea-level and along the banks of the Kaituna River, where the- swamps are drained and dry, but also upon the terraces overlooking the riverflats, and consequently upon a variety of soils, such as clay, loam, and peat. Throughout the whole of this dry area the larvae have not yet made an appearance. The hill flax of Southland; is mostly free from Xanthorhoe, while, owing to the steepness of the ; ground, the country is well drained. This is also characteristic of - many other swamp areas, such as those at Koputaroa (in the Manawatu), Otaika (near Whangarei), Hikutaia (near Te Aroha), &c. On the other hand, in the Makerua Swamp, which has of late been subject to few floods and has been well drained, the larval depredations have become extremely serious. Again, in the swamps at Kawakawa (Bay of Islands), which are low-lying and undrained and

subject to severe floods (there being no fall), Xanthorhoe is absent. After a flood these swamps are not only free from all insects, both winged and otherwise, but owing to the ■ heaviness of the floods even the flax in parts is almost exterminated. The condition of the grub being absent from wet or flooded swamps is also seen at Gore, and two small areas in the vicinity of Te Aroha, and in a few other isolated cases. Although Xanthorhoe exists throughout New Zealand, its attacks on the flax are at present of no moment except in the Makerua Swamp. With few exceptions it was observed that Xanthorhoe confined its attacks to those bushes around and within the roots of which was an accumulation of decaying leaves and other rubbish, which apparently afford a suitable shelter to the grub during the day. Also, as a rule, there were no attacks on those bushes free from rubbish at the roots and having the fans well separated at the base, the butts being exposed to the light. In most public and private gardens where phormium is grown rubbish is not allowed to accumulate at the base of the plant, and invariably in such cases the attacks were totally absent. However, in part of the Dunedin public gardens the flax-bushes have been neglected and rubbish has accumulated, in which case the attacks of the grub are well marked. If the larvae be placed in the direct sunlight they become helpless. They are nocturnal in their habits, but have been noticed to commence feeding operations on dull and wet days. Temperature seems to affect, their movements; practically few or no fresh attacks were noticed in the North Auckland districts, a few in the Manawatu, many at Waikanae, while great activity was observed in Southland, where the weather was very . cold. It should be noted, however, that a month intervened between the observations of North Auckland and those in Southland, which last district was visited in April, about six weeks after the observations made in the Manawatu and at Waikanae. The grub exists in districts where severe frosts occurfor example, in parts of Southland; while in other districts also subject to frost the grub does not exist— e.g., on the margins of Lake Tutira, which has an elevation of about 800 ft. above sea-level. But the absence of the grub in this locality may be due to the presence of large numbers of swamp-hens and other native birds. The presence of birds may also cause the absence of the grub at Wairoa, where the flax grows at sea-level and is not subject to frosts which occur in the surrounding valleys.The presence of . birds (such as the swamp-hen) in the swamps probably plays an important part in keeping such pests as Xanthorhoe in check. I have noticed that in most of the Auckland, Hawke’s

Bay, Wairarapa, and Southland flax areas, where the grub does no material damage and is in many places totally absent, the swamphen is present and in several cases very abundant. Also, where there are large flocks of starlings the grub is by no means serious. It is said that the grub became more apparent in the Makerua Swamp after the extermination of the swamp-hen. Though this may be one of the causes of the increase of Xanthovhoc in this district, it will probably be found that other, changes in the environment have influenced the insect directly or indirectly. . It would prove invaluable to secure swamp-birds from various localities and dissect the crop in order to ascertain what forms their staple diet, and whether they are partial to X author hoe when other foods exist, as it is well known that certain insects are unpalatable to birds. I have also observed several small brown lizards amongst the flax in those districts where no Xanthorhoe exists, and the same may apply to these animals as to birds. It would be a simple matter to confine two or three swamphens within an area of flax badly attacked by the grub, and keep them under close observation. A wire-netting fence would be suitable if the wings of the birds were cut. Lizards could be kept under observation within a much smaller area than the birds. Live-stock would no doubt play an important part in freeing the swamps of undergrowth and rubbish. As already indicated, the larvae, being nocturnal, doubtless require some such shelter as decaying leaves, &c., to protect them from the sun and daylight. In the Makerua no stock are run, but on the other side of the Manawatu River —where the grub is not serious, there is a dense growth of grass, and stock are allowed access to the flax area. In many parts where the grub is not' serious stock are allowed to graze upon the grass of the swamps. Such is the case in Auckland, Southland, Nelson, &c. ; but at Wairoa and other large blocks, though the grub is practically absent,. no stock are grazed, since the flax is so dense that beasts would be unable to move about without damaging the leaf. If stock were employed it would be necessary to fence in each area as it is cut until the young flax had attained a sufficient growth not to be eaten by the animals. This would necessitate fencing off the newly cut areas, and - for this reason I suggest that sheep would prove the most satisfactory in this case, as they are not so liable to inflict damage as cattle frequently do, chewing off the succulent parts of the leaves to about half their length and leaving the fibre stripped. Besides, the outlay on fencing for cattle would no doubt be prohibitive. It would also be necessary to know at what periods the stocking of the swamps

would be most likely to damage the pest, and consequently a thorough knowledge of the life-history of Xanthorhoe would be required. In the Manawatu swamps the river, and to a less extent drains, form an impregnable barrier to the advance of the larvae of Xanthorhoe, but of course would hardly check the distribution of the moth. The fact that two areas such as the Moutoa and Makerua, so closely —being only separated by a rivershould differ so much as regards the attacks of the larvae points to a probable local difference, such as a more palatable leaf in the Makerua than the leaf of the Moutoa. This might be caused by some latent element in the plant-food of the soil made available by the heavy draining of the Makeruathe peat sinking, becoming more- compact and undergoing decomposition in contact with the atmosphere. The lack of this element may account for certain patches of the. Makerua being almost free from the attacks of the larvae, though surrounded by badly affected leaf. Not only is the Moutoa Swamp free from serious grub-attacks, but also the other areasthe Lockwood Swamp-on the same side of the. river. That the river should separate two classes .of soil seems very probable; but none of the areas on the Moutoa side of the river have been so severely drained as the Makerua. Again, the heavy draining of the latter swamp, combined with the peculiarity of the soil, may present some favourable condition for the pupation of the insect, affording perhaps greater warmth and protection to the pupa. But the large flax areas in the Bay of Plenty have also been well drained, and the peaty portions have sunk considerably, as in the Makerua, yet Xanthorhoe is practically absent in the former locality. However, ' though the one condition may occur in .both localities, they may differ in certain other features necessary for the increase of the insect. Or, again, in the Makerua the environment may have been so changed that some parasite or predaceous enemy has found the new conditions unfavourable. It is apparent that Xanthorhoe does not abound on hill country except where some rubbish or decaying matter affords a suitable protection. But here, again, the succulent parts of the leaf may be found unpalatable to the- larvae, as it is well known that it takes less leaf of the hill flax to obtain a ton of fibre than is required for the same amount of the swamp . flax, the former taking much longer to mature than the latter, and apparently having less edible tissue and more fibre. - ■ ■ Before any definite conclusions can be arrived at as regards pupation, it is necessary to know whether the insect pupates in

the ground, and, if so, at what depths; it would also be necessary to know where the female deposits the eggs, whether in sheltered or exposed conditions. The extinction of a natural enemy, whether parasitic or predaceous, would release a very important check on the ' increase of Xanthorhoe, and if this be so it would be necessary to discover what was the enemy, and what the cause of its —whether due directly to new conditions or indirectly through those conditions causing an increase of a secondary enemy preying upon the first ; . also what stage of the metamorphosis of Xanthorhoe is the most vulnerable, and if any stage is yet attacked by parasitic or. predaceous enemies, or if each stage is liable to be attacked by its particular enemy. As the greatest mortality to insect-life occurs in the wingless stages— i.e., egg, larva, and pupa —it would be most reasonable in this case to apply a check during these periods. However, by the use of birds a considerable decrease of . the moths might be brought about if it were known the extent of their flight, and whether the moth be agile or sluggish, how far and high it is capable of flying, also whether it usually alights near the ground or otherwise, as it often is to be found with'wings expanded on the flax-blades, its white colour rendering it conspicuous under these conditions. The larval attacks of Xanthorhoe. have been noticed upon other plants, such as bullrushes, and the absence within a particular area of an abundance of food plants other than flax would cause the larvae to concentrate their attention upon the remaining available food material. I have been unable to ascertain whether the grub selects one or more varieties of flax in preference to all others, but, so far as has been observed, most varieties* are liable to be attacked. • The insect fauna of the flax areas does not vary to any great extent throughout New Zealand, although I cannot definitely report on the insects of Southland, as the weather was extremely cold when the observations were made there. However, of the predaceous forms, the Libellulidae (dragon-flies) exist in large numbers in all areas of flax; but during the period of my visit (FebruaryMarch) they were more abundant in the North Island than in the South (visited during April). It is curious that the Asilidae (robberflies) abound in the Makerua Swamp, which in many places is very dry and the ground bare — natural habitat. But across the .river in the Moutoa Swamp, which is moister and carries a growth of grass, those Diptera do not exist. They may have some secondary bearing on the increase of Xanthorhoe in that they prey upon some other insect or insects which, before the advent of the

Asilidae, held Xanthorhoe in check, the new drier conditions of the swamp being more favourable to the Asilidae than formerly. These flies were not nearly so abundant, and were in most cases absent from the other swamps, both in the north and south. Another predaceous family, the Therevidae, with similar habits to the Asilidae, is represented in the Makerua but not in the Moutoa. It is evident that these predaceous insects do not prey upon Xanthorhoe, or at least to no marked extentperhaps because the latter are nocturnal and the former diurnalelse the pest would not be so marked. The parasitic insects are also well represented throughout the flax areas, and some of these insects will require to be investigated. In action taken to control Xanthorhoe it will probably be necessary to apply not only one method of checking, but a combination of methods, since the insect presents four different stages of development, each with its own peculiarities and, probably, enemies.

* Mr. David Miller, well-known in the Dominion as an entomologist, was some months ago attached to the Biology Section of the Department for the purpose of conducting a complete investigation of the pests and diseases of New Zealand flax, especially with a view of devising methods of control of the grub Xanthovhoe praefectata in the interests of the flax-milling industry. Editor. ..