January-February, 1952 NEW ZEALAND SCIENCE REVIEW Entomological Aspects of Yellow-Leaf Disease of THE yellow-leaf disease of Phor­ Phormium the insects belonging to the vector mium has been recognized for at group one in particular fell under least forty years. In many areas, it R. A. CUMBER suspicion because of its almost com­ has been responsible for the termina­ which might cause the condition by plete restriction to Phormium as a tion of fibre production, and half of indirect means or through toxic host plant and because of its preval­ those remaining in the North Island effects. ence in the badly affected areas. are seriously threatened by inroads of the disease. In the South Island Oliarus atkinsoni Myers—Vector the outbreaks have been much less of the Yellow-leaf Disease extensive. Both Phormium tenax J. G. Myers (1924) described a Forst, and P. colensoi Hook. f.. are Cixiid bug, O. atkinsoni, from a affected. series of specimens collected at Wai- There have been numerous at- kanae and Wellington, stating: "This temps to discover the cause of the large and sluggish species has gone yellow-leaf condition. Physiological a step further than Oliarus oppositus factors and the question of fungal, in its preference for ground herbage bacterial, and virus pathogens have as distinct from bushes, and is found been studied. A. H. Cockayne, in the only in flax swamps, apparently at­ early stages of his association with tached to the flax itself, where it may the problem, reached the conclusion he found sitting ... on the shaded that the condition was a physiologi­ side of the leaf." Myers dedicated cal one, but later the rapid spread the species to its discoverer E. H. ol yellow-leaf in some areas con­ Atkinson who was engaged on Phor­ vinced him that a pathogen was at mium research at this time. But the work. study of virus diseases was very much Only recently has it been shown in its infancy then, and no import­ that the condition is due to a virus ance was attached to the insect. which is transmitted by an insect. O. atkinsoni has been observed in Mechanical transmission using the the high country at Mangamuka. carborundum abrasion technique, North Auckland, feeding as adults and transmission by means of root on the leaves of Arundo sp., and the Adults of O. atkinsoni on a blade of grafting have also been obtained, and absence of Phormium in the vicinity Phormium tenax. The lining up of suggests that there may be host it is suspected that seed transmission individuals (males on each side of the may occur (Boyce et al. 1951). female) is a very characteristic beha­ plants in addition to that from which viour prutr to mating. As many as five the species was originally described. Entomological Surveys individuals (four males and one female) have been observed in this in November, 1949, adults of O. Kirk and Cockavne (1909), Coc­ attitude. atkinsoni were discovered at Moutoa. kayne (1915), and'Miller (1916, 17. and soon they appeared in consider­ 18, 20, 30) carried out surveys of The Phormium areas at Moutoa able numbers—especially in the Phormium areas, studying the insects were selected for special study. badly affected yellow-leaf areas. Pre­ which caused direct injury to stands. These include plantation and, in liminary experiments showed that More recently (1947), Boyce studied addition, what have been termed bugs caged on 7-8 months old seed­ the Cicada populations and attempted "induced areas." These latter com­ lings (in pots) fed readily, and the transmission of the condition by prise those through which Phormium transmission trials were set up early transferring Cicada nymphs from the has spread following the draining of in December. These first trials were roots of diseased plants to roots of stagnant swamps. Associated with the entirely successful in incriminating healthy ones. Phormium, more especially in the in­ 0. atkinsoni as a vector of the yel­ An intensive survey of the insect duced areas, are a large number of low-leaf disease (Cumber, 1952). A pop illations of Phormium areas was plant species—both native and intro­ high percentage of transmissions was commenced in June, 1949. The pur­ duced. The accompanying insect obtained; excessive yellowing was pose of this survey was to ascertain species are likewise of a high order apparent in from 4 to 7 weeks, and if the yellow-leaf condition might be and run to at least 500. Included in on final examination (May. 24, 1951) due to mechanical injury, and also these are approximately fifty species 16 of the 20 test plants, were diseased. to determine possible vector species of true bugs (Hemiptera) of which and none of the 20 control planis about twenty could be considered as si 10wed any symptoms. Dr. R. A. Cumber, F.R.E.S.. graduated possible vector species. B.Sc. from Victoria University College in 1942, subsequently gained his doctorate at The survey gave no grounds for The Ecology of the Vector and the University of London. He is at present suspecting that direct injury might Its Relation to the Spread Entomologist, Entomological Research Sta­ be the main causal factor of the yel­ of the Disease tion, D.S.I.R., and has been attached to the Phormium Research Station at Moutoa, low-leaf condition, and at an early The incrimination of the vector- where he carried out the important work stage attention was directed to the species and the subsequent study of described in the article. field of insect transmission. Amongst its biology have thrown light on the 4 NEW ZEALAND SCIENCE REVIEW January-February, 1952 history of yellow-leaf outbreaks. It has been common knowledge for many years that over-draining and stop-banking of rivers have resulted in outbreaks of the disease, and this more especially in those areas which no longer receive the periodical inun­ dations. Adults of 0. atkinsoni are present on the upper portions of the plants from November to March. For the remaining months the bugs occur in the nymphal stages only, and as such are restricted to the basal por­ rf-%r tions of the plants. Here they live in galleries amongst the dead leaf mate­ rial, feeding upon the roots which pass through or ramify therein. The nymphs secrete a filamentous mealy Flood ivatqrs covering the Phormium bushes to a depth of two feet promise some measure of control of the bug. The nymphal stages, which are restricted fluff which issues dorsally from the to the base of the plant, are killed by a submergence lasting 14 days. three posterior abdominal segments. This material is brushed off by the history study of 0. atkinsoni is provest­ . The rate of expression of symp­ walls of the spaces in wrhich the ing of considerable interest. Itomt s may well be affected by geneti- nymphs live, producing white, fluff- appears to occupy two years of whichca l constitution and the physiological lined galleries where the bugs alarl ebu t one month is spent in conditionthe s under which the plant is protected from predators and also egg and nymphal stages hidden away growing. from the weather—the fluff possess­ in complete darkness at the base of Experimental transmission work ing anti-wetting properties. the plant, reminding one somewhat with 0. atkinsoni to determine virus- Laboratory tests have shown that of the life-cycle of the cicada. This vector relationships is being con­ the nymphs possess considerable relon­ g nymphal cycle will be an ad tinued. In addition, seven other insect vantage in any attempt to control sistance to submergence, a small per­ species are being tested to ascertain the bug by flooding, but likewise a centage being able to survive this for if they play any part in the transmis­ disadvantage in any attempt to con­ a period of a week. When this period sion of the disease. These include trol the bug by the insecticidal spray­ •of submergence is doubled, however, the other New Zealand Cixiid species, ing of adults. a complete kill of all nymphs present Oliarus oppositus (Walk.), which is is obtained. It is known that the plant The yellow-leaf disease, as stated commonly found on Phormium and can withstand flooding to a depth previously of , is present both in thoccure s throughout both North and 2 to 3 feet for periods of up to Nort4 h and South Islands, but is moreSout h Islands on a very wide variety weeks during the winter months. prevalent in the North. 0. atkinsonio f host plants. 0. atkinsoni has become adapted is very widespread in the North to the conditions under which Phor­ Island, but has not been reported Acknowledgment mium tenax occurs naturally—i.e., from the South Island. marginal river and swamp areas— Miss D. Oakley, Grasslands Divi­ and is able to withstand the periodic sion, D.S.I.R., Palmerston North, Some Additional Points of floodings which these places receive, kindly prepared the photographs. Interest and Further Research provided they are not of too long a duration. Since the plant is able The toCixiida e or plant-hoppers References withstand considerably more inunda­ have only once (and this since the BOYCE, S. W., et al. (1951) : Prelim­ tion than the nymphal populations present work commenced) been in­ inary Note on Yellow-leaf Disease of the bug, it is anticipated that criminatea d as a disease-transmitting of Phormium. N.Z. Jour. Sci. & satisfactory measure of control ofgrou the p of insects. Thus, although the Tech., 33 (A), 76-7. bug may be obtained by flooding. prevalence of the bug in the diseased Thus a basis for the long-held belief areas immediately suggested that iCOCKAYNEt , A.