Farm Forestry New Zealand

PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Eriococcus orariensis, Causal Agent of Manuka Blight

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Forest and Timber Insects in New Zealand No. 23.
Causal agent of manuka blight.

Based on R. Zondag (1977)

Insect: Eriococcus orariensis Hoy (Hemiptera: Coccoidea: Eriococcidae)


Fig. 1 - "Blighted" manuka.
 

Type of injury
Feeding by large populations of the scale insect Eriococcus orariensis over a period of 2-5 years can debilitate and kill manuka. The insects suck sap and excrete large amounts of excess sap sugars as fine droplets of solution called honeydew. Where the honeydew fails on stems and foliage it provides food for a soot fungus which blackens infested plants, so they appear "blighted" (Fig. 1). Although the photosynthetic activity of leaves covered with the soot fungus is reduced, this alone has little effect on the health of the plant. Debilitation and death are believed to be the direct result of massive withdrawal of sap nutrients by nymphs and adult females.

Hosts
Only Kunzea and Leptospermum species are attacked by E. orariensis. In New Zealand there are two - K. ericoides (kanuka) which is little affected, and L. scoparium (manuka) which may be weakened and killed.

Distribution
This insect, an accidental introduction from Australia, is present throughout New Zealand. The blight, first seen in Canterbury about 1937, had by 1948 spread at the most only 145 km from its original place of discovery. During the following 10 years many farmers who regarded manuka as a major weed deliberately disseminated infested twigs. As a result, the scale insect became widespread throughout North and South Islands by the late 1950's with consequent death of manuka. However in 1957 an entomopathogenic fungus, Myriangium thwaitesii Petch, was found attacking E. orariensis, and this seems to have resulted in the decline of this species, and its displacement by Eriococcus leptospermi Maskell, which rarely causes the death of manuka plants (van Epenhuijsen et al. 2000). Eriococcus orariensis is now rarely found, and it appears that its range has contracted (van Epenhuijsen et al. 2000).

Economic importance
As manuka is a natural nurse crop for seedlings of indigenous forest trees, its loss because of scale insect attack was deplored by some foresters. On the other hand, farmers welcomed this means of ridding their lands of manuka. However, since the displacement of E. orariensis manuka has recovered in most areas and the scale is now of little importance from either point of view.

Description, life history, and habits
Most features used to identify scale insects can only be seen by using a microscope. However, most members of the family Eriococcidae can be recognised by the presence of a pair of small hardened lobes at the rear end of the bodies of females and immature males, and these lobes can be seen with a hand lens.

Adult female E. orariensis are reddish to light brown, oval, approximately 1.25 mm long, and 1 mm wide tapering to the rear (Fig. 2). Like all female scale insects they are wingless. They have a pair of antennae each composed of six segments and, although possessing three pairs of well-developed legs, are incapable of movement at full maturity because of their body size. Waxen threads are produced from pores on the insect's abdomen; after mating, the female uses these threads to construct a white felted sac which encloses the whole of her body except for an opening at the rear. Each female lays about 50 eggs over a period of several weeks. The eggs hatch within half an hour of being laid and the minute nymphs or "crawlers" wander over the plant until they find a suitable place to insert their stylet-like mouth-parts. The female nymphs moult twice before becoming adult, and usually remain in the bark crevice into which they originally settled.

Behaviour of male nymphs is similar to that of females up to the end of the second moult. They then disperse over the host and construct sacs in which they pupate. These sacs are more cottony and not so closely felted as those of the females. The female sacs are always hidden in fissures under strips of dead bark and, although males favour a similar position, high populations may result in the male sacs being made on the bark surface, on leaves, or even on seed capsules. The adult male is a delicate insect with one pair of iridescent wings and a pair of long waxy filaments protruding backwards from the abdomen. The body is 0.75-1.00mm long and the last segment is developed into a spike. Males do not feed and probably live no longer than 48 hours. A male may mate with several females, but females mate only once.

The development from egg to adult takes several weeks, depending upon temperature. In summer the minimum generation-time known is 11 weeks and in winter 25 weeks. The generations overlap, all stages can be found at the same time, and there can be from two to three-and-a-half generations in a year. Natural transfer to new plants is mainly by wind dispersal of the young crawlers. Some transfer may also occur through crawlers being carried between plants on the bodies of other insects; or on birds, or on the coats of browsing animals.

The soot fungus which gives infected plants their fire-blackened appearance is Capnodium walteri. The mat of fungus may be as much as 3-4mm thick, and other plants growing under or near “blighted” manuka may also be covered if honeydew has fallen on them.

Several other scale insects occur on manuka. One, also introduced from Australia, is E. leptospermi. The white felted sacs of this insect are always made on the surface of bark or leaves, and often occur near the tips of twigs. They are not hidden like those of E. orariensis. The native scale insect, Coelostomidia wairoensis (Maskell) (family Margarodidae), may also be present. Adult females of C. wairoensis are orange-red, oval, 3mm long, and have a hard waxy covering. This species produces a copious flow of honeydew and there is a consequent heavy growth of soot fungus, usually Capnodium elegans. Manuka is not known to be killed by either E. leptospermi or C. wairoensis.

Eriococcus orariensis has little effect on kanuka probably because too few of the young of the scale insect are able to become established on a plant. As it grows, kanuka (in contrast to manuka) tends to shed its bark and the resulting smoother surface presents fewer establishment sites. It is not known whether other factors in the physiology or the way of growth of kanuka contribute to its apparent resistance to attack.


Fig. 2 - Adult females and nymphs of E. orariensis on stem of manuka. Bark strips covering the scales have been removed. Length of adult female is about 1.25 mm.

Control
Although parasites and predators keep E. orariensis under control in Australia, no attempt has been made to introduce them into New Zealand. No native parasites attack it in this country, but on a few occasions the predatory ladybird beetle Rhizobius ventralis, introduced for control of gum-tree scale Eriococcus coriaceus, has been found feeding on E. orariensis.

Main control is exerted by the parasitic fungus Myriangium thwaitesii, the hyphae of which penetrate the scale insect and kill it. This fungus can be seen protruding through the bark as brown, wart-like lumps approximately 2-3 mm in diameter (Fig. 3). It attacks only E. orariensis. 

Chemical control in forests is unnecessary and impracticable. Ornamental manuka can be sprayed with a systemic insecticide, such as an organophosphate, or with summer oil. Contact insecticides will not penetrate the waxy coating of the scale insect.


Fig. 3- The fungus Myriangium thwaitesii, parasitic on E. orariensis, appears as wart-like lumps (arrowed) on the bark of manuka.

Bibliography
Epenhuijsen, C.W. van; Henderson, R.C.; Carpenter, A. and Burge, G.K. 2000: The rise and fall of manuka blight scale: a review of the distribution of Eriococcus orariensis (Hemiptera: Eriococcidae) in New Zealand. New Zealand Entomologist 23(1): 67–70.
Hoy, J.M. 1961: Eriococcus orariensis Hoy and other Coccoidea (Homoptera) associated with Leptospermum Forst. species in New Zealand. New Zealand Department of Scientific and Industrial Research, Bulletin 141, 70p.
Zondag, R., 1977: Eriococcus orariensis Hoy (Hemiptera: Coccoidea: Eriococcidae), causal agent of manuka blight. New Zealand Forest Service, Forest and Timber Insects in New Zealand No. 23.

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