Pests and diseases of forestry in New Zealand

 Puriri moth

Puriri moth
Forest and Timber Insects in New Zealand No. 16

Revised 2009
Based on P.J. Alma (1977)

Insect: Aenetus virescens (Doubleday) (Lepidoptera: Hepialidae)
 
 
 Fig. 1 - A "7-shaped" tunnel made in a stem of Carpodetus serratus (putaputaweta) by a
caterpillar of the puriri moth. Length of the tunnel is 120 mm; length of the caterpillar is 58 mm.

Type of injury
The caterpillar of the puriri moth bores a 7-shaped hole (Fig. 1) in the growing trunks and
branches of host plants and feeds on the callus tissue at the opening of the hole.
 
Hosts
The puriri moth may be found in a wide range of indigenous and exotic hardwoods (Grehan 1984).
It will attack living eucalypts, but complete development has only been recorded in Eucalyptus
saligna (Grehan 1984). Early stage larvae feed on a range of fungal fruiting bodies (Grehan 1984).
 
Distribution
This native insect occurs only in the North Island.
 
Economic importance
Although caterpillars are usually found in relatively small-diameter stems and branches (up to 30
cm), the tunnel system persists indefinitely after the adult has emerged. The entrance hole is
eventually occluded by callus tissue and further growth, but the old tunnel serves as a focus for
core rots. The old holes are often used for shelter by other insects, particularly wetas, and so the
entrances may be enlarged and kept open for long periods. Timber sawn from the cores of
infested trees is unlikely to be free from defects, caused either by the holes themselves or by
secondary rots (Fig. 2). Damage caused by the caterpillars of the puriri moth in Nothofagus
(beeches) has restricted their potential for productive forestry in the North Island. Many exotic
species, for example Acacia melanoxylon (Tasmanian blackwood) and Eucalyptus species,
which are planted to enrich partially logged or poorly regenerated native forests, also act as
attractants to the insect. Attack on living eucalypts is usually abortive, the caterpillars being
flooded out by heavy gum flows. The kino, or gum reaction, produced by some species (for
example E. delegatensis) in response to this attack, may cause serious defects in sawn timber or
necessitate increased quantities of bleaching agents if the wood is used for pulping.
 
The puriri moth also attacks poplars and fruit trees, but so far this has been on a limited scale. It
is unusual for attack to kill trees, but small- diameter stems and branches may be ring-barked and
die. Damaged branches may be broken off in high winds.
 
 
 Fig. 2 - Defects in Nothofagus fusca (red beech) resulting from damage caused by caterpillars of
the puriri moth. (The small dark-stained holes and tunnels were made by native pinhole borers,
Platypus beetles).

Description, life history, and habits
The moth is the largest in New Zealand, with a wingspan up to 150 mm in the female and 100 mm
in the male. The moths are usually green, but the intensity of colour and wing patterning is very
variable (Fig. 3). Miller (1971), who describes the moth and its life history, records the
occurrence of bright-yellow, brick-red, almost scarlet, and even albino specimens. The females
have a dark-brown or black mottled pattern on the forewings and the hind wings are usually buff
coloured. The markings of the male's forewings are white and the hind wings are greener than
those of the female.
 

Fig. 3 - Female puriri moth above, male below. The wingspan of
the female can be as great as 15cm.

Moths may be found throughout the year, but are rare in winter and most abundant in September
and November. They are nocturnal and males in particular are attracted to light. The females
scatter their eggs indiscriminately in forested areas, and may lay up to 2000. The eggs are round
and pale yellow when first laid, turning black a few days later. They hatch in 12-14 days.
 
  
Fig. 4 - Fully grown caterpillar of the puriri moth in a stem of N. menziesii (silver beech).

The length of the life cycle varies from one to four years (Grehan 1987). Larval development involves
two phases: an initial period of approximately three months in forest litter, generally under logs and
pieces of dead wood or on fungi, before moving into live tree host for the second phase where the
feeding tunnel is formed (Grehan 1983). The tunnels are typically 7-shaped but frequently there are
extra vertical shafts (Fig. 4), presumably occupied by the caterpillar when it was smaller in size. The
caterpillar rests in the vertical shaft, but ascends to feed on living callus tissue at the entrance. The
tunnel slopes up from the entrance, presumably preventing the entry of rainwater. The opening is
covered with a tough, fibrous, silken web, which so well camouflages the holes (Fig. 5) that they
are often difficult to detect.
 
 
 
Fig. 5 - The first photo shows webbing in place camouflaging a tunnel entrance in silver beech.
In the second photo the webbing has been removed to show the entrance.
 
The caterpillar grows to about 100 mm long and 15 mm in diameter. It is a delicate transparent
purplish-pink with a hardened dark-brown head capsule.  The last-stage caterpillar (Fig. 4)
assumes a more creamy colour as fat accumulates in the body. The prothorax is hard and shining
brown. The remainder of the body segments bear pairs of light-purple plates. The caterpillars
pupate at the bottom of the vertical shaft after closing the top with a fibrous disc (Fig. 6). Prior to
emergence the pupa wriggles up the shaft, pushes up the disc, and protrudes through the
camouflaging web. Movement up the shaft is aided by twelve horny ridges, armed with hooklets,
on the upper surface of the abdomen, and five similar ridges on the underside. Pupae may be
found from August onwards, and the pupal stage is thought to last about three months.
  

Fig. 6- Pupa of the puriri moth in a stem of putaputaweta. Length of this pupa is 55 mm.

Control
No means of controlling this insect on a large scale are known. The caterpillars within their
tunnels are protected from insecticides, unless these can be introduced through the webbed
entrance. It may be possible to destroy the caterpillars in individual trees with a probe or toxic
liquid, if their tunnels can be located.
 
Attack on eucalypts usually occurs where branches have been pruned flush with the stem. If a
short stub of branch is left then this soon dies and prevents entry of the caterpillar.
 
Three fungi (including Beauveria bassiana) and one bacterium has been recorded attacking this
species (Grehan & Wigley 1984). Kakas are known to tear away the wood in their search for
the caterpillars, and moreporks capture the moths on the wing. It is probable that most newly-hatched
caterpillars exhaust their food reserves and die before finding or becoming established on their host plants.
 
Bibliography
Alma, P.J. 1977: Aenetus virescens (Doubleday) (Lepidoptera: Hepialidae), puriri moth. New
Zealand Forest Service, Forest and Timber Insects in New Zealand No. 16.
Grehan, J.R. 1982: Infection of Aenetus virescens (Lepidoptera: Hepialidae) larvae by the fungus Beauveria
bassiana. New Zealand Entomologist 7(3): 327–329.
Grehan, J.R. 1983: Larval establishment behaviour of the borer Aenetus virescens (Lepidoptera: Hepialidae)
in live trees. New Zealand Entomologist 7(4): 413–417.
Grehan, J.R. 1984: The host range of Aenetus virescens (Lepidoptera: Hepialidae) and its evolution. New
Zealand Entomologist 8(1): 52–61.
Grehan, J.R. and Wigley, P.J. 1984: Fungal and bacterial diseases of puriri moth, Aenetus virescens (Lepidoptera:
Hepialidae), larvae. New Zealand Entomologist 8(1): 61–63.
Grehan, J.R. 1987: Life cycle of the wood-borer Aenetus virescens (Lepidoptera: Hepialidae). New Zealand
Journal of Zoology 14(2): 209-217.
Miller, D. 1971: Common insects in New Zealand. A.H. & A.W. Reed, Wellington, 178 p.