Exploitation of sapling Banksia marginata by Cyrioides imperialis (Fabricius 1801) (Coleoptera: Buprestidae) in Tasmania

Abstract

First record of the use of sapling Banksia marginata by larvae of the jewel beetle Cyrioides imperialis.

Full text

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27
Exploitation of sapling Banksia marginata by
Cyrioides imperialis (Fabricius 1801)
(Coleoptera: Buprestidae) in Tasmania
Karen Richards & Chris P. Spencer
141 Valley Road, Collinsvale, Tasmania 7012
spenric@gmail.com
The impressive Cyrioides imperialis
(Fabricius 1801), is a widespread jewel
beetle species, occurring across the
Australian east coastal region, including
Tasmania. The common name, Banksia
borer, implies it is host specic to this
genus of Australian plants; species
known to be visited by the beetle include
B. marginata, B. serrata, B. integrifolia and
B. spinulosa; Tepper (1887) reported the
adults and larvae inhabiting B. marginata
with adult activity around Christmas.
More recently, records of the species
utilizing Leptospermum polygalifolium, (= L.
avescens) (Williams 1977) and perhaps
other Leptospermum spp. (Williams &
Williams 1983) were reported in a review
of larval host plants for the species
by Hawkeswood (2007). In that same
paper, B. marginata, B. integrifolia and
B. serrata are listed as larval hosts, and
Hawkeswood comments that the beetle
is becoming less common, speculating
this is possibly due to bushre. The
most detailed account of C. imperialis
[as Cyria] includes illustrations of adult,
larva and larval bore, provided by the
Victorian Government entomologist
French, who portrayed the species as a
destructive pest and suggested it may
destroy all the Banksia forests if not kept
in check (French 1900). Here we present
new information on the utilization of
juvenile B. marginata as an important
larval host plant.
A frequently encountered species,
particularly along the Australian
eastern sea-board, C. imperialis was once
considered to be coastal in Tasmania,
extending from Bruny Island, north
along the east coast, and west along the
north coast to Burnie (Cowie 2001). In
2014 the authors located a population
of the species inland near Conara and
another near Cleveland in 2015. Natural
Values Database and Atlas of Living
Australia searches revealed a previous
record from the Epping Forest region
in 2009 by S. Fearn. A further, most
intriguing record was reported from the
Lake Augusta area by J. Wood in 2009; at
this altitude in Tasmania, (1140 m.a.s.l.)
B. marginata occur as scattered alpine
dwarves, often possessing large bulbous
root boles. Cyrioides imperialis has also
been recorded from similar elevations
in the Blue Mountains, NSW. A study
to ascertain the population density of

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28
C. imperialis from the Tasmanian Central
Highlands would be most interesting.
Banksia marginata is widespread across
Tasmania and given the diverse range
of conditions from which the beetle is
currently documented it is likely that
C. imperialis has a much greater area of
occupancy than previously assumed.
Although French (1900) illustrated the
species, limited information is available
on its biology. French noted oviposition
occurred in the wood of B. integrifolia,
where, after hatching, the larvae began
feeding on the wood, tunneling into the
tree, often descending 20-25 cm below
ground level into the tap-root; one such
observation was recorded in a “smaller
Banksia bush”. Only one observation of
copulation has been published (Wilson
1977). References identifying larval host
plants are limited; aside from French
(1900), Hawkeswood (2007), reporting
an observation from NSW, describes
the position of an emergence hole in
Banksia serrata, with a height of 6 m, stem
diameter of 16 cm and an approximate
age of 15-20 years. The distribution of
B. serrata in Tasmania is limited to the
coastal region of the central north-west;
however, there are no conrmed records
of its usage by the beetle in this State.
We have recorded abundant C. imperialis
emergence holes in B. marginata regrowth
saplings and established trees in dry
Eucalyptus amygdalina forest at Cleveland
in central Tasmania. Despite having
multiple emergence holes, mature
banksias in this area appear healthy
and are obviously capable of sustaining
successive generations of larval attack.
However, this is not the case for small
regrowth saplings, which, though seldom
Plate 1. Cyrioides imperialis emergence hole in Banksia marginata. Scale button 10 mm diameter.

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29
having more than one emergence
hole, usually die from the attention of
the larva. Emergence holes in dead
juvenile B. marginata plants have been
recorded in stems of 45 mm diameter
(measurements taken at the level of
emergence); the smallest live tree bearing
an emergence hole had a trunk diameter
of 62 mm (Plate 1). In another live
tree, forked at the ground, each branch
bore an emergence hole at ground level,
the stem diameters were 80 mm and
120 mm; in both instances the trees
appeared healthy. Invariably, emergence
holes in small trees were within 30 cm
of ground level, the positioning of
emergence is apparently haphazard and
no preference of directional orientation
of holes was evident.
Oviposition sites appear to be low on
the stem, at or below the emergence
hole, as dissection of several previously
occupied root boles and stems revealed
no larval excavation above this point.
Examination of larval tunnels indicates
that the larvae descend, usually into
the root bole. Feeding is conned to
the conductive tissue, which is reduced
to frass packed into the wide shallow
bore that is often only 3 mm deep and
located immediately beneath the bark.
Though larger trees are able to survive,
small saplings are inevitably killed by
the voracious larval feeding which
effectively ring-barks the tree. The much
attened ultimate instar larva, which is
usually curled inside the bore, possesses
an enlarged pronotum 13 mm wide; the
body is very slender with a total length
of 55 mm (Plate 2).
The fully fed larva excavates a central
frass-packed tunnel connected to the
Plate 2. Cyrioides imperialis ultimate instar larva, dorsal view.

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30
pupal chamber (Plate 3) formed beneath
the stem surface, leaving as little as 1 mm
of intact bark covering the emergence
site. Adult female C. imperialis average 30
mm in length, 12 mm width and body
thickness of 10 mm, which parallels the
average emergence hole dimensions of
13 x 11 mm.
The larvae of the buprestid Castiarina
rudis (Carter 1938) feed in Orites revolutus,
as reported in Richards & Spencer
(2017). The larval habits of this species
are supercially similar to those of C.
imperialis, including feeding in the root
bole, the position of the emergence
hole, which is mostly low on the stem,
and larval occupation often causing
death in smaller stems. Over the course
of the current investigation many dead
saplings were uprooted to establish the
patterns and extent of larval occupation.
Larval activity was conned to the bole
of the tree, not extending into the larger
roots and no evidence of subterranean
emergence of C. imperialis was recorded,
though beetle exit points were observed
in several exposed living roots with
diameters as small as 30 mm on mature
trees. Saplings at the Cleveland location
occur in thickets, despite no obvious
signs of recent re damage. Cyrioides
imperialis appear to prefer the peripheral
2-3 m of these stands, since emergence
holes were seldom seen deeper in the
thickets. More beetle usage was apparent
in the younger cohort of B. marginata
trees with stem diameters under 12
cm, compared to mid-age regrowth of
20 - 25 cm diameter.
Captive reared adult C. imperialis
emerged in December and January,
while a fully hardened female imago was
collected from the pupal chamber in
December in the eld (Plate 4). Cowie
(2001) documents the adult ight period
as occurring from December to March;
specimens held in the authors’ private
collection were obtained between
December and February. Adults are
heavy, somewhat cumbersome iers
and only active in hot weather with little
or no wind. Despite their apparently
aposematic patterning the beetles are
cryptic amongst the foliage and new
growth of the food plant.
It is worth noting that not all dead B.
marginata are populated by C. imperialis
larvae. Lamprima aurata (Latreille 1817)
Plate 3. Male Cyrioides imperialis emergence
hole (A) and exposed pupal chamber (B).
Chamber dimensions 40 x 13 x 8 mm.

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31
has also been recorded in decaying
roots and stems, however, this species is
known to only utilize dead and decaying
wood (Fearn 1996). Fruiting bodies
of the saprotrophic fungus Mycena
clarkeana (Grgurinovic 2003) were also
occasionally found around the stem
base of dead saplings, with or without
beetle emergence holes. This fungus is
not pathogenic and likely inltrated the
tree post-death (G. Gates pers. comm.).
Larvae of the large cerambycid beetle
Paroplites australis (Erichson 1842) co-
occur in B. marginata but display a
preference for dead and dying mature
trees. The emergence holes of this
species may be found in the trunks of
living trees, but the greatest abundance
occurs where there is evidence of decay
or a dead branch. Emergence holes of
P. australis are ovoid, as opposed to the
D-shaped holes of C. imperialis, measure
up to 25 x 15 mm and are conned to
the trunk and larger branches.
Emergence of adult C. imperialis occurs
before the dead host tree loses its
leaves. As noted above, larval feeding is
restricted to the conductive tissue, and
deeper excavation is only undertaken
to provide a larger gallery leading to
the pupal chamber. Excavation of the
tree bole from which a male C. imperialis
exited revealed that the larva utilized
less than 20% of the available wood
and feeding activity was conned to
the sapwood immediately beneath the
bark. This limited use of the wood
resource suggests that the larval stage
is relatively short, perhaps 1-2 years,
which is probably similar to that of
other buprestid species currently under
investigation (Spencer and Richards
unpub. data). Cyrioides imperialis is the
largest jewel beetle in Tasmania, thus
one might expect a protracted larval
period and extensive use of the larval
host plant. It is surprising to discover
the small extent to which it excavates
Plate 4. Female Cyrioides imperialis imago emerging from exposed pupal chamber in
Banksia marginata (inset), and fully emerged imago.

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32
its host, suggesting that B. marginata
is a particularly nutritious food plant.
Although it is usually fatal to sapling B.
marginata, multiple attacks on mature
trees appears to have little or no effect,
therefore, contrary to the concerns
expressed by French (1900), the future
of B. marginata would seem to be assured
despite the attention of this magnicent
beetle.
Acknowledgements
Sincere thanks to Dr. Genevieve Gates
for advice and fungal identication, and
to Jill Richards for her enthusiastic eld
assistance.
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