|
Parkinsonia biological control: Seed-feeder
(Bruchidae: Penthobruchus germaini)
|
Parkinsonia biocontrol agent
Penthobruchus germaini
|
Summary
Penthobruchus germaini was introduced from Argentina
and released throughout Queensland and the Northern Territory,
and in the Kimberley Region, from 1995 to ca 2003. It is now
established and relatively abundant in most parkinsonia infestations
across northern Australia, with the exception of those in
the Pilbara region in Western Australia. Research is currently
underway to better quantify impacts, but available evidence
suggests high rates of parasitism may be responsible for limiting
impacts.
Taxonomy and Origin
The genus Penthobruchus contains two species, P.
germaini which is host specific to P. aculeata,
and P. cercidicola which has only been recorded from
the closely related genus Cercidium. Penthobruchus
germaini is native to Argentina and Chile.
Lifecycle
Females only oviposit onto mature (or very nearly mature)
pods and free, mature seeds. Oviposition preference is for
pods on trees, when they are available. Eggs are glued onto
the surface of the pod or seed and covered with a fine membrane.
Larvae hatch within 8-9 days and tunnel down through the pod
and then into the seed. Pupation occurs within the seed and
adults emerge by cutting a hole through the seed coat and
through the pod wall. Generation times at 30oC are 35 to 45
days (Briano et al. 2002). Adults can live for 2-3
months and produce an average of 348 eggs under laboratory
conditions and when provided with a honey and pollen food
source (Cordo and Briano 1987).
|
Parkinsonia pods
|
|
P. germaini eggs
|
Host-specificity
Host specificity tests have been conducted by USDA at their
Biological Control of Weeds Laboratory in Buenos Aires (Cordo
and Briano 1987) and by QNRME in their quarantine facilities
in Brisbane. As a result permission was obtained to release
P. germaini in Australia in 1995.
Mass-rearing and Release
P. germaini is easy to mass-rear on harvested mature
pods kept in polystyrene boxes, together with a mixture of
pollen and honey as an adult food source. However, care must
be taken to ensure that colonies are free of both egg and
larval parasitoids to prevent their unintentional spread.
Large scale releases and redistributions were made by QNRME,
NT DIPE and community groups at many of the main parkinsonia
infestations in Queensland and the Northern Territory from
March 1995 to ca 2003.
Establishment, Damage and Impact
P. germaini is established throughout Queensland, the
Northern Territory and the Kimberley region of Western Australia.
Only small releases have been made in the Pilbara region of
Western Australia (in 2002-03) and there is not yet any evidence
of establishment there. High levels of seed predation have
been reported in parts of Queensland.
Current Research
Research is currently underway at sites across Australia to
determine what proportion of seeds produced each year are
being predated by seed-feeders, including P. germaini,
and to determine what factors might be limiting seed predation
rates. Possible factors include egg and larval parasitism,
and pods not being available for long enough for P. germaini
to consume sufficient seeds.
Where results have already been analysed (east Kimberley
and the Victoria River District) seed predation rates have
been low (< 10 % of total seeds produced in a year). Egg
parasitism has been very high (at least 70%) and parasitoids
are therefore likely to be an important factor limiting seed
predator populations. Also, most seeds are only available
to seed-predators for a few months in this region because
pods rapidly decay through the wet season, releasing the seeds.
Seed availability is even briefer in wetland habitats that
are inundated soon after pods are produced.
References
-
van Klinken, R.D. (2004). How important is environment?:
a national-scale evaluation of a seed-feeding beetle on
parkinsonia, a widely distributed woody weed. The Eleventh
International Symposium on Biological Control of Weeds,
Canberra, Australia.
|
