Parkinsonia biocontrol agent, the seed-feeding beetle Penthobruchus germaini.

The seed-feeding beetle, Penthobruchus germaini was introduced to Australia to help control parkinsonia.

Parkinsonia biocontrol agent: seed-feeding beetle

The seed-feeding beetle, Penthobruchus germaini has been introduced to help fight the invasive woody weed parkinsonia throughout northern Australia.

  • 8 January 2007 | Updated 14 October 2011

Summary

Penthobruchus germaini (Bruchidae) was introduced from Argentina and released throughout Queensland, the Northern Territory, and the Kimberley Region of Western Australia. It is now established and relatively abundant in most parkinsonia infestations across northern Australia, with the exception of those in the Pilbara region of Western Australia.

Research is currently underway to better quantify the beetles impact, 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 Parkinsonia aculeata, and P. cercidicola which has only been recorded from the closely related genus Cercidium. P. 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 eight to nine days and tunnel down through the pod, then into the seed.

The seed-feeding beetle, Penthobruchus germaini is the only parkinsonia biocontrol agent released in Australia to become widespread.

Pupation occurs within the seed and adults emerge by cutting a hole through the seed coat and through the pod wall.

Adults can live for two to three 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).

Generation times at 30 °C range from 35 to 45 days (Briano et al. 2002).

Host-specificity

Host specificity tests have been conducted by the United States Department of Agriculture (USDA) at the Biological Control of Weeds Laboratory in Buenos Aires, Argentina, (Cordo and Briano 1987) and by Queensland Department of Natural Resources and Water (QNRW) in their quarantine facilities in Brisbane, Queensland, Australia. As a result of these tests 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. 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 QNRW, the Northern Territory Department of Infrastructure Planning and Environment (now Northern Territory Department of Planning and Infrastructure) and community groups at many of the main parkinsonia infestations in Queensland and the Northern Territory from March 1995 to approximately 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
  • pods not being available long enough for P. germaini to consume sufficient seeds.

Where results have already been analysed (in the east Kimberley and the Victoria River districts) seed predation rates have been low (less than 10 per cent of total seeds produced in a year). Egg parasitism has been very high (at least 70 per cent) and parasitoids are therefore likely to be an important factor limiting seed predator populations.

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.

Learn more about Parkinsonia: an introduced woody weed.

van Klinken RD. 2004. How important is environment?: a national-scale evaluation of a seed-feeding beetle on parkinsonia, a widely distributed woody weed. In: The Eleventh International Symposium on Biological Control of Weeds. Canberra, Australia.

Briano JA, Cordo HA, DeLoach CJ. 2002. Biology and field observations of Penthobruchus germaini (Coleoptera: Bruchidae), a biological control agent for Parkinsonia aculeata (Leguminosae: Caesalpinioideae). Biological Control. 24: 292–9.

Briano AE, Cordo HA. 1987.  Establecimiento y dispersión en Castelar (Prov. de Buenos Aires) del gorgojo Rhynocyllus conicus agente de control biológico del 'cardo pendiente' y del 'cardo'. Acintacnia. 26: 64-66.