Monday, 16 August 2010

Featured insect: Pantorhytes plutus (Coleoptera: Curculionidae)

The weevil genus Pantorhytes is a large genus placed in the tribe Pachyrhynchini in the subfamily Entiminae. It consists of over 74 species found primarily in New Guinea, but also being found in the Solomon Islands and Queensland. The species pictured here, P. plutus is found in the Bismarck Archipelago. A map showing the distribution of specimens in the Australian National Insect Collection (see here also) can be found here

Pantorhytes plutus and a number of other species in the genus have become major pests of cacao trees, particularly in PNG. All the species have fairly limited ranges, such that P. szentivanyi, P. albopunctulatus and P. healyi are pests in the Northern Province of PNG, P. torricellianus is a problem in the Sepik region, P. plutus through the Bismarcks, and P. biplagiatus through Bougainville and the Solomon Islands. The genus has had a surprisingly large amount of study done on their biology, including egg development, and control. A couple of studies have looked at their dispersal, including one study that used a radioactive isotope tracing technique, which provided theoretical insight into mathematical models of insect dispersal. A parasitic wasp, Pristocera rufa is known to parasitize P. szentivanyi, though not to such an extent as to be a reliable biological control agent.

They are such a threat, they have made it onto a page detailing the world's worst cocoa problems (though I cannot find any other evidence that Pantorhytes are in Tuvalu), and accordingly there's been a number of studies dealing to their control (such as this one and this one).Biopesticides, including Beauveria bassiana have also proved to be of use in their control. A photo of an infected beetle is shown above.

A circular detailing their control in the Solomon Islands recommends using ants as a form of biological control. Unfortunately, two of the species they recommend for this control are the yellow crazy ant (Anopolepis gracilipes) and the little fire ant (Wasmannia auropunctata). Both these species are highly invasive generalist predators and scavengers which have adverse effects on more than just Pantorhytes weevils in cacao plantations. Should they already be present in the area, their use as a control agent may be encouraged, but they should NOT be introduced anywhere for that purpose if they aren't already there.

Gressitt JL. 1966. The weevil genus Pantorhytes (Coleoptera) involving cacao pests and epizoic symbiosis with cryptogamic plants and microfauna. Pacific Insects 8(4):915-965.
Setliff GP. 2007. Annotated checklist of weevils from the Papuan region (Coleoptera: Curculionoidea). Zootaxa 1536. 296pp.
Stibick JNL. 1978. The genus Pantorhytes (Coleoptera: Curculionidae) Division A. I Addistions and changes to the common and major cacao species. Pacific Insects 18(3&4):115-136.

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