Palm leaf beetle (Brontispa longissima) is a serious pest that is capable of killing small palms and weakening larger ones, making them more susceptible to other disorders.
This pest was initially subject to internal quarantine controls within Queensland and on the movement of palms interstate. These have now been removed. However, anyone moving palms out of Queensland should check current conditions with the relevant interstate authority. Western Australia maintains its own intrastate controls on palms and palm materials out of the Broome area.
The adult palm leaf beetle is about 10 mm long, 2.5 mm across, very flat and mostly black with an orange head and shoulders (top third of the wing covers). The larvae are white. Unlike most beetle larvae, the 'grubs' don't curl up and have two pincer-like spines at the rear end of the body.
The adults are very light sensitive, slow-moving and can be found hiding in young leaflets emerging from the centre of the palm. Flight distances are small, often only a few hundred metres, so natural spread is slow.
There are some native beetle species which could be confused with the pest including Anadastus sp. and Pleispa sp., however, they are found in fewer numbers, do less damage and have wing covers that are completely black.
Palm leaf beetle is a native of Indonesia and possibly Papua New Guinea and occurs from Indonesia to the Pacific Islands. It was first reported in Australia in Darwin in 1979 and since then in Broome (Western Australia), and from Moa Island in the Torres Strait through to Cooktown on Cape York Peninsula. In December 1992, it was found in the northern suburbs of Cairns, North Queensland. It has since spread in the Cairns area and to Innisfail, and in 2000 was detected in Marcoola on the Sunshine Coast and Townsville. It is predominantly a tropical pest of palms, although its climatic range was predicted to extend from Cape York into Northern NSW.
|Host range||Palm leaf beetle attacks more than 20 palm species with the coconut (Cocus nucifera) being a favoured host. Other common hosts include: the Royal palm (Roystonea regia), Alexandra palm (Archontophoenix alexandrae), Queen palm (Arecasrum romanzoffianum), Betel nut (Areca catechu), Dwarf fan palm (Livistona muelleri) and Fijian fan palm (Pritchardia pacifica). The range of palms occasionally affected includes: Fishtail palm (Caryota mitis), Golden cane palm (Areca lutescens), Date palm (Phoenixdacylifera), Dwarf date palm (Phoenixroebelenii), Washington palm (Washingtonia robusta), Foxtail palm (Wodyetia bifurcata) and others.|
Palm leaf beetle attacks the youngest leaf still in the throat (the spear leaf). This is the tightly packed spear that eventually opens out into a frond. The larvae chew off large areas of the surface of the leaflets causing the underlying tissue to die. Narrow, linear chew marks are characteristic of adult feeding.
As the leaf emerges, the leaflets curl and turn brown giving a characteristic scorched effect. As the spear unfurls, the beetles move to other plants or to the next emerging spear. Leaves that emerge undamaged will not be attacked by the palm leaf beetle.
The damage is often seen during the dry season in Northern Australia, when the pest is active and palm growth is slowed. The beetle rarely causes plant death in mature plants, however it can weaken the plant, predisposing it to other stressors such as disease or drought.
The palm leaf beetle spreads mostly through the movement of infested palms. If you are buying or moving palms from known infested areas, check to see if they are free of palm leaf beetle. A simple inspection for the pest on young leaves in the throat of the palm will be sufficient.
Palm leaf beetle in low numbers can be difficult to detect, so an additional safeguard that can be used in conjunction with inspection is chemical application. Before spraying, the spear leaf should be gently twisted to open up the leaflets and allow good penetration of the spray. A mixture should be prepared by adding 2 ml of a liquid carbaryl product (containing 50% carbaryl) per litre of water, plus a commercial wetting agent used at the manufacturer's recommended rate. The throat and unopened frond of each palm should be flood sprayed with this mixture.
Carbaryl kills the adults and larvae but not the eggs or pupae. A repeat spray is necessary about one week later to kill insects that have emerged from eggs and pupae. Care should be taken when using carbaryl as it is lethal to bees.
Destroy seriously affected palms.
A few natural enemies of the palm leaf beetle have been found in Darwin. For example, green muscardine fungus (Metarhizium anisopliae) can infect various stages of the beetle and cause death, especially during wet spells.
A small parasitic wasp (Tetrastichus brontispae) was first released in affected areas in 1994/95. There is incidental information suggesting this parasitoid does have an impact where it has established, however no recent surveys have been done to determine its distribution or effectiveness.
Foulis, S. and K. Halfpapp, (1997), Biological control of palm leaf beetle, Project NY 317, HRDC, Sydney, Australia.
Halfpapp, K. H. (1999), Further releases of the palm leaf beetle parasite and assessment of present distribution, Project 521, HRDC, Sydney, Australia.