Banana weevil, Banana weevil borer, Banana root borer
Worldwide. It is present in all banana-growing areas of the world (Asia, Africa, North, South and Central America, the Caribbean, Eurolpe, Oceania. It is native to Malaysia and Indonesia. It is recorded from American Samoa, Australia, Cook Islands, Fiji, French Polynesia, Guam, New Caledonia, Niue, Palau, Papua New Guinea, Samoa, Solomon Islands, Tonga, Vanuatu, and Wallis and Futuna.
Banana, including all dessert and cooking types, and Manila hemp (Musa textilis).
The grubs or larvae do the damage. They bore into the corm below ground to feed. When there are many weevils, tunnels occur throughout the corm, and into the pseudostem.
Female weevils lay 1-3 eggs a week; the eggs are oval, about 2 mm long. They are laid singly in pits chewed between the leaf scars on the corms near or just above ground level. They are also laid on plants blown over in the wind, where the corm and leaves meet, and on the side facing the soil. The eggs are very hard to find as they become covered in plant sap. Eggs hatch in 5-7 days, and the stout, legless grubs that emerge are creamy-white with reddish-brown heads. After 20 days of tunnelling through the corms they are fully-grown. They then make oval chambers near the corm surface and pupate. Eight days later, adults emerge. They are reddish brown at first, later black, about 12 mm long, with a typical weevil snout (Photo 1). They feed on plant tissues or crop debris, and the damage done is slight. In Fiji, the life cycle is about 50 days.
The adults have well-developed wings, but rarely fly, and when disturbed pretend they are dead. During the day they hide under debris or in the soil around banana plants; at night they walk short distances over soil and vegetation. Spread is mostly in infested planting material not by other means. The weevils live up to 2 years.
The tunnelling by the larvae: (i) causes rots, and stops the flow of nutrients to the leaves so that they appear unhealthy and die prematurely; (ii) weakens the plants so they are easily blown over; (iii) leads to suckers withering and dying; and (iv) produces small bunches of undersize fruit. It is often difficult, however, to tell if the weevil is the cause of poor growth, if it is poor nutrition, or nematodes are attacking the roots (see Fact Sheet no. 08). In Samoa, the weevil is a serious pest in some areas only. In Fiji and Australia, yield losses are more likely in poorly maintained plantations.
Look for larvae and their tunnels in the corms just above and below ground level. Cut discs (10 cm thick) from freshly harvested banana stems and put out about 50 discs per ha within a plantation, placing each on the soil at the base of a banana stool, and covered with leaves to prevent drying out. After 5 days, count the number of weevils on the discs. An average of four weevils per disc indicates treatment is required.
In countries where the weevil is already established, planting material should be carefully checked before it is used to established new plantations (see below). Those countries yet free from the weevil should follow the FAO/IBPGR Technical Guidelines for the Safe Movement of Germplasm. No. 15. Musa. 2nd Edition, (http://www.bioversityinternational.org/uploads/tx_news/Musa_spp.__2nd_edition__502.pdf).
Early last century, entomologists from Fiji went to Indonesia, the presumed origin of Cosmopolites sordidus, and found several beetles preying on the weevil. The most important was a histerid beetle, Plaesius javanus . Later, this beetle was introduced into Fiji, along with another histerid, Plaesius laevigatus, which looks similar. Plaesius javanus is also established in Samoa, and many other Pacific island countries, but not Tonga. The larvae and adults of Plaesius javanus attack both grubs and pupae of Cosmopolites sordidus. In Fiji, it appears to have reduced the damage done by the weevil, but this needs to be confirmed. The ant, Tetramorium bicarinatum, is reported to control the weevil in Cuba, but its effect in Papua New Guinea, and elsewhere in the Pacific, where it also occurs, is unknown.
Cultural practices are important in preventing outbreaks of this insect. Do the following:
There are several insecticides in different groups that are registered for use by the Australian Pesticides and Veterinary Medicines Authority, and they can be found at www.apvma.com.au. Do not use insecticides from the same group for more than one year. Examples of those chemicals registered include chlorpyrifos, fipronil, bifenthrin and imidacloprid. An aggregation pheromone (sordidin) that attracts both sexes is available and used for monitoring (4 traps/ha) and mass trapping (20 traps/ha). (Note, chlorpyrifos is under review in Australia).
AUTHOR Grahame Jackson
Information from Waterhouse DF, Norris KR (1987) Biological Control Pacific Prospects. Inkata Press. Photo 1 Gerald McCormack, Cook Islands Biodiversity & Natural Heritage. (http://cookislands.bishopmuseum.org/).
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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