Banana weevil borer

Scientific name

Cosmopolites sordidus

Description of adult

Adult weevils are about 10–12mm long, hard shelled and have the pronounced snout typical of weevils. The newly emerged weevil is reddish brown but soon becomes uniformly dull black. The weevils are nocturnal and hide during the day in or around corms or in moist areas near the plant and in the trash. Unusually sluggish in their movements, they feign death when disturbed and seldom fly. Natural spread is very slow. Dispersal is primarily by the introduction of infested suckers and bits for planting.

Immature stages

Eggs are laid singly in a shallow pit at the base of the pseudostem. They are elongate, oval, about 2mm long and pearly white. The eggs are very hard to find because the oviposition site becomes covered by congealed sap. The soft, creamy-white, stout (up to 10mm in length), legless larvae have a distinctly curved body, are swollen in the middle and have a hard brown head. Pupae are about the same size as the larvae. Inside the white pupal skin the structure of the future adult with its snout, wing buds, legs and antennae is visible.

Life history

An average life cycle is completed in 12 weeks in north Queensland. The eggs hatch in 8 days. The larvae immediately burrow into the corm and, after 4 moults lasting a total of 3–14 weeks depending on temperature, pupate close to the surface of the corm. The short (8-day) pupal period is followed by the emergence of the reddish-brown adult. The pre-oviposition period varies and may extend to 40 days. Adults are long lived.

Two peaks in adult emergence and activity have been noted-the first in spring during September and October, and the second in March and April. These peaks are particularly evident in south-east Queensland where activity almost ceases in winter. In north Queensland, where winter temperatures are higher, activity continues throughout the year although at a reduced rate in winter. In north Queensland, dry conditions greatly reduce adult activity, whereas rainfall may be a major factor in increasing adult activity (as determined by adult weevil borer traps).

Distribution

Throughout coastal Queensland and New South Wales, associated with cultivated and wild bananas.

Host range

Hosts include members of genus Musa including banana (M. sapientum) and abaca (M. textilis).

Damage

Usually minor and frequent in northern Queensland but has a greater impact in southern areas. It can become an important pest in poorly managed plantations.

The larvae tunnel within the corm that lies below the soil surface. When there are large populations, tunnels are found through most of the corm tissue and a short distance up the pseudostem. This tunnelling weakens the plant and renders it susceptible to 'blowdown' in windy weather. Heavy infestations interfere with the movement of nutrients to the plant, and plants appear unthrifty. In severe cases the young suckers whither and fail to develop.

In some situations it is difficult to determine if weevils are the main cause or a symptom of plantation decline, since other factors (primarily burrowing nematodes) may be contributing to poor growth. Because of slower growing conditions in south-east Queensland, this pest can become a major pest, reducing plant growth and causing extensive plant fall outs.

The economic impact of weevil borer varies considerably with rate of growth of the plant. In northern Queensland, populations are rarely of economic importance and only neglected or poorly managed plantations suffer production losses attributed to this pest. In southern Queensland, where it is difficult to maintain a high level of plantation hygiene, plant growth is slower due to cooler conditions and where plantations are maintained for many years, the economic significance is much higher leading to yield losses. Typical symptoms of a severe infestation are reduced plant growth, choking of the bunch in the pseudostem, yellow leaves and weak or dying suckers.

The development of insecticide resistance has increased the potential threat of weevil borer.

Control

At monthly intervals during March to April and September to October monitor populations using 100mm thick pieces of cut pseudostem as baits during periods of adult activity. At other times 6-weekly intervals would be sufficient. The pseudostem material selected for making baits should be taken from the lower part of the stem of freshly harvested plants. Place at least 20 baits at the base of 20 randomly selected plants and with the cut surface in full contact with the soil after removing any weeds and debris. They should be covered with leaf material to prevent drying out. Count weevils after 3–4 days and treat if the average count is greater than 2 adults per trap (in south east Queensland) (i.e. greater than a total of 40 adults over 20 trap baits). In northern Queensland an average count above 4 per trap would be more appropriate. Concentrate baits in known 'hot spots'.

Adult weevil activity increases during warm and/or wet weather and decreases during cold and/or dry conditions. Consequently, weevil borer numbers at baits placed during adverse conditions may not accurately reflect actual population levels.

Regular monitoring of adults will give information on weevil borer trends that can be useful in targeting the correct time for treatment. Chemical control measures will achieve the best result if applied during periods of high adult activity, because more weevil borers will come in contact with the lethal dose of insecticide.

Cultural

It is essential to use weevil-free planting material. Ideally, planting material should be obtained from weevil-free blocks, plant nurseries or from tissue-cultured plantlets.

When replanting into old banana land, all banana residues must be destroyed and the land left to fallow for at least 6 months after all residues have rotted down to prevent carry over of adults.

Ensure good plantation hygiene by removing all trash from the area around plants and suckers by raking all leaves into the interrow. Cut up all fallen and harvested plant pseudostems to increase the rate of breakdown and destroy breeding sites. This is more important in south-east Queensland where breakdown is slower due to lower temperatures and drier conditions.

Biological

A large range of general predators including ants, beetles and cane toads assist in reducing weevil borer numbers.

Chemical

Apply strictly in accordance with label directions. Select the most appropriate for your area based on known chemical resistance history.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.