Mango seed weevil

Mango seed or nut weevil (Sternochetus mangiferae) has only one known host, cultivated and wild Mangifera spp. Although, S. mangiferae is the only species of weevil found in Australian mangoes, several other species including S. frigidus and S. olivieri are found in some other mango producing areas around the world.

Scientific name

Sternochetus mangiferae

Description of adult

Adult weevils are dark brown to black with grey markings and are 6–9mm long. They possess typical weevil features and a tough exoskeleton. Weevils spend the winter living under loose bark around the base of mango trees or in the forks of branches. They may also live in leaf litter around the tree and approximately 25% of the adults over-winter in the seed. Adult weevils can live for 2 years, so even with a crop failure in 1 season some weevils can survive into the following year.

During flowering the adult weevils leave their sheltered areas under loose tree bark and litter under the trees and move into the outer canopy of the tree to feed on new growth and to mate prior to egg-laying.

Immature stages

Eggs are first laid on small young fruit about 30mm in size. Female weevils lay eggs over a number of weeks and egg-laying can continue in the orchard until a month before harvest. The eggs are elongated and creamy white. The egg has a protective brown covering with 2 tiny tails at one end. At laying, the weevil cuts a very small nick into the fruit and the resulting sap flow covers the egg case. This helps to protect the eggs and adheres them to the fruit. Several eggs may be laid on each fruit and each female can lay up to 15 eggs per day and up to 300 eggs during the season.

Each egg produces a white, legless and elongated slender grub which appears atypical of weevil larvae. Older larvae (instars) are compact and C-shaped, typical of weevils. The larvae tunnel into the fruit through the flesh and reach the seed in 1 to 2 days where they complete their development into adults. In the seed the larva grows as it feeds on the cotyledons or seed leaves, causing conspicuous damage. Pupation occurs in the seed and adults emerge after chewing through the seed coat any time up to 2 months after fruit fall. Although many eggs have been seen on the surface of fruits, usually only 1 to a few larvae develop in the seed, indicating that significant numbers of eggs fail to hatch or young larvae die. No parasites or predators of eggs or larvae have ever been recorded.

Adult weevils can fly, but are not known to be strong fliers and do not move far from the tree where the fruit drops. All the evidence suggests that weevils spread into clean areas through the movement of infested fruit for propagation and consumption. Young orchards planted from weevil-free nursery stock, even in areas known to have seed weevil, have been shown to be free of seed weevil infestation for a number of years after establishment.

Life history

The time from egg to adult takes 5–8 weeks and only 1 generation is produced each year.

Distribution

Mango-growing districts of Queensland and Northern Territory.

Host range

Mango

Damage

The mango seed weevil is considered a minor pest as it causes no significant economic damage to fruit. The minute egg-laying scars are barely discernable at harvest and the weevil is present only within the seed. The fine tunnel in the flesh the young larva causes as it burrows towards the seed heals, leaving no sign of its earlier presence in the flesh. However there are quarantine restrictions on the movement of mango fruit infested with seed weevil into a number of markets.

Control

Monitoring for egg-laying on young fruit is the best way to detect adult activity during fruit growth. If weevil eggs are detected, chemical control can be used to reduce weevil populations to low levels. Treatments timed to coincide with early fruit set when weevil adults are in the canopy will give the best results. Target known 'hot spots' to maximise the benefits of treatments.

A weevil control program should use 3 strategies to control weevil infestation; quarantine, hygiene and chemical control.

Orchard quarantine

To maintain a young orchard's seed weevil free status, avoid bringing any mango fruit suspected of harbouring weevils within the seeds into the orchard and surrounding areas. A strict policy of not bringing mango fruit onto the property will greatly reduce the chance of infestation. It is also advisable to remove all non-commercial mango trees in the immediate vicinity.

Orchard hygiene

The removal of all fruit and seed material from the orchard will aid in minimising the infestation in following seasons. Infestation levels in orchards where fruit is regularly removed at harvest are far less than in non-commercial trees where much of the fruit is left to rot underneath the tree.

Farm-house trees located near orchards are generally untreated and pose a constant threat of infestation. Either remove these trees or treat them with insecticide to suppress weevils.

Chemical control

Chemical sprays used to control other mango pests can also control seed weevil. Specific weevil treatments can also be used to kill adult weevils using 1 or 2 targeted sprays at the start of egg-laying. The best time to apply a spray is when the first eggs are noticed on the fruit. At this time the adults are active within the canopy, moving onto the fruit and can be targeted together with the newly laid eggs. Good spray coverage to run-off is critical.

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.