Bactrocera minax

Crop: Citrus

Why is it a problem? Chinese fruit fly is the most serious insect pest of citrus. UP to 70% losses through late fruit drop have been reported. It remains difficult to manage.

Where and when is it a problem? It is most important in orchards above 1200 m asl and shady orchards at lower altitudes. The majority of orchards are affected in those areas. Without management it can be a serious problem every year.

IDENTIFICATION

It is a big fruit fly. Adults have a general wasp-like appearance, are almost 1.5 cm long (wingspan up to 1 cm) and brownish in colour with yellow markings. Wings have a dark band along the outer margin. Females possess a long and exposed ovipositor. The larvae (maggots) are creamy white with black mouth parts. They measure 12-15 mm when mature. The puparium is yellowish brown and 8-11 mm long.

SYMPTOMS

From June onward oviposition spots can be observed on the upper half of fruit over about 11 mm in size. Later, fruits show round, brownish, hard spots of 1-2 mm diameter just under the skin. The maggots feed inside, rendering the fruits useless. Fruits turn prematurely yellow and eventually drop prematurely. By October the maggots become easily visible. Dropped fruit have 1-2 mm brown-black oviposition punctures, dry or rotting segments, often maggots and later exit holes. Affected mandarin parts become dry and later turn rotten.

 

 

 

 

 

 

Confusion with other pests: It is the only fruit fly recorded from citrus. Fruit can yellow and drop prematurely for a range of causes, but the role of Chinese fruit fly can be confirmed by the symptoms and the presence of maggots.

BIOLOGY

Lifecycle: Female flies lay their eggs in mandarin fruits from June to August. Egg-laying commences about when the first mandarin fruit reaches 11 mm in diameter, which is the first stage susceptible to fruit fly oviposition. Eggs are laid just under the skin of the fruits. The larvae (maggots) feed on the pulp and develop inside the maturing fruits. When the maggots are matured they leave the fruit by making an exit hole, and enter into the soil to pupate. They pupate in the soil at a depth of few centimetres. Puparia remain in the soil through winter. Adult emergence occurs from April to May.

Dispersal: Adults are good fliers and are likely to move easily between neighbouring orchards.

When can damage be expected? Greatest losses are seen in orchards above 1200 m asl and in shady orchards at lower altitudes. Losses can be very high. 55% of fruit drop was the result of Chinese fruit fly in Lenkhar (Tashigang) in 1993, but this was expected to represent an upper limit in that area. Most orchards suffered from attack.

Hosts: It utilises a wide range of wild and edible citrus (Rutaceae) including sour orange, pomelo, wild lime, lemon, citron, mandarin, orange, tangerine and cumquat.

MANAGEMENT PRACTICES

Successful management requires an integrated management approach throughout the year. As adults are good fliers management needs to be coordinated across neighbouring farms within a locality. The single most effective management action is to pick up and destroy all fallen fruit to prevent populations surviving into the next season. Farmers obtain best results if collection of dropped fruits, together with bait spraying, are carried out for two to three years in a row. Monitoring is critical for assessing the need for, and success of, the management programme.

Monitoring

Dropped fruit should be assessed to estimate percentage losses resulting from fruit fly attack. Collect 50 from across the orchard in September or October and split open to look for maggots. Estimates of fruit loss will allow you to monitor the success of your management programme, and to modify it if necessary.

Effect of variety

Not known.

Non-chemical management

Collection and destruction of dropped fruits is the single most effective management action. However, everyone in the community needs to participate to prevent pockets of fruit fly from surviving.

• This method will reduce the fly population in the following year.
• From mid-October dropped fruits should be collected every 2-3 days. Doing it less often will allow maggots to leave the fruit and enter the soil.
• Fruit should be buried in pits 1-2 m deep and cover with soil once all fruit have been collected. Ants and the rotting process will destroy the larvae and pupae. Alternatively fallen fruits can be fed immediately to pigs and cattle.

Chemical management

• Insecticides can be used to manage adult fruit fly populations. They should be applied as bait sprays. Cover sprays are not recommended. They are also not feasible where citrus trees are tall. There are no sprays available that can kill larvae, which are well-protected within the fruit.
• Bait sprays are applied as spot treatments. It is therefore preferred over cover sprays because it is effective, less costly and is less disruptive to natural enemies of other pests. Bait sprays work because both male and female flies are attracted to a protein source from which ammonia emanates.
• To make bait sprays:
  • Mix 1 litre of water, 5 grams of protein hydrolysate, 2 ml Malathion (insecticide) and a few drops of Sandovit and stir well. This one litre mixture can be used for about 25 trees.
  • Contact NPPC for advice on procuring protein hydrolysate.
• To apply bait sprays:
  • Bait should be splashed onto trees with a locally made broom or something similar. Apply the mixture to every second tree, using two or three splashes on each.
  • Splashing should be done on dry and sunny days.
  • The first splash should be done around 15 April, when flies start emerging. Repeat the treatment every week, until mid-May; and then every two weeks until 15 August. Where infestations are low level, splashing can be stopped after 15 May.

 

Version: NPPC 2017. Chinese Fruit Fly V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Contact: nppcsemtokha@gmail.com

Image acknowledgements: NPPC.