Agronomist's notebook: Fruit flies are on the prowl this dry spell

Saturday February 9 2019

Tomato crop affected by fruit flies.

A tomato crop attacked by fruit flies. The pest attacks and damages most of the soft-skinned and some harder-skinned fruits. PHOTO | ANN MACHARIA | NMG 

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Nothing can be as disappointing to a consumer as cutting a fruit like a mango or an orange only to notice white maggots inside.

The maggots are the larvae stage of fruit flies. There are different species of fruit flies, such as Bactrocera invadens, Bactocera curcubitae and Ceratitis cosyra, that affect a wide range of fruits and vegetables, making them unsuitable for human consumption.

The pest attacks and damages most of the soft-skinned and some harder-skinned fruits. They include citrus, tomatoes, capsicum, cucumbers, apples, watermelons and butternuts

The adult fruit fly is about an eighth-inch long and has red eyes. It is pale yellow to brown in appearance with transparent wings, and it feeds on the sap of the decaying materials. The pest is highly productive with the female laying up to 500 eggs.

The adult fruit fly causes damage to ripening fruits by puncturing the skin to lay eggs. The eggs hatch to larvae (whitish maggots), which begin to feed on the fruits within.

During the laying of the eggs, bacteria from intestines of the fly are introduced into the fruit causing rotting of the tissues surrounding the eggs and resulting in rotten fruit.

Sometimes the larvae are found in large proportion of the harvested fruits. For home consumption, one may cut away the small portions of the affected fruits, but this is not possible for commercial produce.

The flies can be a problem all-year-round especially to a farmer with a wide range of fruits and vegetables, which mature at different seasons.

However, the population of the fruit flies tends to be abundant during the hot seasons and multiply rapidly causing higher damage within a short period.

During the ripening of the fruits, the infestation is quite higher since the fruit flies are attracted to the ripening and fermenting fruits.

The symptoms of the damage vary from one fruit to the other though the affected produce shows small holes visible when the larvae leaves the fruits.

In tomatoes, affected fruits develop lesions accompanied by a clear oozing sap with a bad odour and eventually, the fruits fall on the ground.

This is unlike when the tomatoes are affected by Tuta absoluta, which also pierces the fruits but does not result in fruit rot. The same signs are displayed in cucumbers.


In melons, the fly attacks young fruits, sucks the juice and breeds its eggs inside deforming it. The fruit rots while still young.

Inside the fruits, the larvae are visible. Sometimes the fruits mature but have deformed shape lowering the quality. Butternuts and pumpkins will show the same signs as watermelons.

Fruit flies also have the potential to contaminate food with bacteria and other diseases causing organisms, which pose risks to human health.

The most-effective, preventive and control methods of fruit flies, do not rely on the use of chemicals as a priority since safeguarding the environment is critical.

They include eliminating the source of attraction of the fruit flies by sorting and grading, eating ripened fruits and discarding the affected fruits appropriately.

Maintaining high levels of farm hygiene by removing the affected fruits is paramount, since leaving them creates a conducive breeding site for the fruit flies increasing the population

Early and timely harvesting when the fruits are mature green helps to protect crops from fruit damage.

The larvae can be killed by burning or burying the affected fruits in disposable bags and exposing them to the sun for a few hours. In case one decides to bury them, then this should be at least 40cm deep.

Reusable fabric garden protection bags can be used in fruits such as mangoes as they create the barrier preventing the pest infestation.

The bags also prevent mechanical injuries thus maintaining the quality of the fruit. This method works best with melons, mangoes, and bananas where the whole bunch is bagged.

Biologically, they can be controlled by use of metarhizium spores, which is a fungus that occurs naturally in the soil and acts as an insecticide to the pest.

Parasitic wasps can be used to feed on the larvae stage of the fruit flies thus controlling their population.

Pheromone and indigenous bait traps can also be effectively used for fruit fly capture. Bio-pesticides such as pyrethrum extracts and neem are used to control the flies.

Further, chemicals can be used to control the fruit flies like those that contain ememactin benzoate indixocarb and lufenuron.

However, care should be taken to minimise the residue effect in the soil and reduce pest-resistance to chemicals.