The Fruit Fly Scourge

If you live in WA and you have a fruit tree that produces a fruit, you will need to be worried about fruit flies. You are required by law to control fruit flies on your property and ensure that none are propagated there to spread elsewhere.

There are over 250 species of fruit fly in the family Tephritidae known to occur in Australia, but only about ten are pests. Many of these species are native to Australia. The one of concern in WA is Ceratitus crepitata, otherwise known as Medfly, the Mediterranean fruit fly, which was accidentally introduced from Africa. It was first seen in Claremont in 1895, and is now found from Esperance to Derby. In eastern Australia, in Queensland, N.S.W. and Victoria, there is a different species, Bactrocera tryoni, called Queensland fruit fly or Q-fly, which attacks a wider range of fruits than the Medfly, including tomatoes. It is native to Australia. Both of these do serious damage to fruit.

Do not confuse these destructive fruit flies with vinegar flies, Drosophila melanogaster which also get called ‘fruit flies’. These are the little flies that are seen congregating around rotting fruits; they are often used for scientific research.

Female fruit flies use their ovipositors to insert their eggs into fruit which is just ripening. Wriggling white larvae hatch out in 2 to 4 days, and proceed to burrow through the fruit until they are ready to pupate. They then cut holes through the fruit skin and fall to the ground where they burrow into the soil, become pupae, and eventually release the newly adult fruit flies, ready to do it all over again. The speed at which this cycle takes place is governed by the temperature: the warmer it is, the faster the whole cycle proceeds, which means that in a long, warm spell, there can be multiple generations of fruit flies. The adults can live for 3 or 4 months. When cold weather comes, the pupating larvae in the soil endure until it becomes warm enough for the new adults to emerge. Also, there is evidence that Medflies can continue their breeding cycle through the winter by way of fruit trees like ornamental kumquats and calamondins growing out of doors in big pots.

So, how to protect your fruit from turning into a nasty-smelling brown mush? There are various approaches.

1. Pre-emptive strike. Some people pick all the fruit before it is ripe, cook it and add sweeteners. If larvae are already in the fruit, you must destroy them carefully. Cut out the parts that are infested; the larvae can then be killed by freezing or cooking them (the remaining good parts of the fruits can be used as usual). Infested fruits can be submerged in water for a week, but you need to put a layer of kerosene on top of the water to suffocate the larvae, or you can put the damaged fruit plus larvae in plastic bags, especially black bags, and put them in the sun for a week or so (they need to be safe from curious animals). Do not bury the fruit and larvae! as they will just go on developing.

2. Trapping. Trapping is not very effective. It is mainly used to detect the presence of the Medflies. Traps consist of a container that holds a food source or pheromones plus water and/or insecticide. There are various recipes for baits in books or on the internet: usually they are a combination of a protein plus sugar. Vegemite may be used for the protein source. There are also instructions for making your own traps from plastic milk or cool drink bottles. There are holes in the containers that allow fruit flies to enter; usually they can't find the way out again, and will drown in the water. An insecticide such as Maldison, can be put in the traps to make sure there are no escapees.

If you buy commercial traps and baits, it is very important to read the labels! All too often you will find they are really traps for the Q-fly. Dak pots are the most common ones, and they are baited with Q-fly pheromones; Medflies are not the least bit interested in Q-fly pheromones. For Medflies, there is the Lynfield trap used for monitoring male Medflies (female Medfly pheromones) and the McPhail trap used to monitor female Medflies (male Medfly pheromones).

3. Foliage baiting. This is done with a mixture of protein and the insecticide, Maldison. It is not necessary to spray the whole tree. Rather, the bait is splashed or painted sparingly on the leaves, especially those in the middle and interior of the tree canopy. Apply it in the morning, rather than in the heat of the afternoon. Avoid getting it on the fruits. Rain will wash it off, so reapply after rain. Both male and female Medflies are attracted to the baited leaves.

4. IPM. Some organic commercial fruit growers use IPM - Integrated Pest Management. This involves very careful assessment of conditions, constant monitoring, and the use of special methods, such as introducing predatory insects, using acceptable sprays, attracting birds and insect-eating bats, and the like. Methods used are very site specific. Organic growers realise that they will inevitably lose some of their crop to fruit flies, but they will have the benefit of knowing the fruits are not contaminated with toxins.

5. Cover spraying. The whole tree is sprayed with an insecticide which either kills Medfly adults on contact and can destroy eggs and larvae and will also kill other insects, or is a stomach poison which must be eaten by the flies. Another class of insecticide is systemic, meaning that all parts of the plant become poisonous.

Currently, there is a conversation going on between commercial fruit growers in the Perth hills and the Australian Pesticides and Veterinary Medicines Authority (APVMA) about the use of the insecticide, Fenthion. The state Senate is considering whether to ban Fenthion, or perhaps phase it out over a period of time. There are fears about its safety, even though it has been in use for at least 40 years, and there is little evidence of harm to humans. There have always been regulations about having a withholding period of several weeks between spraying and consuming fruit sprayed with it. The growers say that Fenthion is the only insecticide that can reliably kill the Medfly larvae inside the fruit, and that there will be 100% infestation of fruit if they can’t use it, and they will be forced to go out of the orcharding business. They fear that Australian-grown fruit will become scarce and very expensive, and that it may become necessary to import all our fruit from overseas (but who knows what overseas fruits have been treated with?)

On a small scale, we, as home growers, can protect our trees and our fruit in several ways. We should always practice orchard hygiene: pick up all dropped fruit, inspect it, and if it is infested, treat it before disposing of it. We should keep our trees small so that we can reach all the fruit and ensure sprays effectively cover the whole canopy. We can choose to plant cultivars that flower and fruit early, with the hope that we will have the fruit before the fruit fly circus is fully under way in the warm season. Warmer winters may make this problematical.

Bagging the fruit is labour-intensive, but reasonably effective. Quite a lot of research has been done on different materials for the bags, their colour (which influences the colouration of the fruit), different ways of attaching them and managing problems such as water collecting inside. There is a good article about bagging here: Protective Fruit Wrapping on Page 10. Old nylon stockings can be made into bags, for example. If there are local problems with rats and birds, especially parrots, it becomes more difficult, as rats will just chew through the bags and the parrots will pull the bags off. I once went to the trouble of making flywire containers for fruit: very difficult to get them over the fruits, and the parrots just pulled them off anyway.

Shade cloth can be draped over smallish trees during the vulnerable fruiting season to exclude Medflies, and it is not too difficult to construct frames to hold the netting up. Tie off the net around the lower trunk to prevent flies from getting in and larvae from getting to the soil to pupate. It seems to me that large-scale netting may eventually be the only way to ensure a good crop, safe from birds, other animals and insects. There is a splendid example of a small enclosed fruit orchard on the website of the Rare Fruit Society of South Australia. Click the button labeled "Espalier" in the public section. The wire mesh of the enclosure appears to be too large to prevent fruit flies from entering, but probably it would be possible to attach panels of shade cloth around the sides when needed.

There are far too many fruit trees that are neglected and the Medflies are free to breed and increase their territory. And accidents happen: there was an accidental infestation of Q-flies in the apple growing areas of southern WA in the 1990s and an impressive emergency operation swung into action. We had quarantine areas and internal checkpoints for fruit inspection. We were not able to transport apples and pears across certain boundaries. Fortunately, it all worked out, and the Q-flies were eliminated in WA. There are very good reasons for fruit quarantines, both here in Australia, and when we return from foreign places!