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Controlling Fruit Fly

Mediterranean fruit fly is regarded as one of the most destructive agricultural pests worldwide but even for the backyard gardener it is a cause for disappointment and frustration when the ripened fruit on a tree is found to be inedible.  The only way to control Medfly is for all members of the community, whether home gardeners or fruit growers, to be diligent and effective in controlling this devastating pest. 

The Mediterranean fruit fly or Medfly was first found in Claremont in 1895 but is now found throughout the south-west and as far north as Derby.  It is known to infest over 200 fruit and vegetable species with stone fruit, apples, pears and citrus being particularly susceptible but it will also cause damage to tomatoes and eggplant.  The eastern states also has to contend with Queensland fruit fly but at present there are no known infestations of this pest in WA although monitoring is always in place.

The adult Medfly is quite small, between 3 to 5mm long.  The body is light brown and the wings have distinct brown bands extending to the wing tips. The abdomen has two light coloured rings and the middle has irregular patches of black and grey.  It can be mistaken for vinegar fly or metallic-green tomato fly.

Like most insects, the life cycle of the Medfly is dependent on the temperature.  During winter the fly is inactive but as the weather warms up in spring, adults begin to emerge from the ground and from over-wintering and become more active.

Adult Medfly may live for two to three months and are usually found in the foliage of fruit trees.  As long as fruit is on the tree, the adults do not move very far away.

After mating, the females search for suitable fruit in which to lay their eggs with apricots being their favourite however, if populations are high or preferred fruit is not available, they will also use apples, pears and even olives.  The larvae or maggots hatch within 2 to 4 days and this is what most home owners see when they harvest the fruit only to find them inedible.  The larvae feed on the flesh of the fruit causing it to decompose and when fully grown they leave the fruit and burrow into the soil to pupate.

The pupae of Medfly resemble small brown capsules and within this the adult starts to form.  When ready, the adult cuts through the casing and burrows up through the soil.  In summer this full cycle may take just 30 days and in winter it can take between 60 and 115 days.

Medfly is most active in late spring, summer and autumn.  If control is not started in spring, Medfly populations will begin to increase and cause problems later in the season.

In urban and outer-urban areas, all fruit trees should be sprayed with a bait starting around August when the weather begins to warm.  Baits or fruit fly controls are now available that contain a combination of low toxic insecticide and a lure and these do not need to be sprayed over the whole tree as the fly will be attracted to the spot from a few metres away.

The insecticide is spinosad which is derived from naturally occurring beneficial soil bacteria which kills fruit fly and the bait is a protein and sugar combination to attract them in.  This is sprayed on the trunk or lower foliage of fruit trees, vegetables and ornamentals avoiding the ripe fruit and can be applied fortnightly or weekly depending on the level of infestation or if rain occurs.  These types of baits have no withholding period, are easy to apply and are suitable for an organic garden.

It is estimated that without control methods, Medfly would infest 100% of susceptible fruit across the state including citrus in warm winters.  Only early maturing varieties of stone fruit or fly tolerant varieties such as some lemon cultivars and avocados can be grown without control methods.

For trapping and chemical control methods to be effective, fly-infested or unwanted fruit must be destroyed including that which has dropped on the ground.  Simply putting them in the bin to be taken to the rubbish tip is not enough to control the problem and burial is not recommended since Medfly larvae can survive in soil.

Fruit can be destroyed by soaking in water topped with kerosene for five days. The kerosene layer prevents oxygen exchange between the air and the water, thereby suffocating the larvae.  Freezing fruit for a few days or cooking it to feed to poultry is also effective.

The main benefit of traps is that they are an indicator of Medfly in the area and also of the level of infestation but they will also help to control smaller outbreaks in the home garden.  However, traps need to be placed throughout fruit trees in spring when the fruit fly just begin to emerge if there is any hope of controlling numbers before they get too great.

Natural and non-toxic sticky traps are available as plastic tubes or on waterproof card that can last for several months depending on the weather, level of infestation and the design of the trap.  Every susceptible tree needs to have at least one trap hung in it from the beginning of spring and larger trees may need several.  Their yellow colour attracts the fruit fly and the sticky substance traps them however other insects can be trapped by these also including aphid and whitefly.

A synthetic lure is also available at Better Pets and Gardens and is commonly used by orchard growers in the south-west.  This is made from a solution which is a feed attractant particularly for female fruit fly and so captures fewer non-target insects than other baits.  Where possible it is best to target females since these are the ones that do the most damage to fruit and also produce future generations of fruit fly.  Synthetic lures can be used either wet or dry with the latter being far easier as they are simply placed in the shady area of a susceptible tree about 2 metres from the ground.  The number to use depends on the tree size and infestation level but they have very good capture power and leave no traces of chemicals on the fruit.

To find out about the Mediterranean fruit fly baits available and how to use them, drop into any Better Pets and Gardens store.

Wet food traps consist of a food source such as protein or sugar in which an insecticide has been added.  There are also insecticide-free traps which the flies enter but are unable to leave so eventually drown in the lure at the bottom.  Wet traps are generally only effective for male fruit fly as the female is usually reluctant to enter them but these will also trap other insects such as bush flies, ferment flies and wasps.  These need to be rinsed and the lure replaced every few weeks.

Commercial versions of wet traps and lures are available which are clean and easy to use but the trap can also be made from plastic drink or milk bottles.  Simply drill or cut four holes at least 1cm in diameter half way up the bottle and fill the bottom third with the lure.  For extra potency, hang a 2cm strip of pet flea collar from the lid inside the container with a piece of wire.  Replace the lid and hang by wire or string to a branch in the tree but if ants become a problem, smear the string with Vaseline to stop them from entering.

Cover spraying differs to bait spraying in that the chemicals in the spray are used to kill all stages of the Medfly on contact and it is sprayed over the whole tree.  However, this mean that other beneficial insects will also be killed and withholding periods need to be closely adhered to with these types of products as the fruit will also be sprayed by the chemical.  These should be considered a last resort in the home garden.

Exclusion bags are waxed paper fruit bags available in two sizes to suit different fruit.  Early in the season, the fruit is thinned and the bag is used to cover each remaining cluster and secured with a built-in twist tie.  The sun can still get in and the fruit will ripen successfully.  These will also control codling moth and some birds and with care, may last more than one season.

Cloth fruit bags are used in a similar way to exclusion bags but are also strong enough to stop possums, fruit bats and birds.  These are made from washable calico cloth with a drawstring and can be used year after year with the fruit ripening in the bags.

Although fruit fly do have predators such as some wasps, ants, spiders and birds such as Willy Wagtails these are rarely enough to keep them in check. However, poultry are enormously helpful if they have the chance to forage at the base of fruit trees.  Design the orchard to incorporate chooks so that these feathered friends can spend hours scratching beneath the trees looking for fruit fly pupae, hatching adults and maggot-infested fruit or simply use a portable fence or cage to allow them a few hours under a tree every day and this will help enormously.

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