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Fighting Fruit Fly


Neville Passmore


27 November 2018

Why protect your fruit from fruit fly?

The mediterranean fruit fly is the single most devastating pest of fruit in WA.  If you grow peaches, nectarines, mandarins, guava, feijoa or Aztec fruit without protection you are unlikely to eat one piece of clean fruit for the year.  Not a very profitable or worthwhile exercise.  From Esperance to Derby, the cursed fly has extended its range, only the Ord River is currently unaffected. The epicentre of this infestation, which has been with us for more than a century, is the Bunbury to Perth region. 

How do fruit flies attack crops?

Male fruit flies gather under leaves, particularly citrus, and call the females.  Once the eggs have been fertilised the female looks for ripe fruit to deposit the eggs through an ovipositor.  Up to 300 eggs are injected into the fruit and as these hatch, the grubs feed voraciously on the flesh causing this to brown-off and turn to rot.  The damage from breaking the skin and the subsequent rotting send a signal to the tree that this particular fruit is ripe and should be ejected. When the fruit falls to the ground the advanced grub then burrows into the soil to pupate.

Which fruit fly?

This is a good question as the Medfly has been joined on a couple of occasions by the Queensland Fruit Fly.  If possible this causes even more devastation as its host range goes beyond fruit and includes vegetable-fruits such as tomato and capsicum. The previous QFly invasion cost millions of dollars to eliminate through the use of purpose-bred sterile male Qflies. A 2017 outbreak centred in Como near Perth is still being monitored and baited.

What can a home gardener do to combat this serious pest?

The main insecticidal spray Lebaycid has been withdrawn from sale, as I understand it, because of the devastating toll taken of Carnaby?s cockatoo.  Alternative forms of protection that don?t involve spraying are listed below.

Exclusion netting has worked well for me.  I drape shadecloth over the entire tree and tie this off around the base of the trunk.  I wait till the fruits are nearly full size but before any colouring of the fruit. You may need to reduce the size of your trees to make netting feasible. If planting new trees, look for dwarf varieties, as these are easy to keep low. Exclusion bags made from shadecloth, mosquito netting or nylon fly screen can also be used effectively. The idea here is to enclose bunches or even individual fruit.  If you are not aiming to pick the fruit on your property it?s best to either strip the tree of green unripe fruits or remove the tree entirely. 

Install hanging fruit fly traps at the rate of two traps per tree.  Commercial types come with a bait solution.  Homemade traps are usually created from empty two litres plastic cool drink, or milk bottles. 

Here are a few bait recipes.

  • No 1, Mix 5mL imitation vanilla essence, 20mL household ammonia with 1L water.
  • No 2, Add 1 teaspoon borax, 1tsp sugar, 2tsp bran to 1L water.
  • No 3, Mix 80gms white sugar, 1.5g dry brewer's yeast with 920mL water.

What can you do with infested fruit?

Solarizing fallen fruit in heavy-duty black plastic bags by placing these in a full sun position for 3 to 5 days. At this point it is safe to feed the solarized content to chooks or add to an active compost bin. Alternatively, place it in the garbage bin.