Fruit Fly: Protect Your Crop

Traps placed in fruit trees with chickens grazing underneath is an effective management technique, Robyn Rosenfeldt

Australia is home to more than 150 native species of fruit fly, but only a few of them pose a threat in the garden.

There are two main types of fruit fly. The Drosophilidae family, often called the vinegar fly which is the one you see around compost bins and fruit bowls. It’s tiny, between two and four millimetres in length, and can range in colour from pale yellow through to black.

The other is from the Tephritidae family which includes the two main flies affecting Australia’s backyard gardeners and commercial growers. There’s the Queensland fruit fly (Bactrocera tryoni), which is found on the east coast of Australia and, although it can’t fly too far, spreads easily by jumping from backyard to backyard. It’s hugely adaptable to its environment, so as climates change and backyards become more productive, this insect is becoming increasingly invasive and can’t be ignored. It’s larger than the vinegar fly, around seven millimetres in length, it’s reddish-brown in colour and easily identifiable by its distinct yellow markings.

The other invasive species is the Mediterranean fruit fly (Ceratitis capitata). Originating in Africa, it is now endemic to the west coast of Australia but has been found sporadically in South Australia, too. It sits in the leaf canopy of citrus trees and attacks fleshy fruit. Smaller than its Queensland counterpart, it’s between three and five millimetres in length, yellow in colour with black markings on its body and brown bands on its wings. There is also the lesser-known cucumber fruit fly (Bactrocera culumis), which loves pumpkins, zucchinis and melons.

Fruit flies love warm weather and high humidity, and their activity increases when temperatures rise above 12 ºC. Warmer autumns have extended their activity over recent years, to the point where they are now a problem for eight months of the year.

Fruit flies are a gardener’s number-one enemy and, if left unmanaged, can decimate an entire season’s crop. For many novice home gardeners, the first sign of a problem is when maggots are already inside the fruit, which is why commercial growers prioritise fruit fly management to stop the insect before the female can lay her eggs.

fruit fly
Queensland fruit fly is currently Australia’s most problematic species, PYP

Fruit Fly Life Cycle

To effectively manage fruit fly in the home garden you need to understand the fruit fly’s four-stage life cycle (see diagram on page 35). In summer, the life cycle takes between 28 and 34 days to complete, which increases to between 60 and 115 days in winter. Fruit flies can live for up to three months, a staggeringly long time when you think that one female can lay up to 100 eggs in a day.

Organic Management

Management involves finding ways to successfully interrupt the life cycle at either the maggot, pupa or adult stage. A really effective interruption is chooks. Let your chickens scratch around the base of your fruit trees, where they will peck at the maggots, pupa and newly emerging adults. However, if you don’t have chooks, there are other effective strategies.

Hygiene

It’s really important to maintain good garden hygiene. Remove fruit as it ripens, both from the tree and what has fallen to the ground, and dispose of rotting or unwanted fruit. If you see maggots inside your fruit, solarise them by sealing the damaged fruit inside a plastic bag and ‘cooking’ it in the sun. Alternatively, heat the fruit in a microwave to kill the maggots before feeding it to your chooks.

Early Harvest

Depending on your crop, harvesting early to remove the host can also be a good option. Some fruit needs to ripen on the tree, but a lot will ripen after it has been picked.

Exclusion

Keep the fly away from fruit by using a bag or fine UV stable mesh netting to cover your produce. This can be tricky as your fruit must not touch the netting where a female fruit fly can reach your fruit with her ovipositor.

Effective control is easier once you understand the fruit fly’s life cycle.Fruit left to over-ripen on the tree is inviting fruit fly into your garden;

Traps

Traps are important to identify adult fruit fly activity. One option is a pheromone trap which hangs in the canopy of your fruit trees. The trap will attract the male fruit fly which becomes stuck on the sticky surface. By trapping the males you are limiting the breeding capabilities of the fruit fly. Pheromone travels considerable distances, so a single trap for a backyard use is ample.

Another trap is a protein-based one that will attract both male and female fruit fly. These are readily available and are both an easy and safe way to deal with all types of fruit fly. The protein liquid lures the flies into the chamber, where they become trapped and drown. This type of trap is also hung in the canopy of your trees and will cover an area of 20 square metres, so multiple units may be needed depending on the size of your garden. The liquid protein will remain effective for up to three months, so replacing the liquid will extend the effectiveness of the trap across the whole season.

Home Remedies

Fruit flies are attracted to warm colours like red, orange and yellow. Another remedy is to paint the trunks of your fruit trees in these hues with a water-based paint, just up from soil level. The once dry, paint the coloured areas with honey. Fruit flies can’t resist the sweet scent and will become trapped on the sticky surface.

Speaking of, sticky bait strips are also popular to hang in fruit trees and vegetable gardens. Some local councils will even distribute the strips for free, but they’re easy to make yourself by cutting up bright- and warm-coloured paint samples or plastic pots before painting them with a coating of vegemite and petroleum jelly mixed together. The flies are attracted to the yeast in the vegemite and will get trapped in the sticky mixture.

An effective way to manage vinegar flies in the home is to pour some apple-cider vinegar into a glass and cover with baking paper or cling film. Poke a couple of small holes in the top for the vinegar flies to find their way to the attractive liquid. Once inside, they won’t be able to find their way out.

A combination of organic fruit-fly controls, good garden hygiene and neighbourly awareness will go a long way in preventing fruit fly rampaging its way through your edible garden. It’s a good idea to synchronise your fruit fly control program with your neighbours, too, because if your neighbour is leaving fruit rotting on the ground, you’ll be fighting an uphill battle.

Homemade liquid trap

Ingredients
½ tbsp cloudy ammonia
½ tbsp vanilla essence
100 g sugar
1 tsp dishwashing liquid
1 tsp vegemite
1 L hot water

Method
Dissolve all ingredients together and allow to cool. Meanwhile, cut four 1.5-centimetre holes around the neck of a 1.25-litre plastic bottle and make a hanger in the lid using wire. Pour cooled liquid in your bottle and hang in your fruit trees to attract and trap fruit fly. For best results, replace the liquid every two weeks.

Brent Whiter is a registered horticulturist, an accredited specialist and has owned a garden centre for 35 years.

This article represents the permaculture principle OBTAIN A YIELD.

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