Fruit flies are some of the most significant pests of fruit and vegetables throughout the world. The fruit flies that cause damage in our orchards are known as tephritid flies. They are quite different to the drosophila group that many people will know as small brown flies that hang around decaying fruits in a fruit bowl. The drosophila flies are not agricultural pests and are mostly only a nuisance where fruit and vegetables are stored. Australia has very many tephritid fruit fly species but the most well known and damaging are Queensland fruit fly and Mediterranean fruit fly. Queensland fruit fly occurs in eastern Australia through much of Queensland and New South Wales. Med fly is present in Western Australia.
In this article the term ‘fruit fly’ refers to Queensland fruit fly although management techniques are similar for most of the tephritid group.
Female fruit fly lay their eggs into healthy, ripening fruit on the tree. Queensland fruit fly has been recorded from a wide range of native and introduced fruits (around 250 species). Around 6 or more white banana shaped eggs are deposited just beneath the skin. A ‘sting’ mark usually appears on the surface of the fruit. It may be accompanied by a gum-like exudate. Fruit subsequently drop to the ground and very heavy losses can be incurred if control measures are not taken.
Eggs hatch in 1-2 days. The maggots commence feeding on the flesh of the fruit and a localised rot develops causing the fruit to drop to the ground. After a week or so mature maggots leave the fruit to pupate in the soil. Around 2 weeks later adult flies emerge to mate and resume the cycle.
To manage fruit fly in orchards there are several important things to understand about their biology:
- The female fruit fly must feed on a source of protein before her eggs will mature
- She must wait about 5 days before she can commence egg lay
- Adult flies usually mate and feed in the host tree
- They are strong fliers and can travel some distance
- The males of many species are strongly attracted to specific chemical compounds
- A female fruit fly only mates once in her lifetime
- Adult flies can survive winter but females will re-sorb their eggs during extended periods of cold weather
- Fruit fly threat is greatest while susceptible fruit are available, the weather is warm and conditions are moist
- Flies are most active from dawn for the first few hours of the day
- Continued availability of suitable host plants will favour fruit fly populations
Fruit fly management
Fruit fly is potentially a very serious threat and can cause extensive crop loss. It is also an important quarantine pest placing severe restrictions on fruit movement from areas where it occurs naturally. There are several strategies which, when combined and carried out diligently can provide excellent control of fruit fly in commercial or domestic crops.
Protein baits consist of a liquid protein that is then mixed with a toxicant. They must be applied regularly to the foliage or trunk of the host tree. Baits are susceptible to weathering and must be applied more frequently during periods of high rainfall.
Bait applications must commence before fruit fly establishes in the crop and regular treatments are essential for this method to succeed. Increased baiting frequency may be necessary during periods of high pressure from fruit fly. Thickeners can improve the life and efficacy of protein baits and are recommended during periods of high pressure.
An organically approved protein bait is now available (see Naturalure ™ Fruit Fly Bait below).
Counting and recording fruit fly populations helps us understand fruit fly activity in an area, and we can use this information to fine tune control programs. Male traps contain a cotton wick impregnated with a powerful male sex attractant (cue lure), and an insecticide. These traps can be a useful monitoring tool but trap counts must be interpreted carefully. It is important to note that:
- These traps only catch male fruit flies
- The number of flies caught does not necessarily indicate the size of the population but can indicate population trends
- Fruit fly traps do not control fruit fly – you must use protein bait sprays also to achieve this
Female traps are generally not very effective. Research is continuing into developing better female traps. They are usually of little value in commercial orchards but may be useful for domestic situations. Female traps require high maintenance and captured flies are difficult to identify.
Male annihilation technique (MAT)
MAT cups are designed to reduce the male fruit fly population. They contain the same impregnated cotton wick used in the male fly traps. Cups are placed at 10-20 per hectare 3 times per year. MAT cups are most effective when used over large areas or entire cropping regions. (Note that MAT cups compete with male fruit fly traps, affecting the results of traps set up for the purpose of monitoring.)
Conventional insecticides may be applied to kill eggs and young larvae in the fruit. However currently registered insecticides are very toxic to beneficial insects and are not compatible with integrated pest management (IPM). Currently registered products are also under review and their continued availability is in doubt.
Cover sprays should only be used as a last resort if all other measures fail. Thorough coverage is essential.
Sterile Insect Technique (SIT)
The technique relies on releasing huge numbers of sterilised male flies in a geographic region to eradicate fly populations. It is only of use in geographically isolated or marginal areas. It is very expensive and usually carried out by governments.
- Removal or slashing/mulching of fallen fruit can reduce pressure
- Remove all fruit from trees when mature
- Remove second-crop fruit to reduce opportunities for fruit fly
- Removal of alternate hosts in an area can reduce opportunities for fruit fly to breed