Now is the time to plan your fruit fly area-wide program, before the insects commence a new season of activity, Richard Bull* writes.

Understanding fruit fly population dynamics throughout the year enables you to plan ahead and anticipate when extra action is required. The graph below shows data collected in a SE Qld kiwifruit orchard and the red points indicate the natural forest population compared to numbers recorded within the orchard during the area-wide program, which resulted in nil stung fruit in a 600-fruit sample.

It is typical of Qfly populations of SE Qld and coastal NSW. Of importance is the early season data where over-wintering adult flies disperse to find new host fruit sources in August. Successive bush generations show clearly as spikes in September, October and November, thereafter declining, but still remaining a threat if uncontrolled.

Strategies for area-wide suppression of fruit fly populations

DacLure CL fruit fly stations and DacGel /HyMal protein sprays were used throughout. With a field life of 4–5months, I now advocate commencing placement of DacLure CL stations through refuge vegetation in August – long before orchard foliage and flowering commences, to intercept and suppress the early season dispersion of adults.

An area-wide management program involves placement of DacLure CL stations throughout fruit crops and surrounding lands, coupled with regular protein/insecticide bait spray applications to suppress both in-crop and wild fruit fly populations. Wild populations breeding and sheltering in non-cultivated habitats continually invade commercial or domestic fruit crops and mature female flies coming from these refuges are responsible for most fruit losses in commercial crops where fruit fly management practices may have already been established.

The larger the area treated, the better the result, is the essential rule, and may be optimised by coordinated group involvement by growers in intensive commercial fruit growing districts or even urban environments. By conducting simultaneous treatment programs using DacLure CL stations and protein/insecticide bait sprays to target males and immature females, combined effects can significantly reduce general district fruit fly populations and minimise fruit losses.

In high population localities, the addition of one or two rows of DacLure CL stations spaced at about 30–40 m intervals outside the crop in non-host vegetation will provide a protective barrier to intercept immigrant fruit flies.

On many farms, strategic placement of DacLure CL stations on flight paths can intercept fruit flies moving from known breeding areas or refuge habitat such as forested creeks, or regrowth scrub patches containing host fruits (guava, loquat or feral fruit trees) with significant benefits.

Open grassed fields or pasture should not be included in the treatment area as these do not constitute favourable habitat for fruit flies. Enhanced district-wide suppression of populations will result from neighbouring growers conducting simultaneous programs to link up treatment effects over large areas as well as employing cultural control practices.

Refuge vegetation beside orchards is preferred habitat for fruit flies and should be targeted with DacLure CL and bait sprays. 

Population Monitoring by Trapping

Successful fruit fly management in commercial fruit crops using a program of DacLure CL and protein bait sprays requires a reliable method of monitoring population changes. Two or three traps widely spaced in a small commercial orchard will provide quite reliable information on population changes within the orchard and also indicate the direction from which most immigrants may be entering. Large (25+ ha) orchards with several blocks may require one or two traps per block.

Modified Steiner design fruit fly trap baited with a DacLure CL station. 

There are many fruit fly traps available, but their efficacy varies greatly and those with proven high performance such as modified Steiner design should be used. These can be baited with a DacLure CL station and should always be hung in dense shade within the canopy of a fruit or refuge tree as a substitute for a DacLure CL station on the grid layout and be positioned at least 25m inside the orchard.

To provide a reference indicator of district populations, place several traps in bushland or non-cultivated vegetation 300–500m outside the treated orchard area and record fly numbers there and within the orchard for at least one week before commencing distribution of DacLure CL stations.

This will provide valuable ‘before and after’ data once treatments commence. To optimise the value of trapping data, traps should be emptied on a weekly routine and numbers recorded, enabling control responses to be followed and good management decisions to be made.

Rubber gloves should be worn when loading a DacLure CL station in a trap and care taken to avoid contaminating the trap’s outer wall or lid with lure, or fruit flies will be distracted by these minute traces and not enter the trap.

Remove fallen fruit from the orchard. Exit holes in fruit indicate mature larvae have left
the fruit to pupate in the soil and re-emerge as a new generation of fruit flies.

Crop hygiene and cultural practices

Over-ripe or damaged fruits are most attractive to female fruit flies and the first to be ‘stung’ and should always be removed from the orchard or destroyed to prevent a new generation establishing. If removal is impractical, pickers in commercial orchards should be instructed to throw all discard fruit to the centre of inter-rows where they can be regularly pulverised by a mower/mulcher, or at least exposed to direct sun to heat them above 53°C (lethal temperature for eggs and maggots).

Place collected damaged fruit in sealed, black garbage bags and leave in direct sun for three days before burying. Abandoned host fruit crops (tomatoes, strawberries, capsicums etc.) should be destroyed by spraying with herbicide desiccant, slashing or ploughing under, as these will otherwise breed massive populations of migrating adults to invade and infest crops many kilometres from their source.

Uncontrolled breeding in late summer will also ensure survival of large over-wintering populations of fruit flies to attack the following seasons’ crops.

Untended, wild or feral host trees can provide major breeding nuclei for fruit fly populations and can weaken the impact of area-wide programs. Roadside guava bushes, loquat and feral stone fruit trees are common sources of large numbers of wild fruit flies and should be destroyed in the interests of local or district-wide crop protection.

*Richard Bull is a Tamborine Mountain, Qld based entomologist and a director of Agripest has been involved in fruit fly research for 25 years with close involvement with Griffith University Tropical Fruit Fly Group, University of Hawaii Cooperative Extension Service and USDA Hawaiian Fruit Fly Area-Wide and eradication programs throughout the Pacific islands. He has had extensive experience in field and laboratory fruit fly research and grower advisory services and has been an entomology consultant since 2003. Contact: 0408 884 589.