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May 23, 2018

Winter and the fruit fly

It is starting to get quite cold now in the Goulburn Murray Valley (GMV) and fruit flies are doing one of two things:

  1. Searching for food and water to build up reserves during periods of immobility caused by the cold
  2. Looking for refuge from the cold, wind, rain and predators

One thing they are not spending energy on is looking for mates. This means that fruit fly trapping numbers in the current male-targeting fruit fly trapping grid in the GMV will bottom out from now until late August. However food-based traps and baits, that is, those based on protein, are still effective, especially during warm parts of the day. Such traps and baits can control hungry fruit flies in the backyard and orchard.

Most immature fruit flies (eggs and larvae in fruit and pupae in fruit or the soil) will die during the winter. Many adult flies will die – but not all. Some will survive the winter (called “overwintering”) and be the cause of next year’s urban and rural fruit fly problems in the GMV. These flies are typically newly bred flies from late-ripening fruit (e.g. apples, plums, feijoas and late-hanging Valencia oranges) that have found themselves in localised warm spots on the otherwise cold landscape.

Survival of these adults depends very much on the weather over the next couple of months – and a large number must survive, otherwise there will be no fruit fly problems next year (unless flies are brought in to the GMV in infested fruit from other locations).

Associating fruit fly risk with weather patterns

Growers of deciduous fruit crops are familiar with “chilling hours” where timing of flowering and fruit set can be predicted once a set number of chilling hours below a certain threshold temperature has accumulated. The same idea can be used for predicting if fruit flies can survive the winter and when they are likely to infest crops. The main difference between “chilling hours” for fruit set and “degree-days” for fruit fly activity is that the degree-days measure the number of days above a certain threshold temperature.

Degree-days have been worked out for Queensland Fruit Fly (QFF) but because temperatures vary so much from location to location, even within a single orchard or urban backyard, and the fact that QFF is so sensitive to temperature they can only be used accurately on-site. Regional average temperatures, such as those produced by the Bureau of Meteorology (BOM) can be used but they give only an overview of the whole area and should only be used for general indications of fruit fly risk.

Adult female QFF that have survived the winter need about 82 degree-days above 12.4˚C from 1 July to be able to find a mate, mate and lay eggs. There is another factor that needs to be addressed and that is the temperature at sunset. This is the time of day when fruit flies mate and if it’s too cold (less than about 15˚C), then they won’t.

Using BOM data from various locations within and around the GMV, estimates of the dates of first egg-laying by adult QFF that survived the 2017 winter for the season were:

Tocumwal (the nearest BOM site to Cobram): around mid-September

Kyabram (near Shepparton): mid-September

Echuca: mid-September

Strathbogie (near Euroa): early to mid-October 2017

This information can be used to plan purchases of fruit fly control products and for strategic timing and placement of fruit fly controls. It also shows how varied the interaction is between the weather and QFF.

BOM’s outlooks for June, July and August 2018, based on data from 1981 to 2010, indicate:

  • A 55% to 65% chance that maximum temperatures will be higher than average – therefore, it is unlikely to be much different from normal (i.e. 12˚C to 15˚C in those three months in the GMV)
  • A 70% to 80% chance that minimum temperatures will be higher than average – therefore, higher winter minimum temperatures are likely to happen (i.e. higher than the normal range from 3˚C to 6˚C in those three months in the GMV)
  • A 50% chance that rainfall will be higher than average – therefore, it won’t be any wetter or drier than normal (i.e. it won’t be much different from the usual 100mm to 200mm over the three-month period in the GMV)

What does this mean for QFF in the GMV?

Usually, a milder winter, where minimum temperatures are higher than normal, means that more QFF adults will survive into spring and these flies will mate earlier in the year than normal. This points to a larger and earlier QFF population than normal. However, this will be tempered by a lack of rain and subsequent levels of relative humidity that QFF need to survive comfortably.

If the BOM forecasts turn out to be correct then the 2018/2019 QFF season will not be much different from those of 2016/2017 and 2017/2018.

It is quite possible that the improved and accelerated fruit fly programs set up in and around the GMV since mid-2017, such as public awareness workshops and programs, QFF host plant removal, greater use of traps, baits and netting and the destruction of unwanted fruit may impact on QFF numbers coming into the next season. If these activities have bitten into the QFF problem and the BOM forecasts are right, we will see a reduction in QFF in the GMV next season.

Advice to home gardeners

The number of fruit flies trapped in the GMV trapping grid dropped sharply in urban locations. This is most likely due mainly to the weather, the fly’s habits during cold periods and the lack of suitable host fruit in urban areas at this time of year. It is highly likely, though, that there are spots within urban areas that remain suitable for QFF survival throughout the winter.

Even though, from now on in the GMV, QFF eggs, larvae and pupae are unlikely to survive the winter it is still advisable to pick up fallen fruit and harvest late-hanging fruit just in case they are situated in a position that is warm enough for QFF survival. Such orchard hygiene is useful for other reasons, too, such as reduction in fungal pathogen load in the garden.

Please do not simply collect this fruit and throw onto the compost heap. Quite a lot of heat is given off as microorganisms break down organic waste. Fruit flies can survive there. It is best to cook, freeze, incinerate, mulch or solarise this fruit. Your QFF Co-ordinator can give you more information about this.

You could get a good indication of the presence of overwintering fruit flies in your garden by placing out food-based fruit fly traps. In winter these traps should be placed in a spot that catches the morning sun. They are best placed high in the canopy of evergreen trees in the warmest position in the yard. Lemon trees are particularly favoured by QFF to overwinter in.

If food-based traps catch flies you can apply fruit fly baits and that should control the flies. Your QFF Co-ordinator and representative from your local produce stores can help here.

The basic idea is that, if your garden is one of the few hot spots for QFF in the GMV, and this is evidenced by finding flies in traps in May/ June, you can help stop flies surviving the winter and building up into damaging populations in spring. It is these flies that then spread out into neighbouring gardens and commercial orchards.

Don’t forget that the removal of unwanted QFF host plants is a very useful and effective way of controlling future fruit fly populations.

Advice to commercial growers

Much of the advice to home gardeners mentioned above is relevant to commercial growers. Orchard hygiene, disposal of unwanted fruit and food-based traps and fruit fly baits are useful techniques for fruit fly management in commercial orchards.

The number of fruit flies trapped in the GMV trapping grid dropped sharply in rural locations. This is due mainly to the weather and the fly’s habits during cold periods rather than human-mediated fruit fly control. This supposition can be tested if minimum daily temperatures jump up about 3˚C to 5˚C following a week or so of low daily minima. If the drop in fruit flies being trapped was due only to the weather then traps will experience a mini-peak a couple of days after the temperature spike. If there’s no mini-peak, then this could indicate effective fruit fly controls have been implemented.

Control of fruit fly for commercial orchards, in addition to trapping and baiting can be augmented with application of pesticides but these must be approved for use against fruit fly in an approved crop and region. All label requirements must be followed. Your QFF Co-ordinator and representative from your local produce stores can help here.

For assistance in managing QFF contact the GMV Fruit Fly Coordinator, Ross Abberfield by phoning (03) 5871 9222 or emailing gmvfruitfly@moira.vic.gov.au

This report was produced by Andrew Jessup and incorporates an analysis of regional trapping data supplied by the GMV Fruit Fly Project.