Queensland Fruit Fly Goulburn Murray Valley Outlook September 2020

This information has been commissioned by the Goulburn Murray Valley (GMV) Regional Fruit Fly Project and is funded by the Victorian Government’s Managing Fruit Fly Regional Grants Program. Use of this material in its complete and original format, acknowledging its source, is permitted, however unauthorised alterations to the text or content is not permitted.

Situation as at 27 August 2020

Queensland Fruit Fly (QFF) trapping rates bottomed to just 9 QFF trapped over the whole of the Goulburn Murray Valley (GMV) over the second two weeks of July 2020. This is due mainly to:

  1. It being too cold in the GMV for QFF to be attracted to the lures inside the traps; and
  2. Successful implementation and maintenance of the GMV Fruit Fly Area Wide Management Project.

Since 2012, when QFF started to become an economic problem in the GMV, the 2018/2019 and 2019/20 QFF seasons are the smallest. Figure 1 compares trap capture rates over the last three years. It shows that there has been a decline in QFF over the past two years compared with 2017/18 which was the start of the current Area Wide Management project.

Despite this decline complacency should not be allowed to set in as there are places within the GMV where fruit flies are starting to stir in response to warm patches on the landscape. During the first two weeks of August 2020, a total of 47 QFF were trapped from about 380 traps. There is no doubt that there are adult QFF that have survived the winter by taking refuge in warm spots.

If vigilance is not maintained through trapping and fruit inspection throughout spring, and fruit fly control measures (e.g. host removal, baits, netting, pruning, windfall clean-up, approved pesticides) are not in place or ready for use, QFF that survived winter will find host fruit resulting in population explosions in December 2020 and January 2021 which will then wreak havoc in home gardens and commercial orchards.

Figure 2 shows the normal pattern of QFF population peaks and troughs, as measured by fruit fly trapping grids, within the GMV over the last few years from July, when QFF activity is at its lowest level, through summer and autumn, when it is highest and into the June of the following year as activity declines. Peak QFF activity varies between urban sites (generally commences in late September) and rural locations (peak QFF activity in late January) in the GMV (Figure 3).

Figure 3 shows that the first seasonal increases in QFF populations occur in urban and, to a lesser extent, peri-urban locations in the GMV. Rural levels of QFF activity do not commence their seasonal expansion until mid to late summer.

In urban and peri-urban areas September is the month where QFF activity starts to increase as the weather warms up, so it is very important to ensure management activities (see Table 1) are in place or ready for activisation. If urban and peri-urban QFF are controlled then their spread to rural locations will be reduced. Control activities in urban locations will assist our growers in their production.

September 2020 outlook

The optimum weather situation for QFF survival into spring is for September to receive above-average rainfall and above-average maximum and minimum temperatures. This situation occurred in 2016 which resulted in extremely high spring, summer and autumn QFF populations all over central and northern parts of Victoria.

Weather patterns forecast for September 2020, provided by the Australian Government’s Bureau of Meteorology show a very high chance (75% to over 80%) of being higher than the normal amount of rain received in September (25mm to 50mm). This means that if adult QFF have survived the winter they will have food (bacteria and fungi) in abundance due to the wet weather.

In September 2020 maximum temperatures are forecast to be average (15 ̊C to 18 ̊C) and minimum temperatures for the same period may be slightly higher (60% to 65% chance) than average (3°C to 6°C).

Likely QFF activity in September

The above-mentioned conditions are very suitable for the expansion and spread of cold acclimatised QFF – which is the case for populations that are now present in the GMV. Urban sites are susceptible to higher levels of QFF that survived winter as they are, on the average, warmer and more humid than rural locations so it is in urban areas that QFF populations will first expand.

A study of temperature ranges over the GMV (see Table 2) shows that March and April rains were, generally much higher than average. This means that QFF adults were able to survive the approach of the oncoming winter. It is likely that adults will have found warm refuges on the landscape well before June and July and early August, when minimum and maximum temperatures were lower than average.

September is the time to start checking fruit for sting marks, setting traps for monitoring purposes and ensuring you have adequate QFF control material in stock and on-hand. Removal of unwanted fruiting plants or pruning trees to manageable/nettable height are also good strategies.

Until sunset temperatures reach the mating threshold of 16°C there will be no QFF egg-laying although there will be an upsurge in the numbers of male fruit flies being found in traps in late September. There will be patches of the landscape that will reach this mating threshold sometime in September. This is more likely to occur in urban situations as QFF overwinter there in greater numbers. These sites are potential ‘hot-spots’ for QFF population expansion.

Control activities for urban and peri-urban locations

QFF populations will start to increase in September 2020 in urban and peri-urban locations of the GMV. This will be as a result of:

  1. Overall warming temperatures during early spring
  2. Forecast higher than average rainfall
  3. Higher minimum daily temperatures
  4. The presence of some early fruiting weed, feral and garden plants
  5. Dusk temperatures of 15 ̊C to 16 ̊C will encourage mating and egg laying

Control activities that home gardeners can undertake to reduce the impact of QFF on their gardens are summarised in Table 1. In short, home gardeners should commence their monitoring for QFF early in the season – August/September. If QFF were present during the previous 2019/20 season and caused damage to fruit they will be a problem again this coming 2020/21 season unless controlled.

When applied to the home garden successfully, these activities in urban and peri-urban areas will assist growers in rural locations.

Control activities for rural locations

Rural locations that are further than 1km away from built up areas will experience fewer impacts of QFF during September and October than urban areas. QFF populations will increase to damaging levels in rural areas from mid-January 2021.

Current control activities commercial growers should be conducting are summarised in

Table 1 and include:

  1. Deploy and maintain fruit fly traps and monitor them weekly
  2. Ensure supplies of trap lure replacements, baits and pesticides are on site, or have been ordered
  3. Ensure unused on-site lures, baits and pesticides are still within their use-by dates and approved for use against QFF on your crops in your state

Table 1

Figure 1

Figure 2

Figure 3

Figure 4