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Fruit Flies in the GMV – Outlook for November 2019

November 18, 2019

Current trends in Queensland Fruit Fly populations and November outlook.
Spring overview:
● Trapping data indicates that the usual spring peak in Queensland Fruit Fly (QFF) numbers occurred across the Goulburn Murray Valley (GMV) in late September/ early October 2019.
● Currently about 88 per cent of the GMV’s QFF population is located in urban sites and only 12 per cent in rural locations, based on trapping data.
● Weather conditions are now suitable for QFF to mate and lay eggs.
● Over the next few weeks ripening fruit will be susceptible to QFF infestations.
● An additional 80 trap sites were set up in 2018/19 to add to the 314 traps deployed two years ago. Many of these new sites are registering high QFF numbers.
● Aerial releases of about 2 million sterile QFF per week commenced on 10 September 2019, as part of a pilot SITplus (Sterile Insect Technique) project, over a defined ‘drop-zone’ of the Cobram township.

There are now 394 QFF traps deployed in the GMV. Of these 80 were set out during late 2018 to mid-2019. Table 1 shows the number of QFF trapped in traps in each regional centre of the GMV recorded on the GMV Fruit Fly Trapping Grid over the past two months. Some of the newly trapped areas show relatively high rates of QFF trappings indicating that these places have significant QFF populations. Most of the high-level trapping rates come from urban areas. However, there is a sizable population at Katamatite rural suggesting that this population is established on-farm rather than migrating in from urban areas. The same could be said for Numurkah rural. These sites could be assessed for the presence of non-crop fruit fly hosts. Alternatively, a large range of different fruit fly host crops may be planted close to each other in these locations.

Figure 1 compares the numbers of QFF trapped per reading (weekly from 1 October to the end of May and fortnightly for the rest of the year) over the GMV from mid-August 2018 to the mid-October 2019 between traps placed in rural locations and urban sites. It can be seen that, generally there are more QFF trapped in urban areas than in rural areas during the start of the fruit fly peak season with the two populations coming together when most rural commercial crops are being harvested in autumn.
Figure 1 also shows that the spring peak, which is normally seen in urban traps only, was larger in 2019 than it was in 2018.     It will be necessary to wait until the summer to see if this means that the GMV is under more QFF pressure this year than last year.

Figure 2 compares peak season fruit fly trap captures for 2018/19 with the previous season (2017/18). More data on this is shown in Table 2. When the two seasons are compared
it is seen that 2018/19 trapped far fewer flies than the previous year, especially from January 2019, onwards. In 2017/18 a total of 8,834 QFF were trapped while in 2018/19
only 3,452 were trapped – a reduction of about 6 per cent. The total number of traps registering zero QFF (the ultimate aim for any area-wide management program) rose from an average or 57 per cent to 74 per cent. The number of traps with more than 10 QFF per fortnight, indicating the presence of a sizable QFF population, has decreased from 21 per cent in 2017/18 to only 7 per cent in 2018/19.

When looking at the opportunity for QFF adults that have survived the GMV’s winter to mate and then infest fruit early in the season data in Figure 3 indicate that the lower mating threshold temperature for QFF of 16°C at sunset was reached in more than 2 days in a row on about 2 October 2019 although there were some single day sunset temperatures at or above 16°C across the GMV from mid-September with one in early September but only in Tatura. This demonstrates the range of temperatures across the GMV and the consequent difficulty in predicting population surges based only on regional weather.

Figures 4 to 6 show the chances that rainfall and daily maximum and minimum temperatures for November 2019 will exceed average levels based on the last 10 years. Weather forecasts for November 2019, accessed from the Bureau of Meteorology website (www.bom.gov.au/climate/outlooks) on 22 October 2019, indicate that rainfall over the GMV is likely to be less than the average (which is 25-50mm for November based on the past 10 years); maximum temperatures will be higher than the average of 24-27°C and minimum temperatures will be not be different from the average of 9-12°C.

These forecasts, especially with regard to the lower likely rainfalls, are generally not favourable to QFF population increases. However, in gardens near the main house or buildings on a farm or orchard, where weather conditions are modified by irrigation, home heating and the presence of evergreen plants for refuge and early fruits and vegetables for egg-laying, QFF will thrive if allowed to.

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.