Queensland Fruit Fly Goulburn Murray Valley Outlook September 2019
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 2019 – ‘Buds are swelling and fruit flies are stirring’
Queensland Fruit Fly (QFF) trapping rates bottomed out at zero QFF trapped over the whole of the Goulburn Murray Valley (GMV) over the first two weeks of August 2019. This is largely due to being too cold for QFF to be attracted to lures inside the traps. When compared to trapping rates leading into winter for the years from 2012, when QFF started to become a massive problem, the 2018/2019 QFF season is the smallest. This winter was not the coldest on record so QFF remain comparatively unaffected. There was high heat stress for QFF in January 2019 but none for the rest of the QFF season after that. The implication is that the GMV Area-wide Fruit Fly Management program is contributing to QFF reduction, too.
Complacency should not be allowed to set in however, as there are places within the GMV where fruit flies are starting to stir in response to warm patches on the landscape. During the second fortnight of August 2019, a total of 18 QFF were trapped from about 390 traps. There is no doubt that there are adult QFF surviving the winter in warm spots.
If vigilance is not maintained through trapping and fruit inspection, as we approach spring, and fruit fly control measures (e.g. host removal, baits, netting, pruning, windfall clean-up, approved pesticides) are not ready for use, overwintering QFF will find host fruit resulting in QFF population explosions in December 2019 and January 2020.
Fig. 1 shows the progress of QFF populations within the GMV over the 2018/2019 QFF season which conventionally starts on 1 August and ends 30 July the following year. Peak QFF activity, however, generally commences in late September (urban sites) or late January (rural sites) and ends in late May in the GMV.
It can be seen that QFF trapping rates, during autumn and winter were very similar in both rural and urban sites but, during the spring and summer, trapping rates in urban areas were higher than rural sites. This pattern has been repeated in each of the last seven years since comprehensive trapping of QFF in the GMV commenced.
September 2019 outlook
The optimum winter 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 the winter of 2016 which resulted in extremely high spring, summer and autumn QFF populations all over the central and northern parts of Victoria.
Weather patterns forecast for September 2019, provided by the Australian Government’s Bureau of Meteorology show only a 40 – 45% chance (Fig. 3) of being higher than the normal amount of rain received in September (25mm to 50mm; Fig. 2).
Temperatures also have only a 45 – 50% (maximum temperatures; Fig. 5 and minimum temperatures; Fig. 7) of being higher than the medians of 15°C to 18°C (maxima; Fig. 4) and 3°C to 6°C (minima; Fig. 6).
Although temperatures may rise a little when compared with the average, the lack of extra rainfall will not be sufficient for a mass-scale QFF overwintering event. However, this may change if September rains and temperatures increase. Urban sites, though, may be susceptible to higher QFF overwintering levels as they are, on the average, warmer and more humid than rural locations.
A study of temperature ranges over the GMV on single winter days shows that, even though average temperatures are below those for significant QFF movement (less than about 12.5°C), the range is plus-or-minus 5°C about that mean (see Fig. 8). If the average temperature in Cobram is about 11°C at 10:30am (as in Fig. 8) there are places nearby which are much lower – down to about 6°C. At these temperatures (less than 12.5°C) QFF barely move, and to survive they need to find refuge from cold, wind and desiccation. However, at the same time, there are spots in Cobram that are between 12.5°C and 16°C. QFF will move and even fly within this range and will definitely survive on dew and the odd protein source supplied by naturally occurring bacteria, fungi and yeasts and sugar from flowers that might be open.
Likely activity in September
September rainfall and temperatures are unlikely to be significantly different from normal. This suggests that, because temperatures are now, basically, on the rise the numbers of QFF surviving over winter will not decrease due to weather. Numbers may go down caused by predation (birds, insect predators). Added to that, QFF populations have gone down in the GMV over the last few years so the level of QFF inoculum (the base for the next QFF population explosion) is relatively small. The ability for males and females to find each other to mate (they really need to mate again after the winter in order to lay more eggs) is decreased unless groups of them are overwintering together.
Even though this low level of inoculum is good news the community cannot afford to be complacent. Despite low QFF numbers, if they can find each other, the temperature at mating time (sunset) is high enough (16°C) and there are available ripe or ripening fruit (e.g. loquats, winter oranges) a significant upsurge in QFF numbers in their first post-winter generation and subsequent generations can result.
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 egglaying, however 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.
accessed 27 August 2019
FIG. 2. Median rainfall for September
FIG. 3. Likelihood of exceeding median September rainfall
accessed 27 August 2019
FIG. 4. Median maximum temperatures for September
FIG. 5. Likelihood of exceeding median September maximum temperatures Minimum temperatures
accessed 27 August 2019
FIG. 6. Median minimum temperatures for September
FIG. 7. Likelihood of exceeding median September minimum temperatures
Ground temperatures – Cobram township