This information has been commissioned by the Goulburn Murray Valley 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.
Fruit fly population snapshot
- Queensland fruit fly numbers remained higher than normal during May across the region. This population increase can be attributed to the La Niña weather system and the presence of more unharvested and fallen fruit than usual.
- Higher than normal May numbers were due to high numbers in rural locations and, to a lesser extent, peri-urban sites. This was likely due to large volumes of unharvested and fallen fruit.
- May fruit fly numbers in urban sites were slightly higher than previous years but well down compared with rural and peri-urban sites.
- Individual trap captures, rather than widespread activity, skewed data with 45% of fruit fly trapped in May found in 7 of the 395 traps on the grid.
- The combined sterile insect technique (SIT) and Area Wide Management projects in Cobram urban areas continue to reduce numbers, despite the current status of a ‘bad’ fruit fly season.
- Area Wide Management + SIT appear to also benefit nearby rural locations.
- Weather outlooks favour the survival of overwintering fruit fly in the Goulburn Murray Valley region.
- The Goulburn Murray Valley fruit fly trapping grid is an essential tool for identifying hot spots, timing of fruit fly management activities and evaluating the efficacy of Area Wide Management programs.
Regional update and activity overview
Fruit fly numbers remained unusually high during May with over 2,000 Queensland fruit fly trapped per month from December to May. A number of factors may have contributed to this expansion in numbers including:
◦ Higher than normal mid-morning temperatures for this time of year. Fruit fly tend to gravitate to traps in the morning.
◦ Higher than normal dusk temperatures. Fruit fly mate at dusk, as long as temperatures are above about 16˚C. There were more days with dusk temperatures above 16˚C in the Goulburn Murray Valley during 2021 than in previous years, allowing adult flies to extend their activity season.
◦ Higher availability of ripe and ripening fruit possibly due to fruit remaining unharvested, dumped or windfallen due to COVID restrictions on pickers and warmer daily temperatures.
◦ Lasting impacts of the La Niña weather pattern which saw more rain and higher autumn temperatures all of which favoured fruit fly survival and expansion.
Rural locations
Queensland fruit fly trap numbers in rural locations of the Goulburn Murray Valley were high during 2020/21, particularly from February onwards. Peak fruit fly numbers were less than at the start of the Goulburn Murray Valley Area Wide Management program which commenced in 2017/18, however numbers remained high for a longer period – March and April in 2018 compared with February to May 2021 (Fig. 2).
When considering fruit fly caught per trap for February, March, April and May, captures were higher in 2021 than other years, with the exception of April 2018 (Table 1).
Peri-urban locations
Queensland fruit fly numbers in peri-urban locations reached high levels during 2020/21 – higher than those reached at the start of the Goulburn Murray Valley’s Area Wide Management program (year 1 – 2017/18). Numbers have dropped to moderate levels, although are still higher than years 2 and 3 of the project, during April and May (Fig. 3).
Peri-urban fruit fly peaks are generally higher than rural peaks and occur a little earlier than rural peaks in March rather than March to April.
Urban locations
Queensland fly numbers peaked in urban sites during December 2020 (Fig. 4 and Table 3), while last year’s peak occurred unusually in March 2020 (Table 3).
Tables 1, 2 and 3 depict when fruit fly peaks occurred. It appears fruit fly peaks occur early in urban areas, mid-season in peri-urban locations and later in rural sites. One conclusion is that Queensland fruit fly move from urban, through peri-urban and into rural locations during the season, possibly due to fruit availability. Another possible explanation is that fruit fly stay in each of their natal sites and are not attracted to fruit fly traps until there are ripe or ripening fruit nearby. This hypothesis seems unlikely, however it is a question that needs to be answered as this knowledge would be useful for strategic fruit fly management.
Towns and urban areas
Table 4 provides a ranking of the highest to lowest averages of fruit fly per trap over the period from 1 July 2020 to 21 May 2021. It shows that some areas have high averages, however, fruit fly is not evenly spread over these areas. Of the 16 traps servicing Merrigum and Nagambie 3 account for 67% of Qfly trapped during 2020/21.
Looking at it another way – 45% of fruit fly trapped in the Goulburn Murray Valley during May 2021 were from just 7 of the 395 traps on the grid.
Figures 5 (Cobram), 6 (Mooroopna), 7 (Shepparton) and 8 (Tatura) – all urban areas are captured to show that each town varies in fruit fly pressure and timing of populations.
Cobram and the impact of Area Wide Management and sterile insect technique (SIT)
Queensland fruit fly numbers have declined significantly in the area of the Cobram township since September 2019 when the SIT project commenced. Despite a very ‘bad’ year for fruit fly across the region and throughout much of southern, eastern and western Australia fruit fly numbers in Cobram urban areas are down significantly. When compared with Mooroopna, which has been targeted as the ‘control’ town (i.e. no SIT) and Tatura (also no SIT) it can be seen that 2020/21 fruit fly trapping rates are higher than those for most of the previous years, whereas those for Cobram are less. When Shepparton data is assessed there is a rise in Queensland fruit fly numbers compared with all previous years from December 2020 onwards.
The impact of weather on fruit fly
Male Queensland fruit fly are attracted to para-pheromone based traps which are used on the Goulburn Murray Valley trapping grid. Fruit fly are attracted to these traps most often during morning hours up to approximately 1pm. However fruit fly won’t go to traps if it is too cold – i.e. about 14˚C (the exact lower threshold temperature for trap attraction is not known precisely and more work is required to allow for better trap usage). Figure 9 shows the midday temperatures at four different locations in or near the Goulburn Murray Valley. The graph shows that fruit fly could be attracted to traps over most of late April and May in all four centres. Midday temperatures were particularly high in early May and this is the reason so many fruit fly were trapped during this period.
Fruit fly numbers were extremely high going into the end of May and into winter. If the coming winter is not cold or dry enough fruit fly will survive winter by overwintering in warm spots on the landscape in high numbers and become extremely problematical in the coming spring and summer.
In order to lay eggs into ripe and ripening fruit Queensland fruit fly have to mate, however they only need to mate the once and can lay eggs into many fruit for up to two months, although it’s generally only for 4 weeks or so in the warm weather. They always mate at dusk and only when the temperature is higher than about 16˚C. Figure 10 shows dusk temperatures at two locations in the Goulburn Murray Valley. Fruit fly had the opportunity through ideal weather to mate most of March 2021, and up to mid-April and then again into early May. Normally, at this time of year dusk temperatures are well below the minimum mating threshold. This weather anomaly has contributed to the large numbers of Queensland fruit fly found in the Goulburn Murray Valley during May 2021, which points to the probability of the survival of large numbers of fruit fly through winter and into spring.
Weather outlook for June and July 2021
The Bureau of Meteorology gave the following outlook for the Goulburn Murray Valley for the month of June 2021.
Rainfall: 60% to 65% chance of receiving more rain than average (25mm to 50mm).
Maximum temperature: 75% to 80% chance of higher than average maximum temperatures (12˚C to 18˚C).
Minimum temperature: 75% to 80% chance of higher than average minimum temperatures (3˚C to 6˚C).
This weather outlook is bad news for the Goulburn Murray Valley as it means that Queensland fruit fly will be able to survive at least this part of winter in greater numbers than normal.
Goulburn Murray Valley Fruit Fly Project
For assistance in managing Queensland fruit fly, contact the Project Coordinator at the Goulburn Murray Valley Fruit Fly Office by phoning (03) 5871 9222 or emailing gmvfruitfly@moira.vic.gov.au. For more information on fruit fly control and Area Wide Management strategies visit www.fruitflycontrol.com.au