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.
The La Niña weather event that impacted South Eastern Australia from March 2020 has now neutralised and our weather is no longer being affected by the La Niña or El Niño weather patterns.
Bureau of Meteorology forecasts predict average rainfall, average maximum daily temperatures and average minimum daily temperatures for May. These conditions will be detrimental to the survival of Queensland fruit fly eggs and larvae that may still be found in fruit (feijoa, apples and oranges) whether on-tree or fallen onto the ground at this time. Fruit fly pupae in the ground or in fruit (e.g. quinces, apples and jujube) will not survive these conditions either.
Infested fruit may still be found on-tree or on the ground under warmer microclimates where temperatures will vary significantly from the household back yard or the commercial orchard. Adults flies will be able to survive by flying to warmer microclimates especially those found in evergreen trees (lemon and orange trees) near packing sheds, heated houses, fowl yards, animal pens or compost heaps so extra vigilance is required in these areas. The importance of crop hygiene, now by picking up, removing and destroying fallen and on-tree fruit, in reducing next season’s fruit fly damage is paramount.
Autumn populations and trends
The Goulburn Murray Valley regional trapping grid is made up of 377 traps, each of which were assessed five times from mid-March 2021 to mid-April 2021.
Over this time the total numbers of flies trapped in the Goulburn Murray Valley ranged from a weekly high of 1,066 (the week to 6 April) to 625 (the week to 15 April). The total number of trappings during the first two weeks of April (1,692) was much higher than that from the last two weeks of March (968). This shows that the Queensland fruit fly season for 2020/21 has been longer and more severe than usual.
Over the same five weeks in March-April 2018, the total number of fruit fly caught was 3,406 (from 303 traps; average of 2.24 fruit fly/trap/week) and 1,462 in March-April 2019 (from 364 traps; average of 0.8 fruit fly/trap/week). For March-April 2021 4,458 flies were trapped (from 377 traps; average of 2.4 fruit fly/trap/week).
Increased Queensland fruit fly pressure is due to warmer and wetter weather than average, more fruit produced and the presence of more unharvested fruit still on-tree or fallen to the ground.
Rural and urban trap variations
A breakdown of trapping data identifies a difference in trap capture rates between urban and rural locations. A large increase in Queensland fruit fly numbers in February/ March 2020, combined with the La Niña weather pattern from then to March 2021 allowed more fruit fly adults to survive the 2020 winter than usual. These factors caused the large spring peak of Queensland fruit fly identified in urban traps in September/ October 2020 which in turn, caused the large fruit fly population explosion in late 2020 and early 2021. These urban flies then moved, it is believed, to rural commercial orchards and are now causing large population increases as of late January 2021. Rural Queensland fly populations are very high at present, with trapping numbers higher than usual.
Newly emerged adult flies
Newly emerged adult fruit fly mate only at sunset and only when the sunset temperature is above approximately 16˚C. Sunset temperatures for a number of centres around Victoria recorded by Bureau of Meteorology for the three days from 18 April to 20 April 2021 indicate temperatures were below the minimum mating threshold temperature as of 20 April 2021 (providing this cold weather pattern continues into autumn/ winter) with the possible exception of Mildura and Albury. This data indicates Tatura, Shepparton, Yarrawonga, Wangaratta and Kyabram are at the point where newly emerged adult fruit fly will no longer infest fruit in the orchard or home garden.
NOTE: There may be warmer spots in and around buildings and other heat sinks where ambient temperatures are warmer than the Bureau of Meteorology data – which will allow for fruit infestation if flies are nearby.
Older adult fruit fly may still pose a problem
Older Queensland fruit fly that have already mated will still be able to lay eggs if the weather is warm enough. They can lay eggs at quite low temperatures, when it is sunny – as low as about 10˚C or possibly lower (the science is not complete on this subject as yet). However, this ovipositional activity is generally due to the female adult trying to off-load eggs before it gets too cold. Prolonged exposure (e.g. overnight) of eggs and larvae in fruit and pupae in the ground to temperatures below about 3˚C for eggs and larvae and 10˚C for pupae will result in complete mortality.
NOTE: Adult fruit fly may move into locations with warmer microclimates and may lay eggs into fruit that is also in that location. In this case the likelihood of eggs, larvae and pupae surviving are increased. Picking up or harvesting fruit that is still in the orchard or back yard is of high importance to reduce the likelihood of this occurring.
It is likely now, that the major Queensland fruit fly activity will be adult flies seeking, and possibly finding, protected refuge in which they will attempt to survive over winter.
Sterile insect technique project in Cobram town
Trap captures in the 33 traps located in Cobram town continue to show very low numbers of flies since the start of the SIT pilot trial managed by the SITplus Consortium and the Moira Shire Council. Current trap numbers are generally down when compared with previous seasons. It is likely that the slight rise in numbers of wild Queensland fruit fly for 2020/21 compared with the previous season is due to the much higher fruit fly population pressure in the region.
Interestingly, the same downward trend is seen in the total numbers of fruit fly trapped in rural traps located up to 5km or more from Cobram urban traps – where sterile Queensland fly are released. No sterile Queensland fly are released over traps in rural locations around Cobram town.
It seems that the SITplus program is impacting on nearby Queensland fruit fly populations. These observations may justify the hypothesis that fruit fly do in fact move out of the town and into rural locations during the late summer and early autumn, when commercial crops are being harvested.
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 firstname.lastname@example.org. For more information on fruit fly control and Area Wide Management strategies visit www.fruitflycontrol.com.au