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 mid-September 2020

Queensland Fruit Fly (Qfly) trapping rates across the Goulburn Murray Valley (GMV) have continued their seasonal upward trend since the end of August, after bottoming out during early August. This is the start of the new Qfly season, and this happens when morning temperatures reach about 13 ̊C to 15 ̊C when male Qfly can fly and are attracted to traps.

These flies were able find suitable refuge during the cold GMV winter and survive. They are the survivors which may be the cause of this season’s fruit fly problems, if not kept in check.

Growers and home gardeners should be on the look-out for early ripening fruit such as loquats and mulberries, or late-hanging fruit such as navel oranges and persimmons. These fruit will be targeted by fruit flies during the spring peak. These fruit should be removed from trees as they ripen and not left to rot on the tree or ground.

Understanding the spring peak

Spring is the time when many of the adult Qfly that survived the winter come out of their slow-moving winter mode and with increasing day temperatures start to mobilise and search for protein, other fruit fly to mate with and fruit to lay their eggs into. Males are attracted to traps at this time. Once mating has occurred, the females lay eggs in early season fruit one to three days later. These adults then die out and it is their offspring that become the problem. The number of Qfly trapped in late October and early November typically drops as by this time adult flies have died out – leaving their young, the first new generation of the new fruit fly season in fruit as eggs and larvae or in the soil as pupae.

Keeping watch for fruit flies by trapping and fruit examination, together with dealing with them as soon as you find them through garden hygiene, netting and letting your neighbours know that Qfly is about is the major area-wide management strategy used by home gardeners during spring.

Advice to home gardeners

Complacency should not set in because the vast majority of the Qfly trapped in late August and early September have been found in urban locations. It is important that home gardeners are aware that urban fruit flies, if not controlled sufficiently, will expand over the next two generations and spread into commercial orchards in late summer/autumn.

Home gardeners should be aware that their efforts to control Qfly in spring 2020 will be of immense use to commercial growers and hence the entire community in autumn 2021.

If vigilance is not maintained through trapping and fruit inspection, in early spring, and fruit fly control measures (e.g. host removal, baits, netting, pruning, windfall clean-up, approved pesticides) are not in stock and ready for use, overwintering Qfly will find host fruit resulting in fruit fly population explosions in December 2020 and January 2021.

Fig. 1 shows how different Qfly trapping rates are between urban and rural areas in September. This graph also shows that fruit fly numbers are on the increase, now that winter has finished. In every year since 2012 (when trapping commenced) rates of Qfly captures in September in urban areas were always much higher than in rural areas. This graph suggests that urban areas are the likely reservoirs for later Qfly fruit fly problems more widely spread throughout the GMV. It means that urban-based home gardeners should be targeted as major contributors to Qfly problems later in the commercial fruiting season.

Advice to commercial growers

It is unlikely that commercial growers, outside of urban locations, will have problems with Qfly right now as numbers in rural traps are very low. However, if you live within approximately 1km of the hot spot towns shown below it is advisable to ensure that you have removed all unwanted fruiting material and have traps, baits and pesticides on supply. It is important to ensure any pesticides in storage are within their use-by dates and are still approved for use in your State and for your fruit type.

The message to home gardeners from commercial growers in the GMV is simple: “Please be on the lookout for fruit flies in your garden and if found please deal with them otherwise their populations will increase and cause us, and you, too, via their impact on the whole community, severe damage.”

The sterile insect technique project (SITplus)

The SITplus project that was carried out from mid-September 2019 to April 2020 recommenced for the 2020/21 fruit fly season on 13 September 2020. There will be weekly releases of about 2 million sterile fruit flies dropped from a locally owned aircraft over Cobram town every week until April 2021. Sterile flies have already been trapped in the Cobram fruit fly trapping grid.

October 2020 outlook

The optimum winter weather situation for Qfly survival into spring is for October 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 Qfly populations all over the central and northern parts of Victoria. It looks like this situation will occur in 2020.

Qfly numbers will increase through October 2020 due to forecasted higher than normal rainfall. October is therefore an extremely critical time for QFF management in the GMV.

Weather patterns forecast for October 2020, provided by the Australian Government’s Bureau of Meteorology show >80% chance (Fig. 2) of rainfall being higher than the normal amount of rain received in October (25-50mm).

Maximum temperatures have a 65-75% chance (Fig. 3) and minimum temperatures a 50-60% chance (Fig. 4) of being higher than the medians of 21-24°C (maxima) and 6-9°C (minima).

Warmer than normal temperatures plus extra rainfall means that there will be more fruit and more fruit flies in the GMV this coming season. However, if home gardeners and growers manage fruit flies adequately this trend can be reversed.

Qfly mate when the temperature at sunset reaches approximately 15-16°C. In many parts of the GMV this happens in September.

As can be seen from Table 1, there have been several days, across the GMV, where the sunset temperature exceeded 15°C. This means that adults surviving the previous winter may have mated. Successful area-wide management programs and home garden fruit fly control ensure that these surviving flies are few in numbers and dispersed widely from each other.

These surviving flies lay into susceptible fruit in the spring and their offspring, the first generation for 2019/20 emerge as new adults in December. These flies damage home garden fruit and vegetables and produce a new, second generation in January/ February – and these flies are the ones that move from urban gardens into rural orchards.

It is essential to continue checking fruit for sting marks, setting traps for monitoring purposes and ensuring you have adequate Qfly control material in stock and on-hand. Removal of unwanted fruiting plants or pruning them to manageable/nettable height are also good strategies.

Temperatures are not uniform across any location. For example, Table 1 shows that sunset temperatures in Strathbogie are not yet high enough for Qfly to mate. There are patches throughout towns and outlying areas that reach the mating threshold earlier in September than other places. This is more likely to occur in urban situations as Qfly overwinter there in greater numbers. These sites are potential ‘hot-spots’ for Qfly population expansion.

Fruit fly hot spots for September/October 2020

The highest numbers of Qfly trapped so far this season are all in urban areas of the GMV.

TABLE 1. The number of days since the end of winter with sunset temperatures above 15 ̊C at various GMV locations in mid-September 2019.

(Data collected from Bureau of Meteorology website:
www.bom.gov.au/vic/observations/vicall.shtml – accessed 22 September 2020 and analysed by author.)

RAINFALL
www.bom.gov.au/climate/outlooks/#/rainfall/summary accessed 24 September 2020

FIG. 2. Likelihood of exceeding median October rainfall

MAXIMUM TEMPERATURES
http://www.bom.gov.au/climate/outlooks/#/temperature/summary accessed 24 September 2020

FIG. 3. Likelihood of exceeding median October maximum temperatures

MINIMUM TEMPERATURES
http://www.bom.gov.au/climate/outlooks/#/temperature/summary accessed 24 September 2020