The Goulburn Murray Valley Fruit Fly Area Wide Management (FFAWM) Program has been monitoring rural and urban Queensland fruit fly (QFF) trapping grids established across the Goulburn Murray Valley (GMV) region In Victoria for five years.

Program coordinator Ross Abberfield said the data obtained from these trapping grids is analysed and interpreted by program researcher Andrew Jessup of Janren Consulting.

“This data is used to drive the program’s communications strategy, which involves creating awareness, education and engagement in industry, the community and government to promote best practice and reduce the impact of QFF.

The program commissioned a scientific study titled ‘The Habits of Queensland Fruit Fly in Winter’, which was compiled by Andrew Jessup and focussed on Victoria’s GMV region.

The results of the study have been used to guide education activities and help inform people about how fruit fly may survive winter and what to do to suppress emerging populations in spring.

“Each month the program provides urban and rural updates, forecasts and newsletters, along with technical reports for growers while also responding to ‘hot spots’ each week during the season. This provides vital knowledge to the community, industry and government, enabling them to take ownership and unite in managing fruit fly,” Mr Abberfield said.

Andrew Jessup recently shared a discussion paper with the GMV program titled – Off Season Management of Pest Fruit Flies in Southeast Australia. This was a self-funded paper and has been condensed to facilitate this publication.

Background

QFF problems generally begin in the spring after surviving through the winter when fruit fly control is not generally practiced. This discussion paper describes the idea that successful winter (i.e. off-season) management of QFF can reduce its impact further into the season.

Normally, pests and diseases develop in at-risk orchards only during suitable climatic occurrences and the flowering and/or fruiting of host plants. This means that pests can be controlled by timely reaction to early indications of pest build-up.

Scouts are often employed to look for early indications of pest build-up. Responses are facilitated because growers and scouts recognise the set of weather patterns and fruit phenology that are likely to give rise to pest population initiation and expansion.

QFF does not fit this category. Although adult QFF can be present in large numbers in areas where QFF populations have become established or are in the process of becoming established, growers and home gardeners often remain unaware of them until fruit is infested with larvae. This is because adult QFF move into refuge microclimates to wait-out the winter and they are mostly unresponsive to the usual methods of detection such as male-targeting traps and the presence of infested fruit.

This means that low winter trap catches do not necessarily indicate the absence of QFF populations. At this time of the year, temperatures are too low for general fruit fly activity. QFF might walk around, if it’s warm enough during the day to look for moisture and feed. They might even fly short distances but, basically, during cool to cold winters:

• QFF will not infest fruit

• QFF will not survive winter as eggs, larvae or pupae – only as adults

• QFF will not be attracted to parapheromone (male-targeting) lures

• QFF will not mate

• QFF will not move at night.

Sizable numbers of QFF adults could still be alive in warm refuges such as heavily canopied evergreen trees near heated houses. These are the flies that will start the QFF population explosion next spring.

Good refuges include evergreen plants near heat sources and sunny evergreen orchards. Heat sources include houses, lighting, compost heaps, fowl and stock yards and evergreen trees surrounding open sunny ground (Fig.1). Heat sources can be:

• Active – i.e. they produce heat (e.g. heated houses, compost heaps) or

• Passive – i.e. they warm up in the sun and slowly release heat at night (e.g. woody evergreen plants).

Survival of these adults depends very much on the weather over the next couple of months – and a large number must survive, otherwise there will be no fruit fly problems next year (unless flies are brought into the target region in infested fruit from other locations).

If sufficient QFF survive the winter and evade any fruit fly control strategies that are employed, such as destroying fallen fruit and fruit that are still on-tree, pruning trees to reduce the number of out-of-reach fruit and covering trees with netting, fruit fly populations will increase significantly with warming temperatures from the beginning of August.

If there is no mating, then there will be no egg laying. During the winter months fruit is generally safe from being stung by female fruit flies as female QFF need to find a male and mate and this does not happen until dusk and when sunsets are warm enough (about 15˚C to 16˚C).

This is a rare event in winter in southern Australia although it is possible if microclimates in and around buildings are warm enough at that time of day for fruit flies overwintering in these warm spots to mate. It is therefore very important to make sure that fruiting trees near the house, or packing sheds, or similar heated places are protected with netting or food-based baits or traps.

Temperatures vary across the landscape. These can be very different from local Bureau of Meteorology data whose weather stations may well be some kilometres distant. In some areas of SE Australia QFF have been trapped from late August through September, about a month before other areas. Early season outbreaks in these areas coincided with localised high minimum daily temperatures and the presence of autumn-maturing QFF host plants.

There can be significant variation in temperatures within small areas on the landscape – from the orchard, and within the orchard, to the farm residence, heated packing sheds and other outhouses and even compost heaps and fowl yards. Even within a single citrus tree it can be 7°C– 8°C higher near the trunk than in the air outside the canopy (Fig. 2).

Lemon trees situated near heat sources such as residential houses are popular QFF overwintering sites. It’s possible that more than one fly can find refuge in one spot. This facilitates spring re-mating as the flies don’t have to fly far to find each other. It depends a lot on the initial fruit fly population size. For example, have a look at your own back yard OR, if you have a commercial orchard, have a look at your house yard. It may be similar to the situation pictured below.

Fig. 1. Design of typical urban backyard or farm house yard showing areas that might
be warmer than others during winter in SE Australia which could act as winter refuges for QFF.

The red patches in the diagram above show possible warm spots during the winter. These would attract adult QFF during the onset of winter. These are suitable overwintering sites for adult QFF. Evergreen trees that are close to warm spots in the yard are attractive to overwintering QFF adults as refuge sites.

These sites should be monitored with traps (winter and spring) or treated with baits (spring). Even if there are no fruit on these trees, they will still harbour QFF adults and be the source of fruit fly damage to your crops and possibly your neighbour’s too in the following fruiting season.

Temperatures vary considerably even within evergreen trees. For example, air temperatures (dark blue regions) cooled down from 3˚C to 1˚C during the evening (from 5:30pm to about 9:30pm) in the orange tree (Fig. 2) BUT parts of the trunk and branches of the tree were still at about 11˚C at 5:30pm and about 8˚C at 9:30pm. Ambient air temperatures around the avocado tree were as low as -1˚C while branch surfaces were as high as 8˚C at the same time.

If the evergreen tree has a thick canopy, such as you would find in a citrus, avocado, loquat and others, these warm temperatures may persist most of the night.

This is an excellent overwintering site for adult QFF because:

1. It is protected from weather extremes

2. It is warmer than the surrounding air temperatures

3. It is more humid than outside the canopy

4. Fungal and bacterial food is more likely to be present here than outside

5. Dew will often settle on the canopy for flies to survive on

6. It is protected from many predators (e.g. birds and other insects).

Fig. 2. Variation in surface temperatures in and around a single
orange tree at night during winter in SE Australia (Somersby, NSW).

CONCLUSION:

If QFF can be controlled in winter, they will be more easily controlled under area-wide management strategies during the rest of the year. This can be achieved with an increased knowledge of specific-site climatic and fruit phenological attributes and the development and implementation of off-season management options.

For more information about the Goulburn Murray Valley Fruit Fly Area Wide Management Program and fruit fly control in the GMV visit www.fruitflycontrol.com.au 

SOME BASIC QFF FACTS:

What cold temperatures kill fruit flies?
- Eggs – 6 to 8 days at a constant 3°C, shorter at lower temperatures, longer when it warms up during the day and cools down at night
- Larvae – 10 to 14 days at a constant 3°C, shorter at lower temperatures, longer when it warms up during the day and cools down at night
- Pupae – Zero survival at a constant 4°C to 10°C and below
- Adults – Zero survival if there are 5–7 days in a row where daily minimum temperatures are -10°C to -7°C, some survival after 5 consecutive days where temperatures fluctuate between -5°C and 15°C.

How long do adult QFF live?
- Summer – QFF are very active and spend a lot of energy on flying, mating and egg-laying – 2–3 months
- Winter – If they find suitable refuge, they go into a type of hibernation mode – 6–9 months
- Total – 2– 12 months