We’re always trying to learn more about fruit tree pests and diseases, and learned a lot of new stuff about Queensland Fruit Fly (QFF) at a recent workshop by Andrew Jessup sponsored by Mount Alexander Shire Council.
Why are QFF such a bad pest?
QFF are a very successful insect, and in fact are one of the most adaptive insects known because they function in new environments and adapt to new fruits very well.
Other things in their favour are the fact that they lay lots of eggs (a single mating leads to 2,000 eggs), and that a high percentage of the eggs go on to become adults.
QFF are just one of many different types of flies, and many of them can get caught in traps, so just because you’ve caught something in a trap doesn’t mean it’s QFF – it’s important to get a positive ID.
They only mate at sunset, and only on days when the temperature at sunset is over 16C. The males gather together, sing, and put out a cloud of pheromones.
The males can mate many times, but the females usually only mate once – unless it’s not successful the first time. After mating the male gives the female a ‘nuptial gift’ of a protein parcel, which helps her to fertilise and lay the eggs.
On a side note, one of the unintended consequences of the Sterile Fly experiment being trialed in parts of Australia is that the sterilised flies are less likely to deliver this nuptial gift, which leads many of the females that have mated with sterile males to go in search of wild males and re-mate.
Do they survive over winter?
At the end of summer, any flies that are still alive will look for somewhere to try to survive the colder months. Eggs, larvae and pupae are very intolerant of the cold and will die, but mature flies can live for up to 4 or 5 months through the winter when their metabolism slows down if they can find a warm spot to shelter.
They tend to congregate in warmer places such as evergreen trees (e.g. citrus), because even in very cold temperatures and frosts, evergreen trees will retain some warm patches within the leaf canopy that will provide protection for overwintering. Most of the time over winter the flies won’t be moving around much, unless daytime temperatures warm up to more than 12C.
Other warm spots near houses like hot water systems may also provide protection, but are not the hideout of choice for the flies because they don’t provide any protection from predators. They also like chook pens because they’re warm and give off the scent of ammonia which attracts them.
After hatching, flies are not sexually mature at the start, and need to find protein, and this is also true of flies in winter. Therefore you can put protein baits or female biased traps in potential over-wintering spots because in winter their metabolism slows down and they’re more interested in eating than sex. Don’t bother putting out male traps because they’re really not interested in sex.
These protein baits traps don’t pull flies in from a long distance so you’ll need to put one in every tree or potential hiding spot. The flies smell the protein and are attracted to it. The traps also contain a poison to kill the flies, and may also include something sweet to encourage them to the bait.
Chickens are one of the best defences against QFF, and only need to be active during the day as this is the only time the fruit fly larvae will be actively coming out of the ground.
Broad leaf weeds provide an excellent resting space for fruit flies, so it’s a good idea to keep them mowed and short. Mulch also provides a perfect place for fruit flies to pupate.
In Victoria fruit flies have been in East Gippsland for more than 100 years, and Melbourne suburbs Ascot Vale and Moonee Ponds are riddled with QFF. Why does this matter? Because controlling fruit flies in urban regions helps rural areas. Flies move from urban, to peri-urban regions, to orchards.
What’s the best monitoring regime?
Step 1 – Put out monitoring traps to detect males early in the season before the sunset temps reach 16C. They contain pheromone lures, so theoretically 90% of the catch will be QFF (not other types of fruit fly). They attract flies from up to 300-400m away. The lure lasts for about 4 months after you put it out, but you can buy replacement lures, so don’t throw the trap out at the end of the season because they’re re-usable. (TIP: write the date on the side of the trap in permanent marker the day you put it out.)
Step 2 – if you catch one, make sure you get positive ID. Here in Harcourt we’re lucky enough to have an Emergency Outbreak Plan (EOP), so a positive identification of a QFF will trigger the EOP being put into place. If you don’t have one of these and your community is at risk from QFF, ask your local council to prepare one (use the Harcourt one as a template). Alternatively check out our Free QFF Resource Pack for more response ideas.
There are many different types of trap – how do you know what are the good ones?
- it’s better to have a yellow base and a clear lid, rather than the other way around because if they land on the lid and can’t easily get into the trap they fly away as they prefer being under things than on top;
- check the concentration of lure in the trap, as it needs a minimum of 2g per lure to work properly);
- sticky traps catch everything because they don’t use a pheromone to attract the flies, they use a fruit smell as the lure, and this attracts a wide range of flies and insects including bees. They also don’t work as well because they’re competing with other fruit smells, and they only attract flies from about a 15m radius;
- protein traps also attract other types of insects. It’s best to put this type of trap out if your monitoring indicates that QFF is present. Put them out at least 6 weeks prior to harvest. Don’t put them in fruiting trees, but in the tops of lemon trees is OK because lemons are not a major host of QFF, particularly Eureka or Lisbon, though they don’t mind the sweeter lemons like Meyer and Lemonade;
- home-made traps have limitations in that they attract all types of insects, so you can accidentally be killing beneficial insects. In addition they will often only work for 2-3 days;
- Don’t leave protein-based traps (home-made or commercial) in the hot sun, as high temperatures will denature the proteins and make the trap ineffective;
- place traps on the north-eastern aspect of the tree as this is where the flies are most likely to be active in the morning.
What fruits do QFF prefer?
- Loquat – the perfect fruit for them at the beginning of the season because they ripen so early
- Vegetables: tomato (cherry tomatoes and Roma are slightly resistant as they seem to have more slippery skins), red chillies, red capsicum
Which ones don’t they like?
- Lemons, especially the sour lemons like Eureka and Lisbons (but being opportunistic little buggers, they’ll even infect lemons if none of their preferred fruit are available);
- Avocados – they won’t attack them on the tree, especially Hass, though they’ll attempt to get through the skin if they’re short of better options – this (or Fruit Spotting Bug) is one of the causes of the small hard masses you sometimes find in avocados.
- Purple passionfruit
- Persimmon – the astringent types, but they will attack non-astringent ones
In America they spray Giberellic acid onto citrus trees to try to make the skin harder to give resistance to fruit flies.
Hot hurricane-type winds can blow QFF very long distances, but in windy conditions they’ll often just hunker down and hang on in the tree they’re in. Very hot conditions (especially dry heat) will also make them stay relatively immobile in a cool spot in a tree.
Preventing QFF getting to your fruit
Netting needs to be maximum 1.3 mm hole size to prevent the flies getting through, but they can still sting any fruit that is next to the net, so it’s not a perfect solution. However it’s still one of the best lines of defence, so if you need some help with figuring out the logistics of setting up your netting system, download this short course.
Netting can also cause an increase in humidity that can actually provide better habitat for other pests.
Bagging individual fruit or branches is more reliable, for example some people use white, waxy paper bags. Either cut the base out or put a hole in the bottom of the bag to allow drainage of condensation, and then tie the bag around the fruit. It’s time consuming but effective.
In the Pacific Islands growers use wet newspaper to protect fruit. They fold it around the fruit like a nappy. When the paper gets wet it turns into a type of papier mache which remains intact and lasts for the entire season.
Does companion planting work?
There’s no good hard evidence for this, but plants like artemis, marigolds, mint and wormwood (and other strong-smelling herbs that would have been used in Mediaeval times as ‘strewing herbs’) are reputed to repel fruit fly. Let the plants grow under your fruit trees, then slash and let them lie on the ground.
What eats fruit flies?
QFF have many natural predators, including:
- Robber fly
- Assassin bug
- Jumping spiders
- Parasitoid wasps (but only if the fruit is already infected)
Like all pests, it’s futile trying to completely eradicate the pest population, as the only way you can attempt this is with chemicals. Chemicals might reduce the population for a while but the pest inevitably comes back – farmers who rely on chemical control of any pest find themselves having to constantly spray for the same pest, and in fact they may be inadvertantly be making the problem worse. Plus chemicals have other unwanted consequences like killing predator insects, and harming human health.
So what’s the best solution? As well as the monitoring and protection mechanisms discussed above, the QFF population will have more trouble building up in a bird-friendly, biodiverse garden with many different types of insects and a few resident chickens.