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Bring in the (Fruit Fly) Rellies

12 Apr 2018

By Penny Ossowski

 

I started writing this article twelve months ago when as usual I had been hand pollinating my pumpkins, but, last year they would grow for a couple of days and then start to shrivel and drop off, as if they haven’t been pollinated. This could have been the case and maybe the pollen being transferred was of poor quality or maybe it was too hot for them to set, I wasn't sure. As a result this led to both Chris and I checking them out quite regularly. Towards the end of January in the late afternoon/early evening, Chris called to me to come and look at these strange insects on a very small pumpkin (only pollinated a day or two previously). It was well hidden behind a few pumpkin leaves, a small pumpkin with four insects attacking it, they looked similar to the Queensland Fruit Fly but had different markings. I quickly got a glass jar and captured two of them for closer inspection.

 

Then to ‘Professor Google’, Chris found out a little and I researched further after dinner, mmmmm, a very close relative of the Queensland Fruit Fly, Bactrocera cucumis, aka the Cucumber Fly, a Pumpkin Fruit Fly, a Melon Fly. These little creatures like to sting and lay their eggs in cucumber, melons, pumpkin, squash, tomatoes, pawpaw, edible gourds, eggplant, watermelon, zucchini and possibly other fruits and vegetables.

 

I think they may have been responsible for the brown spots/sections in my cucumbers and zucchinis last year. I checked my Wild May fruit fly traps but they only contained Queensland Fruit Fly, so it didn’t seem to attract them.

 

I said then I would not grow pumpkins again but those who know me quite well know that I can't let anything get the better of me without a fight. I have already resigned myself to not growing some plants that the fruit fly attacks in the summer months so I was not impressed when the fruit flies invited their rellies into my garden.

 

This year I decided to grow butternut pumpkins instead of Jap pumpkins and to only grow a few plants in a different garden bed. I also decided to grow them on the ground and not on a trellis and over the roof of the pergola as I had done for the past 5 or 6 years.

 

I had already seen evidence of the Cucumber fly in my cucumbers and zucchini but for some reason they seem to be able to get past the stings and mature with a couple of brown spots in their flesh. Maybe this is because of the speed with which they mature by comparison to a pumpkin.

 

When the first female flower appeared on the pumpkin vine, I hand pollinated it and waited. Late that afternoon Chris called me to come and have a look and there again  were a couple of Cucumber Flies attacking the small pumpkin and within the next day or two it shrivelled and fell off. These pumpkins needed protection while they were immature and their skin still soft. So I conceived a plan and was ready when the next female flower opened. After hand pollinating the flower I wrapped the small pumpkin in half a Chux Wipe and tied it with string leaving the cover lose enough for the pumpkin to start growing. After 6 or 7 days I made a largish bag out of curtain mesh. I removed the Chux wipe and replaced it with the new bag. Now about a month later the pumpkin is fully grown and looking good, still in the bag. I now have five butternut pumpkins in curtain mesh bags. You can make your own or buy fruit fly bags from Eden Seeds.

 

Clean up any affected fruit and dispose of it by immersing in water or cooking it in a sealed plastic bag in the sun before binning it.

 

My observations indicate they attack young, immature fruit NOT damaged mature fruit. Here is the information about them from the

 

Department of Agriculture, Fisheries and Forestry Queensland web site.

 

Scientific name:          Bactrocera cucumis

 

Description of adult:   Cucumber fly is wasp-like, yellow-brown with yellow marks on the sides of the thorax, and about 8 mm long. The yellow mark running down the centre of the dorsal surface of the thorax between the wings distinguishes the cucumber fly from other fruit flies.

 

Immature stages:        The female pierces (stings) the maturing fruit and lays a clutch of white, banana-shaped eggs just below the surface. Hatching takes place after 2-3 days and the resulting larvae are carrot-shaped white maggots (about 7 mm long when mature) that tunnel in the flesh for about a week. They carry bacteria to aid in breaking down the fruit. The mature larvae can 'jump' or 'spring' by bending into a 'U'-shape and then releasing back to a straight carrot shape.

 

Life history:                  Larvae mature in 7-10 days in summer and emerge from the fruit to pupate in the soil. The pupal stage lasts about 10 days. In summer the life cycle takes about 2.5 weeks. Adults can survive for up to six months. The adults are attracted to and feed on bacterial colonies on the surface of fruit and foliage. They mate at these feeding sites and later lay into the fruit.

 

Distribution:                 Cucumber fruit flies are coastal and sub-coastal native insects occurring from the tip of Cape York to just south of the southern border of Queensland

 

Host range:                  Cucumber fly damages cucurbits and papaya.

 

Damage:                       It is a major pest of cucurbits north of Rockhampton where it also attacks ripening papaya in late spring. Populations are smaller in southern Queensland, but can cause appreciable damage to cucurbits, especially zucchinis, during late summer and autumn in years of good rainfall. Unlike the Queensland fruit fly, it is attracted to packing sheds and can infest produce at the time of packing.

In the field it prefers to lay into ripe or damaged fruit. With cucurbits, eggs are laid in the fleshy green fruit stalk. Insecticidal control may therefore be required at fruiting for the more susceptible crops, such as zucchini, during summer months. Papaya fruit ripening on the tree or in the shed may be attacked but mature green to a quarter colour fruit escape damage. Adult flies leave a puncture mark in fruit when they oviposit and the larvae destroy the edible flesh. Affected fruit are readily recognised since rots develop rapidly and the skin around the sting marks becomes discoloured. Fruit-fly damage is more severe during mid and late summer than at other times. Large numbers of flies can be expected after good falls of summer rain or periods of high humidity as this is when they become active

 

Control options:           Avoid spraying if at all possible. If infested fruit are found while picking, consider field spraying.

 

Cultural:                        Ensure good farm hygiene, e.g. destroy crops immediately after last harvest and do not leave reject fruit around the shed Harvest at first colour. Avoid papaya hybrids or varieties with a lengthy ripening period.

 

Biological:                    Some chemicals can be very disruptive to parasitoids and predators of many pests and may actually increase other insect pest problems e.g. mites. Spray only when damage is serious enough to cause commercial loss and where possible use soft chemicals that have minimal impact on beneficial organisms.

 

Peter from the DPI recommends putting old beer or a marmite solution in traps to catch the female Cucumber Fly and the Queensland Fruit Fly


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