Fruit flies can breed rapidly, disperse widely and successfully infest many fruiting vegetables. They not only destroy fruit, but are a market access barrier in domestic and international markets. In May 2017, Dr Jenny Ekman from Applied Horticultural Research provided an update on a vegetable levy-funded project that focused on in-treatment solutions to control fruit flies.

Fruit flies used to be effectively controlled with pre-harvest cover sprays, however de-registration of dimethoate and fenthion (Lebaycid) means vegetable growers have had to find other ways to manage these pests.

One option is to replace the chemical barriers with physical barriers. According to Applied Horticultural Research (AHR) scientist Dr Jenny Ekman, flies rarely – if ever – enter greenhouses.

“Theoretically, they could get in through open roof vents or doors, but they rarely do. If they can’t see or smell the plants inside, they have no reason to try,” Dr Ekman said.

“Also, fruit flies are forest dwellers that tend to fly close to the ground, or dart from tree to tree, rather than venturing into the open sky looking for a roof vent.”

Netting solutions

Net houses are a potential solution – they can protect crops from rain, hail, wind and sunburn, and keep out many pests. White hail netting that includes sidewalls is surprisingly good at keeping flies out of orchards. However, net houses are expensive to erect and can be inconvenient. Unlike apple trees, vegetables are not necessarily grown in the same place all the time.

Many of the benefits of net houses are achieved using simple ‘floating row covers’. Netting or frost protection fleece is simply draped over plants and secured at the base with soil.

An AHR project that was completed in 2017, led by Dr Ekman, tested how well floating row covers work.

“We used to think that keeping fruit flies out required fine mesh with no holes or gaps. However, even fairly coarse netting has achieved great results, despite there being holes that the flies could wriggle through if they really wanted to,” Dr Ekman said.

“Flies use a lot of visual cues to find host fruit and netting obscures the crop surprisingly well. It also offers many of the advantages of net houses – it reduces irrigation requirements and gives plants some protection from extreme weather and other pests.”

Field trials

In the Sydney-based trials, large numbers of mature fruit flies were deliberately released into sacrificial crops of capsicums and chillies. Monitoring traps were placed under different types of netting to see how readily the flies could get under the covers. In addition, samples of fruit were harvested weekly to check for larvae.

The trials tested Vent Net (5mm x 4mm mesh, which is really a windbreak material) as well as VegeNet (1mm x 3mm mesh) and a fine Insect Net (0.8mm x 0.8mm mesh) as floating covers.

“Insect Net worked well at excluding flies, but was a bit heavy for the plants and also excluded the predatory insects we were relying on to control aphids. VegeNet weighed only 45g/m2 , and also proved very effective at keeping fruit flies away from the plants,” Dr Ekman said.

“Even though some flies got through the Vent Net, these were mainly males attracted by cue lures in the traps, with infestation in the fruit remaining extremely low.”

Meanwhile, trials in Bundaberg focused on the effect of netting on capsicum plants.

“It’s hard to quantify, but the plants under the netting just looked healthier,” Dr Ekman said.

“There was less wind damage, and sunburn was avoided in fruit under netting. The result was a moderate, but potentially important, improvement in yield and quality that was consistent across all the trials we did.”

Further considerations

The best results were achieved when netting was applied to young plants. However, even applying nets only two weeks before harvest still provided significant benefits for fruit quality.

The research also found that fruit grown under nets tended to have more consistent colour, as there was a smaller range of fruit colour on each plant.

Although nets can be used many times, re-use creates potential issues with weed and disease management. Cleaning large nets is no simple matter.

“We thought one solution would be to use disposable frost protection fleece,” Dr Ekman explained.

“This material is cheap, presents an excellent barrier to fruit flies and could potentially help plants grow faster during cooler months. However, it is easily damaged by wind. In the end, we decided it just isn’t suited to use on upright plants such as capsicums.

“Nets aren’t going to suit every producer of fruiting vegetables, but they are definitely good for managing fruit fly, and can have other benefits as well.”

Find out more

The final report for New in-field treatment solutions to control fruit fly (2) can be found on the InfoVeg database.

This article first appeared in the May/June 2017 edition of Vegetables Australia. You can view the full article and graphics on page 26.

New in-field treatment solutions to control fruit fly (2) was a strategic levy investment under the Hort Innovation Vegetable Fund.

This project was funded by Hort Innovation using the vegetable research and development levy and contributions from the Australian Government.

Project Number: VG13042