Hail nets may have a dual purpose of hail protection and insect control.
Myriam Siham, entomologist, says that several growers are experimenting with using hail nets to keep insects out of apple orchards.
The hail nets used extensively in France to cover apple orchards may have an added bonus—keeping insects off the fruit.
Entomologists at the Technical Research Center for Fruits and Vegetables in Lanaxde near Bergerac are evaluating the efficacy and economics of extending the hail nets to the sides of the orchard, completely enclosing the orchard with hail cloth.
"Most of the orchards here are already covered with hail nets," said Myriam Siham, scientist at CTIFL. "The idea was that you already have the hail nets above, so why not just close the sides to keep the insect out. So that's what we did."
Several growers have already installed the closed system, she said, adding that they are still evaluating data to determine the system's effectiveness against codling moth and other insects.
The orchard under trial was enclosed after flowering to avoid interference with pollination, and opened only when tractors and equipment entered for spraying, thinning, and such. CTIFL began the trial in 2008 and is collecting data on factors beyond insect control, such as flowering, color, fruit quality, and microclimate.
"We think that by closing the sides of the orchard, it might change the microclimate and orchard ecosystem, affecting conditions for pests and diseases," she said. "We don't know if it will, but that's why we want to study it for four years."
The researchers are considering releasing predators and beneficial bugs inside the nets as a way to keep them captive inside. Previous research has shown that mesh size of the net is not that critical as long as fruit is not touching the net, allowing the pest to sit on the fruit and deposit eggs through the mesh.
The block in the trial was planted in 2005, and thus far, has had low pest pressures. In fact, the scientists had to release codling moth outside the nets to ensure that a codling moth population was present. She notes that the cold, wet spring of 2008 may have discouraged codling moth activity. Scab incidence throughout France is high this year because of the cold spring conditions.
Initially, they will focus on codling moth and aphids and on apple scab and powdery mildew diseases.
The costs of enclosing the entire orchard are difficult to determine because the designs vary greatly, Siham said. Some growers have used simple designs, but the cost can be high if the nets are installed professionally.
The additional cost is in the door openings, which can be simple buttons or mechanical openings at the row end of the block, she noted. In their trial, there is room under the net to turn the tractor around between rows, but most growers are adding side walls to an existing system.
A grower wouldn't put up this kind of system, because it would have too much space under the netting without trees, she said, explaining most growers have installed a drop door at the end of the rows to open and allow tractors turnaround space outside the netting.
The concept of using the hail nets started in organic orchards but has now expanded into conventional orchards, Siham noted.