In Quebec, about 25 different pests—insects, fungi or bacteria—can invade orchards and damage apple trees and their fruit, from the moment the buds open in April until the apples are picked. It is therefore very difficult to produce apples without pesticides in Quebec, says a researcher at the Institut de recherche et de développement en agroenvironnement (IRDA).
Due to the multiple threats to fruit, 95% of Quebec apple growers use a whole series of phytosanitary treatments, from insecticides to fungicides, acaricides and herbicides.
However, experiments are being conducted at IRDA to develop strategies to reduce, or even eliminate, the application of these substances in apple growing.
In a traditional crop, about a dozen fungicide applications are made during a season. They begin as soon as the first leaf appears to prevent apple scab, an infection caused by a fungus, explains Mikaël Larose, project manager at IRDA. Two applications of products that attack mites are also made at the beginning of the season. And five insecticide applications are necessary, on average, to prevent damage that can be caused by multiple insects — the tarnished plant bug, the apple sawfly, certain butterflies (such as the obliquebanded leafroller and the codling moth), among others. Not to mention, at the end of the season, the apple maggot.
However, the majority of apple growers in Quebec apply the concept of “integrated fruit production,” which consists of applying the right product at the right time to act against the right pest. “We do not use broad-spectrum pesticides at all and we do not apply them immediately. Instead, we proceed with targeted applications when an intervention threshold is reached,” emphasizes Mr. Larose.
“Agronomists will detect pests in the fields using traps. And there are capture thresholds [d’insectes dans les pièges] from which the intervention must be carried out. If this threshold is not reached, no treatment is required.
Nets instead of pesticides?
At IRDA, we have been experimenting since 2012 with the installation of exclusion nets — from mid-April, just before the buds burst, until harvest time in mid-September — in the hope of eliminating the application of all pesticides. Which would be even better than organic farming, which uses pesticides, but of natural origin.
The experiment involves wrapping a row of a dozen apple trees in a polyethylene net with a mesh size of about one millimeter by two millimeters, a size that keeps most pests out. The net is tied to the base of each apple tree’s trunk to exclude the soil from the protected area. “You have to create a closed microcosm around the trees, because most insect pests, to complete their life cycle, have to spend their larval stage in the soil — and once the adult has emerged, it climbs back up the tree.”
“By excluding the soil with the net, the insects cannot complete their life cycle. If they are in the net, the larvae cannot reach the ground, and if they are on the ground, the adults cannot climb the trees,” explains the biologist specializing in apple plant protection.
In the fields where this experiment is taking place, each row of apple trees under netting is next to a row of apple trees without netting subjected to the same phytosanitary regime (i.e. without pesticides or fertilizers). The latter serve as controls in order to evaluate the effectiveness of the netting by comparison. The data collected between 2012 and 2018 showed that “there is practically no difference in terms of yield and fruit quality,” says Mr. Larose. “We noted the same sugar content, the same firmness, the same coloring, the same size of fruit under netting as without it.”
We were able to close the door to most pests. “Only the oblique-banded leafroller played a trick on us; it easily completed its life cycle under the net. And since its natural predators cannot cross the net and reach it, we saw an increase in this pest,” notes Mr. Larose. “We should have treated the trees before installing the nets to eliminate the individuals that were present on the trees, but we were really looking to not put any [pesticide] in our system.”
Projects are underway with engineers from Polytechnique Montréal to find solutions to eliminate this pest, he said.
The researchers also observed a decrease in the incidence of apple scab on fruit under netting. Even though the spores of the fungus responsible for the disease can easily pass through the mesh of the netting, “they find fewer places to attach themselves to the apples, given that the latter have fewer microfissures in their epidermis thanks to the netting that has protected them from abiotic elements,” namely hail, wind and sun, the biologist says.
Still challenges to overcome
After experiencing poor results with normal-sized apple trees at the Centre for Expertise and Transfer in Organic and Local Agriculture (CETAB+), researchers are now opting for dwarf apple trees. This makes it easier to install the nets and they are also less likely to be shaken by the wind. These narrow-canopy trees can also be planted very close to each other, which makes it possible to densify the plantation.
IRDA researchers are currently experimenting with the use of very large nets that can cover up to eight rows of apple trees. “This makes it easier to enter the plot with tractors and the nets are installed more quickly. However, the soil is not excluded,” says Mr. Larose, while specifying that they are evaluating whether this method has an impact on the fruit.
The main challenge facing apple growers in adopting nets is the workload, because installing and handling the nets, especially when they have to be attached to the base of the trunks to exclude the soil, is labour-intensive, Larose notes. He says researchers are developing a mechanized system to facilitate these manual operations.
Pierre-Manuel Plante, owner of Verger Sainte-Marguerite in Trois-Rivières, has placed a small plot of his apple trees under netting. “It’s an interesting technology,” but for now, it’s not a panacea, says the apple grower, because he still has to intervene against apple scab, fire blight and certain insects, such as spider mites, the mites that ants carry back to their nest in the ground under the netting. And, of course, the time it takes to install the nets and their cost are not within the reach of all producers.
Mr. Larose still very much hopes that apple growers will opt for netting. He believes that this strategy will become more relevant in 10 years in the face of a climate scenario that will likely include more hail, heat waves and generations of insects. “Netting protects against hail, it can moderate temperature differences and it keeps insect pests away. In addition, we lose pesticides every year in apple growing that are no longer effective due to the development of resistance. And there are no other molecules that are developed by the industry. So, in 10 years, our toolbox will be empty. Blueberry fields are already cornered into this situation, and netting has become the only economically viable option for them.”