Allow parasites to control leafhopper
The beneficial Anagrus wasps are susceptible to pesticides.
A decade ago, the western grape leafhopper was known to exist in British Columbia only on the east side of the Okanagan Valley, from Penticton south to the Canada-U.S. border. Today, reports of the pest are coming from the west side of the valley and north of Penticton, highlighting the failure of traditional control measures to stop the pest's spread.
While several pesticides are effective against the Virginia creeper leafhopper, the same products aren't necessarily effective against the western grape leafhopper and in some cases may even eliminate natural enemies such as members of the Anagrus group of wasps.
Dr. Tom Lowery of the Pacific Agri-food Research Centre in Summerland told growers attending the annual B.C. Wine Grape Council conference in Penticton last summer that the situation demands greater attention to biological controls such as the Anagrus parasites.
A leafhopper infestation can reduce the photosynthetic ability of grapevines, limiting productivity. One of the best species for controlling the western grape leafhopper is Anagrus erythroneurae, a parasite of the leafhopper's eggs. It's also one of the species killed off by Sevin (carbaryl), which has proven effective against the Virginia creeper leafhopper.
Lowery told growers that if they can manage the population of Virgina creeper leafhopper—one option is by removing basal leaves in early June, when the eggs are hatching—then it reduces the need for chemicals, and parasites of the western grape leafhopper will have a chance to do their work. Anagrus erythroneurae is a particularly effective beneficial because it overwinters on the eggs of leafhoppers infesting rosaceous plants, then is able to get started on mint pests in the spring before moving on to grape leafhoppers.
"If you don't screw things up with chemicals, it will keep the numbers down to very low levels," Lowery said.
Virigina creeper leafhopper has its own Anagrus parasite, Anagrus daanei, Lowery said, but it hasn't been as effective as A. erythroneurae because it lacks species that can host it over the winter. Lowery believes there may be hosts on the Naramata Bench just north of Penticton, however, because Virginia creeper leafhopper hasn't been as great a problem for some growers in that area. The challenge is finding which species is hosting the parasitoid, and developing a system that will work in favor of growers.
Lowery also noted a third species, Anagrus tretiakovae, that parasitizes the eggs of both types of leafhoppers. Unfortunately, it's not known to be in British Columbia, though reports indicate that it's in Prosser, Washington, and working its way north.
"We don't know where it is between Prosser and here. It's likely moving up; how long it will take to get here, we don't know," Lowery told growers.
Depending on the speed of the wasp's travels, there's interest in importing it into British Columbia in order to address the leafhopper problem, Lowry said. However, an import permit would take years to get, and the parasite might actually move faster than the bureaucratic machinery handling the import request.
Growers can also take measures to control leafhoppers, Lowery said.
While early-season leaf thinning is effective—reducing Virgina creeper leafhopper populations by as much as 70 percent (with little impact on grape quality)—plants that are well fed and watered (but with a regular thirst) also stand to be more resistant to the leafhoppers.
Lowery told growers that controlling leafhoppers comes down to a few key points.
"Don't overfertilize and don't stress the vines. Add appropriate physical and cultural controls," he said, citing the presence of diverse vegetation around vineyards for the use of beneficials and sticky tape in hot spots as key elements of control strategies.
Lowery also urged growers to keep records that will help them determine which species of leafhoppers are in their vineyard, where they're located, and the intensity of infestation so that sprays can be applied effectively. Insecticides should address the species in question, and only where required and when required.
Looking ahead, Lowery told growers a key objective for researchers will be surveying Okanagan Valley vineyards for western grape leafhopper and A. tretiokovae populations, with a view to determining whether or not there's a need to import the latter.
Further research is also planned into products that repel or discourage leafhoppers from feeding on vines, as well as cultural practices that will make vineyards less hospitable to the pests.