Puzzling outbreak

Adult beetles have dark reddish brown or black heads, and the body is covered with short gray hairs.

A mysterious outbreak of an endemic pest that threatens grapevines in Kelowna, British Columbia, Canada, has put local growers and government entomologists on alert.

In May 2008, beetles causing severe damage to grapes and Floribunda roses on the east side of Kelowna were identified by experts with the Insect Identification Service, Agriculture and Agri-Food Canada, as the western grape rootworm (Bromius obscurus).

Although this pest is native to North America and has been reported previously on Vancouver Island, this appears to be the first record of serious damage by this insect in the Okanagan Valley.

"In Europe and California, the western grape rootworm is a pest of grapes, and it could potentially become a problem for the B.C. grape industry," said Dr. Tom Lowery, entomologist for the B.C. Ministry of Agriculture and Lands.

"It is unclear if this is the result of a new invasion, favorable environmental conditions or the introduction of a damaging biotype from another region."

Lowery said the outbreak is especially mystifying because the western grape rootworm is not a pest that B.C. grape growers normally have to deal with, and it’s not even listed in the B.C. Grape Production Guide.

"It hasn’t typically been a problem. We get a little bit of chewing on leaves up here, but it isn’t something we typically worry about."

In California, however, the western grape rootworm can be a major pest.

"If you look in the production guide for California, you’ll see that in some cases they say it’s even high enough numbers that can kill vines because the larvae also feed on the roots of the grapevines," Lowery said.

Adults feed by cutting distinctive linear or slit-like holes in leaves; severely damaged leaves look like lace. Damage on roses leads to drying and death of leaves. On grapes, adults may feed on the bark of tender shoots, leaf petioles, and berries. Larvae feed on roots and can cause serious damage to roots.

Full-grown larvae are C-shaped white grubs, about seven millimeters long, with a yellowish brown head, brown or black mouth parts, and three pairs of legs near the head. Adult beetles are about four millimeters long, dark reddish brown with a black head. The body is covered with short gray hairs.

Rosebush connection?

The outbreak was identified after a homeowner notified Dr. Susanna Acheampong, provincial entomologist with the Ministry of Agriculture, about damage to Floribunda roses. The outbreak was extensive enough to kill one of the rosebushes. Lowery got a call about the same time from a vineyard and winery operation that had discovered the same pest.

"The grapes also had roses planted at the end of the rows, so we’re not certain if it could be a link to the roses more than to where it was found in the newly planted vineyard. It was on a couple of properties," Lowery said.

"The most severe damage appears to be associated with recently planted rosebushes, but we do not presently know if there is a link or simply a host preference. Although it is possible that the outbreak was due to the introduction of a new biotype of the western grape rootworm, it might be an anomaly caused by some other factor.

"What is curious, though, is why we had such a large outbreak and, in the backyard location, it was actually enough to kill that rosebush. These were very limited infestations, so it was plantings of maybe a dozen rosebushes at both locations. You can find it on grapes as well, but the serious infestation was on roses."

Although the Canadian Food Inspection Agency inspected the sites, the agency deemed no quarantine would be established because the rootworm is indigenous to the area. Without the assistance of the CFIA, Acheampong said it was almost impossible to trace the origins of the infested rosebushes through the nursery they came from.

"Right now, we can’t say for sure that it was introduced from anywhere else," Lowery said. "It could be just an unusual occurrence, that it was actually just our endemic species. My gut feeling is that that’s not the case. It was isolated. We haven’t had any experience with this previously, for a number of years."

Lowery said a subsequent survey of vineyards around the Kelowna area led to the discovery of the rootworm in a number of vineyards at low levels.

Information was provided to growers through a pest fact sheet and at the Enology and Viticulture Conference in Penticton. They were asked to notify the ministry if they suspected damage from this pest.

Follow-up research

"We’d like to go back out in March-April and dig up some vines and rosebushes and see if we can find larvae feeding on the roots of those plants," Lowery said.

"We now have sites identified in the Kelowna area where we have quite a problem. The main focus will be to go back to those sites and see how the population looks this year."

Depending on the outcome of site visits and surveys this spring, follow-up research may be required to better understand the biology of the pest, and develop damage thresholds and appropriate control strategies.

A small number of adult beetles were collected for the Pacific Agri-Food Research Centre in Summerland, B.C., in hopes of establishing a colony that could be used to evaluate susceptibilities to insecticides.

"We want to find out how big of a problem this is before we really invest too much time and money into it, so we’ll start off with surveys, and then we’ll go from there," Lowery said. "That would involve some of the growers who would be willing for us to dig up some of the grapes and find out if we have high infestations of larvae feeding on the root.

"The beetle has been flying under the radar all this time, so it’s a little bit surprising to see such an outbreak that it’s actually killing plants. Definitely, we’re a little bit alarmed, and we do intend to follow-up on it."