Berry growers on lookout for rose stem girdler

SALEM, OREGON – Unpredictability makes the rose stem girdler a challenging adversary, and the fact that it has set up camp to the south and east of BC is all the more concerning.

The pest’s arrival in commercial berry fields in BC’s Lower Mainland is a matter of when, not if, and research south of the border presented at the 17th annual Northwest Berry Foundation’s Caneberry Production Workshop at Chemeketa Community College in Salem, Oregon this past spring helps growers identify ways to prepare.

“Rose stem girdler, they sort of have this ‘now you see them, now you don’t’ fluctuating pattern of population increase and decrease,” says Washington State University regional agriculture specialist Justin O’Dea. “What that leads to is damage intermittently and unpredictably severe. You may have nothing one year and the next year is just incredibly bad.”

O’Dea says growers are often blindsided by yield losses of 60% to 90%.

According to BC Ministry of Agriculture and Food entomologist Tracy Hueppelsheuser, rose stem girdlers are already in BC but are not yet an issue.

“It does exist in BC,” she says. “But I have only seen it in dry areas like Kamloops and south-central BC. I have never seen it in commercial raspberries or any wild Rubus species in the Fraser Valley, where most of the berry production is.”

ES Cropconsult owner Heather Meberg says her berry monitoring team continues to look for rose stem girdler but hasn’t seen any in berries yet. However, just as SWD made its way into berries in the Lower Mainland, she feels this pest will as well.

“They have a girdling effect on the host plant and in this case that’s Rosa family brambles and Rubus family brambles,” says O’Dea. “So, obviously, it’s a pest of both caneberries and ornamental roses in the nursery industry.”

At less than a quarter inch long, the adults are slender, flattish beetles with green faces and copper wings. There is only one generation per season, but that’s cold comfort.

Adult beetles arrive around bloom and lay eggs within a week or two. Larvae enter the canes and by late summer, dieback is apparent. By October, the larvae will be well into the middle of the cane to overwinter. In May, the adult emerges, leaving a D-shaped hole, and begins the cycle again.

“We just have abundant hosts,” says O’Dea. “Himalayan blackberry is No. 1, but also evergreen blackberry, wild rose, thimbleberry. They all host this. Eradication is really unlikely and it’s probably completely unrealistic.”

Assessing risk is challenging. The rose stem girdler beetle can move hundreds of feet to a quarter mile, O’Dea says. The damage can also look like other issues such as phytophthora, water stress, nutrient deficiencies or biennial cane dieback. From mid-summer to fall, top growth may be wilted, foliage will be nutrient-stressed and fruit may be mushy, in addition to the expected girdling above galls.

“It’s hard to scout in real time. It’s easy to miss it,” he says. “Especially in floricane fruiting varieties, damage may appear after summer harvests. Key damage symptoms … are spherical galls, but they might also have a more spiral shape.”

For some farmers, the first indication of rose stem girdler comes later in the season when they start tying canes.

“They lift up the canes and they snap right off,” he says.

The challenge is that spraying has a tight, difficult window between emergence and egg laying. Spraying commonly needs to occur during bloom, when pollinators are also present, and it may time with SWD, pushing spray limits beyond regulations.

Pruning a few inches below the lowest gall and burning the pruned canes is a non-spray control method. Digging into canes will reveal spiral tunnelling and the larvae inside, which can mature and emerge even from dead canes, which is why burning is the best practice.

“Robust pruning alone can provide significant in-field control, only if nearby wild hosts are also removed,” he says.

At WSU, alternative control steps are being explored. One is a model to help growers determine when pre-emptive spraying should be done. Growers can sign up for access to the model.

Another method is diluted tree paint and kaolin clay with treatments like azadirachtin to disrupt egg laying.

There is also the promise of a naturally occurring parasitoid wasp being deployed in the future.