Filling in the Blanks About Rose Stem Girdler

Berry farmers are all too familiar with the rose stem girdler (RSG), an invasive insect from Eurasia that came to North America over a hundred years ago. They know that the copper-colored beetle has migrated to the Pacific Northwest. They know it attacks caneberries — blackberries and raspberries — sometimes ravaging up to 90% of a crop and forcing farmers to replant fields.

What they may not know is when or where it will strike or how to contain it.

Now, however, researchers at Washington State University (WSU) may be on the verge of providing a breakthrough by developing a coordinated approach to control RSG; an approach known as integrated pest management. The project was funded by the Northwest Center for Small Fruit Research, an  Agricultural Research Service-led consortium, to gather information about RSG and then develop tools that growers could employ against it.

According to Justin O’Dea, a WSU regional agriculture specialist, one such tool is a pest emergence model that will help farmers precisely time their insecticide sprays to be most effective. The model helps predict when RSG, which overwinter as larvae inside the stems of berry plants, emerge into the outside world as adult beetles.

A major concern with controlling RSG is the insect’s innate unpredictability, O’Dea said.

“Damage from this pest is commonly intermittent and variable, which blindsides berry growers when infestations flare,” he said. “This can lead to growers making ineffective, pre-emptive sprays to try and make sure they are not blindsided again.”

The recent discovery of a natural predator of RSG — a parasitoid wasp known as Baryscapus rugglesi – is also now in the integrated pest management toolkit, although its efficacy is not yet fully known. Parasitic wasps lay their eggs inside other insect species, killing the host insect in the process. As a result, these wasps are used extensively, worldwide, to control pest insect populations in agriculture.

“We know now that we have a parasitoid of RSG [in the Pacific Northwest] and have observed periods where RSG appears to decline inexplicably,” O’Dea said. “This phenomena of RSG pest pressure decline may be at least partly due to parasitism. If so, perhaps parasitoids will eventually lead to RSG becoming less of an important threat to caneberries in the Pacific Northwest, but only time and further research will be able to confirm that.”

In the meantime, or at least until late spring when the emergence model predicts RSG adulthood, O’Dea said there is something that farmers can do to help prevent infestation: prune. This step is important because, after hatching, RSG larvae burrow into the canes where they feed on water, nutrients, and sugars moving through the plant’s vascular system. The larvae will eventually bore into the middle (the pith) of the cane.

Thoroughly pruning out all canes that show symptoms of RSG damage (cane swelling, wilting, or breakage) can be done at any point before the pest emerges. The damaged canes should be removed from the field and burned.

“This strategy [is] part of an effective integrated pest management program,” O’Dea said. “Research in Utah found this method to be about 80% effective. When combined with insecticides, RSG control could be as high as 98%.”

Successful development of an integrated pest management program for RSG means that growers will have a better chance at minimizing the need for insecticide applications, O’Dea said. That equates to reduced time, energy, and money spent on ineffective and unnecessary insecticide applications, as well as reduced risk to pollinators and other inadvertent impacts to the environment.

“Based on historical records of how RSG has played out in other regions of the country and our own observations, I’m cautiously hopeful that it will become a limited threat in the long run,” O’Dea said. — by Scott Elliott, ARS Office of Communications