Dr. Tom Unruh checks if strawberry leafrollers in wild rose bushes are parasitized by the wasp Colpoclypeus florus.
Wild multiflora roses planted near orchards can be a haven for a tiny wasp that parasitizes and kills the orchard pests, pandemis and obliquebanded leafrollers.
The parasite Colpoclypeus florus can be an effective biological control, but it can’t survive year-round on the leafrollers that infest orchards. It attacks fourth- or fifth-instar leafroller larvae, but those species of leafrollers overwinter as young larvae, a stage at which they are too small to serve as hosts for the wasp.
Dr. Tom Unruh, entomologist with the U.S. Department of Agriculture in Yakima, Washington, has been exploring the idea of planting patches of roses near orchards and infesting them with the strawberry leafroller. The strawberry leafroller, which is an alternate host of Colpoclypeus, overwinters as a fully grown larva or pupa. It is not a pest of apples.
Unruh has been monitoring more than 40 rose gardens that have been planted in Washington and Oregon, some of which are reliably producing overwintering populations of Colpoclypeus that can move into the orchard in the spring to attack leafrollers. Research has shown that the adult wasps can fly a considerable distance and will move throughout the orchard from the rose beds. In the absence of pesticides, leafroller parasitism can reach 100 percent in April and May and continue through the summer.
Unruh said the need to spray for leafrollers may decline dramatically if there are enough strawberry leafroller hosts in rose patches near orchards. However, the strategy only works well in orchards that aren’t sprayed with insecticides.
Sensitive to pesticides
Colpoclypeus is extremely sensitive to pesticides, even the newer ones like Success (spinosad) or the organic formulation Entrust. "Unfortunately, these newer products are softer on codling moth than on the predators," Unruh said. "The organophosphates, of course, would wipe everything out, including the codling moth. Success or the neonicotinyls will do a moderate job on codling moth and a pretty darned good job on several of the natural enemies. That’s what’s destroying the potential in some of these sites."
Dr. Jay Brunner, entomologist with Washington State University Tree Fruit Research and Extension in Wenatchee, first discovered Colpoclypeus in Washington in 1992 in two unsprayed orchards. Brunner and colleagues began releasing the parasite in orchards. No one knew how the wasp was managing to survive the winter, and they began looking for overwintering hosts.
Unruh has been involved in the rose research since it began in 1999. He’s done experiments that involved taking potted apple trees up into the dry hills around Yakima in September and October, at least a kilometer from an orchard. The trees were infested with obliquebanded leafrollers raised in the lab, which soon became parasitized.
"We’ll get 50 to 60 percent parasitism by Colpoclypeus in desert habitats," Unruh reported. "We think there are lots of Colpoclypeus being generated in orchards in the summer, and they’re looking everywhere for an overwintering host, and that’s the bottleneck in the system."
In some orchards, planting roses has produced striking results in terms of parasitism of pandemis and obliquebanded leafrollers, in others, the parasite has had no effect, and for no apparent reason, Unruh said. "We haven’t figured out why it works in place A and not in place B that seems to have a very similar program," he said. "Part of it is just the topography and where the garden is, the soil, and how much water the grower gives it."
It also appears that strawberry leafroller populations are cyclical. There can be lots of them in a rose garden for a couple of years, and then the numbers drop precipitously for another couple of years before they rise again. Unruh thinks strawberry leafroller larvae are attacked by hornets and birds.
But he still thinks the strategy has promise. "I’m still optimistic," he said. "We’ll continue to work on the garden concept, but I’m also more realistic in the sense that I don’t think it’s going to work in all situations all the time.
"I would not recommend that everyone plant roses," he added. "It kind of depends on their program. Roses are something that organic growers can invest in with confidence, but people who are going to spray for their problems are going to kill out the parasites."