Michigan researchers found plum curculio to be the most challenging pest to control in tart cherries.
A federally funded project designed to encourage tart cherry growers to transition to more benign pest and disease controls showed that plum curculio, a key cherry pest in Michigan, was difficult to control with the limited range of reduced-risk pesticides available. However, more new pesticides are becoming available and will be used in a second phase of the project starting this year.
The four-year project, which ended last season, was funded by the U.S. Environmental Protection Agency's Risk Avoidance and Mitigation Program (RAMP) with the goal of developing reduced-risk pest management strategies for tart cherries. It involved a partnership between researchers, extension educators, and individuals from the states of Michigan, Wisconsin, and Utah. Dr. Larry Gut, entomologist with Michigan State University, presented results at the Great Lakes Expo in Michigan last December.
Gut said one of the goals was to demonstrate pest management using only pesticides that are classified by the EPA as reduced risk or organophosphate alternatives.
Pesticides classified as reduced risk must have low impact on human health, low toxicity to nontarget organisms (such as birds, fish, and plants), low potential for groundwater contamination, low potential for pest resistance, compatibility with integrated pest management (meaning they are soft on beneficial insects), and must be effective at relatively low rates.
Trials were conducted in ten-acre blocks in nine commercial orchards in Michigan, and results were compared with ten-acre blocks in the same orchards where standard pest control strategies were used.
During the project, SpinTor (spinosad) was the only reduced-risk pesticide registered for use on cherries, Gut said, and that was used to target green fruit worm and leafrollers.
The researchers received an Experimental Use Permit allowing them to apply another reduced-risk pesticide Avaunt (indoxacarb), which had not yet been registered for cherries. That product was used for plum curculio.
Although Actara (thiamethoxam) does not meet the EPA's reduced-risk criteria, it is classified as an organophosphate alternative, Gut said, and that was used in the trials to target cherry fruit fly and plum curculio.
Provado (imidacloprid) was used to control cherry fruit fly towards the end of the season.
Plum curculio proved to be the most challenging to control with that limited range of products, Gut said. "That was probably the monster pest for the entomologists in the project and the one that really terrorized our project."
Avaunt and Actara did a fairly good job, he reported, but year by year plum curculio became more difficult to control. In 2007, the researchers did not obtain an EUP for Avaunt, leaving them with only Actara to target the pest.
"It didn't work out very well," Gut said. "Growers actually made a rescue spray with an organophosphate to avoid plum curculio damage."
Diane Alston, extension entomologist at Utah State University, has been researching alternative controls for plum curculio and has reported good results with two nematode species, Heterorhabditis bacteriophora and Steinernema feltiae, which feed on plum curculio larvae in the ground. Gut said Alston would continue to explore this while she is in northwest Michigan for a sabbatical this summer.
During the last two years of the project, MSU entomologist Dr. Mark Whalon felt it important to look at the impact of the reduced-risk program on beneficial insects, and he monitored natural enemies using sticky traps and beating-tray samples. Surprisingly, he found that natural enemy levels were almost equivalent in both the reduced-risk and conventional blocks, Gut reported. "We need to continue working on this."
Whalon's data were presented to the EPA and were part of the package that was critical in extending the use of Guthion (azinphos-methyl) until 2012, he added.
Cherry fruit fly
Cherry fruit fly is a critical pest to control because of the zero tolerance for fruit infestation, Gut said. The species found in Utah is the Western cherry fruit fly (Rhagoletis indifferens), which is similar to the Eastern cherry fruit fly (Rhagoletis cingulata) found in Michigan orchards.
Alston reported success in controlling cherry fruit fly in Utah with the GF-120 bait, which contains spinosad. However, research in Michigan suggests that the bait is not effective enough to control cherry fruit fly there. The fly must eat the bait in order to die. If there is movement of flies into the orchard from outside that are ready to lay eggs before they fly around and eat anything, the product won't provide control, Gut said.
In the Michigan trials, Provado was the mainstay of cherry fruit fly control, and the final season of the project, 2007, was the best in terms of cherry fruit fly control, he reported. "We detected no cherry fruit fly in any of our plots prior to harvest, which was great."
In all four years of the project, there was very little fruit fly activity before harvest, he said. Few flies were active in time to damage the fruit. Most of the larvae were found in fruit after harvest, so the entomologists considered whether it would be worthwhile controlling the flies postharvest to reduce the pressure for the following season. In 2006, they applied Provado after harvest, at the same time as the fungicide spray for cherry leaf spot. Gut said timing of the postharvest spray is critical, and it is best applied within seven days after harvest.
"It's right after harvest that the activity occurs," Gut said. "If you wait too long, all the fruit are infested, and you can't get rid of the larvae. It has to go on soon after harvest."
The bottom line of the project was that pest management with the reduced-risk products was a struggle for some pests, Gut said. "It did a pretty good job with cherry fruit fly, but plum curculio was a real problem. The real problem was it was expensive."
The project's management team was able to convince the EPA that it needed to continue the work to try to make it more economical, he said, and the EPA is funding a second RAMP project.
More options will be available in terms of reduced-risk pesticides, he said. For the coming season, Avaunt now is labeled for use on cherries, so no EUP is required. A label change to Actara allows twice as much material to be used each year. Up to 11 ounces per season can be applied, so two applications can be made instead of just one. In addition, Assail (acetamiprid) will soon be registered for use on cherries. In tests, Assail has provided excellent control of plum curculio and is also effective against cherry fruit fly.
"It looks like we have a shot next year with some more options to really make this happen," Gut commented.