Leafroller challenges cherry growers

Tart cherry growers need to use a modern, effective insecticide to control obliquebanded leafroller about two weeks before ­harvest—or risk delivering a contaminated crop that may be rejected by the processor.

That’s the advice of Dr. Nikki Rothwell, the entomologist who coordinates activities at the Northwest Michigan Horticultural Research Station northwest of Traverse City, Michigan, the heart of the nation’s tart cherry production region.

Obliquebanded leafrollers don’t feed on cherry fruit, as they do on apples, nor do larvae develop inside the fruit, as do cherry fruit flies, she said. They do some damage from defoliation, but the major concern is they roll themselves into the leaves, and, during mechanical harvest using trunk shakers, they are shaken out with the ­harvested fruit. While many are washed away in the process of cooling the fruit, any remaining can cause problems.

“If we don’t control OBLR, growers face downgrades or rejection of loads of mechanically harvested cherries if these larvae are found floating in tanks,” she told growers at the Great Lakes Fruit, ­Vegetable, and Farm Market Expo in Grand Rapids, Michigan, in December.

Working with Michigan State University entomologist John Wise, Rothwell tested several insecticides and application times. The most effective treatments were the diamides Altacor (rynaxypyr) and Belt (flubendiamide), the spinosyn Delegate, and the diamide-neonicotinoid combination Voliam Flexi (chlorantraniliprole and ­thiamethoxam) applied at 400 to 450 degree-days after biofix. In tests last year, the applications were made on June 2.

Materials approved for use by organic farmers include the spinosyn Entrust, which was somewhat less effective, and Bacillus thuringiensis (Bt) materials such as Dipel, which are not recommended for application around ­harvest time because they act too slowly.

“Bts are better suited to target overwintering larvae in the spring,” she said.

The likely reason for the rise in obliquebanded leafrollers in cherry orchards is resistance to organophosphates, Rothwell said. Organophosphate resistance was documented in obliquebanded leafroller in Michigan in the early 2000s, primarily in apple orchards. Most apple growers have moved away or minimized organophosphate use in their pest management programs, primarily because of resistance of codling moth, but organophosphates are still the backbone of the insecticide programs in many sweet and tart cherry orchards.

“Primarily, it appears that apple growers are controlling obliquebanded leafroller because they are using new materials, while cherry growers are seeing increasing problems, likely as a result of resistance,” she said.

The leafrollers overwinter as small larvae in cracks and crevices in trees. When temperatures warm in spring, they move out to feed on buds and emerging leaves, and as leaves expand, larvae web and roll leaves where they remain concealed except when feeding, Rothwell explained.

Scout

“Targeting this overwintering generation is an effective strategy, particularly as these larvae are small, hence easier to kill, at this time,” she said. “At early petal fall, growers should scout their orchards by looking at 20 clusters per tree in five trees per orchard for larvae or feeding sites. An insecticide should be applied if they observe more than two larvae or feeding sites per tree.”

The materials that target this life stage are Delegate, Belt, Altacor, Voliam Flexi, Entrust, and Bts. In most cases, growers should not expect organophosphates (or pyrethroids due to cross resistance) to provide effective control because of insecticide resistance. The insect growth regulator Intrepid (methoxyfenozide) might also have some level of cross resistance and will not be effective. A second insecticide application might be needed at the preharvest timing.

The Michigan State University entomology team will be further investigating this pest in both apples and cherries in 2012, Rothwell said.