The strawberry sap beetle was one of three species identified in cherry orchards. The other two were dusky and picnic sap beetles.

The strawberry sap beetle was one of three species identified in cherry orchards. The other two were dusky and picnic sap beetles.

Photo by Stephen Luk

Cherry growers around Traverse City, Michigan, were plagued last season by an unusual attack of sap beetles. Three species of them were found, all having the potential to infect fruit with small white larvae or to directly contaminate tanks of fruit containing harvested cherries.

There is no tolerance for worms or adult insects in tart cherries harvested for canning or freezing. The beetles also attacked ripening sweet cherries, some of which are produced for fresh market or freezing, and some of which go into brine for processing.

Dr. Nikki Rothwell, an entomologist who is coordinator of the Northwest Michigan Horticultural Research Station, has a hypothesis for why it occurred and doesn’t think this will become an ongoing problem for growers.

The problem grew out of the large cherry crop in 2009. The tart cherry industry is regulated by a federal marketing order that restricts supply. In some years, like 2009, growers leave cherries in the orchard.

If they leave them on the trees, it sets the stage for infestations of cherry fruit fly and brown rot, so most growers who participate in the diversion program shake the cherries onto the ground or catch them and take them out of the orchard and dump them. In 2009, that resulted in a large supply of rotting fruit that attracted sap beetles and became a breeding ground for them.

The problem was most serious on the Old Mission Peninsula. It is a long sliver of land, 18 miles long and three miles wide at the widest, stretching north from Traverse City into the Grand Traverse Bay, where lots of tart cherries are grown. The sap beetles there really had no place to go, Rothwell said. Sap beetles overwinter and emerge in May and June, laying eggs and feeding as ­cherries ripen in June and July.

Sap beetles are a primary invader of strawberries and other soft fruit and prefer it very ripe, even fermenting.

Rothwell doesn’t expect a repeat of the situation in 2011, but she does suggest that growers be watchful.

“Target adults for control and kill them before egg-­laying,” she said. “Monitor ripening fruit. There are no traps or pheromone lures, but the beetles do aggregate if they’re present.”

Several insecticides will knock down adults, and growers need to choose those that are fast-acting and have short preharvest intervals—both to prevent egg-laying and to eliminate adult beetles that fall into the tanks of water growers use to receive and transport cherries taken off the trees with trunk shakers. Both pyrethroids and neonicotinoids are options for control, but based on ­efficacy work in strawberries, these insects are simply ­difficult to control, Rothwell said.

In the event of further fruit diversions in future years, she recommends that growers remove the fruit from the orchards and either bury it or till in into the soil if they spread the unwanted cherries on the ground.

She spoke to growers during the Northwest Michigan Orchard and Vineyard Show north of Traverse City.