In this Issue:

VEGETABLE NEWS

Vegetable Insect Pest Update

A RISK MANAGEMENT AGENCY FACT SHEET

APPLE NEWS

Summer Pruning

Apple Weekly Trap Counts

Apple Scab Infections

STRAWBERRY NEWS

White Grubs in Blueberries and Strawberries

Please Note: the next newsletter will be published August 11.


Insect, Pest Fact Sheets

Vol 3 No. 9   July 21, 2006

White Grubs in Blueberries and Strawberries

Thaddeus McCamant, Specialty Crops Management Specialist, Northland Community & Technical College

This past week several berry growers reported that some of their blueberry and strawberry plants were dying. I told one strawberry grower to inspect the roots of the dead plants. He said that the dead plants had no roots, and we knew that he had grubs. White grubs, or grub worms feed on the roots of strawberries, blueberries and cranberries. Grubs feed on the roots of many different types of plants, but in a good berry planting, the only roots available for the grubs are berry roots. Grubs will move down a row of strawberries and kill every plant, just like a gopher in a potato field. In a mild infestation, grubs may only kill three or four plants per acre. In a severe infestation, a third of the plants can be killed.

In blueberries, grubs eat the soft roots while leaving the woody roots. We determined that grubs were causing the damage, because we saw the grubs on the roots, and the plants suddenly died in the severe heat.

White grubs are the larval form of the June beetle (Phyllophaga spp.), one of the largest insects in Minnesota. June beetles have a three year life cycle. Adult June beetles emerge in late May to feed on tree leaves and lay their eggs on bare soil. First year grubs typically feed on dead plants. In September, the larvae burrow deep into the soil and overwinter below the frost line. On the second year, larvae move towards the surface to feed on live roots. Most of the damage we see in berry fields is from second year larvae. The third year, the grubs continue eating, but by August, the grubs burrow into the soil to pupate. By September of the third year, they have turned into beetles.

 

Control: The most effective grub control occurred in the winter of 2003, when deep frost levels killed overwintering grubs and kept the population in check for two years. After two mild winters, the grubs are back. Skunks will eat all the grubs in a field, but skunks also damage strawberry plants while digging for the grubs.

During a minor infestation, the most effective way of killing grubs is to dig up wilting plants and removing the grub. Always look for the grubs in the wilting plants and not the plants that have already died. If you catch the grub while the plant still has a fourth of its roots, the plant will survive. If you find an average of two to three grubs per plant, chemical controls may be necessary.

Probably the most effective grub spray has been from home made sprayers that inject the insecticide into the soil near infected plants. Broadcast sprays of diazinon or chlorpyrifos kill grubs, but act very slowly. I have surveyed several fields that were sprayed with diazinon, only to find that half of the grubs were still active in the soil.

 

 

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison (hutch002@umn.edu), Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (woldx018@umn.edu), Department of Entomology, University of Minnesota

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jean Ciborowski, 651-201-6217, jeanne.ciborowski@state.mn.us, MDA, 625 Robert St. North, St. Paul, MN 55155-2538. You can access the Newsletter at the U of MN web site in htm format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf format at: http://www.mda.state.mn.us/ipm/ipmnews/

Partial funding for this publication is provided through partnership agreements with the Minnesota Fruit and Vegetable Growers Association (MFVGA) and the United States Department of Agriculture – Risk Management Agency (RMA). These institutions are equal opportunity providers.

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