MN Fruit & Vegetable IPM News - Vol. 4, No. 9

Strawberry fields in northern Minnesota finished picking this week. In the rest of the state, fields are renovated and strawberry growers are either taking a break or paying attention to fields planted this year. In spite of the hot weather, most new strawberry fields are growing quite well. Some fields already have enough runners to completely fill the rows. When the rows in the new fields have filled in with runner plants, weed control becomes less critical. Few weeds can compete with established strawberry plants.

Many new fields have potato leafhopper injury (IPM newsletter, June 29), but in most fields the damage is old, having occurred several weeks ago. In fields observed this week, older leaves had symptoms and younger leaves were healthy. The leafhoppers appear to have matured and flown to other areas. The only sign that the leafhoppers were there were variegated leaves and white exoskeletons left behind after molting. Once the leafhoppers leave, strawberry plants quickly recover. When controlling potato leafhopper, spray the insects and not the symptoms.

June beetle emerging from soil in late May

In July, some new plants wilt and suddenly die. The most common cause for the sudden death of plants is the white grub, the larval form of the common June beetle (Phyllophaga spp.). Grubs feed on the roots, starting at the tips of the roots and move towards the crown. Normally, grubs eat the roots up to 1” below the crown. Grubs can move down a row of strawberries and each grub can kill several plants. If you suspect grub damage, gently tug on the wilted plant. If the plant pulls easily out of the ground, you have grubs. Dig around plants that are wilting to find grubs. Grubs are about 1 ½” long, with a white with a brown head and small legs.

Grubs live for three years. The beetles emerge from the soil in May and June, mate and deposit eggs in the soil. The larvae hatch in July, and feed on fine roots and decaying vegetation. In the fall, the grubs burrow below the frost line and hibernate. When they wake the second year, they grow rapidly and feed extensively on many different kinds of roots. Second year grubs cause most of the damage in strawberry plants. Third year grubs feed for two months, burrow into the soil and pupate, emerging the following summer as adults.

Previously, we thought that first year grubs did no harm to strawberry plants. Last August, I documented two fields where first year grubs were feeding on strawberry roots. First year grubs only ate the fine roots of strawberries, leaving the tough roots. The plants survived, but were weakened and produced poorly this year.

If grub damage is minor (less than 5% of new plants), grubs can be killed by hand. Dig up plants, remove the grub and replace the plant. If most plants have 2-3 grubs, an insecticide may be necessary. The only insecticide labeled for grubs in strawberry is Imadicloprid (Admire).

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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 Jeanne Ciborowski, 651-201-6217, jeanne.ciborowski@state.mn.us , MDA, 625 Robert St. North, St. Paul, MN 55155. 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: www.mda.state.mn.us/plants/pestmanagement/ipm/ipmnews.htm

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.

DISCLAIMER

Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current manufacturer directions.

Last Revised July, 2007 by woldx018@umn.edu
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