Ask the Masters: Let's Talk Plants


by Marianne Duffy Hohenner

Master Gardener of Winona County


Have gardening questions? Connect with a Winona County Master Gardener today! Call 507-457-6440, visit our Facebook page at winona, or send a text to We will call you back within two days.

You may also diagnose a problem through online resources at or call the Yard & Garden Line at the Minnesota Arboretum at 612-301-7590 and a Minnesota Master Gardener will call you back to discuss your gardening problem(s).

Many people are familiar with Master Gardeners as reliable resources for questions about flowers or grass because horticulture skills are a cornerstone of our work. But sometimes we surprise people.

Did you know that Master Gardeners also focus on local foods and sustainability? Or that we teach practices that restore Minnesota’s native prairies and protect our water? Or that volunteering as a Master Gardener gives people a chance to spread their wings in photography, writing, community speaking and more? 


Recently a friend asked ‘what is causing holes in my raspberry leaves?’

There is a beetle that is common from June to July, especially in Southeast Minnesota. A brilliant metallic green with copper-brown wing, five white patches of hair on each side of its abdomen and two patches of white hair on the tip of the abdomen. If you guessed Popillia japonica or Japanese beetle, you are correct. 

Originally from Asia, they were first detected in the U.S. in the early 1900s. The beetles were found in Minnesota in 1968. In recent years, Japanese beetle populations have been increasing significantly, primarily in the southeast regions of Minnesota.  Growers have noticed infestations on a variety of plants, including raspberry, blueberry, apples, roses, plums and wine grapes. 

Adult Japanese beetles feed between leaf veins of plants, resulting in a lace-like appearance (i.e. skeletonizing). 

Gardeners are thinking about what steps will protect their plants from Japanese beetles. Maybe treat the soil and kill the Japanese beetle grubs. Well, the best time is July through mid-September when they are small or moderate-sized. As they get larger, it is more difficult to kill them and, by fall, it is no longer practical to manage them. 

Grub insecticides include:

•    Imidacloprid — use from July until early September; minimal risk to mammals and fish.

•    Halofenzide — use from July until end of August; best applied when adults are active and laying eggs. 

•    Trichlorfon — fast-acting; degrades rapidly in very hard or alkaline water or in high-pH soil. 

Just remember, even if you effectively treat grubs, this does not have any impact on how many adult Japanese beetles you will see later. Why? Because Japanese beetle adults are good fliers and can travel miles. 

The best control method is to fill a bucket or jar with soapy water, handpick adult beetles early in the morning when they move slowly, and drop them into the soapy water. 

Sprays of contact insecticides such as carbaryl, acephate, pyrethrins, and pyrethroids are used on adults. Follow label directions and avoid spraying under windy conditions. Never spray when bees are foraging.  

A good option may be to select plant species that Japanese beetle adults do not like to feed on. 

•    American elder    

•    Arborvitae

•    Ashes    

•    Box elder

•    Common lilac    

•    Euonymus

•    Fir    

•    Hemlock

•    Holly    

•    Juniper

•    Maples    

•    Oaks

•    Pine  

•    Rhododendron

•    Spruce    

•    White poplar

•    Yew 

If you have a gardening problem, let us help you. Call or text the Winona County Master Gardeners at the contact information given earlier in this column. 


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