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Don't treat Japanese beetle grubs during spring

Spring is finally here but some gardeners are already thinking ahead to summer and what steps they want to take to protect their plants from Japanese beetles. For those thinking about treating grubs now to reduce Japanese beetles later, don’t do it, it isn’t effective.
The best time to treat Japanese beetles is
in the summer, not the spring.  Photo:
Jeff Hahn, UMN Extension

Unfortunately, grubs are too large to treat now. The best time to treat Japanese beetle grubs 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. When spring arrives, these grubs are definitely too large to control.

Even if you could effectively treat grubs in the spring, this does not have any impact on how many adult Japanese beetles you will see later. This
is because Japanese beetle adults are good fliers. They
can travel up to several miles and can easily enter your property from adjacent areas.

However, do treat grubs to protect your lawn from damage. If your lawn has suffered damage within the last few years, be on the watch for symptoms this summer.

For more information on dealing Japanese grubs and adults, see Japanese beetles in yards and gardens.


Jeffrey Hahn, Extension Entomologist

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