Do not treat Japanese beetle grubs in spring

Jeffrey Hahn, Extension Entomologist

It is too late to treat white grubs now.  Photo: Jeff Hahn, U of M Ext.

People who experienced Japanese beetles last summer are looking to take action against them this year before they become a problem. However, if you are thinking of treating for grubs this spring it is too late (or too early depending on your point of view).

The bottom line is that grubs are too large to treat now. The best time to treat Japanese beetle white 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 still too large to try to control.

Also, consider why you want to manage grubs. If your hope is to see fewer adults by treating grubs, you will be disappointed. The grubs you kill in your yard has no bearing on how numerous the adults will be. An important reason for this is that Japanese beetle adults are good fliers and can easily enter a property from adjacent areas.

The best reason for treating grubs is to protect lawns from damage. Insecticides can be treated either preventatively, before the grubs are seen, or curatively once grub presence is verified.

Japanese beetle grub damage can be confused with other
problems so check under the grass to see if grubs are present.
  Photo: Jeff Hahn, University of Minnesota Extension

If your lawn has suffered damage the last couple of years, treating grubs preventatively is an option. The best timing is when adults start to emerge, about early July.

 Effective preventative insecticides are chlorantraniliprole and halofenizide, both low risk products as well as imidacloprid and chlothianidin.

If turf damage has been sporadic the last few years, it is worth waiting to see if they are a problem this summer. Watch closely for symptoms of turf damage. You will typically see yellowing or browning grass.

 Because there are other possible causes for discolored turf, check under the grass to verify it is due to grubs. If you verify their presence, effective curative insecticide are triclorfon, clothianidin, and carbaryl.