Extensive lawn damage due to European chafer grub feeding in Sturgeon Bay this spring and last fall may seem like just a bad memory of the past. However the lull we are seeing now is due to the beetle’s lifecycle. Immature European chafer, like the May/June beetle and Japanese beetle, are white, c-shaped grubs that feed on grass roots. They hatch mid-summer, feed in the fall, overwinter, and continue feeding in the spring. In the middle of the summer, the grubs pupate (transform) and emerge as adult beetles. The adult beetles feed on plants in varying degrees, mate, lay eggs in the soil, and then die. That cycle repeats each year.
Over the last couple months, many lawns have recovered from the damage this spring and are filled in with new, healthy grass. This was expected because during the last couple months the adult beetles were active and laying eggs, so there were less grubs in the ground to feed. Now, however, the grubs are beginning to feed on grass roots and grow. European chafer grubs go through three stages of growth, and with each stage they eat more and become less susceptible to insecticides.
What should be done now?
Before any course of action, it is important to check if grubs are present in the lawn. Not only does this confirm whether damage is likely, but it also helps direct what insecticide will be most effective based on the growth stage of the grub. Currently, insecticides are the main way to deal with the grubs. Not all insecticides will work, and even effective insecticides will not work well when used at the wrong time. There are two categories of insecticides: preventive (“pre-emergent” – before the insect is noticeable) insecticides and curative (“post-emergent” – when the insect is getting larger) insecticides. The active ingredients in these products are listed in the table below. A few weeks ago, I found grubs that were about the size of a pencil eraser when curled up. At that point, preventive products may still have worked, but by now curative products are likely the better option. Insecticides containing only the active ingredient gamma-cyhalothrin will not be effective because it binds to the organic material in the grass and will not reach the level where the grubs are feeding. Additionally, alternative-type products including nematodes have not been shown to be reliable. Milky spore only attacks Japanese beetle grubs, so it will not work against European chafer. Make sure to follow all label directions before applying any insecticide. Any insecticide application must be watered in with at least 1/5 of an inch water to move the product through the grass root zone to where the grubs are feeding.
Good lawn care throughout the summer can help the grass grow well and tolerate more damage in the fall and into next spring. Watering the grass, applying fertilizer, and cutting it at a longer height can all help. Beware that too much fertilizer may cause the grass to grow more leaves but less roots, meaning that grub feeding will kill the grass more quickly.
Even with excellent lawn management, this pest may be difficult to completely control or it may take a few years to determine a pest management program that works best in a particular area.
UW-Extension is performing an insecticide trial in Sturgeon Bay in 2017 to determine the effect of preventive versus curative products and if alternative-type products can have adequate control of European chafer.
If you would like more information, visit our website door.uwex.edu/horticulture or call 920.746.2260.
Copyright © 2017 Board of Regents of the University of Wisconsin System doing business as the division of Cooperative Extension of the University of Wisconsin-Extension. All Rights Reserved.