Why You Have Grubs
Sean Breckin, AOLCP

Given the time of year, you may be wondering why you have grubs…

Grub Lifecycle

In the Northeast, damage to turf is a result of four species of grubs : Japanese beetle, European chafer, oriental beetle, and asiatic garden beetle.  Much of the blame tends to fall to the Japanese beetle because it is the one that is active during the day. The others are night flyers, so we’re often unaware of their existence. All specieis tend to be out in their beetle form by mid-June.

Grubs feed on grass roots in April/May, and change into pupae that hatch into beetles in May/June. In May/June they start to feed on garden foliage and begin laying eggs in June through July.  The eggs hatch into grubs that start aggressively feeding on grass roots August into September. This is when you are likely to see the most damage from both the grubs and wildlife as they forage for food.  As weather gets colder, the grubs go deeper into the soil over winter and start the cycle again in the spring.

Scout for Grubs

The best time to scout for, and treat grubs, is the end of August through mid-September when they are small and in their first instar.  Before treating, you should first determine if you have a problem.  One method is to focus on the sunnier areas: cut a square foot of sod on three sides with a shovel, fold it back to expose the soil and count the grubs checking both the root system and soil.  If the turf is dense with a good root system, then it can withstand pressure of 8 grubs per square foot. Should you determine you have a grub infestation that merits treating, we recommend hB nematodes or cedar oil.

Treatment options

Nematodes are effective against all varieties of grubs and applied with water using either a backpack sprayer or watering can.  Because nematodes are aquatic, it is critical that they be put down during times of low light, watered in (preferably in the rain) and the area must be kept moist so the nematodes can move through the soil.  Nematodes enter the grub and kill the grub from the inside out.  The grub will start out as white, begin to appear sluggish and slowly turn black.  During times of heat, a second application may be necessary.  We have found The Green Spot Ltd. has been a reliable supplier.

Cedar Cure:   Grubs – Exposure to the cedarwood oil formulation immediately triggers the erosion of the exoskeleton and is effective on eggs and 1st instar life stages of grubs. Insects – will stifle the ability of the insects’ receptors to detect food, mate, and to reproduce. When these comfort levels are destroyed, insects become overwhelmed and relocate.

Milky spore is a one-time application effective only against Japanese beetle grubs and can be purchased at your local garden center.  It can take 3-5 years to effectively inoculate an area.

Given the drought, cedar oil would be the preferred method and may require two applications.  You don’t need to spray the entire yard, just the affected area and edges using a pump or backpack sprayer.

Cultural Practices

Cultural practices also help to minimize infestation. Accordingly, remove the attractor plants and add shade trees (since beetles like it hot and sunny). In addition, improve organic matter to support bio diversity. Also promote deep grass roots by watering deep and mowing high.  Areas that have been damaged should be top dressed and over seeded in the fall. Over the years we have found that we had to treat very few instances of grubs on our organic properties.  Because, while the beetles may have laid eggs, they never grow into white grubs because there is a healthy predator/prey relationship. Therefore, the eggs are eaten by ants, springtails or earthworms.

Reach out if you need help mitigating grubs in your lawn!