Tips on Operation - Common Problems

The grubs that you see in the lawn are the larvae of Japanese beetles, June beetles, and chafers. These grubs are C-shaped, off-white in color with a dark head. They eat the roots of grass, causing irregularly shaped patches of wilted, dead or dying grass in April and May, and again in August to mid-October. With a serious infestation, the turf can be lifted up from the soil and rolled back like a carpet. If the damage to the grass is not too severe, the grass will recover with normal watering and fertilizing. Lawns that are heavily damaged by grubs will have a yellowish tinge and will feel spongy when walked on.

In small populations, grubs do not represent a problem to a healthy lawn. It is normal for all lawns to have some grubs present. 4-6 grubs per square foot of turf probably won't cause any visible damage in a healthy lawn.

However, when a lawn begins to have more than 6 grubs per square foot of lawn, this would be considered a grub problem. To check the size of a grub population in your lawn, dig out a square foot of grass and turn it over to examine the roots. Count the visible grubs. If you have more than 6 visible grubs, you should consider applying a grub control product.

If animals such as skunks, raccoons, birds and moles are digging up the turf to feed on the grubs, consider treating your lawn. Ten or more grubs per square foot will likely cause damage, especially if the lawn is otherwise stressed.

Knowing the life cycle of grubs is the key to determining whether you have a problem, what to do about it, and when to do it.

In late June and early July, Japanese beetle adults emerge from the ground and begin to search for food and mates. The adults can fly as far as a mile and feed on a multitude of plants; their favorites include roses, grapes, and linden trees. Other scarab beetles may go unnoticed at this time because they are not attacking ornamental plants.

In July, female beetles spend 2-3 weeks laying up to 60 eggs in the soil. Depending on soil moisture and temperature, eggs hatch about two weeks later. These first-stage ("first-instar ") grubs feed on grass roots for most of August. The grubs are small, feeding close to the surface, and vulnerable to insect controls at this time. Control grubs at this stage, before feeding on turf roots is noticed.

From late August through October (depending on your climate), grubs molt into a second and then a third stage. As they grow, grubs consume more roots. Damaged turf often appears in late fall.

As temperatures drop in autumn, grubs move down in the soil. They overwinter as third-instar grubs below the frost line.

In the spring, they move up in the soil to feed on roots for a very short time. (Most of the lawn damage seen in the spring is a result of fall feeding, not spring feeding.)

In late spring, grubs stop feeding and turn into pupae that are resistant to insect controls. In late June or early July, beetles emerge from the pupae and crawl out of the soil, completing the cycle.
Evaluate the Turf to Determine the Thatch Thickness

If the thatch is more than ½ inch thick, and a dense, compacted layer is present, your chances of grub control are rather slim. Insect controls get trapped in dense thatch layers. If the insect controls cannot reach the soil/root layer where the grubs are actively feeding, they will not be controlled. You will need to dethatch the area prior to applying an insect control, then adopt management practices that keep thatch to a minimum going forward. In the long run, this will likely reduce the grub population and allow an insect control to reach it's target when needed.
The Timing of Grub Control Treatment is Critical

Although grubs are present near the soil surface from April-early October, they cause the most damage from July-September. During this time, the eggs are hatching and the voracious young grubs are actively feeding on grass roots. It is also during this time that the grubs are most susceptible to insect controls. Their active eating habits increase their consumption of the control. Also, the smaller the grub, the easier it is to control. Tests have shown that it takes nearly 300 times the amount of an insect control to control a mature grub in April/May as to kill the young grub in August. Since timing is so critical, we recommend that you apply Scotts® GrubEx® when you begin to see Japanese Beetles flying in your area.
Life cycle of the grub
How to Stop Grubs

For the prevention of grubs, apply Scotts GrubEx just before the grub hatch date in your area. The GrubEx has a four-month residual; avoid putting the product down too early in the season. Apply the product at the rate recommend on the bag. For best results, water the lawn the day before you plan to apply GrubEx. This moistens both the soil and the thatch layer, making it easier for the insect control to reach its target area. Apply the GrubEx to a dry lawn. Then, after the application, water again. You will need to put at least ½ inch of water on the lawn to wash the insect control down to the root layer.