When Is It Safe to Apply Grub Killer? How Long Do I Wait?

Detecting a Grub Problem, What Harm They Cause

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Reader, Rita writes, "I just planted some new grass (to fill in spots in an existing lawn) and am wondering how long I must wait till I can safely put down a grub killer, such as Grubex? Is there a certain length of time I should wait until the new grass becomes established?" Here's how Rita's question was answered:

While Not Harmful to New Grass, Perhaps Wait to Apply Grub Killer

It is very important that you always follow label guidelines with whatever product you choose to apply, including when you apply grub killers to new lawns.

Label guidelines will tell you all of the information that you need to know relative to rates, timing, target species, etc. Also, be sure to follow all of the safety requirements and precautions noted on the label.

In general, most insecticides will not harm new grass, given that their chemistry is designed to target insects and not plants (herbicides would be another matter). New grass, though, should be treated carefully to ensure proper establishment, and some maintenance practices that will be needed for proper establishment of grass may conflict with the label guidelines for an insecticide (for example, timing for irrigation). So, it is very important to ensure that label guidelines are followed.

Since most plant protectants require proper application at a specific time relative to the life cycle of the target species, consider this timing and how it relates to the development of your new grass.

If it is possible to wait with your application until after the new grass is established, then that may be the best route to follow.

Source: the lawn care experts at John Deere.

Addendum: What Grubs Are, Why They're a Problem, How to Detect Them

A grub is what the larva (plural, larvae) of various insects is called, especially beetles.

So just as butterflies and moths have larval forms known as "caterpillars," the Japanese beetle, for example, has a grub as a larval (immature) form. The Japanese beetle (Popillia japonica) is that common garden bug that is about 1/2-inch long and has metallic bronze and green coloration. Its grub is about 1 inch long, white in color, with a brownish-orange head.

Having grubs in your lawn is problematic on three scores:

  1. The grubs, themselves do damage, eating grass roots, thereby damaging the grass. If you spot large dead patches of grass in your lawn, grubs could be the cause.
  2. If they are Japanese beetle grubs, once they mature into adult beetles, they may eat the leaves on several of the plants that you are landscaping with, including rose bushes and Japanese maple trees.
  3. Moles eat grubs, so having grubs in your lawn is an open invitation to moles to come on in and set up shop. Thus grub control is a central pillar in mole control, too. Another pest that eats grubs (and that you may, therefore, be attracting to your yard if you allow grubs to flourish in your lawn) is the skunk.

Since grubs live underneath the soil surface (and so their presence is not readily apparent to the casual observer), how can you find out if your lawn is infested with them?

Well, as already mentioned, skunks eat them; they will dig down through the grass to access grubs. If you spot holes in your lawn and cannot account for their presence in any other way, skunks -- and, therefore (indirectly) grubs -- may be the culprit.

Or you can simply remove a square of sod to see what is going on down there in the soil. If the dirt is riddled with grubs, then you probably have a grub problem. How many is too many? According to the University of Minnesota Extension Service, you should start worrying if you find 7-15 grubs per square foot.