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University
of Rhode Island GreenShare Factsheets
White
Grub Control
URI Master Gardeners and Hotline volunteers answer questions,
by the thousands each year, on how to control white grubs in
lawns. Many times people apply too much or too little insecticide,
treat areas that don't need treatment or apply insecticides at
the wrong time of year. Below is a set of procedures you can
follow to successfully control these insects in the lawn.
Not all lawns have grub infestations that warrant control. Our
best estimate is that in our area only 10-20% of lawns have damaging
populations - and these infestations are often limited to part
of a lawn. Furthermore, lawns differ in susceptibility to white
grubs because of differences in grass species, soil health, irrigation,
amount of sun or shade, traffic, etc. A dense stand of grass
with a healthy root system can generally tolerate up to 10 grubs
per square foot, although skunks, raccoons, birds and moles sometimes
damage turf, seeking grubs in lower densities.
To determine how many grubs you have
in your lawn you can use a flat spade to cut back a sample
of turf. Count the grubs in
the top 3 inches of soil and replace and water the turf. If you
take a 6" X 6" sample (1/4 of a square foot), a grub
density of more than 2-3 per sample probably warrants treatment.
Take a dozen or so samples throughout the lawn area to determine
which areas may need treatment.
The time to sample for grubs in Rhode Island is between
August 1 and September 15.
Adult beetles
generally lay their eggs in July and most larvae should be
present by early August,
although in dry years
development may be slowed. Depending on soil moisture, you
may need to sample more than once to make sure you "capture" the
grubs developing at your site. Since grubs are found in patches
and do not generally infest the entire lawn, the more you sample,
the more confidence you will have in treating only areas infested
with grubs.
There are several white grub species that cause similar damage
to lawns in Rhode Island. Japanese beetles and Oriental beetles
are the most common species, but we find quite a few Asiatic
garden beetles and European chafers as well. It is best to get
your grubs identified to the species level to optimize choice
of insecticide against these pests. The URI Cooperative Extension
Education Center personnel can do this for you. If you want to
try it yourself, you'll need a hand lens and some reference illustrations.
Illustrations are available on the web at: http://ohioline.osu.edu/hyg-fact/2000/2510.html.
It is tempting to treat white grubs in the spring when
damage is apparent, but spring treatments are generally ineffective.
The large grubs present in the spring have already done most
of their feeding and caused their damage and are now difficult
to kill with insecticides. It is better to overseed damaged
areas in the spring and wait until late summer to sample for
a new generation (concentrate sampling in previously-damaged
areas - they tend to return to the same area of your lawn).
We recommend treatments between August 1 and September 15 only
to those aesthetically important areas of your lawn that exceed
8-10 grubs per foot. Note that (as discussed below) two
commonly used insecticides: Merit (imidacloprid) and Mach-2
(halofenozide) must be applied early in the season before eggs
are laid in order to be effective against the fall larvae.
With these products, treatment decisions should be based upon
past history of grub infestation.
The choice of products for use against white grubs changes every
year as older products are withdrawn and new ones become available.
Dursban was used for years against white grubs, but it will no
longer be sold for this purpose. Similarly, Diazinon will not
be available for sale for turf use after 2003. If you have already
purchased these materials, you can use existing stocks on your
lawn. Apply these products in the fall. When selecting insecticides,
it is important to look at the active ingredients list for the
chemical name for the product. Some companies have used the same
trade names for products with very different ingredients.
Other choices for fall application include: Dylox (trichlorfon)
and Sevin (carbaryl). Dylox is a fast-acting material but is
susceptible to alkaline hydrolysis. That means that it degrades
in very hard or alkaline water or in a high pH soil very rapidly.
One-half of the active ingredient will be degraded in 30 minutes
at a pH of 9. Many public water supplies in Rhode Island are
at pH 9 or higher. You also would not want to lime the lawn just
before or after a Dylox treatment for the same reason. (If you
are determined to attempt grub control in the spring - perhaps
to reduce bird or mammal damage to the lawn, Dylox is one of
the more effective materials available.)
Mach-2 (halofenozide) is an insect hormone
mimic that is most effective against Japanese beetle grubs. It
should be applied
when beetles are flying (June 21-July 21). Low toxicity to non-target
organisms. Soil half-life: 129 days. Imidacloprid (Trade Name:
Merit) is an effective chemical treatment for grubs but, as noted
above, it must be applied before eggs
are laid. Treatments between April 1st and August 15th are generally
effective.
All chemical controls must be watered in to be effective. Read
the label for specific instructions, but generally 1/4-1/2-inch
of post-treatment irrigation is required.
Non-chemical controls: Milky spore disease
(Doom, Japidemic) is a slow acting bacterium used to control
the grubs of the Japanese
beetle. It is not intended for use where grub populations are
high. Once treated, however, bacteria are supposed to cycle in
your
lawn as new grubs die and release more bacteria. Research
data are inconclusive on the effectiveness of this material.
Nematodes are becoming available for white
grub control. The most promising results to date have been with Heterorhabditis
bacteriophora (the so-called 'HB' strain) at a rate of at
least 1 billion per acre or Steinernema glaseri at a rate
of 2 billion per acre. It is critical that the soil temperature
be close to 70 degrees F and that you water these in with 1/4
inch of water. Several studies have shown very good control;
others
have
demonstrated poor control. Research is continuing and product
availability is limited. For additional information see the web
site: http://www.oardc.ohio-state.edu/nematodes.
Since grubs are not found as often
in shaded lawns, plant shade trees and select shade-tolerant
grasses for long-term
suppression (see URI's Sustainable
Tree and Shrub List).
By Steven R. Alm and R.A. Casagrande,
2002
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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