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Organic Lawn
Grub Control
by Arzeena Hamir
Published April 2001
Beetle grubs can turn a fine looking lawn
into a patchwork quilt of yellow spots.
But before you reach for the insecticide
bottle, there are a number of organic alternatives
that will help you cope with the grubs without
poisoning yourself or your family.
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 the grass to
die and form brown patches. Lawns that are
heavily damaged by grubs will have a yellowish
tinge and will feel spongy when walked on.
The sod itself can be easily lifted, a sure
sign that the brown patches were not caused
by dog urine.
Adult beetles emerge, mate, and lay eggs
from late June until early August. The eggs
hatch in about two weeks and the tiny grubs
grow quickly. The yellowing patches
of sod usually appear in late August and
September, when the grubs are vigorously
feeding and the turf is otherwise water-stressed.
In October or November, when soil temperatures
begin to cool, the grubs stop feeding and
move deeper into the soil, where they spend
the winter. They return to the root zone
and resume feeding early the following spring.
Just a couple grubs per square foot are
not a problem to an otherwise healthy lawn.
Ten or more per square foot are necessary
to justify treatment. Predatory nematodes
are available for use in Canada and the
US as a biological control for white grub.
The use of these nematodes requires that
the soil be kept very moist and it is very
important that the application instructions
for this product be followed closely, as
nematodes are living organisms.
Treat the entire lawn. Do not attempt to
control lawn pests by spot applications.
Water the lawn thoroughly after application
to wash in the nematodes.
The younger the grubs are, the easier they
are to control. The best time to apply grub
control measures is from mid-July to August
and September when the grubs are small and
near the soil surface. Although treatments
can be made after this time, grubs will
be more difficult to kill (because of their
larger size). The second best time is March
to April when the grubs are once again near
the soil surface but a little larger.
Certain species of wasps parasitize white
grubs. They are sometimes seen hovering
over the turf in late summer in search of
green June beetle grubs on which to lay
their eggs. They are not aggressive and
normally will not sting people. The
wasp larva feeds externally upon the grub,
eventually killing its victim before spinning
a fuzzy, brown, jelly bean-size cocoon in
the soil. Predators such as ground
beetles and ants also take their toll on
eggs and young white grubs.
Lawns that are heavily managed and watered
regularly may actually attract beetles.
They prefer grassy areas where the soil
is constantly moist such as lawns, pastures,
and meadows in close-cropped grass. Frequent
irrigation in June and July may attract
egg-laying female beetles to the turf, especially
if surrounding areas are dry.
In contrast, adequate soil moisture in August
and September (when grubs are actively feeding)
can help hide root injury. If grub damage
starts to appear in late August or September,
watering will promote tolerance and recovery.
Deep, periodic soaking of the turf is more
beneficial than frequent, light watering.
Resources:
Terra Viva Organics
http://www.tvorganics.com
Based in Vancouver, BC, this company supplies
predatory nematodes across N. America through
its website.
University of Florida Factsheet
- Microbial Insecticides
A great overview of the advantages and disadvantages
of using nematodes, Bt, and other biological
insecticides.
Ohio
State FactSheet
Good description of grubs, ignore the chemical
advice
Arzeena Hamir is an agronomist and garden
writer based in Vancouver, BC. When she's
not planting peas or harvesting zucchini,
she runs Terra Viva Organics at www.tvorganics.com
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