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Grubs
by Arzeena Hamir
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.
What are these grubs?
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.
Life cycle
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.
What to do
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.
When to treat the lawn?
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.
Encourage natural enemies
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.
Managing your lawn to minimize damage
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:
University of Florida Factsheet - Microbial
Insecticides (http://edis.ifas.ufl.edu/BODY_IN081)
A great overview of the advantages and disadvantages of using nematodes,
Bt, and other biological insecticides.
Ohio State FactSheet (http://ohioline.ag.ohio-state.edu/~ohioline/hyg-fact/2000/2500.html)
Good description of grubs, ignore the chemical advice.
Biography:
Arzeena Hamir is an agronomist (has a
Bachelor's degree in Crop Science and a Master's Degree in Sustainable
Agriculture) and has been working in the field of organic gardening for
the past 8 years.
"I have trained farmers in Thailand,
Bangladesh & India on how to grow food organically and now live
in Canada where I run my own company, Terra
Viva Organics. My real joy is educating people on how to grow food
without chemicals."
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