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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. |
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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. |
