What are 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:
Terra Viva Organics
www.tvorganics.com
Supplies predatory nematodes on-line.
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
Arzeena Hamir is an agronomist and garden writer based in Vancouver, BC. She has a Master's Degree in
Agriculture and specializes in organic vegetable gardening. She frequently writes articles for Suite101.com,
GardenGuides.com and ICanGarden.com. When not tending her own veggie
patch, she runs Terra Viva Organics - www.tvorganics.com
