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.
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.
Terra Viva Organics http://www.tvorganics.com
Supplies predatory nematodes on-line.
University of Florida Factsheet - Microbial
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.