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 Grubs are the larval stage of moths and beetles. There
are certain types of both moths and beetles that are known to be
problematic to turf. MOTHS
The most
common moth larvae (caterpillars) known to cause damage to residential
lawns are the sod webworm, armyworm, and cutworm. They range in color
from gray/brown, greenish, to a dingy white with four parallel rows of
dark brown spots on the abdomen. Sod webworms are anywhere from 3/4" to
1" in length while the armyworm and cutworm are slightly larger, from 1
1/2" to 2 " in length. Detection and management of all three is nearly
the same. The adult moths don't feed on grass, just the larvae. The
female moth lays as many as 200 eggs in late spring or early summer by
flying low over the grass at dusk. The eggs require moisture to develop
and hatch in about 4 1/2 days.
There are six 'instars' or stages of larval growth.
During the first three instars, the caterpillars usually remain on a
single blade of grass. By the fourth instar they are large enough to
construct burrows or tunnels in the layer of thatch that forms on lawns
just above the soil surface. From this protected vantage point the
larvae can feed by cutting off entire blades of grass and pulling them
into their burrow where they feed in safety from predators. They stay
close to their burrows and feed on the grass immediately surrounding it.
Circular brown or straw colored patches about the size of a half dollar
indicate the presence of this type of larvae. This damage might resemble
the damage done by other pests so in order to determine you should check
the 'thatch' layer directly above the soil line which is where these
larvae reside, rather than in the soil itself. If grass blades are
missing in the affected areas, and not just damaged, this indicates the
presence of webworms. When the larvae have completed all 6 instars they
crawl from their burrows and construct a cocoon of silk and bits of
earth in the soil. In 10 to 14 days the adult moth appears and once its
wings dry, which takes mere minutes, the adult is ready to mate and
repeat the cycle. The adult only lives for a few days and takes no solid
food. It is only in the larval stage that these insects are harmful. The
larvae can over winter in the soil and resume feeding in late April or
May when soil temperatures begin to rise. By late June or early July the
adults emerge and the cycle repeats itself. It is only during the instar
stages that damage is done to the turf.
BEETLES Beetle larvae (grubs) can become serious pests
of residential lawns when present in large numbers. The adult beetles may
differ in appearance but typically the grubs of each are similar in size,
shape, and color. They are usually white or grayish in color and when found
in the soil are found curled in a C shape which is characteristic of white
grubs. The most common beetle grub to cause lawn damage is that of the
Japanese beetle, but unlike moths, the adult Japanese beetle goes on to feed
on the leaves, flowers, and fruits of ornamental or orchard plants, causing
extensive damage in the adult stage as well. Japanese beetle grubs are just
one species of a number of lawn dwelling beetle larvae known collectively as
"white grubs" and are considered to be the most damaging insect pest to
lawns. Beetle grubs over winter in the soil, from 4" to 8" below the
surface. When the weather turns warm in spring, they move to the surface and
begin feeding on the roots of a wide variety of grasses by cutting the roots
and loosening the sod. Irregular patches of wilted, dead, or dying
grass indicate grubs feeding beneath the surface. These patches usually
occur in April and May and in August to mid-October. The fall grub
population typically causes the most damage because the grass is
semi-dormant and stressed from hot weather and low moisture levels. The
presence of large numbers of birds feeding on the lawn, or animals such as
skunks and moles digging or burrowing can also indicates a grub problem and
do as much damage as the grubs themselves.
Affected lawns
will exhibit irregular brown patches of turf that might resemble water
stress. The affected areas of lawn can feel spongy when walked on, and the
browned areas of sod are easily lifted and/or removed much like rolling up
carpet. It normally takes 20 grubs per square foot to affect a healthy lawn,
but only 10 grubs per square foot to affect an already stressed one.
Severely damaged turf might yield 40 or more grubs per square foot.
Controlling Grubs |
