With the air
temperatures climbing back into the high 80’s and low 90’s, now will
be a crucial time to monitor for white grubs.
The term “white
grub” refers to a group of insects with a larval stage that damage turf. Subsurface
feeding insects are of major concern in athletic fields because they feed on
roots, cause turf to be easily dislodged, and result in poor footing. Know the life
cycle of underground feeders such as grubs and anticipate when they may become
a problem. The beginning of football season coincides with peak turf injury
from white grubs. Masked chafers, Japanese beetles and May/June beetle are the
most common grub species to attack Iowa athletic fields. Annual grubs such as
masked chafers and Japanese beetles lay their eggs in the spring, and hatch in
the summer. The larvae begin to feed on the root systems in August and these
two species are commonly referred to as “fall grubs”, because a majority of
damage occurs in the fall.
The damage is
best diagnosed by grasping the blades of the grass and lifting. This process is
known as a “tug test”. The grass will break away at the roots. Another option
is to shovel or spade a three sided 1 sq. ft. piece of sod about 3 inches deep.
Slowly peel back the sod and expose the soil as was done in the picture below.
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Figure
1. Several white grubs feeding on athletic field in Fort Madison, IA. Picture
courtesy of Cody Freeman.
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Fall grubs that
sever the root system do not necessarily kill the grass. If it is properly
watered via irrigation or rainfall, it will recover. The drought stress
following grub damage, kills the grass. With the warm temperatures in the
foreseeable future, it is the most likely time to see damage across Iowa,
especially if the rain switch suddenly turns off.
Keep Dylox or another insecticide handy for preventative measures. It is important to
remember that all products need to be watered in. Using nozzles that produce
larger droplets will move the product further into the canopy. After
application, irrigation or rainfall can help move the product down into the
soil, where it can be most effective.
