White grubs are immature scarab beetles (for example, European chafers, Japanese beetles,
and Oriental beetles). Although all grubs cause similar damage, the treatment you choose should
correspond to the species.
White grubs eat organic matter including the roots of plants. Therefore, damage first appears to be
drought stress.Heavily infested turf first appears off color, gray-green, and wilts rapidly in the hot
sun. Continued feeding will cause the turf to die in large irregular patches. The tunneling of the
larvae cause the turf to feel spongy under foot and the turf can often be rolled back like a loose
carpet. Grub populations may not cause observable turf injury but predatory mammals such as
skunks, raccoons, opossums, and moles dig in the turf in search of a meal.
How many grubs are too many? Here’s a guide to treatment thresholds for European chafers,
Japanese beetles, and June beetles, the most common grubs in home lawns. Numbers are
based on grubs/sq.ft.
0-5 grubs: rest easy. Fewer than five grubs per square foot is a low population. You don’t need
6-9 grubs: think about your lawn. Is your grass dense, with a healthy, robust root system? If so,
it can probably withstand grub populations of 6-8 per square foot, or more. On the other hand, if
animals such as skunks, raccoons, birds, and moles are digging up the turf to feed on the grubs
and this bothers you, consider treating highly populated areas.
10 or more: they may cause damage. Ten or more grubs per square foot will likely cause damage,
especially if the lawn is otherwise stressed. In most circumstances, you’d be justified treating
where populations are this high. Several weeks after treating, sample in a few locations to determine
whether treatments were effective.
In the soil, microscopic worms known as nematodes live and breed. Some nematodes infect and kill grubs,
thereby reducing populations. You can also purchase and release nematodes that will kill grubs, if used properly.
Most eggs are a creamy-white in color, about 1/16-inch (1.5mm) long and slightly oval when first laid in the
soil. These absorb water from the soil and swell slightly, becoming more round.
The C-shaped white grubs are thick bodied, creamy-white with brown head capsules and short legs. All
species have three instars, that is, the larvae molt three times.
The pupae are often slightly longer than the adults and are formed in chambers one to two inches in the
soil. The pupae are first cream colored and darken before the adults emerge.
The adults are typical scarabs - robust, oval beetles with the antennae ending in a large club of flattened
plates. Most genera are easy to identify by sight but species identification of May/June beetles and masked
chafers require a specialist.
White grubs seem to be periodic pests, attacking turf areas irregularly from year to year. The major factor
influencing development of damaging numbers of grubs is soil moisture and rainfall. In general, in years with
normal or above normal rainfall, grub populations increase. Well maintained turf next to ornamental plants
favored by the adults seems to be more commonly attacked. However, masked and European chafer adults
do not feed as adults and these pests build up in well
watered and maintained turf.
Certain species of scarab adults prefer specific host plants. Where Japanese beetles are common, do not
plant roses, grapes and lindens around high maintenance turf areas. May/June beetles prefer oaks. The fine
and tall fescues are not as severely attacked as Kentucky bluegrass and perennial ryegrass.
Practically all white grub species require moist soil for their eggs to hatch. The young larvae are also very
susceptible to drying out... In areas where turf can stand some moisture stress, do not water in July and
early-August when white grub eggs and young larvae are present. On the other hand, moderate grub
infestations can be out grown if adequate water and fertilizer is applied in August through September and
again in May when the grubs are feeding. This latter strategy is not preferred because mammals may dig
up the turf or irrigation bans may occur.
Since white grub occurrence is rather sporadic, applying pesticides for control of anticipated grub populations
is not recommended. However, in areas where adult activity has been observed or perennial infestations
have occurred, preventive applications may be warranted. Chemical applications generally perform best when
applied before mid-August, or when white grub egg laying is underway.
Most of the modern soil insecticides have short active residual periods (three weeks or less) and must be
used when the grubs are actively feeding. No registered insecticide is 100% effective; they usually kill 75 to
90% of the grubs present in any given area. This is why re-applications may be necessary when grub
populations get very high. Timing of treatments is critical for success. You should apply the pesticide when
the grubs are small and actively feeding yet late enough to catch all of the population. In general, reducing
thatch and using good irrigation after making a pesticide application will increase control.
As with the late-fall pesticide applications, spring treatments are often ineffective. Though the grubs feed
during the spring, they are quite large and the span of time for treatment is short. If a spring application is
deemed necessary, check to make sure that the grubs are actively feeding at the soil/thatch level.
Populations of annual grub species that are less than six grubs per square foot can usually be masked by
water and fertilizers. Populations between 10 and 15 per square foot can cause significant turf damage in
September and October. Of course, populations occasionally reach 40 to 60 grubs per square foot and
these levels can cause damage by late-August.
In general, irrigating after an insecticide application is made will improve performance for soil insect control.
It is also generally recommended that grass clippings be returned to the lawn for one to two mowings after
a grub insecticide application. Do not wait more than 30 days to recheck the grub infestation, especially if
the original population was high. If the grub population has not been reduced below six grubs per square
foot consider reapplication of another pesticide. Remember, the smaller the grubs the easier they are to kill