The white grubs found in turfgrass are the immature forms of several kinds of scarab beetles. In high populations they can cause serious damage to lawns and golf courses. Late summer scouting for grubs in the soil below turf is the best way to determine whether treatment is necessary. Any treatment should also be done in late summer, while the grubs are small and susceptible to management and before damage is apparent.
THE LIFE CYCLE OF THE JAPANESE BEETLE
The annual life cycle of the Japanese beetle is illustrated here because this beetle is typical of the various scarab beetles in its basic biology and its window of sensitivity to biological and chemical controls. However, in home lawns in New York state the European chafer may do the most damage. Both of these beetles are well adapted to the climate of the northeastern states.
The grub overwinters as a third-instar larva in the soil below the frost line. In spring it moves up in the soil to feed on roots for a short time.
It then pupates for one to three weeks within the cast skin of the grub. This covering splits as the insect matures, and the adult beetle is ready to emerge. Pupae are very resistant to pesticides.
Japanese beetle adults crawl out of the ground in late June and early July. They can fly as much as a half mile per day. The adults will feed on the foliage of 300 species of plants, but they usually exhibit preferences for a few in particular, such as grape, raspberry, or roses. The beetles mate, and females are ready to lay eggs about one week after emergence.
The female beetles lay 40 to 60 eggs in the soil over a two- to three-week period, preferring irrigated turf. Within two weeks the eggs hatch into first-instar larvae that feed on roots for three to four weeks. These grubs molt and become second instars that feed for three to four weeks. The grubs molt again to become third instars by the middle of September. They continue to feed until they reach full size before winter. As temperatures drop in the autumn, the grubs migrate down in the soil.
HOW DO WE KNOW WHERE THE GRUBS ARE AND WHICH AREAS NEED TREATMENT?
Annual grub scouting is necessary to detect shifts in the location of grub populations. It is best to prevent serious damage to turf from grubs and from animals feeding on them by learning where the grubs can be found in high numbers, while they are still young and small.
Samples of the turfgrass and soil are lifted and examined, then replaced. One method is to cut a square foot of sod on three sides with a shovel, then fold it back to expose the soil and grubs. After the grubs are counted, the sod is replaced and watered. A golf course cup cutter can also be used to remove 1/10 of a square foot of sod, making it easy to estimate the number of grubs per square foot.
Select sample sites on a simple map of the area to be scouted. Because the grubs are feeding in the root zone, it is necessary to dig up the sod in order to see them. On a golf course, samples may be spaced across a fairway at 20-foot intervals. Homeowners should sample their lawns at 10-foot intervals if possible. Special attention can be paid to "showcase" areas and to areas with a history of grub damage. If sod is pulled back in sampling, homeowners will need to irrigate thoroughly after replacing it.
With the map mounted on a clipboard, record the number of grubs found at each site and which kinds were present. This information will be used to decide whether the population is high enough to justify treatment. If a treatment is applied, its effectiveness can be determined by doing follow-up sampling.
The grubs that are found can be identified by the shape of their anal slits and rastral patterns. To be certain which species they are, take the collection of grubs to the local Cooperative Extension office for identification.
Fewer than five grubs per square foot indicates a low population
and no need to treat, whatever the kind of white grub. The standard
damage threshold for Japanese beetle grubs is eight to ten per
square foot. However, the specific situation should be taken into
account in the decision whether to treat. If the turf is dense,
with a healthy, robust root system, it can withstand more grub
feeding. If skunks, raccoons, birds, or moles are digging up the
turf to feed on the grubs, a lower threshold can be chosen. If
the owner prefers not to treat with conventional insecticides,
alternatives such as insect growth regulators or parasitic nematodes
can be applied.
The advantage of scouting, determining how many grubs are present
and where, is that treatment can be directed only to the areas
where high numbers of grubs are feeding. If there are very few
grubs present, no treatment is necessary at all, saving considerable
expense, labor, worker exposure, and potential environmental contamination.
Research in New York State has shown that only 20 percent of home
lawns and golf course fairways require treatment.
If pesticides must be used, it is best to minimize the area treated,
restricting the product to spot treatment of areas at high risk.
Protective clothing must be worn, and children and pets must be
excluded from the area, as described on the product label. All
poisons carry some risks, including detrimental health effects
to humans and other mammals, beneficial insects, or other residents
of the soil. White grubs may be found that are infected with natural
diseases and parasites that will control the population without
human intervention. Homeowners who find diseased grubs in high
numbers should let nature do its work.
1. In order to reduce the population before the grubs have
caused serious damage. Eggs laid in late July will hatch by the
third week in August. Heavy feeding begins in September.
2. Treat when the larvae are young and are most susceptible to
biological and chemical products.
3. Treat when the larvae are close to the surface, so that a drench
with a liquid product can physically be in contact with the grubs
and have an effect on them.
Spring treatments are ill-advised. They are a waste of pesticide
and money, as they won't affect fall populations.
"ALL ABOUT WHITE GRUBS" PHOTOS
Also see
Controlling
Insects and Common Pests of Lawns
Insects and Other Common Pests of Lawns
Prepared by Jana Lamboy and Michael Villani. Editing design by M. H. Cowles. All photos provided by NYS Ag. Exp. Station.
From Cornell publication GP7/98 10M/IPM Pub. 412 For copies, contact the Cornell Cooperative Extension office or call the New York State Integrated Pest Management Program at (315) 787-2353.
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law.Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations.The user of this information assumes all risks for personal injury or property damage.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kirklyn M. Kerr, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.