Managing White Grubs in Home Lawns

VL006(1/05)

By Rose Hiskes
 The Connecticut Agricultural Experiment Station
Valley Laboratory
153 Cook Hill Rd.
Windsor, CT 06095

Telephone: (860) 683-4977 Fax: (860) 683-4987
Email: Rose.Hiskes@ct.gov

White grubs are the most destructive of all pests of home lawns in Connecticut. These root-feeding pests are larvae of scarab beetles that were introduced into the United States in the early to mid 1900s. For many years the Japanese beetle, Popillia japonica, was the most common, but the oriental beetle, Exomala orientalis, Asiatic garden beetle, Maladera castanea, and European chafer, Rhizotrogus majalis, have become more numerous in the last decade. Additional indirect damage to lawns results from vertebrate pests, such as skunks and raccoons, digging up large areas of turf to feed on the grubs. Moles also feed on grubs and kill turf by leaving roots exposed to air in their tunnels. Successful, environmentally safe, management of white grubs in home lawns can be accomplished with accurate identification of grub species, correct selection and proper timing of management strategies.

DAMAGE

Damage done by white grubs is aptly described as ‘the lawn rolls up like a carpet’. Grubs eat roots near the soil line causing large dead patches in late summer and fall. Without the water and nutrients supplied by their roots, the grass declines and may die. Damage is more pronounced and widespread if conditions are hot and dry. While they are a major problem in turf, white grubs also feed on herbaceous and woody ornamental plant roots. Oriental beetles seem to be the most common species found in landscape beds. Planting small, bare-root plants into grub-infested soil can result in plant losses.

Adult beetles feed on the leaves and flowers of many herbaceous and woody plants, giving them a ragged appearance. Japanese beetle adults feed during the heat of the day, skeletonizing the leaves of many plants. Their favorites are roses, grapes and linden trees. Asiatic garden beetle adults feed from the leaf margin inward on many herbaceous perennial plants, favoring daisies and their relatives. Oriental beetle adults feed on the flowers of rose, squash, dahlia and daylily and often congregate in swimming pools where they can be so numerous that they clog filters. European chafer adults do little feeding but can become a nuisance, often collecting in chimneys during their mating flights.

DESCRIPTION

All white grubs are cream colored with dark patches because of the soil they ingest. Three pairs of jointed legs are found just behind the reddish-brown head capsule. Grubs can reach up to one inch (25mm) in length at maturity and are usually found curled up in C-shapes, especially when disturbed. European chafers (at 23mm) are the largest and Asiatic garden beetles (at 19 mm) are the smallest. Each species has a characteristic pattern of bristles, called a ‘raster’ (lower lines in drawing) and anal opening (top line in drawing) on the bottom of the last body segment which can be seen using a 10x hand lens and used to identify the species.

Adult scarabs are also different from each other. The European chafer is tan to brown, 5/8" long with a heavy body. Japanese beetles are metallic with copper-colored wings, a green head and patches of white hair along each side of the ½" long body. The Oriental beetle is ½" long and straw-colored with irregular black patches which can cover the entire beetle. Asiatic garden beetles are 3/8" long, heavy bodied and reddish-brown with a faint iridescence.

LIFE CYCLE

These beetles have a one-year life cycle in Connecticut. Winter is passed as partially grown grubs in the soil below the frost line. As the soil warms in spring, grubs move back up into the root zone to resume feeding. Pupation occurs in May and adult beetles emerge in June or July depending on the species. European chafers conduct mating flights at dusk near trees or chimneys. Japanese beetles can be seen mating at their feeding sites during the day. Asiatic garden beetles are nocturnal. Asiatic garden beetles, and to a lesser extent oriental beetles and European chafers, will come to lights at night. After mating, females lay eggs in moist soil below turf or in landscape beds near turf. Adults live from six weeks up to four months.

Eggs absorb moisture during development and hatch in one to two weeks, depending on soil temperature. Small grubs begin feeding on roots. As their mouthparts get larger, they are able to completely sever roots.

MANAGEMENT

Integrated pest management (IPM) uses a variety of methods to effectively monitor for and then manage diseases, insects and weed problems while protecting applicators, other people and the environment. Cultural, biological and mechanical means should be tried, where feasible, before resorting to chemicals or in conjunction with chemicals for management of white grubs.

Cultural

Vigorous turf can usually tolerate some grubs. Maintain good vigor with annual fertilization and periodic liming based on soil test results. Mow on a regular basis, but cut off no more than a 1/3 of the grass blade height each time to avoid turf stress. If the lawn is used as a play area or walkway, aerate it with a core aerator as needed to reduce compaction and allow for good root growth. High maintenance lawns that receive multiple fertilizations and frequent irrigation during the summer months may develop a thick thatch layer, found between grass blades and the soil line, which can limit percolation of water and fertilizers as well as movement of any needed insecticides. Dethatch with a mechanical rake to keep thatch ¼ to ½". Frequent irrigation during the scarab beetle egg laying period in July and August may actually increase white grub populations.

Mechanical

Managing white grubs mechanically is usually unproductive. Lawn aerating spiked sandals puncture grubs as you mow your lawn. The best time to try this is in late spring when the grubs are large and near the surface. Literally, this is a hit or miss situation and at best minimally successful.

Mechanical management of adult beetles can be helpful. Handpicking clumps of Japanese beetle adults into cups of soapy water may help reduce the population. The same can be done to Asiatic garden beetles, European chafers and oriental beetles at night when they come to lighted windows and can be flicked off screens. Using a butterfly net to catch European chafers during their mating flights can result in large captures of beetles which can then be drowned in soapy water.

Floral and sex pheromone bag traps for Japanese beetles, that attract both males and females, should be placed away from favored food plants, such as roses. Catches may help reduce beetle numbers and therefore grub numbers.

Biological

Biological control of white grubs can be effective. Milky disease, caused by native bacteria, can infect European chafer, Japanese, and oriental beetle grubs. Bacterial spores are introduced to the soil as a powder and kill grubs slowly. Annual untreated grub populations are needed however, to keep the disease going. New England soils are usually too cold for populations of these bacteria to establish and become effective natural enemies. Another bacteria, Bacillus thuringiensis var. buibui, is an effective natural enemy of scarab beetle larvae but is not commercially available at this time.

Beneficial nematodes, microscopic roundworms, can be watered in to the soil where they enter white grubs. Once inside the grub, the nematodes release a bacterium that actually kills the grub. These nematodes can be very effective but have limited availability, may not ship well and have a limited shelf life. Because they are living organisms that dry out quickly and die if exposed to sunlight, they need to be applied on cloudy, calm days; or in the early morning; or evening; or in the rain. September, when soils are warm, is the best time to apply these nematodes.

Chemical

IPM typically involves applying chemicals only after early detection of a damaging pest population. White grub larval development, however, limits the success of this practice. The most effective chemicals work only on newly hatched larvae, which are barely visible to the naked eye. Therefore, waiting to apply insecticides until mature grubs are detected in late summer or early fall prevents adequate control. Monitoring the location of adult activity is not always predictive of where grub populations will be. Adult beetles are capable of flight so grub infestations are not in the same place every year. If monitoring detects a damaging population of white grubs that is causing dead patches in the lawn, application of a preventative insecticide to the next generation may be the most effective way to manage white grubs.

Effectiveness of Chemical Control of White Grubs*
Merit- imidacloprid	Mach 2 - halofenozide	Dylox - trichlorfon
Pest Species\description	‘season long’	Insect growth regulator	‘24 hours’
European chafer	90% mortality	Ineffective	50% mortality
Japanese beetle	90% mortality	>75% mortality	>75% mortality
Oriental beetle	90% mortality	Ineffective	Ineffective
Asiatic garden beetle	50% mortality	increased survival	50% mortality
* based on research done by Dr. Richard Cowles, Connecticut Agricultural Experiment Station, Valley Lab

Identification of the grub species is important because of their different responses to insecticides used against them (see table). Residents of Connecticut can bring grubs to the Agricultural Experiment Station in Windsor or New Haven for identification.

Imidacloprid, (Merit), provides excellent management of European chafer, Japanese beetle and oriental beetle larvae and moderate management of Asiatic garden beetle larvae. It should be applied no more than once a year as close to early July as sufficient irrigation or rainfall will allow. This compound breaks down in sunlight and will need to be watered in to get it through the thatch into the root zone where larvae feed. It stays active in the soil for several months.

Halofenozide, (Mach2), is ineffective against European chafer, oriental beetle and Asiatic garden beetle and should not be used if these insects are the most prevalent species. Mach2, which is very effective against Japanese beetle larvae, is an insect growth regulator that has low toxicity to anything other than invertebrates with exoskeletons. It does not stay active in the soil very long and is effective only on small larvae, so applications should be timed for early August for the best success against Japanese beetle grubs.

Trichlorfon, (Dylox), a quick acting organophosphate, provides very effective management of Japanese beetle larvae and moderate results with European chafer and Asiatic beetle larvae. It is most effective against young grubs during early fall when they become visible in the soil. Dylox is not effective against oriental beetles.

Imidacloprid, halofenozide and trichlorfon may be the active ingredients of products other than those mentioned above. Familiar products like diazinon and Dursban, chlorpyrifos, are no longer available to homeowners for control of white grubs.

Killing adult beetles before they can lay eggs is another chemical control option. Many insecticides are labeled for control of beetle adults in landscapes. Practically speaking, European chafers, Japanese beetles and Asiatic garden beetles are the only species that congregate on plants in large numbers where sprays might provide effective control. Older insecticides, such as Sevin, which is very toxic to bees, and malathion along with methoxychlor have good activity against adult beetles. Both are somewhat toxic to the applicator and environment. Pyrethrins, which are derived from plants, are less toxic to mammals but are effective against insects in general. Be careful to protect aquatic insects from contacting any sprays. Many products are available with pyrethrin or related compounds, such as cyfluthrin and bifenthrin, as the active ingredient.

With all applications, be sure to read and understand the entire label and follow all directions. The label is the LAW! Regulations and insecticides change frequently so be sure to use a product that is appropriate for the safe management of your correctly identified pest.

Reference

Turfgrass Insects of the United States and Canada, 2^nd Edition. Vittum, Villani and Tashiro.