ROOT-FEEDING: White grubs, a general name

Figure 1. Grub

Identification: White grubs is a general name for the grub (larval) stage of beetles in the family Scarabaeidae, order Coleoptera (beetles) that feed on the roots of turf (Figure 1). All species of scarab beetles have larvae or grubs that are C-shaped and vary in size depending on the species and larval age (instar). All six legs of the grub are located under the head, and the diameter of the abdomen increases slightly towards the end. In general, the grub’s head capsule is an orange-black color and the end of the abdomen can be darker than the rest. Remember all grubs start small and increase in size as they molt or shed their skin and change into larger instars (larval stage). Do not treat for grubs in the fall because grubs move down into the soil for the winter. An expert can determine the species of white grub by examining the hairs and sutures on the last abdominal segment on the grub’s body. In the field, identification of the grub is more difficult. However, the color and form of the adult scarab beetles are distinctive and species identification is easy. Adults often feed on tree and shrub foliage and then return to the turf for egg laying.

Damage, scouting, and management: Identify a grub problem by examining a square foot sample of lawn along the border where dead or damaged grass meets healthy grass. When grub densities are high, the blades pull away from the roots and the turf rolls back like a carpet. Skunks and moles are known to use grubs for food. However, in Minnesota night crawlers account for a sizable portion of the diet of those mammals. Therefore, grub control often will not correct damage to lawns by skunks and moles (see Extension bulletin FS-1139 for mole control). Remember the grubs turn into adult beetles that emerge from the soil and fly to trees, shrubs, and roses to feed on the leaves before returning to the turf to lay eggs. In some species, control of adults is warranted if they are damaging ornamental plants.

Figure 2. June Beetle

May/June beetles (Phyllophaga species)

Identification: All species of Phyllophaga are called May or June beetles. Adults are about 1 inch long and a chestnut brown color and fly to lights in the early summer. The adult scarab beetle feeds on foliage and lays eggs in the turf in early summer (May beetles) and summer (June beetles) (Figure 2). The grubs are whitish with brown heads and are usually found curled in a "C" shape and range from 1/2 to 1 inch in length. These are the largest grubs found in turf.

Damage, scouting, and management: Grubs feed on the roots of the grass and heavy infestations will loosen the sod so that it can be rolled back. The damage will appear as irregular patches of yellowed or dead grass. In Minnesota, May/June beetle grubs feed on grass roots for three years before becoming adults. The first year grubs grow up to 1/2 inch long and produce little damage. The second year, they range from 1/2 to 3/4 inch in length, and damage becomes more apparent. This second year is the best time to control grubs since damage usually is not extensive, and an insecticide will be effective. Control for grubs is desirable when there are more than 4 grubs/ft2. The third year, the grubs grow to 1 inch long and damage becomes very apparent, particularly in July and August. In late summer the grubs become adults in pupal chambers in the soil and emerge the following spring as adults.

Figure 3. Annual white grubs or masked chafers

Northern masked chafers (Cyclocephala borealis)

Identification: Adults are shiny brown scarabs around 1/2 inch in length with a dark brown mask across the head and a dark spot on each side of the thorax (Figure 3). After overwintering in the soil, adults emerge in late June and females lay egg clusters on top of the soil. Adults are nocturnal and do not feed. Northern masked chafers have a one-year life cycle. Damage is more severe in late summer when the grubs are third instar.

Damage, scouting, and management: The larvae feed on roots, separating crown from roots. The larvae reach maximum size in September and then move down deeper in the soil to overwinter. Healthy turf can tolerate greater than 20 grubs/ft2; while stressed turf can tolerate less, around 10 grubs/ft2.

Black turfgrass Ataenius (Ataenius spretulus)

Identification: The adult is a small, black scarab beetle around 1/5 inch in length that is common around high maintenance golf courses, especially highly watered and fertilized areas (Figure 4). Larvae are very small, around 1/4 inch in length. It is a native insect that has emerged as a turf pest in the last 20 years.

Figure 4. Ataenius

Damage, scouting, and management: Adults overwinter in woodlots and start to fly in May to June to lay eggs in the thatch. The larvae feed and develop over two months with peak damage in late July and early August. Most root injury occurs near the soil-thatch interface. Larvae pupate in the soil and start emerging in August. High levels of infestation of this pest can be tolerated. Control when greater than 50 grubs/ft2 are found.

Aphodius beetles (Aphodius granarius)

Identification: This is a small, black scarab beetle around 1/5 inch in length that can be common around high maintenance golf courses, especially highly watered and fertilized areas. Aphodius beetles primarily feed on decaying organic matter, particularly compost and manure, but also damage turf roots. This European scarab beetle can fly to turf and be found feeding with black turfgrass Ataenius in areas with high organic material. It is also commonly found at dung.

Damage, scouting, and management: Adults overwinter in woodlots and start to fly in May to June to lay eggs in the thatch. The larvae feed and develop over two months with peak damage in late July and early August. Most root injury occurs near the soil-thatch interface. Larvae pupate in the soil and start emerging in August. High levels of infestation of this pest can be tolerated. Control when greater than 50 grubs/ft2 are found.

Figure 5. Green June Beetle

Green June beetle (Cotinis nitida)

Identification: Green June beetle is not established in Minnesota, but may be transported on nursery stock. The green June beetle is green in color trimmed with brown along the edge (Figure 5). The underside is also green, but has a very shiny, metallic look. The adults are attracted to manure for oviposition and grubs can be found in many crops and ornamentals that have manure added to the soil. Adults reach a length of 3/4 - 1 inch. Larvae have typical scarab characteristics and reach 2.0 inches in length.

Damage, scouting, and management: The larvae feed on the roots of turfgrass as well as corn, oats, sorghum, alfalfa, and nursery stock, especially where manure has been added to the soil. Adults feed on a variety of ripening fruits such as apples, pears, and grapes. Tolerance levels for this species have not been set.

Japanese beetle (Popillia japonica)

Figure 6. Japanese Beetle

Identification: The Japanese beetle is an exotic scarab originally established in New Jersey (Figure 6). Japanese beetles are approximately 7/16 inch long. The front of the beetle is dark metallic green and the wing covers are dark tan. There are five small, white patches of short hairs along each side of the dorsal abdomen on the beetle. These white patches are a key characteristic for identification. If it does not have these white hair patches, it is the False Japanese beetle and not the quarantine pest.

Damage, scouting, and management: One of the favored foods of adult Japanese beetles is rose foliage and flowers, although adults feed on over three hundred species of plants. Larvae feed on the roots of grasses. Inspect your plants for skeletonized leaves and the presence of adult beetles. Pheromone traps use a rose-scented lure to attract the adult beetles and can be purchased in garden centers.

False Japanese beetle (Strigoderma arbicola)

Figure 7. False Japanese Beetle

Identification: This native scarab resembles the Japanese beetle (Figure 7). False Japanese beetles are native to Minnesota, but are less of a problem species. The adult beetles are about 7/16 inch (10-12 mm) in length and are a dark tan to brown color with a slight metallic green color on the front third of the body. However, there are no white tufts along the dorsal abdomen edges as found on the Japanese beetle.

Damage, scouting, and management: This insect has not been studied extensively. Larvae feed on plant roots, but a species list is not well known. Adults are found feeding on buds and flowers of wild and cultivated roses and other plants. Control is not necessary. Information on the false Japanese beetle is given so this species can be distinguished from the Japanese beetle, a major turf pest and quarantine pest in Minnesota.

Oriental beetle (Exomala orientalis)

Identification: Oriental beetle is not established in Minnesota, but may be transported on nursery stock. Introduced into Connecticut as early as 1920, this scarab beetle has spread across the mid-Atlantic states. The adults are similar in size to Japanese beetles (7/16 inch), however the adult beetles do not have any green, but vary in color from light brown to black, often with darker mottling on the wing covers (Figure 8).

Figure 8. Oriental Beetle

Damage, scouting, and management: Larvae feed on the roots of turf grasses and adults feed on roses, phlox, and petunias, although they are active at night and more cryptic compared to Japanese beetle. A good pheromone trap is available. As of 1997, pheromone traps did not detect this beetle in Minnesota, but it may arrive on the roots of nursery stock.

ROOT-FEEDING: Weevil larvae

Bluegrass billbug (Sphenophorous parvulus, Family Curculionidae, Order Coleoptera)

Identification: Adult billbugs are long-snouted, 1/4 inch long, gray-to-black beetles with a strongly tapered abdomen (Figure 9). They can be found walking on hard surfaces in spring prior to depositing eggs in grass sheaths. The plump, legless white larvae first feed inside stems and then on roots.

Damage, scouting, and management: Infested lawns have off-colored, irregularly shaped areas that rapidly yellow and finally turn brown. Areas of advanced infestation contain turf offering little resistance when pulled. Larvae complete feeding in late July and emerge as adults in August to overwinter in protected locations.

Billbugs are best controlled in May as the females are laying eggs. Application of an insecticide is suggested about 10 days after adults are sighted. The immature stage can also be controlled when they are feeding on the roots. Scout for larvae by inspecting a square foot sample of lawn along the margin where dead or damaged grass meets healthy grass.

Treatment is not suggested after mid July because the larvae complete their feeding and move 1 to 2 inches into the soil to pupate. Watering and moderate fertilization of grass will help reduce the damage caused by billbugs and may eliminate the need for insecticide treatment. Thresholds vary depending on turf health and vigor.

Figure 9. Bluegrass Beetle