Turf and Landscape - Green June Beetles

Green June Beetles can be a pest of turf and landscape areas. The green June beetle is in the Scarabaieidae family and also referred to as white grubs. Unlike many of the other white grub pest of turfgrass this species is unique in that it will come to the soil surface and crawl on the turfgrass at night. Their larval tunneling activity can damage turfgrass stands. The following is more information.

Identification

The adult green June beetle (Cotinis nitida L.) is usually 3/4" to 1" long, and 1/2" wide. The top side is forest green, with or without lengthwise tan stripes on the wings. The underside is metallic bright green or gold, bearing legs with stout spines to aid in digging. In the Mid-Atlantic region the names "June bug" and "June beetle" are commonly used for this insect. They're called "fig eater" in the southern part of their range. Do not confuse the green June beetle, however, with the familiar brown May or June beetles that are seen flying to lights on summer nights. The green June beetle adult flies only during the day.

The larvae are white grubs often called "Richworms" because they prefer "high" levels of organic matter for food. With three growth stages they develop and are similar to the other annual scarab species. Their body lengths reach 1/4", 3/4", and 2" respectively. The larvae have stiff abdominal bristles, short stubby legs, and wide body. One unique characteristic of this grub is that it crawls on its back by undulating and utilizing its abdominal bristles to gain traction. Other typical white grubs, like the Japanese beetle grub, are narrower, have longer legs, crawl right side up and when at rest assume a "C" shaped posture.

Hosts

The adults generally don't feed but occasionally become a pest of fruit. Any thin skin fruit such as fig, peach, plum, blackberry, grape and apricot can be eaten. The principal attraction is probably the moisture and the fermenting sugars of ripening fruit. They occasionally feed on plant sap. In turf situations egg laying females are attracted to moist sandy soils with high levels of organic matter. Turf areas treated repeatedly with organic fertilizers, composts or composted sewage sludge become more attractive to the female.

The grub feeds on dead, decaying organic matter as well as plant roots. This species is commonly associated with both agricultural crop and livestock production areas as well as urban landscapes. Field stored hay bales, manure piles, grass clipping piles, bark mulches and other sources of plant material that come in contact with moist soil are prime microhabitats preferred by both the female for egg laying and the migrating 3rd instar grubs

Life Cycle

The green June beetle completes one generation each year. Adults begin flying in June and may continue sporadically into September. The peak occurrence of adults is during a two week period in mid-July in Maryland and Virginia. On warm sunny days, adults may swarm over open grassy areas. Their flight behavior and sounds resembles that of a bumble bee. At night they rest in trees or beneath the thatch.

The adult females shortly after emerging may fly to the lower limbs of trees and shrubs and release a pheromone that attracts large numbers of males. Frequently, males repeatedly fly low and erratic over the turf trying to locate emerging females. After mating, females burrow 2" to 8" into the soil to lay about twenty eggs at a time. The spherical eggs are white and almost 1/16" in diameter.

Most eggs hatch in late July and August. The first two instar stages feed at the soil thatch interface. By the end of September, most are third instar larvae and these large grubs tunnel into the thatch layer and construct a deep vertical burrow. The grubs may remain active into November in the Mid-Atlantic region. In the more southern states grubs may become active on warm nights throughout the winter. In colder areas they overwinter in burrows 8"-30" deep. The grubs resume feeding once the ground warms in the spring and pupate in late May or early June. The adults begin emerging about three weeks later.

Damage

The green June beetle grub differs from other white grubs in their feeding behavior. Damage to turf occurs as a result of their unusual habit of tunneling as well as root feeding. Smaller stage grubs tunnel horizontally in the top 4" of the ground, loosening the soil, eating roots, and thinning the thatch. This activity begins in early to mid-August when the disturbed grass may wilt or die if conditions are dry. Damage is minimal when grub density is low or if the grass receives plenty of moisture. As the grubs grow, tunnels become vertical and deeper with turf damage becoming more severe. Tunnels to the surface are kept open by grubs pushing little mounds of loose soil to the surface. The resulting mounds appear similar to earthworm castings. To determine that a mound was made by a green June beetle grub, wipe the mound away and feel for a hole in the ground about as wide as your finger. Earthworm holes rarely exceed the diameter of a pencil. The soil mound will reappear the next day. Fecal pellets about as big as mouse droppings may also be present on the soil surface near the holes. Fresh mounding activity is especially visible after a heavy rain. The mounds and holes are visible by mid-August, but the damage becomes more pronounced in the following months as the grubs continue to grow. The grubs do feed on some roots, but the major damage to the turf is due to the upheaval of the soil, dislodging of roots from the soil and subsequent weed problems.

The large green June beetle grubs come to the surface at night to feed or "graze" on the turf and individuals may migrate long distances (20-30 ft per night). Grubs may also be found in the twilight hours and on overcast days. Their trails through the dew can frequently be seen on golf course greens.

Besides the direct damage, these grubs cause some indirect problems. The mounds and holes disfigure turf while the tunneling kills the grass. Drought stressed turf mowed very short succumbs easily to this damage. As a consequence, spaces open up as the grass dies and allows for weed encroachment. The tunneling and excavation of subsoil brings acidic soil to the surface and this changes the microhabitat that favors grass and broadleaf weed species. Turf managers using reel mowers have complained that the loose soil and grit from the mounds accumulates on the machinery and dulls the cutter blades, especially when the dew is still on the grass. Additionally, predators such as small mammals and birds damage turf as they dig for the grubs.
To date no thresholds are available for landscape turf or lawns. Treatments are recommended on perennial ryegrass/bentgrass golf course fairways when grub counts exceed five per sq ft. Damage thresholds for Kentucky bluegrass and tall fescue based on field observation are slightly higher at 6-7 grubs per sq ft. Kentucky bluegrass will quickly recover with new growth from rhizomes.

To prevent damage to turf, apply controls to grub stages before many mounds become evident. We recommend an action threshold of five 3rd instar larvae per sq ft. Damage cycles historically run for 3-6 years than subside. During these outbreaks, damage may be expected if high populations of grubs were present the previous year and insecticide control was inadequate. An increase in the number of adults over the previous years observations is also a reason to expect damaging populations of grubs.

Biological-Biorational Control

To date there are no effective commercial biological agents available to control this grub. The most common parasite is a type of digger wasp, Scolia dubia (Say). This beneficial wasp enters the grub tunnel, stings the grub then lays an egg on the paralyzed grub. The resulting larva feeds in the grub, eventually killing it. Increased flight activity in late September by the wasps was noticed after green June beetle emergence. Unfortunately, even though these wasps help reduce the grub population, many people are afraid of being stung and consider them a nuisance or threat to life. These wasps are not aggressive and rarely sting humans. Milky disease products effective against Japanese beetle do not control green June beetle grubs nor do any Bacillus thuringiensis (B.t.) products.

Cultural Control

Some turfgrasses recover from damage once stress factors are removed. For example, species having stolons and rhizomes may repair the damage once the grub population is controlled. Also, the damage resulting from the grub tunneling is less severe when the turf receives sufficient moisture, fertilizer and lime. Overseeding in the fall is critical in preventing weed encroachment the following season. It's helpful to remember that tufgrass cut at a higher height (2 1/2" to 3") is less stressed, and therefore the damage is less visible. Also, turfgrass species such as tall fescue with the wider leaf blades also hides damage better than the fine-leaf grasses such as perennial ryegrass, bentgrass or fine fescue.

Chemical Control

Insecticides are effective on all grub stages and applications may be warranted anytime between August and November, as long as damaging numbers remain active. Spring applications of chemicals are not generally recommended since the grubs are active only for a few weeks and many may have pupated by the time damage becomes obvious. Once the grubs reach the third instar in August or September, they migrate freely and can easily move from an infested area to an adjacent area. To protect golf course greens, treat the greens, collars, and a few yards beyond the collars. The insecticides normally used to control sod webworms, cutworms, and armyworms on the greens will generally suppress migrating grubs. Sevin and Turcam insecticides have been effective in controlling the larval stage. If fairways are treated, the rough areas should be spot treated where there are high grub populations. The risk of high grub populations is generally correlated with areas where the adult beetle populations were most concentrated.

Chemical control of the larvae can be achieved preventatively by applications of imidacloprid (Merit), halofenozide (Mach2), thiamethoxam (Meridian), or clothianidin (Arena) in July or August or by curative applications of carbaryl (Sevin). To control the early instars before the migration phase, application of insecticides must be followed immediately by irrigation with 1/2" of water, or timed with rainfall. A word of caution is appropriate when an curative insecticide treatment is applied. After treatment the grubs come to the surface within 12 hours and die causing a foul order as they decay. Finally, monitor treatment areas carefully because migrating grubs may reinfest an area once the insecticide has broken down. It may be necessary to retreat.

Information by Dr. Lee Hellman, Department of Entomology and Dr. J. Kevin Mathias, Institute of Applied Agriculture, University of Maryland. Some control information from the University of Arkansas Turf Tips.