Don't Give up on Ground Pearls

Ground pearls represent one of the most unusual turfgrass insect pests that grounds managers experience. They are a type of scale insect, a group of bugs more typically associated with problems on ornamentals. Ground pearls live in the soil, are for the most part not very mobile and are small in size; yet, they can cause serious damage and even death to the turfgrass and have proven to be a difficult pest to manage.

Ground pearls are members of a unique family of scale insects, and several species exist in the United States alone. However, they are found worldwide and are often associated with acidic soil. They not only cause problems on turfgrass, but damage crops such as sugar cane in Australia, for example. There are two major problems associated with ground pearls in turfgrass: the first is the early detection and proper identification of the problem; the second — and perhaps the most challenging — is the difficulty in providing any level of control.

GETTING TO KNOW THEM

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To better understand the challenge of ground pearl management, it is important to initially review the biology and ecology of this pest. Ground pearls are found from the Carolinas in the East all the way across the West into southern California. At least one species is probably native to the United States and another one may have been accidentally introduced from Australia.

Ground pearls attack the roots of a wide range of warm-season turfgrasses, including bermudagrass, centipedegrass, St. Augustinegrass and zoysiagrass. Damage is most severe on centipedegrass and bermudagrass. The immature ground pearls suck plant juices from the roots, causing the plants to turn first yellow, then brown and usually die in the fall or winter. The ground pearls most likely inject a toxin with their saliva that may speed the death of the plant. The impact of this pest is most pronounced during dry spells. Damaged turfgrass rarely recovers and areas of dead turf frequently are replaced with a variety of weed species.

LIFE CYCLE

The life cycle of ground pearls is quite simple. There are three life stages: the egg, nymph and adult. Adult females are pink, almost sac-like small-scale insects that rarely exceed 1/16 to ⅛ inch in length. They have forelegs and can move about in the soil and on the soil surface. The male is a small, gnat-like creature.

The ground pearl lays pinkish-white eggs in small clusters. The eggs are covered with a white, waxy substance. The eggs hatch into a nymph called a crawler. Subsequent nymph stages are a hard round “pearl” shape that is a yellowish tan color. As the nymphs grow, they increase in size from less than 1/32 of an inch to 1/10 inch in size. Some large nymphs may reach ⅛ inch in size.

The life cycle of the ground pearl is not clearly understood and the exact timing of stages appears to have a lot of variation, even within one site of infestation. In general, the females mature in the spring and early summer. During this time, you can often see the pink, slug-like females crawling over the soil surface. The adults are mobile for only a short period of time (less than two weeks) and then secrete a waxy substance that covers their body. The adults are typically 2 to 3 inches deep in the soil when this occurs and once they are inside the waxy coat egg laying usually begins in the summer. The females can reproduce without mating and can lay up to 100 eggs. Eggs hatch in 9-15 days.

Once the nymphs hatch, they move to the roots of the turf. When they begin feeding on roots, the ground pearls soon develop into the globular cyst stage. Most ground pearls probably complete their development in one year. Previous studies indicate that under unfavorable conditions, the pearls may take more than one year to complete their development. Ground pearls have been found as deep as 10 inches in the soil.

NATURAL PREDATORS

Various predators, such as ants, probably play an important role in suppressing ground pearl populations. As such, unnecessary applications of a broad-spectrum insecticide could potentially increase a ground pearl problem by eliminating some of their natural enemies. Spreading ground pearls from one area to another probably occurs when you move soil, sod, plant materials and possibly as adults “hitch rides” on people and animals.

ALMOST INVINCIBLE

The challenges presented by ground pearls are numerous. Ground pearls are hardy insects that can tolerate unfavorable conditions for an extended period of time. There are cases where grounds managers killed turfgrass with an herbicide application one summer and then reestablished it the following year only to have it attacked again by ground pearls that survived during the period when no host turfgrass was present. In other instances, grounds managers removed the upper 4 inches of topsoil and replaced it with new “ground-pearl-free” soil and sodded or sprigged only to have ground-pearl damage show up the following year. Because ground pearls can be found almost a foot deep in the soil, the removal of a few inches of topsoil is not adequate.

Centipedegrass may be the most aggressively attacked by ground pearls simply because it is a less-competitive and less-vigorous turfgrass compared to bermudagrass. Death in centipedegrass from ground pearl infestations is quite common and the area infected seems to increase each year.

RESEARCH CONTINUES

Efforts to develop effective control measures for ground pearls have been frustrating and produced only modest success. Research has revealed that ground pearls appear to emit an acidic substance and are commonly found in centipedegrass, which grows best under acidic conditions. Studies indicate that liming the soil may create a less-suitable environment for ground pearls. However, the major limitation in these studies is the persistent nature of ground pearls and their ability to survive for years under adverse conditions. Experiments to test this method yielded no improvement in the turf quality and no reduction in ground pearls present in the soil. It may be several years before significant effects of such cultural practices are evident.

Current research is investigating the use of general broad-spectrum insecticides, such as chlorpyrifos (Dursban), on ground pearls during periods when the adults are at or near the soil surface. Although this proves to kill the adults, there are no short-term benefits to this approach and the number of treatments required to cover the time period when adults are present makes such a practice economically and environmentally unsuitable.

Research indicates that even good cultural practices (fertilization, irrigation, etc) simply slow down the inevitable. These seem to have the greatest impact in grasses (such as bermudagrass) that have the capacity to respond to increased nitrogen and moisture. Some grasses (such as carpetgrass and bahiagrass) appear to be less susceptible to the chronic decline and death seen in centipedegrass.

Recently, some success has been achieved in Australia on bowling greens with the use of a mixture of imidacloprid (Merit), a horticultural oil, and wetting agent applied twice a year. Our researchers at North Carolina State University have looked at this program in several locations over the past two years. While we were not able to document a reduction in ground pearls in the soil within a year, we did see a slight increase in turf quality in highly managed bermudagrass. This is not to say we have found a highly effective program, but rather to suggest that in the future some combination of treatments may prove useful in reducing the impact of this pest.

KEEP TRYING

Due to the difficulties in studying ground pearls and the lack of promising results, there are few studies underway on this pest. However, there are at least two points that you should consider if you are forced to deal with ground pearls. A broad range of insecticides and nematicides have been somewhat ineffective for ground pearl control. The problem is not necessarily associated with the toxicity of the pesticide, but rather the hardiness of the pest. The waxy coating of the nymph most likely contributes to its ability to survive almost everything you throw at it.

The second issue is that, in many situations, when all efforts to address a ground pearl infestation fail, the only viable option is to redesign the landscape without turf. Options may include the use of raised beds, ground covers, mulches, shrubs or trees. Keep in mind that the pearls will exist several feet outside of the defined line of damage. These may not be your desired approach, but in some situations they are better than looking at yellowing and dying turfgrass or bare patches of soil inhabited only by a few weeds.

Ground pearls remain one of the most troubling insect pests of warm-season turfgrass. The distribution of ground pearls ranges from the Carolinas to California, but fortunately is generally sporadic in most locations. If it occurs in the turfgrass for which you have responsibility, then it is a serious problem.

Don't discount well-timed insecticide applications in the late spring and early summer when adults are present because they may have long-term benefits. Good turf management practices can also help. Other than that, we can only wait and hope that continued research will come across a cost-effective approach that is practical.

Dr. R.L. Brandenburg is a professor and extension turfgrass entomologist at North Carolina State University (Raleigh, N.C.).

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