7 minute read
Feature
Ground Pearls
Big Headaches with
Few Solutions
By Katelyn Kesheimer, Ph.D.,
Assistant Professor and Extension Specialist,
Department of Entomology and Plant Pathology,
Auburn University
Joshua Weaver, Ph.D.,
Lecturer, Department of Horticulture,
Auburn University
Many turf managers and homeowners are familiar with the frustration of trying to control insect pests in turf. There is, however, a special type of frustration that comes with trying to control ground pearls. These small, soil-dwelling pests present unique challenges that often result in major headaches and dead grass.
Background
Ground pearls are scale insects that belong to the order Hemiptera. These are true bugs characterized by their piercingsucking mouthparts. This is a large and diverse group of insects that can use their straw-like mouthparts for good or evil. On the good side, we have predatory assassin bugs that contribute to biological control by sucking the guts out of pest insects. And on the evil side we have blood-sucking bed bugs, which, while fascinating, are traumatizing and expensive to control.
Scale insects, the group ground pearls belong to, fall somewhere in the middle of good and evil. For example, lac scales produce shellac, a natural glue with a variety of uses; cochineal scales produce a red dye that is commonly used in foods and fabrics. But there is also the crape myrtle bark scale, which has spread across the southeastern U.S. in recent years. Like aphids, crape myrtle bark scales excrete honeydew, a substance that may lead to sooty mold, reducing plant health and appearance.
There are thousands of species of scale insects with lots of variation, but all have straw-like mouthparts and grow underneath a waxy coating. These mouthparts are what enables scales to suck nutrients from different plant parts, and the waxy coating protects them from many control measures. Ground pearls live underground and suck on grass roots, slowly weakening the grass over time; these are added layers of difficulty when it comes to both identification and control.
Damage
Worldwide, ground pearls infest a wide range of grasses and other plants. Here in the United States, ground pearls will infest bahiagrass, bermudagrass, carpetgrass, centipedegrass, St. Augustinegrass, and zoysiagrass. The biggest problems occur in centipedegrass, but we still see serious damage in other grasses.
Initial damage from ground pearls can be difficult to diagnose, as it first appears as yellow patches in the grass. Without additional information, the yellowing can be misdiagnosed as drought stress, nutrient deficiencies, disease, or other insect pests such as grubs. Yellowing from ground pearls is caused by the nymphs (immature form of the insect)sucking juices from the plants. These yellow patches will turn brown as the summer progresses and may even die off at the end of the year. Infested patches may fail to green up in the spring. Symptoms may be subtle in the early days of an infestation but will gradually get bigger as the population grows and continues to feed.
Ground pearls remain underground their whole lives, so accurate diagnoses are not always easy. But there are some common symptoms to clue you in to this pest. The yellowbrownish area of weakened grass will sometimes take on a circular shape, like fairy ring. Narrow, serpentine lines of declining grass are also common with ground pearl infestations. As the grass continues to decline, weeds may invade these areas. Year after year, the damaged area will continue to expand. Other stressors, such as drought or improper fertilization, can exacerbate the damage and make it more obvious.
Biology
There are two species of ground pearls that attack warmseason grasses: Margarodes meridionalis and Eumargaraoides laingi. Additional species of ground pearls within the family Margarodidae are pests of grape vineyards and sugarcane in the U.S. and Australia. Ground pearls have a unique adaptation where the nymphs cover their bodies in a protective wax coating, resembling a pearl. This resemblance gives us the family name Margarodidae, as ‘pearl’ in Latin is margarita. The waxy coating provides protection against extreme weather and other environmental conditions, natural enemies, and even insecticides. Water-based or water-activated insecticides are unable to penetrate the wax layer, making the ground pearl cysts completely protected.
Ground pearls have only one generation per year and overwinter as cysts attached to the turf roots with their piercing-sucking mouthparts (Figure 1). The cysts can be found in the top couple of centimeters of the soil, or as far as 30 centimeters deep. The adult females emerge from the cysts in late April to August, but timing may be influenced by soil moisture and temperature. The adult females are the dispersal stage of ground pearls; male ground pearls are rare and not thought to be pests of turf.
Female ground pearls are pinkish red with a pear-shaped body, measuring about 1.6 millimeters long (Figure 2). They have three distinct pairs of legs, with the first pair modified for digging. These digging legs allow the females to move around and find feeding and egg-laying sites. Once she finds an oviposition site, she makes an egg chamber by secreting a waxy filament that she then fills with pinkish-white eggs. Females do not require mating to produce eggs and over the course of one to two weeks will lay about 100 eggs.
Egg chambers are found 5–8 centimeters deep in the soil where eggs will hatch into nymphs, called ‘crawlers.’ Crawlers move to a nearby root, attach with their mouthparts, and coat themselves with a protective wax layer. Following the transition into cysts, ground pearls can remain over the winter until emergence next spring.
Control Options?
This is the section where we typically summarize efficacy tests and provide research-based recommendations to control the pest in question. However, ground pearls are especially obnoxious in that they do not currently allow us to do that. Despite years of research and trials, entomologists and turf specialists have not found a chemical treatment that can completely control ground pearls. Further, we have not found any other organism (arthropod, nematode, even vertebrates) that can provide biological control.
While ground pearls spend much of their life as protected cysts, they are vulnerable to insecticides as adult females and crawlers. There are limited products registered for ground pearl control on home lawns, and no products registered for ground pearls on turf. Products registered on home lawns are typically pyrethroids or combination products with diamides and pyrethroids. Chemical applications applied when femalesare present and laying eggs may help reduce the infestation but will not completely control it.
Management
Like chinch bugs, ground pearls are often found at the boundary between the infested turf and healthy turf. This is where you have the highest probability of finding ground pearls when diagnosing an infestation. Once diagnosed, the best recommendation is to keep the turf as healthy as possible to prolong the life of the grass. This includes watering as needed rather on a schedule and improving water penetration. Appropriate fertilization will hide some damage, but the turf will ultimately succumb to the infestation.
Eventually severe infestations will take over and kill entire areas of turf. As all warm-season grass species are susceptible to ground pearls, simply replacing with another grass is not a viable option. Further, since cysts can survive for years in the soil without a host, killing the grass and replanting with warm season turfgrass is also not a viable option. Replacement options include ornamental plants, low-growing ground covers, or flowering plants. My favorite option remains flowering plants, which provide food and shelter for pollinators and natural enemies. Most insects are in fact good, so why not grow plants that harbor the good ones rather than the bad ones.
