Box 9.1. Ground pearls

In parts of Africa, the encysted nymphs (“ground pearls”) of certain subterranean scale insects are sometimes made into necklaces by the local people. These nymphal insects have few cuticular features, except for their spiracles and sucking mouthparts. They secrete a transparent or opaque, glassy or pearly covering that encloses them, forming spherical to ovoid “cysts” of greatest dimension 1–8 mm, depending on species. Ground pearls belong to several genera of Margarodinae (Hemiptera: Margarodidae), including Eumargarodes, Margarodes, Neomargarodes, Porphy- rophora, and Promargarodes. They occur worldwide in soils among the roots of grasses, especially sugarcane, and grape vines (Vitis vinifera). They may be abundant and their nymphal feeding can cause loss of plant vigor and death; in lawns, feeding results in brown patches of dead grass. In South Africa they are serious vineyard pests; in Australia different species reduce sugarcane yield; and in the south-eastern USA one species is a grass pest.

Plant damage mostly is caused by the female insects because many species are parthenogenetic, or at least males have never been found, and when males are present they are smaller than the females. There are three female instars (as illustrated here for Margarodes (= Sphaeraspis) capensis, after De Klerk et al. 1982): the first-instar nymph disperses in the soil seeking a feeding site on roots, where it molts to the second-instar or cyst stage; the adult female emerges from the cyst between spring and fall (depending on species) and, in species with males, comes to the soil surface where mating occurs. The female then buries back into the soil, digging with its large fossorial fore legs. The foreleg coxa is broad, the femur is massive, and the tarsus is fused with the strongly sclerotized claw. In parthenogenetic species, females may never leave the soil. Adult females have no mouthparts and do not feed; in the soil, they secrete a waxy mass of white filaments — an ovisac, which surrounds their several hundred eggs.

Although ground pearls can feed via their thread-like stylets, which protrude from the cyst, second-instar nymphs of most species are capable of prolonged dormancy (up to 17 years has been reported for one species). Often the encysted nymphs can be kept dry in the laboratory for one to several years and still be capable of “hatching” as adults. This long life and ability to rest dormant in the soil, together with resistance to desiccation, mean that they are difficult to eradicate from infested fields and even crop rotations do not eliminate them effectively. Furthermore, the protection afforded by the cyst wall and subterranean existence makes insecticidal control largely inappropriate. Many of these curious pestiferous insects are probably African and South American in origin and, prior to quarantine restrictions, may have been transported within and between countries as cysts in soil or on rootstocks.

Ground pearls

→   See all boxes