Introduction

Margarodes are sub-terranean insect pests that attack plant roots. The name derives from Margarodidae, the family name under which this type of scale insect is classified. Larvae, the life stage that actually attacks plant roots, secrete a hard, somewhat shiny wax layer that covers the whole body. The resultant cysts are round to oval-shaped. Species may vary in both colour and size. In the Bahamas cysts are strung to form necklaces resulting in the common name “ground pearls”.

The approximately 70 margarodes species have an almost global distribution and a very wide range of host plants. However, species that attack grapevines occur only in the Americas, South Africa and Namibia. Two of the three species causing economic damage in the Americas are Margarodes vitis in Chile and Margarodes brasiliensis in Brazil. Of the ten species known in South Africa, five attack grapevines, while the rest mainly occur on grasses. Serious economic damage is inflicted on table, raisin and wine grapes (De Klerk, 1985, De Klerk et al., 1982a, 1982b).

 

Identification and distribution (De Klerk, 1983, 1985, De Klerk et al., 1982a)

The five grapevine margarodes species are endemic to South Africa and are not present in other countries, except Margarodes prieskaensis which was recently also recorded on table grapes in Namibia. Cysts are present throughout the year and clearly visible on roots or in the soil with the naked eye.

  • Margarodes greeni (Brain) is small (maximum 2.5 mm), round with a soft, white cyst wall. It is easily confused with fertiliser pellets or even snail eggs. The species occurs in and around Vredendal, Malmesbury, Ceres, Paarl, Stellenbosch, Worcester, Robertson and Montagu.
  • Margarodes trimeni (Giard) is large (maximum 6.3 mm), oval in shape, very hard with clearly visible overlapping scales. These scales are smooth and yellow to bronze in colour. tremeni is the only South African species with an oval shape and with a metallic lustre. This species occurs in areas around Paarl, Stellenbosch, Worcester, Robertson, Montagu and Ceres.
  • Margarodes capensis (Giard) has round cysts that vary in size to a maximum of 6.3 mm. The light brown cysts wall is thick and hard with a bark-like appearance. This species is common in vineyards in and around Malmesbury, Paarl, Stellenbosch, Worcester, Robertson and Montagu.
  • Margarodes vredendalensis (De Klerk) is endemic to the Olifants River irrigation area near Vredendal and Lutzville. The round cysts vary in size to a maximum of 8.6 mm. The light-to-dark brown cysts wall is hard with a wart-like appearance.
  • Margarodes prieskaensis (Jakubski) is common in the Orange River irrigation area in and around Kakamas, Keimoes, Upington, Groblershoop, Prieska, Douglas and Hopetown. This species was recently also recorded on table grapes in Mpumalanga [Lephalale (Ellisras), Mokopane (Potgietersrus) and Groblersdal] and Southern Namibia, near Noordoewer. The spherical cysts vary in size to a maximum of 6 mm. The thick, hard, dull yellow cyst wall resembling a tortoise shell.

Margarodes are therefore present in almost all local viticultural areas and serious damage is caused by all five species.

FIGURE 1. Margarodes occur in most viticultural areas and serious damage is caused by five species.

FIGURE 2. Cysts of the different species differ in size and are easily detected with the naked eye. M. prieskaensis (6 mm in diameter) left and M. greeni (2.5 mm in diameter) right.

 

Life cycle

The larva (nymph) within the cyst has no legs and therefore cannot move around. It has elongated, sucking mouth parts that are inserted into the plant root via a small opening in the cysts wall. The mouth parts are very prominent and in most species its length exceeds that of the whole body.

Inside the cyst the larva develops into an adult female that makes an opening in the cyst wall to move out into the soil. Females have characteristically strong forelegs specially adapted to move in soil. They actively move about for two to four days, becoming stationary at the onset of oviposition. During oviposition wax filaments are secreted to form a pocket in which the eggs are protected. The average female lives for 24 days and may produce up to 1 200 eggs.

FIGURE 3. Females have strong, well-developed forelegs for moving around in the soil.

The microscopic small larvae emerging from the eggs are equipped with legs that allow them to move to the nearest roots, where they insert their sucking mouthparts for feeding. As the free living larvae grow, becoming thicker and rounder, they start secreting layers of wax that form the cyst wall. They then moult to form second instar larvae without legs within the cyst. At this stage the cyst is only the size of a pin head. The cyst keeps growing and the larva moults for an unknown number of times to form an adult female. The total timespan of the whole process is unknown, but apparently it lasts for more than one year indicating that margarodes cannot multiply on annual plants.

Females stay underground and lay fertile eggs without mating with males. For M. prieskaensis, however, mating does take place with winged males present above the soil surface. Male pre-pupae develop from cysts and closely resemble females also with well-developed legs. They move to just below the soil surface and moult to form pupae. When females emerge from the cysts they also move towards the soil surface. Simultaneously, winged males emerge from the pupae and mating takes place. Thereafter the males die, while the females burrow down to lay their eggs on or near the roots.

 

Symptoms and identification of an infestation (De Klerk, 1983, De Klerk et al., 1982a, 1982)

The first symptom possibly indicating a margarodes infestation is poor growth that usually manifests in patches within a vineyard. Shoots get shorter and thinner, while smaller than average leaves curl downwards. Later one or more arms of the vine die before the whole plant ultimately succumbs. Infested patches gradually increase in size. Vines can be killed within four years. No typical symptoms are evident and poor growth could also be ascribed to phylloxera, nematodes or other organisms.

FIGURE 4. Above ground symptoms are poor growth with smaller leaves curling downwards.

No typical galls or other symptoms appear on roots. As noted earlier, cysts on roots or lying free in the soil, are easily detected. If cysts are crushed, only a yellow fluid is evident and the larvae are not visible with the naked eye. Cysts can occur to a depth of 1.2 m in the soil and the highest population is found where most of the roots are concentrated.

Females vary in colour from grey-white to yellow, while size varies greatly, also within a species (3 mm to 12 mm). Females of species that do not mate are present (always below ground) from middle December to the end of May. Females and egg pockets are clearly visible to the naked eye if excavations are conducted during this period.

Females of M. prieskaensis (Orange River irrigation area) are clearly visible on the soil surface from June to August. Males are also present though much less apparent. They look like small midges flying near or walking on the soil surface. Male pre-pupae are present just below the soil surface (approximately 2 cm) during April and May. They develop into pupae that are also covered in wax filaments and clearly visible at the same depth during this period.

 

Chemical control

Recent studies (De Klerk, 2010a) indicated effective control of M. prieskaensis when applying the following products:

  • Actara (thiamethozam) 25% suspension concentrate at 2 mℓ/m2.
  • Confidor (imidacloprid) 35% suspension concentrate at 3 mℓ/m2.
  • Telone II (dichloropropeen) 100% undiluted at 15 mℓ/m2.
  • Rugby (cadusafos) 10% emulsion at 25 mℓ/m2.

Actara and Confidor act systemically and need to be applied as a soil treatment shortly after harvest when the annual population of new cysts starts feeding and translocation in vines is still active. As only a certain percentage of cysts annually develop into females, and cysts can survive in the soil for years without feeding (De Klerk, 1980) follow-up treatments are essential.

For control of M. prieskaensis, Telone II as soil fumigant should be applied during the first week of April so that male pre-pupae are also killed. For all other species it should be applied directly after harvest. Telone II is phytotoxic and can therefore only be used when an infested vineyard is to be replaced or a few infested vines are to be removed.

Rugby applied during the first week of April also gave good control of M. prieskaensis male pre-pupae. However, control of females was poor. This indicates that Rugby cannot be used to control the other margarodes species.

Actara and Confidor are registered for the control of mealy bug on grapevine, while Telone II and Rugby are registered for nematode control. None of the above-mentioned products are currently registered for control of margarodes in vineyards and their use for this purpose cannot be recommended.

 

Resistant rootstocks

No rootstock cultivars with resistance to margarodes are available at present. It is known that 99 Richter, 101-14 Mgt and Rupestris du Lot are killed by these insects.

 

Management

Margarodes are easily spread within and between vine blocks by soil cultivation implements. All soil should be washed from tractor tyres or implements when moving them from an infested to a healthy block. It is therefore important that margarodes infestations are identified early and eliminated by fumigating effected areas and replacing the vines. Annual evaluations are needed to decide on follow-up treatments.

Crop rotation using annual plants could be applied to help control the problem. Cysts, however, may remain inactive in the soil without feeding for up to four years. Newly emerging females may then re-infest the newly replanted vines. Crop rotation plants must therefore be alternated over a number of years. Annual investigations are necessary to decide on further treatments.

When developing new ground, virgin soil in particular, it is important to know which plants act as hosts for the various margarodes species. This will allow a pre-planting estimate of a potential infestation and facilitate the appropriate management procedures prior to planting. The Camel Thorn tree (Kameeldoring) (Acacia erioloba), which is widely spread in the Orange River irrigation area and Mpumalanga, was recently identified as the natural host of M. prieskaensis (De Klerk, 2010b). The natural hosts of the other South African margarodes species are currently unkown.

An additional very important management aspect is to correct all growth restricting factors, such as drought, and especially over-irrigation to create conditions for maximum growth. Die-back of vines from a margarodes infestation is much more rapid in plants growing under unfavorable conditions.

 

Acknowledgements

ARC infruitec-Nietvoorbij, Winetech, Dried Fruit Technical Services, South African Table Grape Industry for financial support of various research projects on the margarodes problem. Mr. A. Vermeulen, ARC Infruitec-Nietvoorbij for photographic support for publications and lectures.

 

References

De Klerk, C.A., 1980. Biology of Margarodes vredendalensis De Klerk (Coccoidea: Margarodidae). South African Journal of Enology and Viticulture 1(1): 47 – 58.

De Klerk, C.A., 1983. Two new species of Margarodes Guilding (Homoptera: Coccoidea: Margarodidae) from South Africa. Phytophylactica 15: 85 – 93.

De Klerk, C.A., 1985. Occurrence of South African species of Margarodes Guilding (Homoptera: Coccoidea: Margarodidae) with special reference to vine infesting species. Phytophylactica 17(4): 215 – 216.

De Klerk, C.A., 2010(a). Chemical control of male pre-pupae and adult females of Margarodes prieskaensis (Jakubski) (Coccoidea: Margarodidae) on grapevines. South African Journal of Enology and Viticulture 31(2): 160 – 164.

De Klerk, C.A., 2010(b). Acacia erioloba: Natural host plant of Margarodes prieskaensis in South Africa. Abstract of research poster delivered at the 32nd SASEV Congress, November 2010.

De Klerk, C.A., Ben-Dov, Y & Giliomee, J.H., 1980. Biology of Margarodes capensis Giard (Homoptera: Coccoidea: Margarodidae) under laboratory and controlled conditions in South Africa. Phytophylactica 12(3), 147 – 157.

De Klerk, C.A., Ben-Dov, Y. & Giliomee, J.H., 1982(a). Redescriptions of four vine infesting species of Margarodes Guilding (Homoptera: Coccoidea: Margarodidae) from South Africa. Phytophylactica 14(2), 61 – 73.

De Klerk, C.A., Ben-Dov, Y. & Giliomee, J.H., 1982(b). Redescriptions of three Margarodes Guilding species (Homoptera: Coccoidea: Margarodidae) found on grasses in South Africa. Phytophylactica 14(2): 77 – 83.

De Klerk, C.A., Giliomee, J.H. & Ben-Dov, Y., 1982. Biology of Margarodes capensis Giard (Homoptera: Coccoidea: Margarodidae) under field conditions in South Africa. Phytophylactica 14(2): 85 – 93.

De Klerk, C.A. & Vermeulen, A.K., 2007. Life cycle of Margarodes prieskaensis (Jakubski) (Homoptera: Coccoidea: Margarodidae) on table grapes in the Northern Cape, South Africa. Extended abstract of research poster delivered at the 5th International Table Grape Symposium, November 2007: 139 – 141.

 

– For more information, contact Dr André de Klerk at annette8@telkomsa.net.

 

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