S. vayssierei is a scale insect found in Equatorial Africa. It feeds on the root system of cassava (Manihot esculenta), and can cause leaf-fall, wilting, tip dieback and ultimately death of cassava plants (
S. vayssierei is a scale insect found in Equatorial Africa. It feeds on the root system of cassava (Manihot esculenta), and can cause leaf-fall, wilting, tip dieback and ultimately death of cassava plants (Ngeve, 2003a; Williams et al., 2010). Plants that develop normally may exhibit small mature tuberous roots that become covered in scales, making them unsuitable for sale (Ngeve, 2003a). S. vayssierei can cause yield losses of up to 100% and could pose a major threat to cassava production in Central Africa if strong control measures are not taken to control its spread (Ngeve, 2003a; Lema et al., 2004). The species could also present a quarantine threat to other parts of the world where cassava is produced, such as parts of southern Asia.
S. vayssierei is the only known hypogeal (below-ground) species in the family Stictococcidae (Tindo et al., 2006). It is a Sternorrhynchan with incomplete metamorphosis. Ngeve (2003a) described the males as rare and the more common adult female as dark-red, circular and flattened. In contrast, Tindo et al. (2006) described the adult females as brown and the first and second instars as purple-red.
The eggs are laid in wax threads secreted underneath the body, where they are protected and hatch into cream-white first-instar nymphs (Ngeve, 2003a).
Three developmental stages have been observed in the females: two immature instars and an adult stage (Tindo et al., 2006). Ngeve (2003a) and Tindo et al. (2006) agreed that there is no pupal stage in the female.
The body size and degree of sclerotization of the dorsal line increases with the development stage. The first instar lacks white waxy secretions, but these secretions are observed on the basal periphery of the second instar and on the dorsum and periphery of the adult females (Tindo et al., 2006).
Authoritative identification requires study of slide-mounted adult females under a compound microscope. Ngeve (2003a) provided a description of S. vayssierei and should be consulted for further details. In the slide-mounted adult female, the dorsal depressions in S. vayssierei are very shallow and sometimes difficult to locate, whereas in all other species of Stictococcus the dorsal depressions are much more conspicuous. (Williams et al., 2010).
In describing the genus Stictococcus, Williams et al. (2010) stated that, although the damage by S. vayssierei has only been reported recently, the species has probably been present but unnoticed for some time because of its subterranean habit. In the literature, S. vayssierei is described as ‘from Cameroon and neighbouring Central African countries’ (Capinera, 2008); however, individual citations have only been found for Cameroon and DRC, and are lacking for neighbouring Central African countries.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
In a seven-year study of S. vayssierei in Cameroon, Ngeve (2003a) stated that this pest is likely to become an epidemic if strong control measures are not taken to control the spread, given that there are ‘poorly enforced quarantine regulations,’ and that there is unrestricted movement of vegetative planting stakes between African countries.
S. vayssieri is found in cultivated land in equatorial climates in Cameroon and DRC. In southern Cameroon, significantly more S. vayssierei were found in nine-month-old cassava cuttings (68 +/- 12.4) than in well-cleared fields, where litter was burned and the ground was ploughed (24 +/- 8) (Dejean and Matile-Ferrero, 1996).
S. vayssierei feeds on the root system of cassava (Manihot esculenta), affecting tuber formation of the plant (Williams et al., 2010); however, there is evidence to suggest either polyphagy or involvement of more than a single scale species (Tindo et al., 2006). Sixteen plant species belonging to 13 families have been identified as hosts of S. vayssierei in the Congo basin (Tindo et al., 2009; see Host Plants/Crops Affected), but this may reflect the involvement of more than one species, as yet unidentified. It is thought that native Dioscorea species may play an important role in maintaining Stictococcus populations during long fallows and in secondary and primary forests. Cassava, an exotic plant in this area, may contribute to the growth of S. vayssierei in fallows less than 8 years old (Tindo et al., 2009).
Young feeder roots of germinating cassava cuttings are attacked by both the nymphs and adults of S. vayssierei. The feeding damage causes premature leaf-fall, wilting, tip dieback and ultimately results in death. Those plants that are not attacked until later develop normally and tuberize; however, they exhibit small mature tuberous roots and become covered in scales, making them unsuitable for sale (Ngeve, 2003a).
S. vayssierei reproduces sexually and was recorded as being viviparous by Ambe et al. (1999); however, Ngeve (2003a) reported that eggs are laid in wax threads secreted underneath the body, where they are protected and hatch into cream-white first-instar nymphs.
S. vayssierei usually appears in clusters or aggregations on the subterranean parts of host plants (Ambe et al., 1999).
S. vayssierei is attended by the ground-nesting ant Anoplolepis tenella. A study by Kuate et al. (2008) it was found that S. vayssierei was the hemipteran most commonly tended by A. tenella in southern Cameroon. Furthermore, Dejean and Matile-Ferrero (1996) reported that S. vayssierei cannot survive without this association.
Field populations of S. vayssierei appear to be influenced by season (Tchuanyo et al., 2000). Ngeve (2003a,b) reported a more severe infestation of S. vayssierei in the dry season compared to the wet, and on flat ground compared to when cassava was planted on ridges. In Cameroon, S. vayssierei is limited to the southern part of the country, which is in the semi-humid zone with a bimodal rainfall pattern (Tchuanyo et al., 2000).
In general, severe infestations occur in lateritic and clayey soils, in fields of depleting fertility and in land where soil preparation is shallow, or planting has been carried out on the flat (Ngeve, 2003a). S. vayssierei is rarely found in newly opened forest farms or monocropped cassava plots (Ngeve, 2003a).
Scale insects move during the crawler stage and as adult males. Newly hatched crawlers move over plants to locate suitable feeding positions. Ants, specifically Anoplolepis tenella Santschi, are the principal agents for the dispersal of S. vayssierei in southern Cameroon (Dejean & Matile-Ferrero, 1996).
Ngeve (2003a) stated that S. vayssierei is a ‘major threat to cassava production in Cameroon and neighbouring Central African countries’. It can cause yield losses in cassava of up to 100% (Lema et al., 2004).
Cassava (Manihot esculenta) is a major source of carbohydrates and extensively cultivated in tropical and subtropical regions. It is a major staple food source in developing countries, particularly in drought conditions, providing a basic diet to over half a billion people (FAO, 1995). Given this important status, it follows that any threat to cassava production, such as S. vayssierei, will have a negative impact on the lives of those who rely on this crop as a source of income and nutrition.
The dorsal depressions in S. vayssierei are very shallow and sometimes difficult to locate, whereas in all other species of Stictococcus the dorsal depressions are much more conspicuous (Williams et al., 2010). S. vayssierei is similar to S. sjostedti, but has more slender bidentate marginal processes; thicker marginal spines; dorsal spines differently shaped; many glands in the pleural zone; a bifid spine on each of segments 3 and 4 of the antenna, and on the dorsal face of the tibia; the tarsus longer than the tibia, and more slender tarsal digitules than S. sjostedti (Richard, 1971). The ventral marginal setae of S. vayssierei are bullet-shaped, as in S. formicarius, but the latter species possesses flower-shaped dorsal setae that are absent from S. vayssierei.
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
In a survey of farmers in cassava-producing zones in Cameroon, Poubom et al. (2005) found that S. vayssierei was only considered to be important in the semi-humid forest zone. However, farmers producing cassava chips in the humid forest zone of Cameroon considered S. vayssierei to be one of the most important constraints in this type of production (Essono et al., 2008).
Cultural control and sanitary measures
Ngeve (2003b) studied the effect of S. vayssierei on cassava production in Cameroon and reported that when cassava was intercropped with maize and groundnuts, infestation by S. vayssierei was more severe compared to when cassava was planted alone. Therefore it is suggested that monocropping is used in areas where the impact of S. vayssierei is severe. However, as mixed cropping is widespread in Africa and used by farmers to insure against crop failure (Ngeve, 2003a), a change in approach for cassava production would be required, especially given that monocropping is one of few control measures available for this pest.
Tindo et al. (2009), working in the Congo basin, suggested that host plants of S. vayssierei from non-crop vegetation should be destroyed and removed to reduce pest infestations and improve cassava yields.
Planting date has also been found to affect incidence and population dynamics of S. vayssierei. In a study carried out by Ambe et al. (1999) in central and southern Cameroon, it was found that higher densities of this pest were generally recorded during the dry season. The start of the main rainy season (April to May) was found to be the most appropriate period for planting cassava, because this meant that the more susceptible growth stages of the crop did not coincide with the period of high pest infestations.
Planting cassava on ridges rather than on the flat results in improved plant vigour and root yield by approximately 22% (Capinera, 2008).
According to Ngeve (2003b), biological control agents such as endomycorrhizae should be studied to determine their usefulness in pest control in Cameroon farming conditions.
Cuttings should be treated with insecticidal bioproducts before planting to reduce the impact of the pest (Ngeve, 2003b).
Host resistance (incl. vaccination)
Lema et al. (2004) investigated six cassava genotypes for their resistance to S. vayssierei. The study included using NPK fertilizer as a component of pest control. Three of the genotypes showed potential because infestations of this pest were delayed on them, with no scales present on them 6 months after planting. The application of fertilizer significantly increased the density of the scale population on an improved clone ‘F100,’ as well as plant height of F100 and a local DRC clone, ‘Kileba’. There was no significant effect on the number of tuberous roots and root yield on F100 or Kileba with fertilizer application.
In a study by Ngeve (2003b), the spread and severity of S. vayssierei from 1990 to 1998 at five sites in the sub-humid forest region of Cameroon was investigated using five cassava genotypes. It was found that the improved clones were more tolerant of S. vayssierei than the local variety.