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In cassava and on Manihot glaziovii, the pest causes stunting, leaf distortion and loss, dieback and weakening of stems used for crop propagation. The insect does not cause any significant damage to its other known host crops/plants which may only serve as a temporary support for 'drifting' populations of the insect that fall on them.
On the basis of the exotic origin and rapid spread of the cassava mealybug in Africa, classical biological control has been the main and most appropriate approach to the pest problem. Among several natural enemies introduced to combat the pest (Herren and Lema, 1982; Lema and Herren, 1985; Herren et al., 1987a; Neuenschwander and Zweigert, 1994), the solitary endophagous parasitoid Apoanagyrus lopezi, specific to P. manihoti, has been the most successful. Herren and Neuenschwander (1991) reviewed the biological control campaign against cassava mealybug in Africa. A. lopezi, collected from South America (Löhr and Varela, 1987; Löhr et al., 1988; Löhr et al., 1989; Löhr et al., 1990), has been the main natural enemy reared (Haug et al., 1987; Haug and Mégevand, 1989; Neuenschwander et al., 1989a, 1989b) and released across the cassava belt in Africa (Herren and Lema, 1982; Lema and Herren, 1985; Bird, 1987; Herren et al., 1987a,b). It was introduced to Nigeria in 1981 and is now established in at least 26 African countries (Ganga, 1984; Herren et al., 1987b; Korang-Amoakoh et al., 1987; Biassangama et al., 1988; Neuenschwander and Herren, 1988; Neuenschwander et al., 1989a, 1989b; Boussienguet et al., 1991; Hennessey et al., 1990; Herren and Neuenschwander, 1991; Neuenschwander and Zweigert, 1994). The biological and ecological impact of A. lopezi has been assessed in several laboratory and field experiments. In some studies, the results indicate a successful role of A. lopezi (Neuenschwander et al., 1986; Neuenschwander and Sullivan, 1987; Sullivan and Neuenschwander, 1988; Goergen and Neuenschwander, 1990; 1992; 1994; Cudjoe et al., 1992; 1993), whereas others are critical of reported success by A. lopezi (Fabres, 1981; Odebiyi and Bokonon-Ganta, 1986; Fabres et al., 1989; Iziquel and Le Rü, 1989; 1992; Le Rü et al., 1990; Souissi and Le Rü, 1997; 1998). Large-scale and sustained field studies have, however, recorded excellent biological control of the pest by A. lopezi (Neuenschwander and Madojemu, 1986; Hammond et al., 1987; Gutierrez et al., 1988a,b; Neuenschwander and Hammond, 1988; Neuenschwander and Gutierrez, 1989; Neuenschwander et al., 1989a, 1989b; van Alphen et al., 1989; Hammond and Neuenschwander, 1990; Neuenschwander et al., 1990; Gutierrez et al., 1993; Chakupurakal et al., 1994; Neuenschwander and Ajuonu, 1995; Neuenschwander, 1996). Ants attending mealybugs for their honeydew are known to defend the pests from natural enemies that would otherwise attack them. They have been observed interfering with biological control of cassava mealybug in Ghana (Cudjoe et al., 1993). It may be advisable to discourage ants in cassava fields if this becomes a problem. The economic impact of biological control of the cassava mealybug, mainly by A. lopezi, has been judged to be excellent (Norgaard, 1988a, b; Zeddies et al., 2001). Nominal costs of the biological control programme 1979-2013 were estimated at US$ 34.2 million, with the peak annual cost of the programme coming to US$ 5.2 million in 1985. The benefit to cost ratio of biological control by Apoanagyrus (Epidinocarsis) lopezi was calculated as at least 199:1. Where the soil is very infertile, however, biological control has been shown to be unsatisfactory, unless it can be complemented by cultural practices such as soil improvement (Neuenschwander et al., 1990; Le Rü et al., 1991; Schulthess et al., 1997) and host-plant resistance (Le Rü and Tertuliano, 1993; Tertuliano et al., 1993; Souissi and Le Rü, 1998). Biological control (particularly using the parasitoid Apoanagyrus lopezi) and the use of resistant varieties to control the pest are briefly described by Calatayud and Le Rü (1997). The coccinellid Hyperaspis notata is associated with the mealybugs P. manihoti and P. herreni on cassava in southern Brazil and the highlands of Colombia. It was brought to Africa to help control the accidentally introduced P. manihoti (Staubli-Dreyer et al., 1997). The parasitoids A. diversicornis, Allotropa sp., and the neuropteran predator Sympherobius maculipennis apparently failed to establish following their releases in Africa (Neuenschwander and Zweigert, 1994).
Organic Chemical Control
Immersion of cassava cuttings in manipueira (a liquid extract from cassava roots) for 60 minutes was found to significantly reduce infestation (Razafindrakoto et al., 1999). Mourier (1997) found that cassava leaves treated with a 1% neem kernel water extract (NKWE) were less attractive to first-instar cassava mealybug than untreated leaves, and those that started feeding died in the second instar. Three NKWE treatments at weekly intervals protected cassava against established early instar nymphs; however, some phytotoxicity was observed.
Cassava contains two significant compounds whose levels increase in response to mealybug infestation. Cyanide content acts as a phagostimulant for the mealybug, whereas rutin has an antibiotic effect on the pest. It was found that the use of mulch and manure increased cassava resistance against mealybug infestation (Tertuliano et al., 1999). The use of resistant varieties to control the pest are briefly described by Calatayud and Le Rü (1997).
Use of manure or other fertilizers can result in a reduction in the mealybug population because improved nutrition results in the production of larger parasitoid wasps with higher fertility levels (Schulthess et al., 1997). Mulch and fertilizer use also enhances the antibiotic properties of cassava against mealybug infestation (Tertuliano et al., 1999).
In 1973, P. manihoti was reported as an introduced arthropod species on cassava in Congo (Sylvestre, 1973; Matile-Ferrero, 1978) and Congo Democratic Republic (Hahn and Williams, 1973). Within a few years after these first reports, the insect became the major cassava pest and spread rapidly through most of the African cassava belt. By the end of 1986, for example, it had reached about 25 countries and covered 70% of the African cassava belt (Neuenschwander and Herren, 1988). In most countries the mealybug caused severe damage by stunting the growth points of cassava plants, sometimes totally defoliating the plants. Storage root yield losses of 84% have been reported (Nwanze, 1982). The pest-induced defoliation reduces availability of healthy leaves which are consumed as leafy vegetables in most of West and Central Africa. After the pest cripples plant growth, weed and erosion problems sometimes lead to total destruction of the crops. Additionally, pest-infested plants produce poor quality stem cuttings for use as planting material. The insect is more abundant and its damage severity is greater in the dry than in the wet season.
Examples of monetary values of damage are given by Norgaard (1988) and Neuenschwander (1990).
Zeddies et al. (2001) analysed the cost benefits of the biological control programme against cassava mealybug in Africa over a period of 40 years. Losses of cassava yield in the year of introduction were estimated at 80%; within 5 years, more tolerant varieties of cassava were cultivated and indigenous predators adapted to a new diet, so reducing annual losses to 20% (in rain forest) to 40% (in highlands and savanna).