Case History Cassava Mealybug in Africa

The cassava mealybug, Phenacoccus manihoti Matile-Ferrero, was accidentally introduced from South America to Africa in the early 1970s (Hahn and Williams, 1973; Sylvestre, 1973; Matile-Ferrero, 1978; Leuschner, 1982). Unaccompanied by its native natural enemies, the cassava mealybug spread throughout Africa (Lema and Herren, 1982; Herren et al., 1987), becoming a serious pest of cassava, a major food staple in Africa (Leuschner, 1982; Sylvestre and Arraudeau, 1983). It attacks the roots and leaves of the plant, causing tuber yield losses up to 84% (Herren, 1981; Nwanze, 1982) and nearly 100% loss of foliage (Lema and Herren, 1985). Despite the effectiveness of chemical insecticides, the low crop value of cassava along with socioeconomic constraints and farmer's inexperience in handling and applying insecticides dictated that other control measures be sought in order to provide a safe and economical long-term solution (Singh, 1982; Lema and Herren, 1982). Thus, a biological control program was initiated in Africa (Lema and Herren, 1982, 1985; Herren and Lema, 1982) to complement existing research and developments in host plant resistance.

In the mid-1970s, scientists began exploring South America for natural enemies of the cassava mealybug (Norgaard, 1988). Although a complex of natural enemies was discovered (Lohr et al., 1990), the parasitoid Apoanagyrus (Epidinocarsis) lopezi Desantis (Hymenoptera: Encyrtidae), found in Paraguay by M. Yaseen, was selected for use in a biological release program for cassava mealybug control. With funding from the International Fund for Agriculture Development, the Africa-wide Biological Control Project (ABCP) was initiated at the International Institute of Tropical Agriculture (IITA) headed by Hans Rudolf Herren in 1980 (Herren, 1987; Herren et al., 1987), and in 1981, parasitoids were imported to Nigeria for propagation and field release (Herren and Lema, 1982; Lema and Herren, 1985). Within 3 years after initial releases, parasitoids had spread over 200,000 km2 in southwestern Nigeria, and by the end of 1985, over 50 releases had been made in 12 African countries (Herren et al., 1987). By 1990, A. lopezi was established in 24 countries covering an area of more than 12.7 million km2 (Neuenschwander et al., 1990).

Based on exclusion experiments (Neuenschwander et al., 1986; Cudjoe et al., 1992), follow up field studies (Hammond et al., 1987; Neuenschwander and Hammond, 1988; Neuenschwander et al., 1990), and a computer simulation model (Gutierrez et al., 1987), it was concluded that A. lopezi was responsible for declines in cassava mealybug populations and damage to plants. Currently, P. manihoti has been virtually eliminated in 30 African countries and no longer poses a serious threat to most cassava-growing regions.

The cost and benefit of the release program in Africa, accumulated over 40 years (1974-2013), was estimated to be $49 million and $9.4 billion, respectively, with a cost:benefit ratio ranging between 1:170 to 1:431 depending on the scenario (Schaab et al., 1996). In a worse case scenario, Norgaard (1988) conservatively estimated a cost-benefit ratio of 1:149. The release program, under the direction of Hans Rudolf Herren, not only saved one of Africa's major staple crops and farmers billions of dollars, but demonstrated again that the classical approach to biological control can be a very successful method for controlling serious insect pests in agriculture.

Because of the program's immense success, Hans Herren was awarded the prestigious World Food Prize in 1995.

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