Approches de lutte phytosanitaire, agronomique et génétique contre la cochenille racinaire Stictococcus vayssierei Richard (Hemiptera : Stictococcidea) du manioc (Manihot esculenta Crantz)

Abstract

Cassava (Manihot esculenta Crantz), a source of carbohydrates for more than 800 million people worldwide, is cultivated for its starchy roots that guarantee food security. But its cultivation faces many constraints including various pathogens and pests. Among the pests of cassava, the African Root and Tuber Scale (ARTS) Stictococcus vayssierei (Hemiptera: Stictococcidae) is a sap-sucking insect that colonizes tubers and roots of cassava plants in the humid forest zone of Africa. Infestation by this pest affects plant physiology and severely reduces the productivity of agrosystems in Central Africa, causing yield losses of 60-100 % in the field. The transport and dispersal of this pest in the field is ensured by the ant Anoplolepis tenella (Hymenoptera: Formicidae). These two symbiotic species of insects have an essentially endogenous way of life, which makes the control of their population particularly delicate. Thus, in order to improve cassava production by reducing yield loss, research of alternative control methods to chemical control of this pest is the objective of this work. The general objective is to test the efficacy of plant extracts, cultivation practices, and varietal selection in controlling S. vayssierei populations. Field experiments were carried out using a factorial design. During 12 months with a sampling interval of three months, the number of scale insects, growth parameters, incidence, and severity of the most representative diseases, as well as yield was evaluated. In addition, in the laboratory, the screening of extracts (aqueous, acetone, and methanol) of Thevetia peruviana seeds and the in vivo testing of the extracts against the ant A. tenella were carried out. Similarly, the content of total phenols and flavonoids, total cyanide, total protein, the activity of oxidative enzymes, and the mode of action of S. vayssierei on its ability to produce hydrolytic enzymes of the plant cell wall (cellulase, protease, and amylase) were determined. The results show that the screening of extracts (aqueous and organic) from T. peruviana seeds shows that the extracts contain many families of compounds such as sterols, phenols, and saponins. The plots treated with the aqueous extract (75.67 scale insects per plant ARTS/P) and the plots treated with the acetone extract (72.78 ARTS/P) had on average fewer scale insects than the control plots (93.66 ARTS/P). The field evaluation in 2016-2017 and 2017-2018 of three cuttings portions from the same stem of four varieties of cassava against scale insect showed a high number of scales on lower portions and middle portions compared to upper portions. In general, the number of scale insects is 112.28 and 42.97 ARTS/P on lower portions, 103.04 and 41.78 ARTS/P on middle portions, and 100.86 and 31.94 ARTS/P on upper portions respectively. For the 2016-2017 and 2017-2018 crop years, the lower portions gave a yield of (16.9 and 19.40 t.ha-1), middle portions (15.72 and 19.58 t.ha-1) and upper portions (10.81 and 13.51 t.ha-1) respectively. The results of field screening of six cassava varieties show that the highest fresh shoot weight is produced by the Douma variety (2.69 ± 0.43 kg) as well as yield (23.8 t.ha-1) compared to the Excel variety which produced the lowest yield (14.1 t.ha-1). In addition to ARTS infestation, two diseases, namely mosaic and anthracnose of cassava, were observed in the plots, with varying incidence and severity. Laboratory testing on some biochemical parameters showed that the improved varieties 96/0023 and 92/0057 are characterized by a high potential for production of total cyanide, phenylalanine ammonia-lyase and catalase in the cortex of tuberous roots. The local variety Douma is characterized by a high production of phenols, flavonoids and total cyanide in the pith. The improved variety 8034 and the local variety Miboutou have a high production of catalase and guaiacol peroxidase in the pith. The filtrate obtened by crushing Scale insect produces cellulase, protease and amylase. Seed extracts of T. peruviana, cuttings portions from the same stem of the varieties and the natural defense of the tested varieties proved to be effective and can be exploited in Integrated Pest Management against S. vayssierei. The improved varieties 96/0023, 92/0057 and the local variety Douma may be the most suitable varieties for the control of this pest as well as the lower portions and the middle portions probably because of the better vigour of the plants at the start.