Interactions between cassava and arthropod pests

Abstract

Study of the interactions between plants and arthropods and especially of the resistance of plants is an essential component of integrated pest management. In the context of interactions between cassava and its three main pests in Africa (cassava green mite, variegated grasshopper and cassava mealybug), it is observed that knowledge in this field is not very advanced except for the cassava-mealybug model. The study of this began in 1987 and has revealed the following features: (i) the resistance to mealybug developed by cassava is partial and is expressed according to the three categories of resistance, i.e., non-preference, antibiosis and tolerance. The results indicate horizontal, polygenic resistance; (ii) the behavioural characteristics of the insect and the sensorial equipment of its antennae and labium (the site of olfactory and gustatory chemoreceptors) suggest that the chemistry of the surface of the plant probably plays a determinant role in the success of the plant recognition phase; (iii) cassava mealybug principally feeds on phloem sap of which the main component is sucrose and which has a low amino acids content, a high cyanide glycosides content and also contains glycosylated navonoids including rutin. Comparison of cassava phloem sap and honeydew excreted by the insect indicates that all these compounds are ingested and metabolised; (iv) the strong positive correlation established between the degree of antibiotic resistance of various cassava genotypes and their phloem rutin contents suggests that this secondary compound contributes to plant resistance to the mealybug: (v) considerable fluctuatinns in the pest numbers observed each year in the field were linked with variations in phloem rutin contents and these variatinns are affected by cultural practices. It was concluded that: (i) research on plant-insect interactions is complex as it requires a multi-disciplinary approach involving entomologists, biochemists, plant physiologists and plant breeders; (ii) there is a need to develop such studies on the other cassava pests while deepening those on the cassava-mealybug model.

Key Words: Cassava, resistance, mealybug, Phenacoccus manihoti

L'étude des interactions plantes-insectes et en particulier de la résistance des plantes est une composante essentielle de la lutte intégrée. Dans le contexte des interactions du manioc et de ses trois principaux ravageurs en Afrique (acarien vert, criquet puant et cochenille farineuse), on contaste que l'état des connaissances dans ce domaine est globalement peu avancée à l' exception toutefois du modèle manioccochenille farineuse. L' étude de ce modèle a permis de préciser que: (i) la résistance du manioc développée vis à vis de la cochenille est une résistance horyzontale et polygénique;(ii) la chimie de surface du végétal joue un rôle déterminant dans le succès de la phase de reconnaissance de la plante par la cochenille; (iii) que la chochenille est un insecte phloemophage se nourissant d'une sève élaborée riche en composés secondaires (cyanure et rutine); (iv) que la rutine participe à la résistance de type antibiose de la plante; En conclusion est soulignée: (i) la complexité des recherches dans le domaine des interactions plantes insectes car elle requiert une approche multidisciplinaire faisant intervenir des entomologistes, des biochimistes, des physiologistes végétaux et des améliorateurs des plantes; (ii) la nécessité de développer de telles études sur les autres ravageurs du manioc tout en approfondissant celles menées sur le modèle manioc-cocbenille farineuse.

Mots Clis: Manihot, résistance, Pseudococcidae, Phenacoccus manihoti