Components of resistance to sweetpotato weevils Cylas Puncticollis (Boheman) and Cylas brunneus (Fabricius) (Coleoptera: Apionidae) in Ugandan sweetpotato germplasm
Sweetpotato, Ipomoea batatas (L.) Lam. Family Convulvulaceae, is an important staple food, feed and raw material for both industrial and non-industrial products. Production in Africa is estimated at over 13.4 million hectares and is second only to China in the world. Sweetpotato production is however constrained by the presence of biotic and abiotic stresses. Cylas puncticollis (Boheman) and Cylas brunneus (Fabricius) are the main field pests of sweetpotato in Africa causing yield losses estimated at between 60-100% during the dry season. The management of these pests therefore, calls for the development of interventions that can be replicated and used across Sub-Saharan Africa and elsewhere in the world. Host plant resistance is one of the valid options especially at small-scale resource poor farmer level, where an affordable and effective weevil management is urgently needed. Previous research indicated variation in levels of resistance among Ugandan varieties to Cylas spp. It was hypothesised previously that bio-chemical content of sweetpotato may influence this variation and provide insight into the mechanisms of resistance. The amount and kinds of these bio-chemicals in sweetpotato varieties with differences in susceptibility to Cylas spp. and their relationships to weevil resistance was unknown. The objectives of this study therefore were to: evaluate levels of susceptibility in farmer-selected sweetpotato varieties (improved and landraces) and sweetpotato breeding lines in Uganda to C. brunneus and C. puncticollis in field trials; determine and validate the modes of resistance to C. brunneus and C. puncticollis observed in the field through laboratory feeding and oviposition bioassays; identify and quantify plant chemicals in sweetpotato varieties that may confer resistance to C. brunneus and C. puncticollis; and determine the effect of selected sweetpotato phytochemicals on C. brunneus and C. puncticollis feeding, oviposition and development. Field evaluation of 136 Ugandan sweetpotato varieties was done over two seasons and at two sites (Namulonge and Serere) in Uganda. Using Principal Component Analysis (PCA) of percentage root damage data and external and internal stem base damage rankings among others, sweetpotato varieties that were consistently less damaged across sites and seasons were identified. These were further evaluated in the laboratory using oviposition and feeding bio-assays and analysed for biochemical content and composition using Liquid Chromatography-Mass spectrometry (LCMS) and High Performance Liquid Chromatography (HPLC). Root surface, latex and whole root content of the varieties were analysed. Artificially synthesized root compounds were incorporated into weevil diets and their effect on the development of 1st instar larvae of both species investigated. Additionally the artificially synthesized compounds were smeared on root surfaces to investigate their effect on adult female weevil biology. Laboratory experiments confirmed resistance of seven resistant varieties including New Kawogo, Anamoyito, Dimbuka2, Orurengo 2, Kyebagambire, ARA228 and APA356. These varieties showed protracted development time and reduced adult weevil eclosion compared to the susceptible control varieties (NASPOT 1 and Tanzania). From biochemical analyses, hydroxy cinnamic acids including hexadecyl p-coumaric acid, hexadecyl caffeic acid and octadecyl caffeic acids were identified in sweetpotato latex flesh and surfaces of sweetpotato varieties. The compounds such as hexadecyl caffeic acids and hexadecyl-p-coumaric acids were found to occur in significantly higher amounts in the latex, root surfaces and flesh of the roots of resistant varieties such as New Kawogo compared to Tanzania and NASPOT 1 varieties. There were significantly (P < 0.05 df= 12) more emergent C. puncticollis adults from susceptible varieties compared to New Kawogo in incubation bio-assays and significant reduction (P < 0.05) in the mean oviposition and feeding by C. brunneus and C. puncticollis on this variety compared to susceptible NASPOT 1. Field resistant varieties when screened in the laboratory gave prolonged adult weevil development time for both Cylas spp. and low susceptibility indices while the reverse was true on susceptible varieties. Diet incorporated bio-assays with hexadecylcaffeic acid and hexadecyl-p-coumaric acids significantly (P < 0.05) reduced larval survival weights and mean survival to pupation in the treated periderms compared to controls; and this response was dose dependent for both compounds on both weevil spp. This study clearly showed that weevil resistance in sweetpotato must be more than simply escape, but is quantifiable and manageable and thus capable of being used in the development of resistant varieties through breeding. It recommends that the recent advances in identifying the sweetpotato genome and success in transforming sweetpotato and incorporating weevil resistant Bt toxins by other researchers involved in transgenic research, could be complemented with the identification of quantitative trait loci responsible for variations in phytochemical composition and thus develop ecologically more stable and sustainable resistant varieties, for sweetpotato resistance to Cylas species in Uganda and elsewhere in the world.