Integrated pest management for sweetpotato in Eastern Africa

Sweetpotato is an important crop in Eastern Africa. Sweetpotato weevils ( Cylas puncticollis Boheman and C. brunneus Fabricius; Coleoptera: Apionidae) cause damage to roots and vinesthroughout the crop's production area. Other insect pests of sweetpotato are of regional importance. The aim of the research project was to gain insight in the biology and ecology of sweetpotato weevils and, based on this insight, develop pest management programmes on sweetpotato in Eastern Africa.In Chapter 1, the role of sweetpotato in the food systems in Eastern Africa is described. The importance of sweetpotato and the way it is produced can be summarized as follows:1. Sweetpotato is mostly grown as a subsistence crop by resourcepoor farmers, who cannot afford inputs, such as fertilizer and pesticides.2. Production occurs on small plots, rarely more than 0.5 ha in size.3. In some regions sweetpotato is grown for food security, while in others it is one of several staple foods.4. Sweetpotato is grown under diverse agro-ecological conditions, ranging from semi-arid to high altitude temperate climates.5. There is almost no storage of fresh sweetpotato roots; farmers practice "in-ground" storage and piecemeal harvesting. As aconsequence, in many regions sweetpotato crops can be found in the field throughout the year.6. Among biotic production constraints, insect pests are considered to be most serious.7. Sweetpotato weevils (Cylas spp.) are the most widespread and damaging insect pest, but they are not everywhere a key pest. Farmers in areas with long dry seasons consider the weevils a major constraint.8. Because roots are primarily destined for home consumption, quality demands are low and high pest levels are tolerated. Until recently, most research on IPM for sweetpotato took place in the USA and Taiwan. In these countries sweetpotato is produced forthe market and modern inputs are used. Therefore, a research programme was developed taking into consideration the specific conditions in Eastern Africa.The sweetpotato weevil species C. puncticollis and C. brunneus are unique to Africa. C . formicarius is the pest speciesin the USA, Asia and some Caribbean Islands. There are only two records of the occurrence of C. formicarius in continental Africa: Msabaha in coastal Kenya, and Natal Province in SouthAfrica. In Kenya and Uganda C. puncticollis and C. brunneus are of equal importance.Virtually all available information on the biology of Cylas weevils relates to C. formicarius. Therefore, the biology of the African Cylas species was studied under laboratory conditions(Chapter 2). C.puncticollisfemales live longer, develop faster, and have a lower oviposition rate than C. brunneus females. The total egg production per female, sex ratio and proportion of eggs surviving to adulthood were similar for both species. During periods of favourable conditions for sweetpotato weevils, such as dry spells which expose roots for egg laying, C. brunneus populations will grow faster than C. puncticollis because of its higher oviposition rate. C. puncticollis seems to be a better competitor during less favourable conditions due to its longer longevity. Females can survive extended periods when no oviposition sites are available and then resume egg laying when conditions improve.In Chapter 3a review is given on the biology (additional aspects), behaviour, dispersal, and mode of infestation of the sweetpotato weevils and the damage caused. Allmost all published studies deal with C. formicarius. This information is supplemented with results of own observations and experiments on the African Cylas species. In the last part of the chapter, pest management considerations based on knowledge of sweetpotato weevil ecology, biology and behaviour are presented. For example, laboratory experiments revealed that weevils cannot dig down through soil. Under field conditions females will reach roots for oviposition through soil cracks or when roots are exposed above soil. Three management options could be considered: a. to plant deep-rooting varieties. b. to cover exposed roots with soil and to fill soil cracks. c. to adjust planting and harvesting dates, so that roots are not present in the dry season, when soil cracks are common.The next part of the thesis deals with the development of appropriate management components. A first step was to make an inventory of traditional agricultural practices and control strategies. A survey, described inChapter 4,was undertaken in South Nyanza District in Kenya, to document farmers' cultural practices and to learn why they were used. Common farmersf practices, which are expected to have a positive influence on pest control, were selection of healthy planting material, crop rotation and earthening up of mounds during weeding and "piecemeal" harvesting. Farmers' practices, which are expected to have a negative influence on pest control, were poor crop sanitation and adjacent planting of successive crops. Farmerswere not familiar with the life cycle of the insects and their means of dispersal and infestation. By gaining knowledge on the biology and behaviour of insects, farmers will be able to understand the effects of their agronomic practices.Traditionally, in most agro-ecologies in Uganda, farmers growing sweetpotato practise in-ground storage combined with piecemeal harvesting. In Chapter 5 the effect of this indigenous cultural practice on yield and quality loss by sweetpotato weevils is described. Several times during the growing period, between three and seven to twelve months after planting, large roots are removed from individual plants and small roots are allowed to remain in the ground to grow. The overall aim of the practice is to maintain a supply of roots in the ground for the longest possible period. In a series of four trials, once-over harvests at different intervals after planting and a simulated piecemeal harvesting treatment are compared for yield and quality loss caused by sweetpotato weevils. For the once-over harvests, the percentage of damaged roots increased linearly with time, losses ranging between 3% when harvested 334 months after planting (MAP) to 73% at 9% MAP. The total yield and undamaged yield for the piecemeal harvesting treatments, which lasted until 9-9,5 MAP, were comparable to the yields at the optimum harvest times for once-over harvesting at 6 to 7% MAP. In-ground storage combined with piecemeal harvesting is a traditional cultural practice that should not be replaced by prompt harvesting from the point of view of weevil control. It is a sensible practice for small-scale farmers because it guarantees a continuous supply of fresh roots with a low percentage of weevil-damaged roots.In Chapter 6 research on sex pheromones of C. puncticollis and C. brunneus is described. In 1994 the female-produced sex pheromones of the two African Cylas species were identified. In field tests, the synthetic pheromone lures proved to be highly attractive to conspecific males, but the synthetic lures for C. brunneus also attracted significant numbers of C. puncticollis. Work aimed at developing a practical trapping system for C. puncticollis and C. brunneus in Uganda is described. Various designs of funnel, water and sticky traps were compared. A 5-liter plastic jerry can trap was the most appropriate design for effectiveness and practicality. A solution of detergent in water was found to be the most effective trapping agent. Fewer weevils were caught in red traps than in yellow, white, green or blue traps. Catches of C. puncticollis increased when the trap was raised above crop height, but catches of C. brunneus were unaffected. When marked weevils were dropped onto the trap, 36% of C. puncticollis and 23% of C. brunneus were captured, and, ofweevils placed in the trap, 88% and 92% respectively, of the two species remained overnight. Lures for the two species showed no significant loss in attractiveness after eight weeks in the field. Chemical analysis showed 19% of the C. puncticollis pheromone and 72% of the C. brunneus pheromone remaining after this time.In Chapter 7 an overview is given of all potential management components for sweetpotato weevil: cultural control, host plant resistance, biological control, sex pheromones and chemical control. Comparisons to management of C. formicarius are made, the state of affairs is described and the potential of the different management components is evaluated critically.Integrating different cultural control practices has specific potential for managing Cylas weevils, taken into account that the insects have a limited flight activity, a restricted host range and a characteristic mode of entry into the plant. A list of potential practices, their mode of action and their potential for use in Eastern Africa, are presented. The following should be taken into consideration, when selecting cultural practices for testing with farmers: a. in general suitable cultural control practices are site- specific and depend on the agro-ecological and socio-economic conditions; b. some practices would require more labour from farm families, which might be a constraint; c. in densely populated areas some practices might require community effort; d. training farmers on the biology and behaviour of weevils is essential to give them insight into the rationale behind cultural control practices.Use of the sex pheromone of C. formicarius in pest management has been investigated for the last ten years. The sex pheromone has proven to be a good monitoring and training tool. In some countries, mass trapping of males with pheromone traps is considered an essential component of the IPM approach to C. formicarius. Research on sex pheromones for the African Cylas species is at an early stage. Experiments to determine whether pheromone traps for C. puncticollis and C.brunneus can be used to reduce weevil populations and subsequent root infestation are ongoing. Restrictions to the approach could be: the availability and price of pheromone lures, the low percentage of males that is caught per night, and the fact that few males might be able to fertilize most females. All aspects are under research presently.In spite of the considerable effort made to identify sweetpotato host plant resistance to C.formicarius, no germplasm immune to the insect has been identified. Research suggests that sweetpotato clones differ in their level of resistance, but these levels are low and do not stand up under high weevil pressure.Similar results were obtained when germplasm was tested against C. puncticollis and C. brunneus in Africa. The biotechnological approach is presently explored to develop transgenic sweetpotato with proteinase inhibitors, resistant to Cylas spp. Some varieties are less susceptible to weevil damage than others based on pseudoresistance: Short-season varieties can be harvested early before the weevil population has built up. Deep-rooted varieties escape weevil damage because their roots are less accessible for females to lay eggs in.Biological control of Cylas species seems to have limited potential. Few parasitic wasps have been reported to attack Cylas species and none sufficiently suppresses host populations, probably due to the weevil's cryptic behaviour. Two predatory ants species are used as biological control agents for C. formicarius in Cuba. Preliminary studies have been started on predators of the African Cylas species. Beauveria bassiana is the predominant fungal pathogen infecting Cylas species. Field experiments have been conducted in Eastern Africa applying the most pathogenic indigenous strains to the soil. However, control was not successful, probably due to lack of appropriate environmental conditions. Field research on entomopathogenic nematodes of C. formicarius was conducted in the USA, but results were inconsistent. Commercially available formulations of the bacterium Bacillus thuringiengis (Bt) showed no effect on adult and larval mortality of Cylas species.The role for chemical control of Cylas weevils in Eastern Africa is very limited. Weevil control after planting is difficult with conventional spraying, as only above-soil adults are killed, and repeated applications would be necessary to kill newly-emerged adults. Systemic insecticides have been used successfully as a dip to control weevils in planting material. However these chemicals are highly toxic and too expensive for subsistence farmers and small- scale commercial farmers.In conclusion cultural control is presently the most sustainable and promising IPM component for Cylas species in Eastern Africa.In the concluding chapter (Chapter 8) the relevance of the findings are being discussed in the light of the socio-economic and agroecological conditions, under which the sweetpotato crop is grown in Eastern Africa. The chapter starts with an explanation of the IPM concept and its relevance to resource poor farmers in sub-Saharan Africa. Strategies for developing and implementing IPM are compared. A 'farmer-participatory-research', approach seems to be the most appropriate strategy. Criteria conditional for the success of a sweetpotato IPM programme in Eastern Africa are: 1. sweetpotato should be a basic staple foodand/or major cash crop; and 2. Cylas attack is important, which can be expected with low rainfall and a long dry season. Such conditions exist in the proposed pilot area in Uganda. In this area sweetpotato production recently became more important and intensive, because the supply of other staple and/or cash crops declined, and demand for sweetpotato increased. For this pilot area organisational aspects are discussed, pest problem assessed and management components developed. We can not expect quick results in developing an IPM programme for the subsistence crop sweetpotato. A holistic approach is necessary involving research, extension and the farming community and considering the whole cropping and farming system.