Interaction of the fluctuation of the population density of sweet potato pests with changes in farming practices, climate and physical environments: A 11-year preliminary observation from South-Kivu Province, Eastern DRCongo
Authors: Munyuli Théodore , Kalimba Yajuamungu , Mulangane Emmanuel Kizungu , Mukadi Théodore Tshilumba , Ilunga Meschac Tshibingu , Mukendi Remy Tshibingu 
Published: November 2017

Abstract
Sweetpotato is a major food security crop grown in eastern Democratic Republic of Congo. Its production is however limited due to high prevalence of pests and diseases among other abiotic and biotic factors. A study was designed to aid understanding the knowledge of farmers about pests and their perception about climate variability impacts, as well as documenting the phenology of sweetpotato pests (pest population dynamics) in relationship with weather factors. The paper aimed at determined which climatic factors may be used as best predictors of the different status of pest populations (declines, outbreaks). Farmer based data was obtained using a semi structured questionnaire administered to several of farmers. Population dynamics of sweetpotato pests were monitored year-round from 2005 to 2015 in South Kivu province, eastern DRCongo. Field monitoring (visual counts) observations (population dynamic of different soil-dwelling and surface dwelling arthropods visiting sweetpotato fields) combined with a survey of farmers’ knowledge on sweetpotato pests and their practices in the management of these pests in South- Kivu Province were conducted for 11 years. Monitoring (with field observations and counts) was carried out in fields under different farming practices (monocropping and inter-cropping) in sites located at different altitudes. Similarly, data for climatic factors, for the same period, were collected from Lwiro Research center. Regression models were applied to understand the linkages between environmental factors (rainfall and temperature) and pest population dynamics. The results indicated that different varieties (local and improved ones) of sweetpotato are grown three times (3 seasons) per annum under various cropping systems (sole crop, mixed crops) in various agroecological zones at different altitudes. Various arthropod species visit the crop at its different stages of development including classically known pests (Acraea acerata, Cylas spp.) or as vectors of diseases (Bemisi tabaci, Aphis spp.). The results indicated a high fluctuation in the population density of different pests. The change in the population dynamics were characterized by gradual increase in the populations during rainy seasons followed by decline during dry seasons (hot months of the year). Significant (P<0.05) differences were observed in the population dynamics between sole sweetpotato and mixed sweetpotato intercropping. There was a synchronization of multiple pest generations (biannual, multiannual cycles of reproduction) built up with early rains. The results indicate that rainfall and maximum temperature were critical to the survival and population built up of the pest population. High rainfall in the previous months caused increases in the population density in the subsequent months within a year. The population dynamic (seasonal occurrence) over months and years was likely to coincide with favorable feeding and breeding conditions available within sweetpotato biotopes when temperatures were sufficiently high or after heavy rainfall. For some species, maximum temperature and dry seasons were associated with declines in the pest populations whereas for other species, heavy rainfall was associated with subsequent outbreaks (high populations) in the following months of the years. It is likely that perturbations in temperature/ rainfall patterns may cause serious changes in the pest population, therefore favoring the build-up of multiple generations within a year, thus. Rainfall and maximum temperatures were reliable predictors of key pest species. In fact, regressions analyses indicated that there were significant relationships (P<0.01) between the fluctuation of the population density of different pest species and the variability of climatic factors (mean monthly maximum/ minimum temperature, average rainfall). The population density of different insect pest species varied according to cropping system and to altitude. For example, a significant relationship (P<0.001) was observed between adult aphid population density and average maximum temperature whereas Cylas spp. correlated significantly (P<0.05) with rainfall at high and mid altitudes in both sole and mixed crops. The population density of Acraea acerata was not related to variability in rainfall because the species seemed to occur in number in crops (mono cropping and mixed crops) in marshland areas in June-July and December-February. The virus pressure (measured as the number of leaves symptomatically showing virus attack) followed the population density of whiteflies and aphids. Population trends of other arthropod groups (millipedes, beetles) were not affected by crop variety (clones) or by the altitude or the climate variability but more by the farming practice (mixed or monoculture) implemented by the farmer. It is possible that the resistance or tolerance of some varieties (bio-fortified/vitamin-rich/orange flesh varieties) may be reduced in the future under changing climatic conditions of crop growing in the region with Maximum temperature as the key driver of changes in the population density. The ability to predict the severity of pest populations from mean monthly rainfall/temperature data will provide a significant input into the development of IPM programme for sweetpotato pest species. The data indicates that building resilient sweetpotato crops will require the consideration of various approaches. It is likely that climate change may affect both the pests and vectordiseases and therefore the yield of the crop in eastern DRCongo. Such situation may endanger food security of small scale farmers in the future. Further investigations would be better focus on the understanding on the interacting of climatic, anthropogenic, environmental and soil factors on the pressure of sweetpotato pest in different agro-ecological zones of DRCongo.