Potential for the establishment of Cylas punticollis Boheman (Coleoptera: Apionidae) as a pest of sweetpotato in Lesotho

Abstract

Sweetpotato, Ipomoea batatas (L.) Lamarck was introduced into Lesotho in 1992 in the hope that it would help alleviate poverty levels. Efforts are being made to learn the potential constraints to optimal production of this crop. Insect pests especially the sweetpotato weevils, Cylas species are a major production constraint worldwide. Hence the main objective of the study was to predict if these pests have potential to establish themselves in Lesotho. Two Cylas species, C. formicarius and C. puncticollis are present in South Africa and the latter is found in the northern Free State and the Eastern Cape both of which border the central and southern lowlands of Lesotho respectively. Cylas puncticollis was chosen as the subject of this study because of its potential spread into Lesotho. Thermal requirements (lower development threshold and degree-days) of this pest were calculated in the laboratory by studying the effects of temperature on its development and survival at six constant temperatures (16°C, 19°C, 24°C, 26°C, 31°C and 36°C). The photoperiod was maintained at 12L:12D for all temperatures, but RH was not controlled. Thermal requirements (r and k) of this pest species were estimated for all the immature stages and for the total life-cycle using the linear regression method. The estimated lower temperature threshold (r) of the total development of the pest lies between 8°C and 12°C and the thermal constant (k) between 360°D and 380°D. The thermal needs of this pest obtained from the laboratory work were used to predict the potential for its establishment in Lesotho as well as determining the possible areas of distribution if it invades Lesotho. Actual soil temperatures to which the pest would be exposed to in Lesotho were recorded for a year. Both the calculated thermal needs of the pest and the field-recorded temperatures were used in the degree-day model to predict potential establishment of this pest. The second approach, climate matching in Geographical Information System (GIS) used the bio-climatic profile of C. puncticollis calculated from the known areas of its distribution in both South Africa and Swaziland. The bio-climatic profiles of the two countries were matched to the climatic conditions of Lesotho to predict the potential for its establishment. The two approaches, linear degree-day model and climate matching approach revealed that Cylas puncticollis is a potential pest in Lesotho. The former predicted the occurrence of this pest throughout the whole country with a maximum of eight generations per year being possible in the lowlands. Fewer generations (two to three) were predicted for the highlands and foothills agro-ecological zones, which are colder than the lowlands. The climate matching approach also confirmed the prediction although according to this method a patchy distribution of the pest was predicted. A survey was then carried out in Lesotho, first to determine if Cylas species were already present in Lesotho, secondly to identify any other pests of sweetpotato and lastly to determine other possible production constraints other than insect pests. The survey was conducted in the form of questionnaire and field sampling. Cylas species were neither documented by the farmers who were interviewed nor by the field sampling. Numerous common pests of sweetpotato were recorded during the sampling survey. These included the following leaf-feeding pests: Bedellia somnulentella Zeller, Acraea acerata Hewitson, Agrius convolvuli Linnaeus and locusts and grasshoppers. The root pests that were recorded were mole-rats, Blosyrus sp. and millipedes (Narceus sp.). According to the sampling carried out in Lesotho there were no insect pests that could be rated as major pests as yet. Sweetpotato farmers did not consider insect pests as an important production constraint for optimal yield of the crop. The major constraint was found to be lack of planting material, which contributed towards a slow adoption of the crop throughout the country.