The response of polyphagous pest (Cyrtomenus bergi Froeschner) to cassava cultivars with variable HCN content in root parenchyma and peel

The hypothesis that HCN in cassava is a defence mechanism against arthropod pests is suggestive, but quality and quantity of empirical data is unsatisfactory and scarce. The burrowing bug (Cyrtomenus bergi Froeschner: Cydnidae: Hemiptera) is recorded as a major pest on cassava in the neo-tropics since 1980. Most major cassava pests are leaf feeders whereas C.bergi feeds on cassava roots, introducing soil-borne pathogens which cause rot lesions on the parenchyma. It is a serious pest on cassava in Colombia and Panama. Whereas most major cassava pests are specialists that have co-evolved with cassava and feed readily on leaves with high HCN content, C.bergi is polyphagous feeding on many food crops as well as weeds. As a generalist that has not co-evolved with cyanogenic host plants, it can serve as a model in the study of HCN levels in cassava roots on non-specialists. Root damage caused by C.bergi was evaluated on 125 cultivars from the cassava CIAT core collection and compared to HCN level in root parenchyma and peel. Severe damage did not occur on clones with high HCN level in the parenchyma and approximately 400 ppm (fresh root parenchyma) was the upper limit to C. bergi feeding. Nevertheless many low HCN clones had low levels of damage indicating other possible mechanisms responsible for resistance to C. bergi in low HCN clones. These mechanisms should be investigated. High HCN levels in the root peel, however, did not prevent pest attack, indicating that high HCN content in the root peel will not protect against sucking pests and may have little affect on feeding pests