Cyanogenic Potential in Cassava and Its Influence on a Generalist Insect Herbivore Cyrtomenus bergi (Hemiptera: Cydnidae)

Abstract

The hypothesis that cyanogenic potential in cassava is a defense mechanism against arthropod pests is one of the crucial questions relevant to current efforts to reduce or eliminate cyanogenic potential (CNP) in cassava. The generalist arthropod Cyrtomenus bergi, which attacks cassava roots, was used in a bioassay relating oviposition and survival to CNP, concentration of nonglycosidic cyanogens, and linamarase (beta-glycosidase) activity in twelve selfed cassava siblings and their parental clone, which has segregated for different levels of cyanogenesis. Electron microscopic evaluation revealed an intracellular pathway of the stylet of C. bergi in the cassava root tissue to rupture cell walls. This feeding behavior causes cyanogenesis and increased linamarin content in the hemolymph of C. bergi while feeding on a cyanogenic diet. This diet resulted in a significant reduction in oviposition, especially at levels of CNP above 150 ppm (expressed as hydrogen cyanide) on fresh weight basis (or 400 ppm on dry weight basis) in cassava roots. An exponential decline in oviposition was observed with increasing levels of CNP, beginning 12 d after exposure to the cyanogenic diet. Cyanogenic potential and dry matter content showed a positive effect on survival. No relationship was found between concentrations of nonglycosidic cyanogens or linamarase activity in the cassava root and either oviposition or survival. According to our results, there is a significant difference between potentially noncyanogen and high cyanogen clones, but there may not be a significant difference between potentially noncyanogen and low cyanogen clones. Consequently, more frequent outbreaks or higher levels of damage might not be anticipated in potentially noncyanogen cassava clones than that anticipated in low cyanogenic clones. The negative effect of cyanogenesis on oviposition concurrent with a positive effect on survival of this pest is most likely the result of a physiological trade-off between survival and oviposition. The question of whether ovipositional rates could be recovered after a long-term exposure to cyanide remains unanswered.

Cassava Plants with a Depleted Cyanogenic Glucoside Content in Leaves and Tubers. Distribution of Cyanogenic Glucosides, Their Site of Synthesis and Transport, and Blockage of the Biosynthesis by RNA Interference Technology

“…In a similar series of experiments, we determined that the cyanide potential was raised in 6-week-old cassava plants following administration of nitrogen. The cyanide potential in leaves and tubers from the same plant may also differ (de Bruijn, 1973;Riis et al, 2003). To minimize the experimental work to identify a transgenic cassava line that does not contain cyanogenic glucosides in leaves and tubers, it was desirable to be able to define the earliest plant stage and the type of plant tissue suitable for robust screening to avoid growing all transgenic lines to the tuber stage.…”

“…Riis et al (2003) have investigated pest attack on cassava varieties possessing different cyanide potential and concluded that a larger pest outbreak or damage level in acyanogenic clones was not to be expected. Insect generalists such as the grasshopper Zonocerus variegatus and the burrowing bug Cyrtomenus bergi do not accept high levels of cyanogenic glucosides.…”

A Review of Cyanogenic Glycosides in Edible Plants

“…Studies have shown that the levels of cyanogenic glycosides in cassava roots are generally lower than that in the leaves and stems [21,22]. Cassava roots have been reported to contain cyanide content of 10-00 mg/kg of dry mater [23] and the leaves were reported to contain 3-1300 cyanide equivalents/kg of dry mater [24].…”

Burrower Bugs (Cydnidae)

“…Basic biological aspects are well known (García and Bellotti 1980 ;Riis et al 2005b ), as well as information about behavior, population dynamics, and food preference (Riis 1997 ;Riis et al 2005a , b ). Chemical, biological, and cultural control was discussed in several works (e.g., Bellotti and Riis 1994 ;Calcedo and Bellotti 1994 ;Sánchez and Bellotti 1997 ;Barberena and Bellotti 1998 ;Riis et al 2003 ;Jaramillo et al 2005 ).…”