Cyanide as a feeding stimulant for the southern army worm, Spodoptera eridania

ABSTRACT. All instars of Spodoptera eridania larvae grow as well or better when cyanide is present in their diet as when it is absent. Concentrations up to 0.05% stimulate feeding in first to fourth instar larvae. Concentrations from 0.1% to 1.0% stimulate feeding in fifth and sixth instar larvae. Three‐day‐old sixth instar larvae pre‐exposed to cyanide are completely resistant to its acutely toxic effects, but previously unexposed larvae suffer reversible symptoms of poisoning when feeding on a diet containing 1.0% KCN. A 1.0% dietary KCN exposure during the sixth instar reduces ecdysis to 17% adult emergence and completely inhibits oviposition. Cyanide concentrations from 0.5% to 1.0% in the diet, although effecting increased growth rates, induce necrotic lesions in larval mid‐gut epithelial cells. Thiocyanate, one of the in vivo cyanide metabolites, at 0.5% in the diet reduces pupation to 23%, delays and reduces adult emergence to 20% and inhibits oviposition. The preferred host plant of S.eridania is the lima bean, Phaseolus lunatus, probably due to its content of the cyanogenic glycoside linamarin. Dietary valine has no effect on the southern armyworm feeding and growth behaviour (Long & Brattsten, 1982) but dietary cyanide does. The lima bean is known to contain up to 31 ppm cyanide in some varieties.