In previous studies, we found that arabinose, mannose or xylose, when added to artificial diet, was insecticidal to sweet potato whitefly (SPWF) nymphs and adults. It was of interest to investigate the mode of action of these toxic sugars. Their effect(s) on alpha glucosidase (converts sucrose to glucose and fructose) and trehalulose synthase (converts sucrose to trehalulose) activity was determined. There was no substantial inhibition of these two main sugar processing enzymes. Analysis of sugars in honeydew produced by toxic sugar-treated SPWFs revealed reduced glucose levels only in the presence of 5% mannose. In other experiments, SPWFs were fed on diets containing U ¹⁴C sucrose and one or none (control) of the toxic sugars for 24 h and the radioactivity (expressed in dpm) in the whitefly body, the excreted honeydew and the excreted carbon dioxide was measured. Five percent mannose, arabinose or xylose reduced total radioactive content (whitefly + honeydew + carbon dioxide) by 83, 37 and 47%, respectively, as compared to controls, and at a concentration of 10%, by 90, 73 and 81%, respectively. Thus, among the three sugars, mannose was the most effective inhibitor; however, xylose and arabinose are somewhat more toxic to SPWF. Mannose, arabinose and xylose, at the concentrations used in these experiments, inhibited sucrose uptake, i.e., feeding, the excretion of sucrose metabolite(s) in honeydew, and the production and/or release of carbon dioxide. It is possible that these sugars may reduce sugar uptake into the body by inhibiting sugar transport.