Differential effect of tomatine and its alleviation by cholesterol on larval growth and efficiency of food utilization inHeliothis zea andSpodoptera exigua.

The effect of tomatine on larval growth ofHeliothis zea andSpodoptera exigua was assessed by rearing larvae on diets with different concentrations of the chemical added. When reared from neonates, linear dose-response relationships were obtained for both species, withS. exigua being three times more sensitive to tomatine thanH. zea. Tomatine toxicity was completely alleviated inH. zea by the addition of equimolar cholesterol into the diet; however, inS. exigua some toxicity was maintained. Larvae ofS. exigua that were started on control diet were insensitive to tomatine after five days; larvae started on diet with an EC50 of tomatine and then switched to control diet after five days failed to recover from toxicosis. Larval growth ofH. zea, on the other hand, was affected both at the neonate and third-instar stage, but normal growth resumed when the larvae were transferred to control diet. Tomatine had little or no affect on food consumption, assimilation, or dietary utilization of the food by third-instar larvae ofS. exigua, except at a concentration 10 times the EC50. In contrast, the efficiency of food utilization ofH. zea larvae decreased with increasing tomatine concentrations. Assimilation of the food tended to increase, although not significantly, as tomatine levels increased. Food consumption ofH. zea larvae also increased when the tomatine concentration was greater than an EC50. The addition of equimolar cholesterol to diets with an EC50 of tomatine restored weight gain and nutritional indices values to control values. These results are related to the utility of using tomatine in host-plant resistance programs.