Insecticidal properties of powdered boldo, Peumus boldus Molina, were evaluated against larvae of fall armyworm, Spodoptera frugiperda (J. E. Smith), and corn earworm, Helicoverpa zea (Boddie). Bioassays assessed development, feeding preferences, and mortality of neonate and third instars. For both species, most mortality (65 and 67.5%, respectively) was obtained with 8% boldo concentration incorporated into an artificial insect diet, and the LC₅₀ and LC₉₀ for fall armyworm were 6.8 and 25.9 g boldo kg^-1 diet and 3.8 and 35.6 g boldo kg^-1 diet for corn earworm. With increased concentration of boldo, larvae were shorter, weighed less, and had anatomical abnormalities, and fewer pupated. Concentrations of 4 and 8% boldo resulted in fewer adults of both species. In feeding preference tests, neonates selected the diet with the least concentration of boldo powder, and larvae fed less with higher concentrations. In tests for insect preference, the greatest concentrations of boldo resulted in the greatest indexes of feeding inhibition and growth, while the least indexes resulted in increases in larval weight and greater efficiency of conversion of ingested food in the diet percentage used to produce new larval biomass.