To assess the impact ofMicrolarinus lareynii(Jacquilin du Val) andM. lypriformis(Wollaston) on puncturevine (Tribulus terrestrisL.), half of the plants in irrigated and non irrigated plots were dusted weekly with 10% Diazinon® insecticide to protect them from weevil attack. The mortality, flower, and seed production, and stem lengths (radii) of insecticide-treated and untreated plants in these plots were then compared throughout the 1974 growing season.Water stress apparently was the principle cause of early-season plant mortality; however, weevil attack caused a 60% reduction in flower production on surviving plants in nonirrigated subplots. Moreover, only half of the flowers on these unprotected plants in nonirrigated sub-plots produced fruit late in the growing season. The mean number of seed/fruit from August onwards also was significantly reduced by weevil attack. Stem growth was exponential during June and July in irrigated plots, but was slower and ceased 2–3 wk earlier in nonirrigated plots where the suppressing effect of weevil attack was especially evident.