Severe defoliation of cassava by Zonocerus variegatus (L.) can occur every year during the dry season in southern Nigeria, but the effect of such defoliation on tuber yield was not known. Experiments simulating different forms of damage and direct observation on damage by Z. variegatus indicated that tuber yield is reduced significantly by defoliation only towards the end of the dry season after natural leaf regeneration has begun, when its effect is to delay regeneration so that damaged plants lag behind those which are undamaged. Continuous defoliation over six weeks at the end of the dry season caused a mean crop loss of 63%, and simulated debarking a mean loss of 49%. Z. variegatus confined in plots until the end of the dry season caused a mean loss of 36%, losses being less than those caused by simulated defoliation because cropping took place sooner after the end of defoliation. Defoliation during the dry season caused no significant crop loss when damage occurred before leaf regeneration at the beginning of the following wet season. Monitoring of natural defoliation by Zonocerus in successive years showed there were considerable differences in amount and timing. It is concluded that crop loss due to Zonocerus damage is significant only occasionally despite defoliation occuring every year.