Trails were carried out at Gemmeiza Agricultural Research Sta­tion, Gharbia Governorate to study the effect of maize plant age (30, 45 and 60 days old), as a natural host of the European corn borer Ostrina nub^-tails on larval survival, as well as , the effect of different timings of maize infestation on grain yield obtained . Considerable number of larvae had been found on the plants infested at the age of 60 days and count­ed 6 , 10 , 18 and 35 days after egg hatching . Maize plants during an-thesis are highly susceptible to ECB moths oviposition . An artificial infes­tation made at anthesis stage of maize plant growth resulted in the highest infestation and the lowest grain yield obtained . The average yield reduction relative to the control was 33.3 %, 15.9 % and 7.2 % for maize plants infested with ECB egg masses at the age of 60 , 45 and 30 days old, respectively. Weight of 100 kernels as one of yield compo­nents was the great affected by the borer infestation and consequently caused yield losses .