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Cerodontha australis Malloch (wheat sheath miner) is a leaf mining fly native to New Zealand and eastern Australia that feeds on grasses and cereals. It is thought to have limited economic impact despite being one of the most widespread and common insects in New Zealand pastures. In this study, data on C. australis were gathered over an 18‐month period from field plots of endophyte‐free Lolium perenne. Observations from pot trials on parasitism and other leaf miner species are also presented. Larval numbers were highest in late spring to early summer. One distinct peak per year of eggs and pupae indicated that this species may not be multivoltine, contradicting previous literature. Damage levels were high in spring/summer, with leaf mining in up to 53% of tillers sampled and, in mid‐summer, extending into the pseudostem in up to 41% of sampled tillers. Measurement of cephaloskeleton lengths confirmed that this species has three larval instars. Generally, only one larva per tiller survives to pupation, indicating density dependent regulation of populations, and larval/ pupal parasitism can reach high levels in summer. This study provides new information on the seasonal biology of C. australis, and its potential impact on L. perenne pastures in Waikato, New Zealand. [ABSTRACT FROM AUTHOR]