AimsA fungal endophyte, Neotyphodium uncinatum, accumulates N-formylloline, which is toxic to Hemipteran insects, in Italian ryegrass. This study aimed to clarify the dynamics of N.uncinatum and N-formylloline in Italian ryegrass, and their relationship to insect resistance. Methods and ResultsChanges in the density and localization of N.uncinatum and N-formylloline in N.uncinatum-infected Italian ryegrass were examined by real-time PCR and gas chromatography, respectively. Neotyphodium uncinatum multiplied on pseudostems at the flowering stage, and then increased on inflorescences at the ripening stage. On the other hand, N-formylloline accumulated heavily in inflorescences and leaf blades, but lightly in pseudostems at the ripening stage. In field experiments, N.uncinatum-infected Italian ryegrass suppressed the occurrence of Stenotus rubrovittatus, which fed on the inflorescences, but was not effective to Laodelphax striatellus, which do not necessarily prefer a particular plant tissue. ConclusionLocalization of N.uncinatum and N-formylloline were discordant in Italian ryegrass. The N.uncinatum-infected Italian ryegrass was effective to only insects that prefer to feed on particular plant tissues containing N-formylloline. Significance and Impact of StudyOur data implies that the relationship between insect habits and the dynamics of alkaloids in plants is important for the effective use of endophyte-infected crops.