In New Zealand, the fungal endophyte Epichlo€ e festucae var. lolii Latch, Christensen & Samuels (Hypocreales) is widely used to enhance productivity and survival of mature perennial ryegrass, Lolium perenne L. (Poaceae) plants. These endophytes produce alkaloids that reduce damage by insect pests, such as Argentine stem weevil (ASW), Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae). However, knowledge of the effect of these fungi on seedlings is limited. High concentrations of grass-endophyte-derived alkaloids occur in endophyte-infected seed and can be translocated into seedlings to provide early protection from insect herbivory. A glasshouse experiment was conducted in which adult ASW were given a choice between perennial ryegrass cultivar 'One50' without endophyte (Nil) or containing endophyte strains AR1, AR37, or NZ CT , and cv. 'Trojan' containing strain NEA2, in three post-sowing time periods. ASW feeding damage was assessed together with plant survival and dry weight production. Plant survival and dry matter production was reduced in all treatments with mortality highest in endophyte-free and AR37 treatments, especially in the first and second time period. Strongest adult ASW deterrence occurred on seedlings infected with endophyte strains that produce peramine (AR1, NZ CT , and NEA2). Strain NZ CT was virtually undamaged in the third time period whereas damage initially increased in AR1 and NEA2 up until day 50-53 post-sowing and then declined. Endophyte-free and AR37 treatments were significantly more damaged than AR1, NZ CT , and NEA2. This research indicated there is a period of seedling vulnerability to adult ASW attack during germination, even for strains producing peramine. Possible explanations for these results are discussed.