Argentine stem weevil larval populations in irrigated pasture

Listronotus bonariensis (Kuschel) is a pest of agriculturally important graminaceous species, with mining larvae that kill the stems of the host plants. In this study, larval populations were measured in spring and summer in irrigated dairy grassland comprising Lolium perenne L. (cv. Nui) and Poa annua L. with and without the endophyte Epichloë festucae var. lolii Latch, M.J. Chr. and Samuels. Larvae were extracted from tillers taken from the swards of these two grass species as well as extracted from turfs. Tiller population densities were used to estimate larval populations on a m-2 basis. Mean larval population densities extracted from the turfs were significantly higher than those from the tillers, but there was no correlation between tiller and turf larval densities. Head capsule -widths of larvae emerging from turf samples showed seasonal effects. The widths of 2nd, 3rd and 4th instar larvae were significantly wider in the spring than the summer, while for the 1st instars the converse was found. Mean head-capsule widths for all instars were significantly greater when extracted from the tillers than the turf ( P