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 <0.001). There was a significant endophyte effect on head-capsule
widths of larvae collected in summer, but the effect was not consistent
across instars or source. No significant endophyte effect on head capsule
width was found in spring populations from either tillers or turfs. This
study shows that in irrigated dairy pasture, a high proportion of L.
bonariensis larvae can live externally of tillers, presumably amongst the
organic matter around the base of grasses in irrigated dairy pasture, and
that density estimates based only on tiller populations will have
significantly underestimated actual numbers. Having a precise indication
of larval population densities is essential when developing life tables or
determining economic damage threshold levels.