Unexpected presence of larvae of Argentine stem weevil, Listronotus bonariensis (Kuschel) (Coleoptera : Curculionidae), in New Zealand pasture thatch

A study conducted in the summers of 1997-1998 and 1998-1999 in Canterbury, New Zealand, revealed that just under one-third of the Listronotus bonarien sis larval population was present in the ryegrass pasture thatch. Hitherto it had been assumed that a much smaller proportion of larvae existed in the thatch, typically in transit from tiller to tiller. Overall, neonate (non- feeding) first-instar larvae comprised 45% of the total larvae extracted fr om the tillers, whereas they made up only 8% of all the larvae extracted fr om the thatch. Conversely, 18% of the total fourth-instar larvae were extra cted from the tillers compared with 27% from the thatch. The frequencies of occurrence of the other instars in the tillers and thatch were intermediat e between the neonate first-instar larvae extremes. The lack of fit of the larval data to a simple ordinal categorical model indicated larval movement into the thatch after the first instar and net migration out of the thatch as late instars. A technique is presented whereby tiller and thatch larval populations can be extracted simultaneously. It is suggested that on the b asis of this study, earlier L. bonariensis larval population data sets coul d be corrected to account for the thatch populations of larvae.