Mark R. McNeill, Russell G. Croy, Chikako Van Koten
Grass grub (Costelytra giveni, Coca-Abia & Romero-Samper), is New Zealand’s most significant pastoral pest. A field trial conducted in 2021 found that larval densities appeared to increase over time, and this result was attributed to first and second instars being missed when soil cores were examined. A new trial was conducted in 2024 to determine if the 2021 results could be replicated. Both trials were sown with six ryegrass (Lolium perenne) – endophyte (Epichloë spp.) combinations. The 2024 trial was sampled for larval populations on three occasions from February to June 2024. Depth in the soil profile, larval weights and head capsule widths, and densities were measured to determine if there was a relationship between populations and ryegrass – endophyte treatments. There was a general increase in densities across sampling dates, attributed to first and second instar larvae being missed in the February sampling. In some plots, larval populations reached damaging populations within six months of pasture establishment while a decline in density was observed in other plots. Overall, 64% of larvae were found in the top third (0–4 cm) of cores across sampling dates, but the proportion decreased on successive sampling dates as third instar larvae moved deeper into the soil. Mean larval weights for the February, April and June samples were 45, 160, and 160 mg, respectively, while mean head capsule widths for the first, second and third instars were 1.21, 1.96 and 3.00 mm, respectively. A small cohort of second instar larvae (c. 11%) were present in June, potentially representing a second adult cohort and subsequent egg laying or as result of poor nutrition. The pattern of C. giveni infestation across the 2024 trial indicates that, overall, the ryegrass – endophyte combinations did not influence the observed larval densities, which is consistent with current knowledge that the endophytes used in the trial generally have negligible impact on larvae.