Phosphorus fertiliser is associated with reduced grass grub (<i>Costelytra giveni</i>) fitness in <i>Epichloë</i> endophyte-infected meadow fescue and perennial ryegrass

BACKGROUND<br>Fertiliser applications are well-established tools in pasture-based agricultural landscapes. This study focuses on the impact of phosphorus (P) fertiliser on grass grub (<i>Costelytra giveni</i>), a major pasture pest. This research investigates the interplay between P, plant growth, and grass grub fitness in <i>Epichloë</i> endophyte-infected perennial ryegrass (Epichloë sp. LpTG-3 strain AR37) and meadow fescue infected with E. uncinata (strain MaxR; AR1017), alongside their endophyte-free counterparts. In a glasshouse trial, plants were grown in P-enriched soil with varying Olsen P levels (9, 18, 28 or 78 mg L⁻¹), and grass grubs were introduced. Their survival and weight gain, and plant performance were measured. In a bioassay, grass grubs were placed in specimen vials with P-enriched soils (Olsen P levels 9, 18, 28 and 78 mg L⁻¹) and provided with identical plant material to assess their diet consumption and weight gain.<br>RESULTS<br>In the glasshouse trial, results highlighted a notable decrease in the survival of grass grub on plants infected with MaxR endophyte, but not with AR37, as well as increasing soil Olsen P levels in both plant species. While grass grub decreased plant performance at the low Olsen P level (9 mg L⁻¹), this effect diminished with increasing P. Likewise, results from the bioassay showed a decrease in diet consumption with increasing soil Olsen P levels. In both trials increasing Olsen P levels correlated with diminished grass grub performance, revealing a nuanced relationship between soil fertility and pest dynamics.<br>CONCLUSION<br>The study underscores the pivotal role of selected Epichloë endophyte–grass associations in mitigating grass grub damage across varying phosphorus levels. This study highlights the potential to integrate P applications for sustainable pest control against grass grub. Further field trials are required to validate these findings.