The Death of Grass: A New Hope

The life cycle and seasonal behaviour of the pasture mealybug is key to management of PD. Field surveys have found H. summervillei associated with dieback symptoms in 24 varieties/species of pasture grass in QLD, NSW and on sugarcane.
Screen house evaluation and rapid assays have been developed and used to demonstrate the relative susceptibility of 15 pasture varieties/species, which show good alignment with other field trial data. More tolerant varieties have been identified. Further research to determine tolerance mechanisms (with transcriptome analysis) is recommended.
Both seed-borne and rhizospheric-competent fungal endophytes are beneficial in reducing mealybug severity. Further research on plant-endophyte and mealybug interactions in combination with pasture legumes mixes is recommended.
Quantitative sampling for the presence of mealybug and its seasonal biology link to their management and the management of PD. Management strategies include:
o Monitoring for early symptoms and seasonal scouting for mealybugs
o Reducing habitat such as dense thatch and opening the pasture of sunlight and airflow reduces refuges for reproduction. Use rotational/crash grazing or slashing to reduce aerial tillering and to reduce run-off
o Systemic insecticides are effective and target leaf and root feeding populations. Spray emerging populations (2 Emergency permits) for incursion management of small areas and early instars feeding on leaf
o Improve pasture using tolerant species and include legumes, brassicas, forbs; include short term break and forage crops
o Improve soil condition and fertility by addressing nutrient deficiency (P)
• Field surveys and rapid laboratory assays have been used to screen 20 pasture grass varieties; verifying field observations that grass varieties display varying resistance to mealybug.
• Proof of concept research initiated showed the efficacy of rhizopheric fungal endophytes (seedborne producing range of metabolites or rhizospheric associations) in resistance and this links to pasture diversity and resilience. Perennial rye grass without endophytes was more susceptible than American Buffel. Rapid (2 week) laboratory screening assays were developed and shown to determine sensitive and statistically robust differences in susceptibility between grass varieties. American Buffel was shown to be statistically more susceptible than Callide Rhodes, while Gatton Panic and Mekong Brizantha were again statistically less susceptible than American buffel. Differences in development and survival of mealybugs were detects on different varieties.
• 39 sites with differing severity of ‘pasture dieback’ were systematically sampled for plant and rhizospheric fungal endophytes across Qld and NSW. Three sites included replicated trials of pasture mixes planted by AHR.
• Soils with greater resilience have measurably more abundant and diverse beneficial fungi, and samples taken from patches of recovered grasses are associated with beneficial fungi. Burning may be associated with reduced beneficial fungi and increased Fusarium spp.. Presence of legumes in sown pasture mixes (sampled from AHR trials) are associated with greater abundance of the beneficial fungal genera and strains. These same trial plots show greater resilience and fewer mealybugs associated with legumes [AHR data].
• Few natural predators (such as the beetle Cryptolaemus) and parasitoid wasps were recovered from field sweeps and mealybug collections of active dieback sites. Parasitism of 1-1.5% was recorded by 2 species of Chalcid wasps (both new records) and seasonal abundance was monitored by repeat sampling at 8 sites (May 20-Mar 22).