B.PAS.0003 Final Report MLA: Glasshouse assays to determine the role of mealybug in pasture dieback

Pasture dieback causes unhealthy growth and death in a range of introduced and native grasses across Queensland and into northern NSW, resulting in large losses in beef production areas. Pasture mealybug, Heliococcus summervillei Brookes, has been identified as the primary cause of pasture dieback. Screening of potential controls for the mealybug, and methods to conduct those screens, are required to identify insecticide control options. Dose response bioassays are also required to support applications to APVMA for minor use permits. This requires data on efficacy and residues.
Two emergency permits (for Imidacloprid and Spirotetramat (Movento, Bayer) were obtained from APVMA. Methods to conduct standardised bioassays to determine efficacy of insecticides were developed. These were used in artificially-infested screenhouse trials, and 4 replicated field trials to determine efficacy of a range of products, and application rates of Movento. Residue tests were established with Eurofins and Bayer. Data will be compiled and submitted to APVMA to obtain a minor use permit for Movento.
The impact and timing of dieback from initial infestation with mealybug to death of grass were determined in laboratory and screenhouse experiments. Symptoms appear rapidly within a week of infestation, and at very low numbers of mealybugs, possibly as low as 1 bug per plant. Death of the grass was slow to occur. Grasses tolerate high numbers of mealybugs through early summer without immediate death.
The population of mealybugs increases from December through to March, a pattern also observed in the field (B.PAS 0004). The actual death of the grass, with the appearance of ‘dieback’ did not occur until the wetter weather in late summer, when the peak in numbers of early and medium, foliar-feeding instars coincides with wet weather. This is when the combination of mealybugs (which disrupt the plant’s immune responses) and conditions favourable to fungal infection coincide, resulting in the dramatic and apparently sudden death of grass with all the symptoms of ‘pasture dieback’. Mealybug numbers declined rapidly in the screenhouses through March and April as cooler, wetter conditions continued and the amount of live grass for feeding decreased.
These findings are important for both management and conduct of research trials. Firstly, there is a very short window in which trials can be conducted, between emergence of summer populations feeding on leaves and the decline in numbers from late February. Secondly, infested grasses, while symptomatic, do not ‘die back’ until the late summer abundance of mealybugs and wet weather combine to create the apparently sudden ‘dieback’ of grass. Finally, leaves with symptoms (yellow, red or purple streaking) don’t recover: grass must be actively growing to recover. These effects combine to give graziers a window to monitor and then manage mealybugs and reduce ‘dieback’.
Pesticides are only one part of possible dieback management. Costs, withholding periods (subject to residue testing) and impracticality of application over large areas limit their overall use. They remain useful in spot-treatment of emerging spring populations to reduce later, more severe, summer infestation. Pesticides are, however, a useful tool for research.
Monitoring for symptoms and presence of mealybugs from spring (September) is important detect early-season populations. Management such as crash grazing, slashing, or insecticides (if appropriate) can then be targeted to disrupt the mating and feeding populations in summer. Timing of management interventions to maximise efficacy needs further investigation.