Final report B.PAS.0004: Biology of pasture mealybug and identification of natural enemies

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, is the primary cause of pasture dieback. This project aims to describe the life history of pasture mealybug, provide an understanding of the link between mealybug and pasture dieback, and identify natural enemies.
A detailed understanding of mealybugs, including species identification, life history and identification by molecular markers, was completed. Laboratory assays determined the sexual reproductive strategy of the mealybug, and its relative survival and development on different grass varieties. Diversity within the mealybug population is very low is possibly a new incursion of a variant of H. summervillei.
Life history studies and seasonal monitoring in the field identified the highly seasonal nature of the mealybug and the windows of opportunity for targeting of management interventions. Seasonal scouting is critical to targeting and management of mealybugs . Seasonal management such as slashing should target vulnerable mating adults and foliar-feeding early instars during late spring and summer. Further research on integrating management strategies (slashing, grazing, insecticides) with the seasonal and reproductive biology of the mealybug to quantify impact on susceptible mealybug populations and benefits within farm practice is strongly recommended.
Transcriptome analysis demonstrated the mechanism of disruption of the jasmonic acid/salicylic acid pathway by which feeding by the mealybug renders the grass susceptible to pathogens such as Fusarium, resulting in ‘pasture dieback’. Interactions between the mealybug, its microbial symbionts, and the plant should be studied further to understand these mechanisms more clearly and to identify pathways of resistance or tolerance in pasture varieties.
Analysis of small RNAs identified a number of viruses in plant material that might be amplified as a result of this suppression. No Closteroviruses, which are amplified in Mealybug-induced Pineapple Wilt disease, were found, but the interaction between disruption of the grass immune response and possible amplification of endogenous viruses requires further study.
Natural enemies of the mealybug were identified by collection of live mealybugs and cadavers, and systematic sampling techniques were evaluated. Two parasitoids were identified, one a new record for Australia and one, the most abundant, a new species of genus Parectromoidella (Hymenoptera, Encyrtidae). Parasitoids are widely distributed and appear to have alternative hosts resulting in greater abundance of the wasps in early spring, but they cannot be relied on to control the pasture mealybug at current levels of abundance. Augmentation of beneficial insects is an important area for further investigation.
The confirmation of H. summervillei as the causal agent of pasture dieback is of critical importance to industry. Graziers and agronomists can now move forward with confidence to identify management strategies that suit their farm operations.
The identification of key points in the seasonal and reproductive biology of the mealybug during which damage occurs and management can be targeted is critically important. These findings and the quantitative methods developed, will inform current management and development of proven strategies suitable for a range of farm operating models and systems.