The Death of Grass: The biology and role of the mealybug

Understanding the seasonal biology of pasture mealybug is key to informing management of dieback. Mealybug behaviour is strongly seasonal with a narrow window for management of leaf feeding instars. The early instar leaf feeding stages through spring and summer, (especially after rain) are the life stages which cause loss of pasture production and induce classic PD symptoms. Mealy bug is not found in dead grass.
• The pasture mealybug Heliococcus summervillei is found associated with many C4 grasses and sugar cane. Molecular characterisation of pasture mealybug using historical samples and fresh material indicates that since 2016 (Atherton- QLD) a new biotype is found associated with the recent dieback infestations. This biotype is similar to the variant of H. summervillei found associated with sugarcane in Pakistan (described by Brooks).
• In laboratory/screen house pot trials using buffel, bisset, Rhodes and panic grass infestations of mealybugs even at very low numbers can reproduce all observed symptoms of dieback (time lapse video).
• Field sampling at 40 sites across QLD and NSW demonstrates that mealybug is found associated with early-stage symptoms of dieback and the abundance of mealybug numbers is correlated with the severity of dieback symptoms. Mealybug is not found at dieback sites where grass is dead. – this relates to the seasonal behaviour and foliar-feeding of early instars of both sexes.
• Mealybug is a sap sucking pest and saliva injected into the plant host delivers molecules which interfere with plant defence pathways. Metabolic analysis shows within 24 hours of instar feeding, the plant jasmonate pathway is down regulated and salicylic acid pathway upregulated. Additionally, plant sugars are also mobilised. [Data not presented]
• Plants are rendered susceptible to opportunistic infection by a range of secondary pathogens, which cause the typical ashy grey symptoms at death. Fusarium species may play a role in this with Fusarium load affecting prospects for pasture regeneration. [Data not presented]
• Mealybug life cycle in the laboratory is exclusively sexual. Mature females are mainly found in the crown and thatch of grasses during warmer and wetter seasons where they mate and produce young. Adult females turn pink when mated, they do not feed, but disperse by crawling to lay eggs, hiding in thatch, crown or soil- especially in response to cold or dry. Males pupate at third instar, they are winged, extremely small and dispersed by flying and wind. They do not feed.
• Early instars move rapidly onto leaves and feed actively during warmer, moist conditions and spring/summer flushes of grass growth. This is the life stage which causes damage to pasture production. They are dispersed by wind.
• Between Jun- Dec mealybugs retreat to under thatch or underground. White females have been observed over wintering with large egg masses. This may indicate parthenogenic reproduction in the field.
• Systematic sampling using 50 m transects X 10 or 5 sampling points (each 25 x 30 cm squares of soil) and hand lens can be used to efficiently to monitor for field populations of mealybug. The peak season for abundance on leaf is in warmer months after rain.
• Application of the pesticides Imidacloprid (Confidor) or Spirotetramat (Movento) in screenhouse or field trials are effective at controlling mealybug and PD symptoms. Rate trials and residue trials have been conducted in collaboration with Bayer (including new active Sivanto). Pasture recovery is correlated with residual numbers of mealybug. Movento at half the recommended label rate is just as effective. Bayer will develop an application to APVMA for a minor use permit for Movento at half current permit rate.