Pasture dieback is thought to be caused by several interacting factors.
Although it is not definitive, there are strong indications that pasture mealybugs (Heliococcus summervillei) are a contributing factor, with research showing that mealybug feeding makes plants more susceptible to disease and drought stress.
Other factors that are detrimental to plant health including disease, fungi, insects and nutrition, are also being explored to understand why episodic outbreaks occur.
The symptoms of pasture dieback, particularly the spread of dead patches and the total destruction of affected plants, broadly resemble a disease. There are many diseases of pastures, and a number of different fungi including Rhizoctonia, Fusarium, Bipolaris, Cladosporium, Nigrospora, Gaeumannomyces and others have been isolated from improved pastures affected by dieback.
However, no pathogen species has been consistently found on dieback affected pasture, and attempts by pathologists to produce symptoms of dieback by deliberately infecting pasture grass with these fungi have so far failed.
Diseases usually impact a limited range of species, whereas pasture dieback symptoms have now been observed in a wide range of different grasses (Rogers, 2017).
Most Australian soils are low in phosphorus. Availability of this element is often considered to be a key limitation on pasture growth. The symptoms of phosphorus deficiency in plants include poor root development and development of reddish-purple tips and striping, particularly on the older leaves. Not only are these symptoms consistent with the early stages of pasture dieback, buffel grass leaves with dieback are frequently lower in phosphorus than normal leaves.
There is some evidence that dieback is worse if soil phosphorus is very low (Rogers, 2018). However, paired sampling indicates little correlation between dieback and soil fertility. While many patches affected by dieback have low phosphorus, so do the unaffected pastures in neighbouring paddocks.
Poor nutrition also does not explain the spread of pasture dieback in expanding patches, its sudden appearance in previously healthy paddocks, or the increased prevalence of dieback in areas with the best pasture growth. Trials inducing phosphorus deficiency in buffel grass produced some of the symptoms of dieback (leaf reddening) but, even when combined with drought stress, did not result in grey and brittle dead plants.
The explanation for the apparent correlation between phosphorus deficiency and dieback may lie with sap-sucking insects. While such insects mainly feed on the phloem vessels (carrying carbohydrates to the roots), they also extract nutrients from the xylem vessels (carrying minerals to the leaves). Feeding on the plant can therefore induce symptoms of nutrient deficiency in the leaves, particularly if availability is low.
Nutrient deficiency is therefore associated with pasture decline, rather than dieback, it likely reduces resilience, making plants more vulnerable to damage.
Insects are a key suspect in the pasture dieback mystery. Mealybugs, nematodes, ground pearls and other soil dwelling insects have all been investigated as potential causes of pasture dieback.
The key insect associated with dieback has been identified as the pasture mealybug Heliococcus summervillei (Schutze et al., 2019). It is found across many areas affected by pasture dieback, with high mealybug density frequently associated with more severe symptoms. Pasture mealybug feeds on both leaves and roots. The species is known to have caused major dieback of paspalum in Queensland during the 1920s (Summerville, 1928), then again in 1938 (Brookes, 1978). In New Caledonia, a major infestation resulted in widespread pasture dieback in 1998 (Brion et al., 2004).
Glasshouse trials have found that introducing mealybugs onto apparently healthy buffel grass caused plants to develop symptoms consistent with dieback, while plants have also been observed to recover when treated with insecticides (Hauxwell and McNichol, 2018).
Field inspections have found young mealybugs several metres from dieback patches, with higher populations at the outer edge of the affected area. The expansion of dieback in paddocks is therefore consistent with an increasing and spreading population of mealybugs (Hauxwell and McNicholl, 2018).
While winged male mealybugs do occur, female mealybugs do not need to mate to reproduce. This means a single female can give rise to a large and increasing population. Females cannot fly but can readily disperse by wind, water, vehicles, animals, equipment and on infested hay. This long-distance travel may explain the sudden appearance of dieback on previously unaffected properties.
Young pasture mealybug nymphs (crawlers) are virtually invisible to the naked eye, being less than 0.3 mm long. Their small size means they are easily dispersed by wind and water. Adults are pinkish, 2mm long with a waxy appearance and disperse within a paddock by crawling. Mealybugs can also disperse in water, on infested hay and through transfer on vehicles and animals.
While mealybugs are strongly suspected of playing a major role in pasture dieback, other insects have also been identified at multiple sites, and their potential role is under investigation.
Ground pearls are tiny, soil-dwelling insects related to scale. Pink ground pearls (Eumargarodes iaingi) are a major pest of sugarcane and turf grasses. However, it is white ground pearl (Margarodes australis) that has been found in areas affected by pasture dieback. The adults appear similar to mealybugs, but without their waxy coating.
Nematodes, such as the root knot nematode Meloidogyne spp., have also been found in pastures affected by dieback. Although nematodes can impact pasture establishment and reduce productivity (Mercer et al., 2008), they rarely kill plants.
While mealybugs are a likely cause of pasture dieback, simply feeding on the plant does not fully explain why dieback affected plants become distinctly grey and brittle, or why low insect populations appear able to kill previously healthy plants. It is possible that a toxin is involved, being introduced into the sap during feeding. This would also explain the observation that dieback affected plants develop xylem blockages, a reaction designed to prevent pathogen transmission through the plant. If a toxin is involved, it may explain the severity of symptoms as well as the lack of recovery in dieback affected areas. Current research work is investigating whether pasture mealybugs introduce a toxin into dieback affected pastures.
Dieback does not appear to be caused by disease or nutrient deficiency, although both do occur on affected pasture and may make symptoms worse. Mealybugs, especially the pasture mealybug H. summervillei, are strongly associated with pasture dieback. However, pasture mealybug has been present in the affected area for at least 100 years. Unless a new mutation has increased virulence, such as by injection of a toxin during feeding, it seems likely that a combination of mealybugs with other environmental stresses is responsible.
The factor that has changed most in the last 10 years is the climate. Drought, high temperatures, soil compaction, waterlogging and other stresses make plants far more susceptible to attack by insects and diseases. Other factors include the biological, physical and nutritional qualities of soil. Any or all of these may be increasing the effects of insect pests which have always been present.