Roadside investigations of psyllids, ants and eucs

A roadside survey of young River Red Gums in southeastern Australia reveals widespread damage: their leaves are discoloured, browned and perforated, and many saplings appear dead or dying. Close inspection suggests psyllids, possibly Cardiaspina or Glycaspis, rather than heat or drought alone. Psyllids feed on eucalypts, and many species shelter as nymphs beneath protective coverings called lerps. They excrete honeydew, which attracts Iridomyrmex meat ants; the ants feed on it and in turn protect psyllids from predators. Seeing ants in the trees alongside a psyllid nymph helps explain how these insects interact.

The outbreak’s cause remains unresolved, but the account challenges an initial theory that recent heat and water stress weakened the trees’ chemical defences. Research on River Red Gums found that leaf levels of nitrogen and the defensive terpene 1,8-cineole did not depend on moisture stress. Higher cineole was associated with less insect damage, while water stress was linked to smaller leaves, which may suffer greater proportional herbivory. Young trees’ juvenile leaves could therefore make them more vulnerable, alongside other plant and insect factors. A second site five kilometres away showed similar damage, apparently at a later stage. The observations offer a glimpse of a complex ecological network, while showing why roadside sightings and limited field evidence cannot establish a single cause.