Host assessment and feeding behaviour of adult female Glycaspis brimblecombei on preferred and non-preferred eucalypts

Insect herbivores assess plants by integrating responses elicited by a variety of external and internal cues and signals mediated by physical and chemical traits. We investigated host assessment behaviours of adult females of the Eucalyptus-feeding psyllid, Glycaspis brimblecombei Moore (Hemiptera: Aphalaridae), on 10 species and subspecies (comprising preferred and non-preferred hosts) using video recordings, electrical penetration graph (EPG) technology, and histological examination of tissues surrounding secondary veins. Our video recordings indicated that the shortest first searching time (mean 7.5 s) occurred on preferred hosts. Females placed on novel (non-preferred) hosts jumped off leaves in <10 s. We found that host acceptance occurred surprisingly rapidly after stylet penetration, e.g., between 7 and 11 s on preferred eucalypts, suggesting a strong role for gustatory cues. Our histological studies provided no evidence of physical barriers to phloem access. Females produced eight EPG waveforms, as observed for other psyllids and aphids, but the durations of PG (xylem activity) and PE2 (phloem ingestion), in particular, were unlike those recorded for any other hemipteran. On preferred hosts, phloem ingestion represented 21.5% (mean duration 4 h 18 min) of individual feeding bouts, whereas on non-preferred hosts, there was almost no ingestion from phloem. Xylem ingestion on non-preferred hosts represented 20.1% (mean duration 4 h 1 min) of feeding bouts, presumably to imbibe water. Thus, contrary to widespread belief, female G. brimblecombei did not feed exclusively from phloem even on preferred hosts, but also regularly ingested from the xylem. This is surprising considering that nymphs construct lerps consisting of approximately 16% sugars, and suggests that adults feed from phloem for shorter periods than offspring. Rapid assessment of hosts could also mean that positive stimuli induced by sugars are opposed by negative gustatory stimuli experienced in non-vascular tissues. The most likely source of such stimuli are water-soluble polyphenolic compounds that are widespread in cell vacuoles.

Automated summary

Insect herbivores evaluate plants by integrating responses to external and internal cues mediated by physical and chemical traits. We studied the host assessment behaviors of the Eucalyptus-feeding psyllid, Glycaspis brimblecombei, on preferred and non-preferred hosts using video recordings, EPG technology, and histological examination. Our findings suggest that gustatory cues play a strong role in host acceptance, and that females can feed from both phloem and xylem. Rapid host assessment could be influenced by positive and negative stimuli, potentially caused by water-soluble polyphenolic compounds in non-vascular tissues.