Behavior of Asian citrus psyllid as influenced by density of conspecifics and damage-induced volatiles

The effects of increasing female density on Asian citrus psyllid (ACP) [Diaphorina citri Kuwayama (Hemiptera: Liviidae)] behavior were investigated.

In four-choice assays, female ACP avoided settling on citrus seedlings that were previously pre-infested with a high density of conspecific females. In olfactometer bioassays, headspace volatiles from ten female ACP were repellent to conspecific females. In a subsequent experiment, the upper canopy of citrus seedlings was confined within glass domes and isolated from the lower canopy with a polytetrafluoroethylene guillotine. These glass domes were attached to a two-choice olfactometer such that only the headspace volatiles of the upper canopy were connected to the arms of the olfactometer. For the first treatment, ACP females were placed in the upper canopy. In this case, conspecific females introduced into the olfactometer did not show preference between volatiles from the damaged plant containing female ACP versus a blank control. In a second assay, ACP females were placed onto the lower canopy, and consequently outside the headspace volatiles connected to the olfactometer. In this case, the females introduced into the olfactometer showed a preference for volatiles from damaged, but without the volatiles from conspecific females as compared with a blank control.  We collected headspace volatiles from the upper canopy using the same experimental setup. We found that psyllid damage induced a marked increase in the release of methyl salicylate (a known ACP attractant), as determined by GC-MS. This increase of methyl salicylate was observed in the upper canopy whether psyllids fed on the lower or upper canopy.  

We show that odors of female ACP repel conspecific ACP at high densities of congregation. However, feeding by ACP induces release of specific volatile (methyl salicylate) that attracts conspecifics. We also demonstrated that the release of methyl salicylate by citrus damaged by ACP is systemic. While causing opposing behavioral effects, these two cues appear to be complementary because they are used by foraging ACP at different ecological levels: 1) at the whole plant level for the herbivore induced plant volatile attractant, and 2) at leaf level for the female-female repellent.