Life history of Stenopsylla nigricornis (Hemiptera: Psylloidea: Triozidae) and phytohormones involved in its gall induction

Many phytophagous insects have an ability to manipulate plant tissue and induce galls, but the mechanism is not yet fully understood. Some insects have multivoltine life cycles, and each generation induces galls on different plant species or different organs in the same host. Such host-use patterns are interesting study subjects to clarify the gall-inducing mechanisms of insects. We focused on a multivoltine and gall-inducing psyllid Stenopsylla nigricornis Kuwayama (Hemiptera: Psylloidea: Triozidae), which is associated with Symplocos lucida Sieb. (Symplocaceae). Based on periodic field surveys in Kyushu, Japan, S. nigricornis is revealed to have a bivoltine life history. Then, we revealed that the spring generation induces galls on leaves, while the autumn generation does so on flower buds and overwintering leaf buds. We also analyzed phytohormones in normal plant tissue, S. nigricornis nymphs, and their galls. As a result, nymphs were discovered to contain much higher concentrations of isopentenyladenosine and its possible precursor, isopentenyladenosine riboside than plant tissues, strongly suggesting that the phytohormone is involved in gall induction by S. nigricornis. Because flower bud galls contained significantly lower concentrations of abscisic acid (ABA) than normal flower bud, the autumn generation nymphs are considered to regulate the ABA level and to promote the earlier opening of host flower buds.