INVESTIGATING THE POTENTIAL ROLE OF TERMITES AS PATHOGEN VECTORS IN THE DECLINE OF IRONWOOD TREES (CASUARINA EQUISETIFOLIA) IN GUAM

Abstract

Termites have been found associated with the decline of Casuarina equisetifolia (ironwood), an important agro-forestry species in Guam. While the ironwood tree decline (IWTD) was first reported in 2002, its association with termites as well as with bacteria Ralstonia solanacearum and Klebsiella species (K. oxytoca and K. variicola) was determined in 2010 and 2012 respectively. In 2019, the termite species attacking the ironwood trees were identified as Nasutitermes takasagoensis, Coptotermes gestroi, and Microcerotermes crassus. However, the role of termites in IWTD is not known. Since termites are repositories of bacteria and consume wood, termite foragers might feed on ironwood trees infested with IWTD-associated pathogens and spread these pathogens from infested to healthy trees. We hypothesized that termite workers are vectors for the pathogens associated with IWTD. The bacterial communities of worker samples collected from sick and healthy ironwood trees were analyzed through 16S rRNA gene sequencing. Relation of the bacterial community of worker samples with tree-, plot-and location-related factors was also analyzed. Feeding experiments were performed to investigate if wood consumption by termite workers was influenced by the bacterial load of ironwood. The ability of workers to ingest and sustain R. solanacearum bacteria was determined through feeding experiments. Ralstonia spp. were not detected in any of the N. takasagoensis samples regardless if they were collected from trees with confirmed Ralstonia infection or healthy trees. One C. gestroi sample and none of the M. crassus samples showed Ralstonia. Four N. takasagoensis samples and one C. gestroi sample showed Klebsiella species albeit in low amounts. Tree Health and Site Management impacted the bacterial community of both N. takasagoensis and C. gestroi workers. The bacterial community of N. takasagoensis workers was also impacted by Presence of Ralstonia, Altitude, Plot Average Decline Severity, Plot Average Decline, Proportion of Dead Tree in the Plot, and Proportion of Trees with Termite Damage in the Plot. The feeding experiments showed that termite workers can ingest R. solanacearum bacteria, however, the bacteria are not able to thrive in the body of healthy workers. We concluded that termite workers are not vectors for the potential pathogens associated with IWTD.