Controlling the Invasive Moss Sphagnum palustre at Ka'ala, Island of O'ahu

Abstract

Sphagnum palustre, a bog moss, was purposely introduced to what is now the Ka'ala Natural Area Reserve on O'ahu in the 1960's from the Kohala area of the Big Island of Hawai'i, where it is indigenous. Though S. palustre cannot produce spores, over a 43 fold increase in the size of the core infestation has been observed between 1997 and 2013. Through vegetative reproduction, S. palustre now occupies an area estimated at 17.3 acres. When exposed to three herbicides SAFER Moss & Algae Killer, Lilly Miller Worry Free Moss & Algae Control and St. Gabriel's Moss Killer (SGMK) in a greenhouse setting, S. palustre only responded to SGMK. It contains the active ingredients clove oil and sodium lauryl sulfate. In a growth chamber, we tested extremely low doses of SGMK (the recommended label rate is 25-33% SGMK diluted in water), between 0.3 - 5.0% concentration SGMK solution in water, on healthy S. palustre. Within 48 hours 100% mortality was achieved at concentrations greater than or equal to 1.2% SGMK solution. In the field, we treated S. palustre with a 2.5, 5 and 7.5% SGMK solution. The solution was applied at a rate of one liter solution per meter square of moss. Results nine months post-treatment showed only the highest concentration SGMK tested (7.5%) significantly reduced S. palustre cover over the control group. Even this dose left close to half of the target pest alive (mean survival = 49% + 11% SEM). From this work, we concluded that higher field doses were needed to achieve adequate moss control. To compare the efficacy of manual removal of moss against that achieved using a 10% and 20% solution of SGMK and to evaluate impacts to non-target species, we established 40, one meter square plots within the infestation (10 replicates per treatment plus a control). Impacts to non-target species were measured using stem counts of plants less than one meter. A significant reduction in Metrodsideros polymorpha was evident at the 20% SGMK concentration. Though differences were not significant, reductions in the other three common native species measured (Cibotium spp., Dianella sandwicensis and Vaccinium calycinum) were, on average, higher in the manual removal and 20% SGMK treatments compared to either the control or 10% SGMK group at six months. At 1.5 years, all three treatments effectively reduced S. palustre cover compared to the control group. Given that the 10% SGMK was found to be as effective as the higher dose and exhibited fewer non-target impacts, this is the treatment currently recommended and being actively used for control of S. palustre.