The factors responsible for the varying levels of UMF® in mānuka (Leptospermum scoparium) honey

The variability in the level of the non-peroxide antibacterial component (UMF®) of mānuka honey produced in New Zealand was studied. A field analysis confirmed considerable variability existed in the honeys, and a number of hypotheses to explain this variability were proposed and examined. Nectar derived from Leptospermum scoparium (mānuka), was confirmed to be the source of UMF®. The dilution of mānuka honey with nectar derived from other floral sources was found to proportionally reduce the UMF® in monofloral mānuka honey. The utilisation of the thixotropic properties of mānuka honey allowed the degree of dilution in the field samples to be established, and an adjustment of the field results to account for the dilution of UMF® by other honey types revealed all monofloral mānuka honey contains UMF®. However, in the monofloral mānuka honey, significantly different levels of UMF® activity were found to come from reasonably well-defined geographic regions. The cause of the variable levels of UMF® activity in mānuka honey would appear to be the different varieties of L. scoparium being harvested by the honeybees, and the environmental parameters influencing nectar production or another species interacting with L. scoparium do not appear to influence UMF® activity. Three methods were used to establish genetic variability within regions of the North Island of New Zealand that gave rise to the various levels of UMF® activity. Analyses of morphological characteristics, chemotaxonomic essential oil profiles, and population genetics of L. scoparium populations were conducted, and the conclusions that were drawn from each of these were very similar. Two major divisions were identified, each divided into two varieties. The northern division, which contained the core populations from Northland and Waikato, represented the previously described L. scoparium var. incanum and L. scoparium var. linifolium. This division yielded mānuka honey with high UMF® activity. The southern division, which contained the core populations from the Central North Island and East Coast, represented the previously described L. scoparium var. myrtifolium and an unnamed variety. The latter, growing principally on the East Coast, uniquely contains triketones essential oils. The southern division yielded mānuka honey with low UMF® activity. Hybridisation between these varieties will occur, leading to a continuum of UMF® activity in mānuka honey. The data indicated multiple dispersions of L. scoparium to New Zealand from the evolutionary centre of the persistent-capsule Leptospermum group in south-east Australia, and later regional dispersal in New Zealand. From this study two hypotheses were accepted: the variability in the UMF® activity of mānuka honey is due to both the dilution of mānuka honey by other honey types and the variety of L. scoparium harvested