Dave Faucette and I have been noticing some differences in naio thip infestations that seem to be related to the morphological characteristics of the naio itself. We are seeing thick pubescent (hairy) leaves with little or no thrip damage compared to the thinner glabrous (hairless) leaves that seem to be easier targets for the thrips. I started talking to other people who have been noticing the same thing; another biologist noted that the deep serrations on the leaves might be another characteristic for recognizing the less susceptible trees.
The naio or myoporum thrip (Klambothrips myopori) is a recently described species that has been found on naio on Hawaii Island since early 2009. They are tiny sucking insects that usually result in galling and leaf curling, sometimes damaging the tree so severely that it eventually dies. Some Australian species and cultivars of Myoporum seem to be resistant to thrip infestation.
Myoporum sandwicense is a really variable species, both morphologically and ecologically. It occurs in almost any habitat you can think of from coastal dry (or wet) to 5000ft in dry, mesic or wet conditions. It can be a dwarf crawling shrub less than 2 feet tall or an enormous 50ft tall tree. They also vary in leaf traits ranging from super shiny, thin and hairless to dull, thick, leathery, hairy leaves. Flowers vary too; some have as few as four petal-like lobes while others have 7, or sometimes 9. The flowers can be white, pink or purple and are often spotted. With all of this variability many taxonomists have suggested that there are different species and varieties but most recent literature suggests that all of the Big Island plants are the same: Myoporum sandwicense var. sandwicense. Other species and varieties occur in Hawaii but are not known on the island. One species or not there is plenty of variety here and it makes sense that some of these combinations of traits might be more or less prone to thrip infestation.
My theory is that the pubescence makes it more difficult for lots of sucking thrips to pierce the leaf surface and feed. This correlation seems to exist in other thrip-plant relationships too. Perhaps by propagating seed from pubescent trees we are selecting for some resistance? So far we have only planted naio that seems to be thrip free but I will keep my eye on them…