The word weed can be a hard one to define. Most people accept that a weed is a plant growing where it’s unwanted, something that’s in the way, or that stops the flower or crop you’re trying to grow from growing, or interferes with valued native vegetation. When you think about it that way, it’s clear that one person’s crop or wildflower can easily be, or become, another’s weed. Unfortunately, the corollary is that one person’s pest might be another’s treasure.
|Ngaio, Myoporum laetum.|
Last week I wrote about the common confusion between New Zealand and Tasmanian ngaio, and how in New Zealand the latter is sometimes planted unintentionally in place of the former. Our native ngaio, although prone to self-seeding in gardens and capable of fast growth, is never really a weed here. But it is a pest plant in California, along with some others of our native flora, like pōhutukawa and cabbage trees. This is the story of the rise and fall of ngaio in California, as told in a recent research paper by Jon Sullivan of Lincoln University (Sullivan 2013).
Ngaio was introduced into California as an ornamental tree and widely planted around the middle of last century, mostly using a California-derived cultivar, M. laetum ‘Carsonii’. It’s the 18th most common street tree in San Francisco and is valued for its fast growth and salt tolerance near the sea. From widespread plantings in Southern California, ngaio has spread into many wild and semi-wild communities from Sonoma County southwards to Baja California in Mexico. It forms a dense canopy that shades out other plants and the dry woody centres of the trees are considered a fire risk. The trees even re-sprout after fire or herbicide spray treatment, so they’re hard to get rid of.
The core of Sullivan’s paper describes the effects of the chance introduction of a tiny insect, a kind of thrips (the singular and plural are both thrips). This thrips, Klambothrips myopori, feeds on the leaves and shoots of plants of Myoporum and seems to have got there from Australia, where New Zealand ngaio isn't native, but where other species of Myoporum are. Although it was first described and named from Californian collections, later a small population was discovered on boobialla in Tasmania. And its closest relative is also in Australia, so it’s likely the insect is a dinkum Aussie and a newcomer to California. Most likely, Myoporum thrips got accidentally introduced to California, maybe via the airline routes that converge on Los Angeles. It probably wouldn’t have become established there, except that there were already large populations of planted and weedy ngaio for it to feed upon.
And it got stuck in. It's taken it about five years to kill about half the ngaios in Southern California, and the remaining live ones are looking pretty sick.
Thrips are small slender insects with fringed wings. They mostly feed on plant sap, which they do through mouth-parts that are modified for piercing plant tissue. A thrips infestation typically produces silvery or bronze patches on shoots and leaves, where sap has been drawn out of the cells. Affected young ngaio shoots turn brown and the leaves are distorted. Sullivan found high densities of nymphs and adults on affected trees in California. Other thrips are pollen feeders and are often seen in flowers, where some botanists believe they can be significant pollinators.
This inadvertent spread of thrips to California is an excellent outcome for environmental managers trying to deal with the Californian ngaio outbreak. To introduce a biological control agent these days involves a paper war of bureaucracy, and rightly so, because they can have unintended consequences. But in California, nature—or at least accident—had already done the job. So, all good, you might say.
The success of Myoporum thrips in California seems to support an idea that ecologists call the New Associations Hypothesis. The idea is that when a host-specialised organism—like a thrips that feeds only on Myoporum—comes into contact with a naive host, one that hasn’t been exposed to it before, then all hell breaks loose (for the host). The best-known historical examples are probably the human populations that hadn’t ever been exposed to European diseases, like smallpox and measles. Because long-distance dispersal to islands is a filter that only some organisms get through, it might be that our ngaio and other native plants have evolved in New Zealand without some or all of the parasites and predators that would damage them in their countries of origin. If they’ve let their guard down, so to speak, then introduction of those parasites and predators by human activity could be a disaster for them.
|A branch of boobialla, M. insulare.|
Sullivan, Jon J. (2013-08-24) Inadvertent biological control: an Australian thrips killing an invasive New Zealand tree in California. . DOI: 10.1007/s10530-013-0532-x