New associations good and bad.

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.

So, what if this thrips ever makes its way to New Zealand?  We now know it can and will happily eat ngaio, and we know it has the potential to hitch rides in aircraft.  It’s yet another pest we need to watch for at the border.  Presumably in Australia, the thrips and the Myoporum have evolved together and the plants have enough defenses not to be wiped out.  But we can see what might happen here by looking at Hawai'i.  There, the Myoporum thrips has already been introduced, again probably unintentionally and perhaps from California, and it’s taken to their native species of Myoporum, M. sandwicense, with gusto.

A branch of boobialla, M. insulare.

If that calamity happens here, we can only hope the thrips prefer the introduced boobialla or Tasmanian ngaio (M. insulare) to our native ngaio, M. laetum.  My guess, and Sullivan’s too, is it’s more likely to be the other way round, because boobialla is likely to have more tolerance to thrips.  Add that to people planting the wrong species, and in the future we might find our ngaio replaced by boobialla almost everywhere.

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  

Wednesday Wildflower: boobialla (Tasmanian ngaio)

Bird-dispersed woody weeds are some of the worst, and many in New Zealand are escapes from horticulture.  Sometimes there are similar native and weedy species that can be confused.

Shoots of ngaio (left) and boobialla.

Ngaio is a case in point.  True ngaio, Myoporum laetum, is a native plant, and a good one to use in revegetation projects because it's easy and quick to grow.  But the problem is there's also an introduced Myoporum from Tasmania, M. insulare, which is also quick-growing.  If the Tasmanian ngaio, often known by its Australian common name, boobialla, is misidentified as the New Zealand native, then it can become widespread in an area by accident.  Because they're both bird-dispersed, both ngaios can spread rapidly.

Plantings between Nelson and Atawhai.

There's a good example of this in Nelson, between the city and Atawhai.  Alongside a new walkway/cycleway, an attractive native revegetation area is flourishing.  Many native trees and shrubs are doing well there, such as Griselinia litoralis, Cordyline australis, Dodonaea viscosa, Phormium tenax, and Coprosma robusta.  Unfortunately, most of the ngaio planted there is the wrong species: boobialla.

New leaves of ngaio (left) and boobialla.

The two are quite similar.  Ngaio has purplish brown new leaves at the tips of the shoots while those of boobialla are green.

Leaves of ngaio (left) and boobialla.

The leaves of ngaio have more obvious glandular dots than boobialla does.

Flowers of ngaio (left) and boobialla.

The flowers are a bit different too: those of boobialla are a bit smaller, more symmetrical, and have fewer and less obvious dots on the corolla.

While we can grumble about an Australian plant taking over the role of ngaio in New Zealand, the New Zealand ngaio isn't wanted everywhere.  It's becoming a weed in California, along with cabbage tree and pōhutukawa.  I'll post soon about an interesting new research paper that deals with this.

Wednesday wildflower: Indian mustard.

Chance can be an important aspect of biological discovery, even very minor discoveries such as I'm describing today, but a prepared mind is an essential complementary aspect to it.  Yesterday I went home from work by a new route, governed by two objectives: to get some eye bolts from the hardware store to support the earthquake-proofing at home, and to collect Veronica hulkeana flowers from the University grounds to photograph.  So I was walking an unusual route between the hardware store and the University when I saw a mustard that looked unfamiliar, growing in a small front yard of a cottage, right up against the house.  

Brassica juncea, Vivian Street Wellington city.

I had a good look over the fence, but I didn't want to trespass, and this morning I went past again, wondering if I should knock on the door and ask permission to collect.  Then I noticed there were half a dozen plants of it, and one or two branches were poking outside the boundary over the footpath: fair game!  So I quickly grabbed a sample and brought it with me to work and spent a happy half-hour keying it out to Indian mustard, Brassica juncea.

Flowering branch, about to be made into a herbarium specimen.

Indian mustard is an Asian Brassica that has been collected only occasionally in New Zealand.  The Flora of New Zealand Vol. 4 describes its distribution as Northland, and the NZPCN website shows several additional records from Auckland City.  These are vigorous plants, up to 80 cm tall.  They're hairless, and have stout stems with wide pith in the centre.  

Brassica juncea: upper leaf

The leaves are bright green; the stem leaves coarsely toothed, the upper ones becoming simple and linear.  The technical details needed for identification include: 

sepals erecto-patent; 

petals bright yellow; stamens 6, the outer spreading (a bit); 

ovary with one vein on each valve, seeds in one row, gynophore absent; style about 4 mm long at this early fruiting stage; stigma capitate.

These plants have coarsely toothed leaves, matching the cultivated variety mizuna.  There seems to be some confusion whether mizuna is B. rapa or B. juncea, and maybe forms of both species are grown under that name.  These ones though are not B. rapa, which, along with B. oleracea and B. napus, has distinctive glaucous and stem-clasping upper leaves.  It's a plant that has become popular in supermarket salads, so I expect more people are growing it these days, and so it's more likely to escape into the wild.  That's good for wild food foragers.

When plant distributions spread southwards, it's tempting to think it might be something to do with global warming.  But weed distributions are governed by all sorts of things, not least by chance, such as the accident of their place of introduction.  It'll be interesting to see if this Brassica spreads further in Wellington and if it's already established in other parts of the city.