Sunday, January 31, 2010

Of Bugs and Herbs: Sharing the Wealth

In my experience, unlike vegetable crops (and flowers), the culinary herbs I grow suffer relatively little from pests. Slugs can be a problem (on basil, dill, and lovage especially), but insect pests are few and far between. The only common one I can think of is the small but rather attractive orange mint moth Pyrausta orphisalis. Fluttering adult moths aren’t uncommon, but I find a caterpillar only every now and then, and I can’t say the mint seems to be much put out. And in a sense, knowing the orange mint moth is feeding away is a comfort, because mints (or at least the members of the genus Mentha – the ones we call ‘mints’) tend to be aggressive and invasive. When a mint runner escapes from the contained bed where I planted it and starts running around the tomatoes, then I consider it a weed. But what would it be called if it escaped from my yard? In the following discussion, I will be relying to two sources for most of my information: the USDA Plants Database and the best source I have on herbs in Canada, Ernest Small’s book “Culinary Herbs” (NRC Research Press, 1997).

When a plant appears in an area where we haven’t seen it before or we think it doesn’t belong, it can attract a variety of descriptors. The USDA Plants Database uses a fairly complex classification of introduced plants that ranges from ‘Waif’ (a plant that has wandered away from the garden, but looks like it needs help to survive) to ‘Garden persistent’ (e.g. peonies that survive for yonks around old homesteads) to ‘Introduced’ (i.e. persisting without help), but recognizes that one can’t always be sure if a plant is any of these classes, including introduced or native. A plant species that successfully reproduces and persists is an area is ‘naturalized’ if it is thought to be introduced relatively recently and ‘native’ if it seems to have been here for a longer period. In Alberta, one must always remember that no plants have been established here for longer than about 9-10,000 years and much of we think of as ‘native’ have been here for an unknown but shorter period.

There are native plants we know are ‘native’ and others that may or may not be, or that may consist of mixes of native and introduced types. Chives is an example of the latter. Although today Allium schoenoprasum is a circumboreal species considered a native in northern parts of both North America and Eurasia, i.e. in two areas that were once part of the ancient ‘supercontinent’ called Laurasia, botantists may recognize several ‘varieties’ (var.). Allium schoenoprasum var. sibiricum, for example, is considered native to North America and A. s. var. schoenoprasum is considered an introduced variety that has escaped from gardens and naturalized in many areas (AK, SK, E Canada, N USA). Is this Eurasian variety an invasive species?

I’m not sure why the plants I use as culinary herbs seem to be relatively immune to insect herbivory – one would expect specialist herbivores to do well on them – but perhaps our herbs, mostly of which seem to have moved to North America recently, have escaped their specialist herbivores. This hypothesis, however, does not explain why my ‘native’ herbs – anise hyssop (Agastache foeniculum), tarragon (Artemisia dracunculus), and chives – also seem to be free of caterpillars, sawflies, aphids, flea beetles, and the other pests that do so well on many other native plants. Perhaps this refuge from herbivory may result in part from the protective function of the essential oils that herbs produce. Although we find these oils tasty in small quantities, in larger doses many are extremely toxic. For example, menthol – that ubiquitous chemical that gives mint its cooling aftertaste – can cause gagging and choking in infants and young children. Field mint (Mentha arvensis), a circumboreal species that grows in Alberta, “may be the world’s most important essential oil plant” (Small 1997) – but the high levels of essential oils make this mint especially dangerous to children.

Whatever the cause, the herbs in my garden have suffered more herbivory from me than from any insect. That is good for me, but surprisingly it is also good for the insect fauna in the Home Bug Garden. The mutual benefit to gardener and bug comes from the entomophilic flowering of most of my herbs. With few exceptions (e.g. tarragon), culinary herbs in flower are a cornucopia for pollinators. Much of this bounty results from the origin of many culinary herbs in two plant families (each of which, confusingly, have two official names): the mints (Lamiaceae, Labatiae) and, the umbellifers (Umbelliferae, Apiaceae). Although both families of plants produce inflorescences with numerous small flowers, the tubular, irregular flowers of the mint family require a bit of tongue action to access the nectar and are of most interest to bees and hover flies. In contrast, the open umbels of the umbellifers are visited by a very wide range of insects. These two groups of plants also differ in how readily they make themselves at home outside the garden. In Alberta, at least, non-native culinary herbs in the mint family appear to be restricted to the Garden Persistent stage at best. Basil, sage, rosemarie, and most thymes succumb to the winter. Lemon thyme, peppermint, and hyssop eke by most winters, and summer savory reseeds itself, but that is the best that can be hoped for. Only the native anise hyssop is persistent and seems able to spread. This is not true of the umbellifers.

Umbellifers (including vegetables like the carrot and parsnip) are easily recognized in flower by their often large, umbrella-like umbels of tiny flowers. The umbel is an easy platform for an insect to land on and the field of small flowers requires little skill or learning to exploit. As a result, umbellifers attract a great diversity of bees, beetles, wasps, butterflies, and flies – a greater diversity of insects than any other plants in the Home Bug Garden except the composites (Compositae or Asteraceae), another family that produces compound inflorescences of small flowers. The only composite herb in my garden, however, is French tarragon – with inconspicuous, wind-pollinated flowers.

The largest and most impressive of the umbelliferous herbs in the HBG is lovage (Levisticum officinale). Lovage requires sun and a lot of space – it is a fully hardy perennial that once established shoots up to 2-3 meters every summer. The leaves have a strong celery-like taste and smell, and the young leaves are tasty in salads, soups, stews, and stir-fries. Once the leaves have lost their light green colour, I find them too tough and too intense, but the young flower clusters oozing with nectar are like celery candy. As you might expect from a plant that attracts numerous pollinators to umbels composed of numerous flowers, lovage sets large numbers of ‘seeds’ (actually each is a dry fruit composed of two mericarps, each with a seed). The seeds are excellent in winter soups, stews, and breads.

Lovage also self-sows with abandon, and so can be a bit weedy. In fact, lovage is ‘naturalized’ in Saskatchewan and much of eastern Canada and the NE USA. Based on how well it is doing in my yard, I suspect Alberta too has fallen to its relentless march. Our other perennial umbellifer herb, Sweet Cicely (Myrrhis odorata), also reseeds freely and is naturalized in BC and ON. This tendency to become part of the local flora seems to be strong in the umbellifers that we use as herbs. Annual members of the family like caraway (Carum carvi), coriander (Coriandrum sativum), and dill (Anethum graveolens) are variously ‘naturalized’ or ‘weeds’, depending on how you feel about them, across much of North America. Caraway is one of the few weeds that infest our quarter section in the bush and all four of these herbs come back year after year in my yard.
I’m still fence-sitting the native-exotic divide, so I haven’t decided how I view these new members of the flora. It was a bit disappointing to spend an hour struggling with the picture-less keys in the Flora of Alberta and then end up with caraway. But, it was an interesting plant before I knew what it was and I don’t feel any need to weed it out. I’m tempted to dither on the divide until I can catch Cane Toads: The Conquest. Perhaps enlightenment awaits in the new 3-D Avatoad.