Remarkably, given their predisposition toward more sensational and sordid fare, the local media recently did some decent pieces about biological controls for weeds on prairie farmland. (Admittedly, the spread of noxious plants on perfectly serviceable grasslands might be considered sordid…sadly, not nearly as interesting as Lady Gaga’s meat dress or Lindsay Lohan’s need for a cup of joe while in rehab. But I digress…). One television story I saw focussed on the use of insects to inhibit the spread of leafy spurge (Euphorbia esula), a particularly nasty invasive that was “accidentally” introduced from Europe and now infests over 2 million hectares of land in North America. It’s a devilish yellow-flowered plant that chokes out native and forage crops, and, to make matters worse, the stems produce a milky white substance that is highly toxic to the touch. Cattle that graze on leafy spurge can suffer from blisters in the mouth and throat; animals may even die from ingesting too much of the plant. Most herds stay away from pockets of spurge, but that reduces the viability of the rangeland for the owner. Furthermore, leafy spurge tends to grow along riverbanks and other “difficult” areas, making chemical applications of herbicide impossible. (As well, most rangelands are very large in area, and chemical controls would be far to expensive to use).
So, how and why do biological controls work? Whereas chemical applications are often non-selective (meaning they’ll kill anything that’s green and rooted, even the good plants you want to save) or fail to kill the whole plant (there are many chemical controls that don’t deal with roots), biological controls are always selective, determined to eradicate a certain species of weed. As well, biological controls don’t cause additional problems such as harmful leaching into water sources. A weed such as leafy spurge happens to have a select group of natural predators – insects, usually, but perhaps bacteria or fungi as well. Once it has been determined (by government agencies) that a weed is completely out of control and requires eradication without the use of chemicals, there are two ways to go about it: inoculative or inundative control. The latter entails the widespread release of a pathogen such as a bacteria or fungus that will actually cause disease in the weed and eventually kill it. (This type of control is usually employed within cropland, because it is more resistant to mechanical processes such as mowing and cultivation). The former concerns the use of a small amount of insects that will feed on the weed. It is a slow process that requires the land to be left relatively undisturbed (to encourage the growth and reproduction of the insect “agents”), but it is widely effective.
There are actually a few insects that can properly dispose of leafy spurge, but the top dogs are Aphthona lacertosa, the flea or brown-legged beetle, and Aphthona nigriscutis, the black dot spurge beetle, both really ugly little beasts (in my opinion) that work very hard in both stages of their lives: larvae feed on spurge roots, while adults attack the foliage. The larvae are particularly effective because the damage they cause to the plant roots makes the spurge more susceptible to diseases, increasing the chance that the plant will perish. Female beetles can produce 150 offspring during a growing season, so if conditions are right, a whole lot of spurge plants can be dealt with in a single year. Over time, the noxious plants will become so crippled by the beetles that they will no longer spread.
Obviously, the key to proper use of biological controls is to avoid release of an inappropriate insect (or bacteria or fungus): in the case of the flea and black dot spurge beetles, they don’t eat other plants and so will not harm crops or native species. In this way, too, the beetles will not themselves become invasive – they will be “controlled” by the presence of their food source, the spurge. Achieving such balances is crucial to successful application of biological controls. Insect controls are being tested on other noxious weeds, such as scentless chamomile, yellow toadflax, field bindweed, cleavers, hound’s-tongue, and purple loosestrife, in both large and small areas. Properly monitored and restricted, biological controls can be employed without great cost to either the wallet or the environment.
For more information, see the document “Managing Leafy Spurge with a Hungry Beetle” at Agriculture and Agri-Food Canada’s website, or The Government of Saskatchewan’s article “Biological Control of Weeds on the Prairies.”