‘Leafless’ spurge?

Photo by Jill Thomson. Spurge Hawk-moth caterpillar feeds on leafy spurge plant.

Jill Thomson, Saskatchewan Perennial Society

The ideas for many of my articles are often triggered by something I notice on my daily dog walks. When walking by the river in Chief Whitecap dog park at the end of June, I noticed a very large, colourful caterpillar feeding on a leafy spurge plant. This black, red and white caterpillar was identified by my entomology friend, Cedric Gillott, as the spurge hawk-moth (Hyles euphorbiae) caterpillar, which is not native to North America, but has been introduced to try and control leafy spurge (Euphorbia esula). Leafy spurge is a noxious weed, introduced to North America from Europe and Asia. I do find the plant has an attractive appearance; it is a shrub with long, pale blue-green leaves and a fresh, bright green appearance when flowering. Many yellow-green bracts forming a flat topped cluster start to appear in May, about three weeks after the plant emerges. Small, green, inconspicuous true flowers emerge two weeks after the bracts. However, all parts of the leafy spurge plant contain a milky-white latex sap that is exuded when the plant is damaged. This latex is reported to be toxic to grazing cattle and horses, and can cause skin irritation in humans. However, it is the invasive nature of leafy spurge that is most concerning. It can compete aggressively with valuable grazing species, significantly reducing the ability of the pasture to provide feed for livestock.

The plant is very noticeable when blooming and should be reported to the local authority responsible for weed control. Small patches may be removed by spraying with herbicide, but in the case of the patch growing by the river a biocontrol agent would be more appropriate. Hawk moths have been released in the dog park area for several years by the City of Saskatoon Entomologist, Sydney Worthy, along with a flea beetle (Aphthona species). So far, populations of these biological control agents have become established but not in sufficient numbers to control the plants.  Another option for control may be the use of goats and sheep, as they are able to eat this weed, with no detrimental effect on their health.

Populations of the caterpillar have been noticed in Saskatchewan in other years, occasionally in high numbers, unlike the single caterpillar I observed on my walk. This year a large infestation was seen in southern Saskatchewan in the Meyronne area, where thousands of caterpillars defoliated the spurge (T. Mulhern Davidson). It seems that the population levels of caterpillars, and their ability to defoliate spurge plants may be determined by summer and winter weather conditions, but generally consistent defoliation and control of spurge is not achieved.

More effective biocontrol may be provided by leafy spurge flea beetles. Adult beetles eat the above-ground parts of the plant, and lay eggs in the root area. The larvae that emerge feed on the roots of leafy spurge, and this exposes the roots to bacterial and fungal infections that can cause death of the plant. Recently, an article in the agricultural newspaper, The Western Producer (August 3, 2023), describes attempts to increase the populations of the beetles by catching adults in an area with a well-established population, and releasing them in other areas where leafy spurge grows. Volunteers catch enough beetles, in sweep nets, to start a new population in another area infested with leafy spurge. The beetles only feed on leafy spurge so other plants are not damaged.

On researching this article I was surprised to find that leafy spurge has many relatives that can be grown as house plants in our climate. The best known relative is the poinsettia, with its attractive coloured bracts and of course this is a plant that can be toxic to pets, because of the latex sap. Another relative is the firestick plant (Euphorbia tirucalli), which again should be handled with care because of the toxic sap.

Jill Thomson is a retired Plant Pathologist who lives in Saskatoon, where she enjoys gardening with her family, including the dogs.

This column is provided courtesy of the Saskatchewan Perennial Society (SPS; saskperennial@hotmail.com ). Upcoming event – Fall Plant and Seed Exchange and Plant Sale Sunday September 10th 2:00pm at Saskatoon Forestry and Zoo Hall. Check our website (www.saskperennial.ca) for more details.