Biological Control of Leafy Spurge

Leafy Spurge Characteristics

In Saskatchewan, leafy spurge (Euphorbia esula L.) is a noxious weed originally from Eastern Europe that infests pasture and native prairie in a diagonal belt from North Battleford to Estevan.

• It's a perennial weed that reproduces by both seed and vegetative root buds.
• The plant grows to a height of one metre
• It has long, thin, dark green leaves and can be identified from a distance by distinctive yellow-green flowers.

Leafy spurge is very competitive and easily out-competes many forage and native plant species. The juice of the plant is a white, milky latex that may cause mouth and throat blistering in cattle and contact dermatitis in people.

Cattle avoid spurge-infested areas, greatly reducing the livestock carrying capacity of infested range and pastureland. Chemical control of leafy spurge in pastures is often time consuming and expensive whereas biological control of leafy spurge has been a biocontrol success in North America.

Chemical Control Options

For chemical control options for leafy spurge in pasture and hayfields see the current addition of the Guide to Crop Protection.

Biological Control

The most successful biological controls of leafy spurge are beetles from the flea beetle genera Aphothona. The beetles have been used as biocontrol of leafy spurge since they were introduced into Canada in the 1980s. There are five beetle species that have been released in Canada for control of leafy spurge: Aphthona cyparissiae, A. flava, A. nigriscutis, A. czwalinae and A. lacertosa. The first three species have brown or gold bodies while the last two are black-bodied. Two species have been shown to have had a greater impact on controlling leafy spurge than the others: the black dot spurge beetle, A. nigriscutis, has been successful on drier sandy sites and the black spurge beetle, A. lacertose (Figure 3), has been more successful in moister locations.

The Life Cycle of Beetles that Control Leafy Spurge

All five beetle species have similar life cycles, producing one generation per year. The adult beetles emerge in late June or early July and feed near the top of the shoot and at the leaf edges. The beetles mate and the females lay their eggs in groups of 20 to 30 below the soil surface near a spurge root. Females lay through the summer, producing up to 300 eggs per season, resulting in about 150 offspring in a growing season.

The eggs hatch in about three weeks and the larvae burrow through the soil until they encounter a small root, which they mine. As the larvae grow in size they feed on the larger perennial roots of the purge. Development to the third and final instar takes about two months. Feeding then ceases and the larvae construct an overwintering cell in the soil and become dormant. In the spring the larvae resume feeding for about three weeks, pupate in the soil and then emerge as adults. Since the Aphthona beetles spend their entire larval stage underground feeding on the leafy spurge roots they are sensitive to soil types and conditions.

The Impact of Biocontrol

Although the adult beetles feed on the spurge leaves, it is the larvae that are primarily responsible for the biological control of the plant. The larvae damage the roots and root hairs through feeding, and the feeding wounds provide an entry for various disease-causing organisms. The plant's nutrient reserves become depleted, the plant is no longer able to flower and eventually will wither and die.

The success of a beetle colony and its impact on the site can be gauged using a number of criteria. Optimal sites, where there is good beetle establishment, will have the following characteristics in common:

• A count of over 25 beetles per five sweeps after one year (a sweep net is used to collect beetles to determine beetle density);
• A noticeable reduction of flowering spurge after two years;
• A temporary increase in the number of nonflowering stems, which may persist for four years;
• A reduction in the dry weight of spurge and an increase in the dry weight of grasses and other herbaceous species;
• The presence of beetles in a five-pace radius from the release point after one year. It is possible to monitor this progress for four to five years, after which the colony becomes so large that it is difficult to establish boundaries.

Beetle colonies that are optimal should be ready for a harvest of beetles to be taken to other leafy spurge sites after three full years. The target for the biocontrol of spurge is to reduce spurge to a maximum of five per cent cover. Field work has demonstrated that this target can usually be achieved, in the release area, within five years. In one case, a release of these insects on leafy spurge has resulted in a 99 per cent reduction in spurge stand density in one area and a corresponding 30-fold increase in grass biomass after four years.