Integrated Pest Management Manual
Leafy spurge (Euphorbia esula L.), a member of the family Euphorbiaceae, is a herbaceous, deep-rooted perennial weed of disturbed lands. Pastures and fields left fallow for long periods, tree rows, waste areas, roadsides, and rangelands or open grasslands are all susceptible to infestation by leafy spurge. Leafy spurge commonly occurs along railroad rights-of-way, water courses, and gullies. It is sometimes found in cultivated lands where infested land has been broken for crop production. Leafy spurge rarely invades fields that have been under cultivation for several years, but long-lived roots can regenerate at any time. The single greatest direct impact of leafy spurge is the reduction of populations of native grasses and legumes and associated ecosystem changes caused by the superior competitive abilities (rapid growth and allelopathic properties) of this species (Steenhagen and Zimdahl 1979).
Indirect impacts of leafy spurge infestations include the loss of food sources for grazing animals caused by competition with native plants in pastures and on rangeland. Leafy spurge infestations may cut pasture production by 50%-75%. Since wildlife and cattle generally avoid grazing in infested areas, carrying capacity may be reduced by up to 75% (Lacey et al. 1984). Leafy spurge is toxic to most grazing mammals, and the milky latex contains substances that act as irritants, emetics, and purgatives for many animals when eaten.
A second indirect impact of leafy spurge is the cost of control; in some cases, the cost of control may exceed the original cost of the land (Lavigne 1984). Due to the extremely deep and hardy root system, control of established leafy spurge populations in uncultivated areas is costly and control measures must take place continuously over several years. Leafy spurge often regenerates when controls are eased. Because of its economic importance, leafy spurge is listed as a noxious weed in many states, with control legally mandated.
Although the competitiveness and toxicity of leafy spurge makes it undesirable, its pest status, as with most introduced species, results from the lack of population suppression exerted by natural enemies (e.g., insect herbivores and diseases). Thus, long term goals in leafy spurge management emphasize biological and cultural controls, although emergency intervention with chemical or mechanical controls may be necessary. Leafy spurge is fairly easy to control within the first two years of establishment. After the third or fourth year, the root system becomes so well developed that the plants are little affected by mowing, cultivation, grazing, or pulling, and herbicides cannot be translocated to the deepest portions of the roots.
IDENTIFICATION AND BIOLOGY OF LEAFY SPURGE
Leafy spurge was introduced into eastern North America from the Old World in the early 19th century. Other introductions were made in the midwestern United States in the late 19th century, probably as contaminants in seed grain (Messersmith and Lym 1983a). Many reintroductions and crosses have occurred in other areas and at other times, resulting in a highly diverse and complex population throughout North America (Lorenz and Messersmith 1981). The extreme plasticity and genetic diversity of this plant has led several researchers to believe that the leafy spurge of North America is a hybrid between two or more Old World spurges (Schaeffer and Gerhardt 1984).
Leafy spurge reproduces by seed and from spreading roots. The roots are deep, woody, and very persistent. Stems are erect, glabrous, branched at the top, and contain a milky sap. Leaves are alternate, broadly linear to narrowly oblong-lanceolate. The inflorescence is a terminal open umbel of greenish flowers, each about 1/8 " high. The petals are fused into a cuplike structure, borne just above the greenish-yellow heart-shaped floral bracts on the top of the stem. See Messersmith (1983) and Eberlein et al. (1982) for complete descriptions and photographs of leafy spurge.
Germination from overwintered seed occurs in early May. True leaves appear 6-10 days after germination. The first pair of true leaves are opposite; later, all leaves are alternate. Stem elongation and vegetative growth occur in mid-May.
Leafy spurge produces vegetative stems from existing roots in late April, making leafy spurge one of the first plants to emerge in spring. Early and rapid growth gives leafy spurge a competitive advantage over most crop and pasture plants.
Yellow bracts form in late May, with maximum display from early to mid- June. Flower development is through mid-June, and the first fully developed seeds occur in early July. Seeds are borne in groups of three within each pod. Seed dispersal is in mid-July, during hot, dry weather. Pods burst violently, scattering seeds up to 15' away from the parent plant. The seeds float and are frequently dispersed by streams.
Leaf loss and late summer dormancy occur during late July to mid-September. Plants renew growth in mid-September with the advent of cooler weather. Several leafy branches are formed off the main stem, which remains leafless. During this period, photosynthesis resumes and additional photosynthates are transported to the root system for storage through the spring.
The root system is extensive, and consists of numerous coarse and fine roots which occupy a large volume of soil. Roots are most abundant in the upper foot of soil, but some roots can extend to a depth of 30'. The root system contains a large nutrient reserve capable of sustaining the plant for years. Root fragments as small as 1/2" long can give rise to new plants. Leafy spurge can withstand repeated mowing and cultivation (Eberlein et al. 1982) due to this well-developed food storage system in the roots. Roots have the ability to regenerate plants from almost any depth.
Leafy spurge usually forms patches that may reach a density of over 200 stems per square yard in sandy soils and higher in heavy clay soils. Patches of leafy spurge usually spread vegetatively at a rate of 1' to 3' per year with allelopathic chemicals secreted by the root to reduce competition (Eberein et al. 1982). Plants emerge in April (from root stocks) or May (from seed) and persist throughout the growing season.
MONITORING AND THRESHOLDS FOR LEAFY SPURGE
Leafy spurge populations are best monitored when the plant is most conspicuous, i.e., when the yellow green flower-like bracts are open in late May to mid-June. Because leafy spurge usually occurs in patches, monitoring usually involves counting or estimating the number of patches per unit area (acre, hectare, etc.). Calculate the average patch size, and count the number of plants per square yard or meter in sample patches. This will give a fairly accurate estimate of the number of plants per area. Careful records should be kept in order to establish a profile of infestation patterns, rates, and treatments.
Leafy spurge can be monitored by aerial infrared imagery using Kodak 1443 color infrared film (for mapping purposes, use large format 9x9 2443 film), a yellow #12 filter, and a film scale of 1:24,000 or larger. Leafy spurge should be in full "bloom" (bract display) and growing vigorously during the second week of June to the second week of July. The image on false color infrared film will be hot pink, which is characteristic of leafy spurge at full bloom and not easily confused with any other plant. Patches as small as 10' x 10' (100 ft2) are easily identified using this method. See Armstrong (1979) for further details.
Economic thresholds for leafy spurge have not been developed. While it is known that heavy infestations can lower range productivity, the cost of mechanical and chemical controls are often considered to be uneconomical in most of the affected areas (Sun 1981). Most ranchers consider spurge to be below injury level if spurge patches do not expand from year to year. In natural areas within a park, leafy spurge management should begin when an infestation is discovered. In areas such as historic or developed sites, or where park lands are adjacent to private or public grazing lands, management techniques should be employed to prevent spurge infestations, and established patches should be controlled to prevent spreading.
NON-CHEMICAL CONTROL OF LEAFY SPURGE
Leafy spurge is difficult to eradicate, but control is possible if a persistent management program is followed. Control strategies should focus on containing the spread of populations by treatment of new populations within their first two years of establishment, and also on concentrating efforts on the advancing edges of established spurge populations. Treatments of well-established plants should receive lower priority. Long-term strategies for weed control depend on biological and cultural controls, while chemical and mechanical controls are useful for short-term suppression. Generally, no one technique will provide adequate control. Currently available biological controls using insects require several years for establishment of the insect, and even longer for control. Most successful programs combine biological control with cultural controls such as timely mowing or reseeding with competitive desirable plants. Suppression of leafy spurge may require altering land use.
Herbivorous insects. Leafy spurge is attacked in North America by only a few generalist native herbivorous insects (Harris 1979). Consequently, natural enemies of E. esula in Europe and Asia have been imported to the United States and Canada. However, hybridization and other factors are believed to have changed the genotype of the North American spurge, and as a result, most natural enemies from its area of origin have had inconclusive results in North America.
The spurge hawk moth, Hyles euphorbiae (L), (family Sphingidae), was introduced into Canada in 1977 (Forwood and McCarty 1980). Populations stabilized at densities considered too low to provide effective control, however, and eventually declined to extinction. Subsequent introductions in Montana and New York have become established and introductions may occur in other states. The spurge hawk moth has one generation per year. Although caterpillars defoliate plants, leafy spurge foliage usually regenerates.
Negative results have been obtained with most introductions. The moth Chamaesphecia tenthrediniformis (Denis & Schoff) was released in Canada in 1970 after promising results in feeding tests. However, all larvae released in the field died without feeding on leafy spurge. The Canadian release of the aphid Acrythosiphum neerlandicum, which is only known from E. esula in Europe, resulted in death on Canadian leafy spurge (Harris 1979).
The stem-and root-boring cerambycid beetle, Oberea erythrocephala (Schrank.), which attacks both E. esula and E. virgata , has been released and established in the western United States (Rees et al. 1986). The main influence on leafy spurge is a reduction in number and vigor of stems produced in the following year. Long-term effects are not known.
The flea beetle, Aphthona flava Guill., feeds as an adult on the leaves of leafy spurge, causing minor damage. The larvae feed heavily on the roots, causing stunting and eventually killing the plant. There is one generation per year. This species has been established in the U.S. and Canada (McClay and Harris 1984, Pemberton and Rees 1990).
The cecidomyid gall midge, Spurgia esulae Gagne, which forms galls over the branch tips that slow growth, stunt the plant, and prevent blossoming, has been evaluated and released (Pecora et al. 1991). This species has several generations per year, making it an excellent potential biological control agent.
Grazing by sheep. Although grazing by livestock has not been recommended in the past, Landgraf et al. (1984) have found that sheep may graze on leafy spurge without ill effects. The diet of sheep can contain up to 50% leafy spurge with no significant difference in weight gain compared to sheep feeding in spurge-free pastures. They conclude that sheep are a viable biological control agent for leafy spurge. Pastures grazed by sheep from May to September for five successive seasons show up to 98% reduction in spurge populations. Utilization of and effects of leafy spurge on lambs and lactating ewes has not yet been quantified. Grazing by sheep may not be an appropriate control measure in natural areas. Some varieties of spurge may be rejected by sheep, and in most cases spurge will regenerate the season after grazing pressure ceases.
If sheep are to be used as a biological control for leafy spurge, the following guidelines from Lacey et al. (1984) should be followed:
should begin in the spring when spurge plants are only
a few inches tall.
sheep grazing rotations so that spurge does not go to
sheep graze after seed set, animals should be held for
five days to allow viable seeds to be passed before
sheep are moved to new pastures.
grazing can be combined with herbicide use around the
fringes of patches for optimal control.
Pathogens. Several plant pathogens have been tested on leafy spurge, including rust fungi, powdery mildews, soil borne fungi, and foliar pathogens. To date, none have been found to be desirable control agents due to wide host ranges (which include domestic crops) or lack of permanent control. Several rusts and Alternaria species have been collected recently in Europe and are undergoing testing at this time (Littlefield 1984, Yang et al. 1990).
In areas where planting of competitive crops is possible, crops such as sudangrass or buckwheat may be utilized. Competitive cropping reduced leafy spurge stands by 50% in the first year of trials, and 80% in the second year when given three cultivations before seeding, and with stubble plowed after harvest (Derscheid 1979).
Elimination of leafy spurge was also achieved in two years following planting of close-drilled forage sorghum or soybeans. A short season of intensive cultivation followed by planting of fall seeded crops of bromegrass reduced leafy spurge populations by 95% (Derscheid 1979). Crested wheatgrass also competes successfully with spurge, but it should be noted that bromegrass and crested wheatgrass are exotic species that are generally considered inappropriate for natural areas. Reinfestation of leafy spurge from seed can be prevented by using soil-building crop rotations. Legumes (such as sweetclover) will prevent establishment by most leafy spurge seedlings (Derscheid 1979).
Use of controlled burning has been attempted in North Dakota and in Wyoming. Although burning has little effect on established plants with deep root systems, fire may be highly effective in reducing seed and seedling viability. Controlled burns in the fall against the wind (burning against the wind results in more complete combustion and hotter fires) resulted in reduced germination rates.
Mowing, especially when used prior to treatments with herbicides, may allow reduced rates of chemicals to provide effective shoot control (Ferrell and Alley 1984b). Hand pulling of leafy spurge while in the bloom stage results in reduced regrowth vigor for two years. Pulling also damages the root, increasing the chance of infection by pathogenic organisms (Maxwell et al. 1984).
Intensive cultivation at 2-3 week intervals will reduce leafy spurge stands by 90% in the first year, and give complete control in 2 years. Similar results have been achieved by cultivation with a duckfoot cultivator every 2-3 weeks or a springtooth harrow each week (Derscheid 1979).
CHEMICAL CONTROL OF LEAFY SPURGE
The use of herbicides provides a quick and easy (albeit expensive on large- scale operations) method of control. Herbicides, applied prior to flowering, give excellent burn-down of top growth but no long-term control of well-established plants. Without a long-term strategy, herbicides often lead to greater problems in the future because of their effect on other plant species, the development of resistance, and the inability to completely eradicate populations.
Consult your regional Integrated Pest Management coordinator to determine which, if any, herbicide is best suited to your integrated pest management program.
To summarize, the following steps are recommended to manage leafy spurge:
1. Monitor leafy spurge by ground checks or aerial surveys using false color infrared film.
2. Determine injury levels based on land usage (local weed ordinances should be acknowledged).
3. Control strategies should focus on containing the spread of populations by treatment of new populations within their first two years of establishment and also to concentrate efforts on the advancing edges of established spurge populations. Treatments of well- established plants should receive lower priority.
4. Use cultural or mechanical controls to reduce small to medium infestations. Consider the use of controlled grazing by sheep as a biological control.
5. Use registered herbicides where appropriate; applications should be timed for best control, and follow-up treatments should be applied when necessary.
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