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Spurgia esulae
(Diptera: Cecidomyiidae)

by Rich Hansen, USDA-APHIS-PPQ, Forestry Sciences Lab, Montana State University, Bozeman, MT 59717-0278.

Leafy spurge is an Eurasian perennial that was introduced into North America in the 19th century. It infests several million hectares of rangelands and riparian areas in the United States and is a serious pest across the northern Great Plains where it displaces desirable grasses and forbs normally consumed by foraging cattle. Cattle and horses usually avoid leafy spurge, but should they eat it, its milky latex may cause sickness and even death. Annual direct and indirect economic losses due to leafy spurge infestation in Montana, South Dakota, North Dakota, and Wyoming are estimated to exceed $120 million. In addition, leafy spurge forms monocultures that often displace native plants and degrade wildlife habitats.

Leafy spurge has a number of biological characteristics that have caused it to be difficult to control with herbicides, and infestations generally occur in remote areas consisting of comparatively low-value land. Thus, classical biological control is envisioned as a potentially valuable spurge management tool in North America. To date, ten Eurasian insect species have been released as biocontrol agents of leafy spurge.

Spurgia esulae was first approved for introduction into the United States in 1985. Through 1995, S. esulae has been released at leafy spurge-infested sites in 19 states across the northern U.S.

Appearance

Spurgia esulae adults are very small (1-2 mm) dark-grey flies with a reddish abdomen, that are somewhat mosquito-like in appearance. Galls produced by S. esulae on leafy spurge stems resemble small cabbages or artichokes, with overlapping warty, pale-green leaves. They are quite variable in size, generally ranging from 6-40 mm in length. The legless orange larvae (1-2 mm) are found within the galls.

Habitat

Leafy spurge-infested grasslands.

Pests attacked

The host range of S. esulae appears restricted to plants in the subgenus Esula of the genus Euphorbia. In Europe, this beetle feeds on leafy spurge and several other closely-related spurge species. There are a few native Euphorbia spp. in the U.S. that could potentially be hosts for S. esulae, though no feeding has yet been documented under field conditions. S. esulae will not feed on poinsettia (Euphorbia pulcherrima), and crop species and native plants outside the genus Euphorbia will not be attacked.

Life cycle

Spurgia esulae adults first appear in late spring to early summer, but are often difficult to find due to their small size and short life span. After mating, females lay groups of 20 or more small (<1 mm) orange eggs on leafy spurge stems or leaves near an apical bud. Newly-hatched larvae crawl into the bud tissues and begin feeding, which causes the plant to produce the gall. Larvae feed and develop within the galls; mature larvae construct silken cocoons within which pupation occurs.
S. esulae appears to complete from two to five generations per year depending on location. During earlier generations, pupation occurs within, and adults emerge from, the mature galls. During the last generation, however, larvae exit the galls, enter the soil, and overwinter. Pupation occurs the following spring, and adults emerge from the soil.

Relative effectiveness

Galls produced by the plant in response to S. esulae feeding rarely kill a leafy spurge plant. However, flower and, hence, seed production may be reduced at high gall densities.

Pesticide susceptibility

The herbicides 2,4-D, picloram, and imazethapyr appear to have no impact on S. esulae populations.

Conservation

For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, or the Feature Article on conservation in the Midwest Biological Control News.

Commercial availability

Spurgia esulae may be available from some state weed management agencies, as well as several commercial suppliers (see the off-site publication, Suppliers of Beneficial Organisms in North America).

References

GagnÇ, R.J. (1990) Gall midge complex (Diptera: Cecidomyiidae) in bud galls of Palearctic Euphorbia (Euphorbiaceae). Ann. Entomol. Soc. Am. 83: 335-345.

Lym, R.G. and Carlson, R.B. (1994) Effect of herbicide treatment on leafy spurge gall midge (Spurgia esulae) population. Weed Technol. 8: 285-288.

Nelson, J. (1994) The gall midge Spurgia esulae Gagn‚ on leafy spurge. MS Thesis, North Dakota State Univ., Fargo, ND. 76 pp.

Pecora, P., Pemberton, R.W., Stazi, M., and Johnson, G.R. (1991) Host specificity of Spurgia esulae GagnÇ (Diptera: Cecidomyiidae), a gall midge introduced into the United States for control of leafy spurge (Euphorbia esula L. "complex"). Environ. Entomol. 20: 282-287.

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