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Biological Control of Leafy Spurge

Biological Control


In Europe and Asia, enough natural enemies have developed over time to control leafy spurge without human help. Insects and diseases in the Old World have put such stress on spurge that it remains an insignificant component of the landscape.

The weed's natural enemies do not occur in the New World, because of its relatively recent introduction.

To achieve biological control, human beings needed to intervene. Small numbers of beneficial organisms that attack leafy spurge have been released in America in the past with limited success. The critical difference between earlier efforts and the current program coordinated by USDA's Animal and Plant Health Inspection Service is one of scale.

APHIS biological control specialists believe that increasing the numbers of beneficial insects released over an extensive area will do better than the modest introductions of the past. The project depends on a broad, coordinated effort with cooperation from many Federal, State, and industry groups.

To start with, USDA officials already have cleared five species of insects from Europe for release in the United States. Together, these spurge enemies make a good team for fighting leafy spurge because they attack different parts of the plant. Some feed on the leaves. Others attack the shoot tips. Still others feed on the stem, the root crown, and the deep secondary roots. Each attack weakens the plant in a different way, thereby exerting a cumulative stress on the weed.

Scientists in USDA's Agricultural Research Service (ARS) are scouting out and evaluating other biological control agents for distribution in North America. Eventually, they hope that there will be enough spurge enemies in North America to achieve the same level of control enjoyed in Europe.


Stem and Root Boring Beetle

The first of the five introduced insects is a cerambyciid long-horned beetle, Oberea erythrocephala Schrank, which was brought in from Hungary and Italy. Females lay an average of 60 eggs and live between 3 and 8 weeks.

JPEG - Long-horned beetle larvae
Larvae of long-horned beetles bore the stem and root crown.
 
JPEG - Flea beetle larvae
Larvae of flea beetles damage spurge by mining the primary and secondary roots as well as the fine root hairs.

The adult beetle feeds on the outside of the plant and girdles the stem. The larval stage of the insect bores into the stem and root crown, feeding from inside the plant. The overall effect is to reduce the carbohydrate reserves stored in the roots.


JPEG - Adult long-horned beetle
Adult long-horned beetles from Hungary and Italy girdle the stems of spurge and feed on the leaves.
 
JPEG - Adult flea beetle
Adult flea beetles feed on the leaves of spurge.


Root Mining Flea Beetles

Three species of chrysomelid flea beetles, Aphthona flava Guill., Aphthona cyparissiae (Koch), and Aphthona czwalinae (Weise), contribute to the attack on leafy spurge. They were brought to the United States from Austria, Hungary, Italy, and Yugoslavia.

Adults of the three flea beetles live up to 3 months and feed on the leaves of the weed. They lay their eggs on spurge stems, about 250 eggs per female. The larvae cause even more damage to spurge, mining the primary and secondary roots as well as the fine root hairs, thereby disrupting the plant's vascular tissues.


Shoot-tip Gall Midge

This cecidomyiid midge, Bayeria sp., is a tiny fly imported from Italy. Female adults, which live only 24 to 36 hours, lay an average of 73 eggs on the shoots of leafy spurge. The species can produce three to five generations in a single growing season.

Larvae feed on the shoots and stimulate the plant to form a tumor-like gall on the shoot tips. This gall keeps shoots from forming flowers or seeds and reduces energy reserves of the weed.


JPEG - Shoot-tip gall midge
The shoot-tip gall midge can produce three to five generations in a single growing season.
 
JPEG - Effects of midge larvae on spurge
A tumor-like gall, induced on the shoot tips of spurge by midge larvae, retards formation of flowers and seeds.


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