How understanding ecological interactions provides tools for conservation biocontrol of the weedy leafy spurge (Euphorbia esula)

Leafy spurge (Euphorbia esula L. subsp. esula, Euphorbiaceae) is a well known invasive species, notonly in North America but also in Europe, its native range. Since the 1990s, its invasiveness has beenreported in the floodplains of Val de Saône in central-eastern France, which are considered the lastand largest European flood-meadows. Growing in dense patches, this latex-rich plant is toxic to cattlewhen present in cut hay. Annually mowed grasslands are losing their profitability as soon as they areinfested. This economic loss may lead to ecological issues such as the shifting of mowed pastures intointensive agriculture (corn) and forestry (poplar). Natural annual floods and various agriculturalpractices such as mowing and grazing promote a rich floral and faunal diversity, which is protectedunder the Natura 2000 network. Our project is an original multidisciplinary approach to the study ofinvasive plant/natural regulator/agricultural practice/biotic factor interactions in two ways: 1) we aredetermining the biological, genetic, ecological, and agricultural factors triggering leafy spurgeinvasiveness, and how these factors interact; and 2) we are proposing an integrated biocontrolprogram combining previously obtained data, by targeting invasive plant/natural regulatorsinteractions, connected with agricultural practices, based upon field and laboratory tests. Preliminaryfield results (Figure 1) show that, as suspected, all factors tested are influencing the plant/insectcomplex. Mowing has a major impact on shoot density (shoots/m2), increasing it by two times onaverage. Mowing also negatively impacts the population size of the insect Oberea erythrocephala(Cerambycidae) by suppressing oldershoots. This beetle is one of thenatural phytophagous regulators ofleafy spurge, and aged shoots are itsmajor oviposition substrate. Grazing,including trampling caused by thispractice, has an opposite effect,mainly on non-mowed patches thatappear to attract cows. Shoot densitydecreased by 50 to 95% in August.This practice occurs only after O.erythrocephala population andoviposition peaks, reducing thepotential impact on the density of itspopulation. The flood factor does notseem to impair plant health. Furtherstudies may show a positive impact onthe increase of shoot density.