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French broom Genista monspessulana (Fabaceae: Genisteae)

Project Leaders: René SFORZA and Gaylord DESURMONT



French broom (FB), Genista monspessulana (L.) LAS Johnson (Fabaceae: Genisteae), is a leguminous shrub native to the Mediterranean region. It has become a noxious alien weed in the Western USA, Chile, South Africa, Australia and New Zealand (Bossard, 2000; Parsons and Cuthbertson, 2001). This perennial leguminous shrub normally starts blooming in the second growing year and produces higher numbers of seeds in the exotic range than in native populations (Herrera et al. 2011), outcompeting and displacing native flora. The plant was first introduced into the San Francisco Bay in California in the mid-1800s as an ornamental (McClintock 1985).  Today, G. monspessulana is highly invasive and widespread in the far western U.S. and Hawaii, and has been classified as noxious weed by the states of California, Oregon, and Hawaii (USDA-NRCS 2009).  French broom is most common in coastal areas and mountain slopes.  It readily invades disturbed places such as river-banks, road cuts, and forest clear-cuts, but can also be found in grassland and open canopy forest.  The ability of FB to form large, dense, monotypic stands and produce large, long-lived seed banks enables it to dominate plant communities in invaded areas and out-compete native plants (Swezy and Odion 1997, Bossard 2000).  Genetic studies have indicated high hybridization level and the presence of various genotypes in California (Kleist et al. 2014).  Most plants in the native range are in cluster 1 (Spain, France, Corsica, Sardinia), whereas most of the invasive FB plants from California are in cluster 3.  Thus, the invasive plants in California do not directly match those that have been analyzed from Spain, France, Corsica and Sardinia.


Location of the invasive range of Genista monspessulana in the united states (CENTER FOR INVASIVE SPECIES AND ECOSYSTEM HEALTH, 2017)


In the western Mediterranean region, two insects have been identified as promising biological control candidates. Research on biological control began in the late 1990s by an international consortium (California, Oregon, USDA-ARS, Australia and New Zealand).  The psyllid, Arytinnis hakani (Hemiptera, Psyllidae) and the seed-feeding weevil, Lepidapion argentatum (Coleoptera, Brentidae) (Sheppard and Thomann 2004). Colleagues from the Commonwealth Scientific and Industrial Research Organization (CSIRO) in France tested the host-range of the psyllid on 27 species of plants (Sheppard et al. 2002, Sheppard and Thomann 2006), based on a draft test list prepared by Akers et al. (2005).  Non-target effects were observed on Lupinus arboreus (Hogg et al. 2019). Quarantine studies were conducted suggesting that the psyllid would be equally effective on a range of FB sizes, particularly at high densities (Hogg et al. 2017), inducing severe negative growth impact to FB plants when infested with psyllids (Hogg et al. 2016). Complementary field bioassays in France with both French broom and non-target lupines would be very helpful to better assess the risk of non-target impact of A. hakani in the field. Concomitantly, the weevil Lepidapion argentatum (Gerstaecker) (Coleoptera: Brentidae) was collected from G. monspessulana, and never on any of the 21 other plant species sampled in the Genisteae tribe (Sheppard & Thomann, 2003). Research has begun on the seed-feeding weevil, Lepidapion argentatum, and it was soon discovered that females can also lay eggs in young branches producing a second generation that develops inside galls (Thomann et al. 2013, Kerdellant et al, 2019). Biological studies have shown reductions in plant size and growth rate induced by weevil galling (Bitume et al. 2019). The weevil could then reduce plant competitive survival to reproduction and also plant population dispersal as seedlings. All results obtained in controlled conditions suggest that L. argentatum has the potential to cause impact to French broom seedlings if released in the invasive range.

3 French broom psyllid Arytinnis hakani female

4 French broom psyllid Arytinnis hakani male