Abstract
Simple SummaryFrench broom is a leguminous shrub that is an internationally significant invasive alien weed in California and nearby US states that competes with native vegetation and increases the risk of wildfires. French broom originates from the Mediterranean region and is the target for classical biological control. Exploration for prospective biological control agents in the native region of Western Europe resulted in the collection of the weevil Lepidapion argentatum. We conducted preliminary field studies showing that this weevil can impact seed production in the native habitat. We also measured the specificity of this candidate biocontrol agent in laboratory conditions on 36 plant species growing in California and Australia. Our data showed that, in no-choice tests, seven species were directly negatively impacted by gall formation induced on plant stems by the weevil, but at different levels, including two lupine species commonly distributed in California. Further experiments are needed to ascertain the safety of this candidate biocontrol agent prior to release.French broom (Genista monspessulana) (Fabaceae) is a perennial species native to the Mediterranean basin. Introduced in the 19th century as an ornamental plant, it is currently invasive in California and Australia. The current research is focused on biocontrol with the use of the phytophagous weevil Lepidapion argentatum (Brentidae). Its capacity to develop both in the stem galls and pods of French broom makes it a promising candidate. The impact on the reproduction of French broom was studied in Southern France and revealed that it could effectively reduce the number of viable seeds by 18.8%, but also increased the number of aborted seeds by 10% within the attacked pods. To evaluate the specificity of L. argentatum, choice and no-choice tests were performed in 2012 and 2015 on a total of 36 non-target closely related species. Results revealed the presence of galls and larvae in the stems of seven species, including two endemic Californian lupines; i.e., Lupinus arboreus blue and Lupinus chamissonis. In the future, new tests will be conducted to determine if L. argentatum is able to complete its entire development lifecycle on the non-target species where galls have previously been observed.
Highlights
Genista monspessulana (L.) LAS Johnson (Fabaceae), is a perennial woody shrub, native to the Mediterranean region commonly associated with regeneration in fire-prone environments
Pods that had at least one exit hole had the number of viable seeds significantly lower than that of pods with no exit hole
There are several examples in the literature showing the negative impact of gall formers on their host plant [28,29,30,31], especially the impact of stem-galling weevils [32,33,34,35], but we found no other examples of a weevil with two modes of feeding that may reduce both seed production and growth
Summary
Genista monspessulana (L.) LAS Johnson (Fabaceae), is a perennial woody shrub, native to the Mediterranean region commonly associated with regeneration in fire-prone environments. In Australia, it has been determined that French broom infests at least 600,000 ha in various ecosystems including some Australian national parks [4,5,6]. The evolutionary history of French broom and its introduction is very complex with many inter- and intra-specific hybridizations in California, such as Cytisus-racemosus [7]. French broom has a lifetime of five to eight years and generates a seed stock of 500 to 900 seeds/m2/year, while, in its introduced range, it can live up to 12 years, may flower more than once a year, and produces more seeds (annual seed rain up to 5000 seeds m2 creating seed banks of between 30,000–100,000 m2) [8]. French broom leads to an increase of the frequency and intensity of fire, and seeds are toxic for some animals (e.g., horses [3])
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