Plant phenology and seed predation: interactions between gorses and weevils in Brittany (France)

Abstract

Exapion ulicis (Forster) and Exapion lemovicinum (Hoffmann) (Coleoptera: Curculionoidea: Apionidae) are seed predators of the three gorse species occurring in Brittany (France): Ulex europaeus L., Ulex gallii Planch., and Ulex minor Roth.(Fabaceae). Host‐plant phenology plays a major role in the relationship between apionid weevils and their gorse species, because larvae develop within gorse pods and adults have to wait for pod dehiscence to be released. We monitored flowering and fruiting phenology of gorse species, weevil reproductive behaviour, and egg‐laying patterns in six natural populations in the native area of these gorse species. At each site, U. europaeus, which flowers mainly in spring, was sympatric with one of two autumn flowering gorse species, U. gallii and U. minor. We noticed that E. ulicis laid eggs in spring and was restricted to U. europaeus whereas E. lemovicinum laid eggs in autumn and was restricted to the two autumn‐flowering species U. gallii and U. minor. Therefore, host specificity depended on gorse phenology, and not on geographic proximity. In addition, the infested pod content showed that E. ulicis laid several eggs per pod and suggested that females chose pods with the highest numbers of seeds. In contrast, E. lemovicinum laid a single egg per pod and showed no preference for pods with many seeds. Finally, the impact of seed predation by E. ulicis was higher than that of E. lemovicinum.

Oviposition Decision of the Weevil Exapion ulicis on Ulex europaeus Depends on External and Internal Pod Cues

“…Mature pods are lignified, brown, and about 13 mm long. In Europe, pods are attacked by several seed predators, of which the weevil Exapion ulicis is the most common [19,24]. In its invasive range, U. europaeus was introduced around the 1800s, without its natural enemies.…”

“…Exapion ulicis is naturally present in Europe and is specific to U. europaeus on which it spends its whole life cycle [19]. It is present in all European gorse populations.…”

“…In some countries, E. ulicis was later introduced as a biological control agent, like in New Zealand in 1931 [18]. This system is particularly interesting because the relationship between U. europaeus and E. ulicis is very specific and very strong [19]. Indeed, the weevil undergoes its whole life-cycle on gorse, and gorse may suffer considerable damage from the weevils, which may infest up to 90% of the mature pods [19,20].…”

“…This system is particularly interesting because the relationship between U. europaeus and E. ulicis is very specific and very strong [19]. Indeed, the weevil undergoes its whole life-cycle on gorse, and gorse may suffer considerable damage from the weevils, which may infest up to 90% of the mature pods [19,20]. However, pod infestation rates are extremely variable in natural and experimental populations, and can vary from 20-98%, even among neighbouring individuals [19][20][21].…”

“…Indeed, the weevil undergoes its whole life-cycle on gorse, and gorse may suffer considerable damage from the weevils, which may infest up to 90% of the mature pods [19,20]. However, pod infestation rates are extremely variable in natural and experimental populations, and can vary from 20-98%, even among neighbouring individuals [19][20][21]. This proportion has been shown to depend on many factors, including plant traits such as plant architecture, flowering phenology, chemical compounds and flower and pod densities [21,22].…”