Phenology and temperature‐dependent development of Ceutorhynchus assimilis, a potential biological control agent for Lepidium draba

Abstract

Lepidium draba (Brassicaceae) is a major concern for agriculture and biodiversity in the western United States. As current control methods do not provide long‐term, sustainable solutions, research has been conducted to find biological control agents. Ceutorhynchus assimilis is one of the currently investigated candidates. Known as oligophagous in the literature, a specialist clade of this root‐galling weevil exists in southern Europe. This raised the question of its ability to survive in colder climates in the target range. We investigated the phenology of C. assimilis in the field in southern France (specialist clade) and Romania (generalist clade) and measured various temperature‐dependent parameters in the laboratory. In both ranges, weevils were univoltine. Oviposition in autumn started later in France compared to Romania, while mature larvae exited galls (to pupate in the soil) earlier the following year. On average, 25% and 32% of galls from France and Romania were completely below the soil surface, respectively, and this appeared to depend on soil substrate. Weevils transported from France to Romania were able to develop, but at a much lower rate than Romanian weevils. Mortality of overwintering larvae of both clades increased with decreasing temperature and exposure time. At −5°C, lethal times Lt50 and Lt95 were 15 and 42 days for the specialist clade and 26 and 72 days for the generalist clade. A higher proportion of third instar larvae compared to first and second instar larvae survived. Pupation time at different temperatures did not differ between weevils from France or Romania. A climate match model (comparing winter temperatures) indicated that the specialist clade of C. assimilis from France has the potential to establish in some parts of the target range (e.g. Washington, Oregon, California). However, temperature extremes and winters without snow cover will likely limit its establishment unless rapid adaptive evolution takes place.

Implications of a phylogeographic approach for the selection of Ceutorhynchus assimilis as a potential biological control agent for Lepidium draba

“…assimilis can also be linked with biological features of weevil populations. Differences in phenology, sex ratio, cold hardiness and overwintering survival have been observed between the different entities (Fumanal et al, 2004b;Virag et al, 2016). For instance, Fumanal et al (2004b) observed in the area where the Lepidium host race is found that sympatric weevil populations on L. draba developed faster and pupation started up to two months earlier than individuals on S. arvensis.…”

“…Lepidium draba is invasive in a large part of North America, including areas with a wide spectrum of climatic conditions (Rice, 2014;USDA-NRCS, 2016). A climate match model suggests that the Lepidium host race may establish and persist in some areas where the weed is problematic, while establishment in more continental climates could be possible, but more difficult (Virag et al, 2016). However, relatively rapid adaptation after release may occur as observed for the flea beetle Longitarsus jacobaeae (Waterhouse) (Szucs et al, 2012).…”

“…Among the known natural enemies of L. draba in its native range in Eurasia, Ceutorhynchus assimilis Paykull, 1792[syn. Ceutorhynchus pleurostigma (Marsham 1802] (Coleoptera, Curculionidae) (Colonnelli, 1993) has been prioritized as a of candidates for biological control (Fumanal et al, 2004a;2004b;Hinz et al, 2013;Hinz et al, 2016;Virag et al, 2016). Adults lay most of their eggs in young and soft roots of the plant, causing the development of galls in which the three larval instars of the weevil develop before larvae leave the galls and pupate in the soil (Hoffman, 1954).…”

“…However, Fumanal et al (2004b) proposed that C. assimilis is probably a complex of morphocryptic entities that differ in their host-plant spectrum within the family Brassicaceae and one of these is a specialist on L. draba with a distribution in the warmer climatic regions of southern Western Europe (Fumanal et al, 2004a). Whilst the host-specificity and life-cycle of this specialist entity have been studied (Fumanal et al, 2004b;Hinz et al, 2016;Virag et al, 2016), population genetic structure and the evolutionary relationships between it and the others remain poorly depicted.…”