Tropical insect diversity: evidence of greater host specialization in seed‐feeding weevils

Abstract

Host specialization has long been hypothesized to explain the extraordinary diversity of phytophagous insects in the tropics. However, addressing this hypothesis has proved challenging because of the risk of over-looking rare interactions, and hence biasing specialization estimations, and the difficulties to separate the diversity component attributable to insect specialization from that related to host diversity. As a result, the host specialization hypothesis lacks empirical support for important phytophagous insect clades. Here, we test the hypothesis in a radiation of seed-feeding insects, acorn weevils (Curculio spp.), sampled in temperate and tropical regions (California and Nicaragua, respectively) with an equivalent pool of oak host species. Using DNA sequences from three low-copy genes, we delimited to species level 778 weevil larvae extracted from host seeds and assessed their phylogenetic relationships by Maximum Likelihood and Bayesian inference. We then reconstructed the oak-weevil food webs and examined differences in alpha, beta and gamma diversity using Hill numbers of effective species. We found a higher alpha, beta and gamma diversity of weevils in Nicaragua compared to California despite similar richness of host species at both local and regional level. By means of Bayesian mixed models, we also found that tropical weevil species were highly specialized both in terms of host range and interaction strength, whereas their temperate congeners had a broader taxonomic and phylogenetic host spectrum. Finally, in Nicaraguan species, larval body size was highly correlated with the size of the acorns infested, as would be expected by a greater host specialization, whereas in California this relationship was absent. Altogether, these lines of evidence support the host specialization hypothesis and suggest contrasting eco-evolutionary dynamics in tropical and temperate regions even in absence of differences in host diversity.

Invasive oaks escape pre‐dispersal insect seed predation and trap enemies in their seeds

“…Moreover, European oaks belong to the white oaks (section Quercus), while northern red oaks belong to the section Lobatae. This has been observed in some previous studies to be a factor for the specialization of acorn weevils (Peguero et al 2017; but see Bonal et al 2016). Host specialization could, however, differ between native predators, thus conditioning their abilities to exploit alien plant species.…”

“…Weevil pre-dispersal predation shapes recruitment potential of trees (Crawley & Long 1995;Espelta et al 2009;Bogdziewicz et al 2017), and severely destroys crops of northern red oaks in their native range (Lombardo & McCarthy 2008;Bogdziewicz et al 2017). While less is known about moth feeding ecology, weevils are feeding specialists, with particular species best adapted to exploit particular hosts (Bonal et al 2011;Govindan et al 2012;Muñoz et al 2014;Peguero et al 2017). Concerning predation upon red and European white oaks, differences may arise because red oaks acorns have higher tannin concentration, as well as thicker shells than European white oaks (Shimada & Saitoh 2006;Steele 2008; but see Łuczaj et al 2014).…”

Role of seed size, phenology, oogenesis and host distribution in the specificity and genetic structure of seed weevils (Curculio spp.) in mixed forests

“…Concerning other seed features, size is a trait claimed to influence ecomorphological diversification in many endophytic insects (e.g. body size and rostrum shape), promoting differences in their trophic niche, ecological adaptations and species radiation (Hughes & Voegler 2004a;Bonal et al 2011;Peguero et al 2017). In addition to chemical and morphological differences, seeding phenology and stochasticity in the availability of this resource have also been suggested as key factors influencing the guild of insect species predating upon a particular plant host (Espelta et al 2008(Espelta et al , 2009bCoyle et al 2012; see also Pélisson et al 2013a).…”

“…Interestingly, for seed-feeding insects a comparison of the genetic structure of their populations and the spatial structure of their potential hosts could provide strong evidence about differences of the trophic niche breadths. Moreover, the use of molecular techniques (DNA barcoding) may help to detect cryptic speciation and trophic niche segregation among morphologically similar species (Peguero et al 2017), and also to establish species specificity in an unequivocal way in comparison to classifications based on the presence or absence of a species on a particular plant, especially when the lack of morphological differences at certain stages (e.g. larvae) make species identification impossible otherwise (Govindan et al 2012).…”

“…Curculio spp. (Coleoptera: Curculionidae) are seed parasites that differ in their dispersal ability (Venner et al 2011), diapause duration (Pélisson et al 2013a,b), oogenesis (with both proovigenic and synovigenic species; Pélisson et al 2013a) and the breadth of their trophic niche (Muñoz et al 2014;Bonal et al 2015;Peguero et al 2017). We conducted this study in Catalonia (northeast Spain) in mixed forests including oaks (Quercus ilex, Quercus pubescens) and common hazel trees (Corylus avellana) with 4 different weevil species present (Curculio nucum Linnaeus, Curculio glandium Marsham, Curculio venosus Gravenhorst and Curculio elephas Gyllenhaal).…”