Phratora vitellinae, the brassy leaf beetle, formerly Phyllodecta vitellinae, is a beetle of the family Chrysomelidae found in Europe and Asia. It feeds on Populus and Salix species. The evolution of its host plant preferences and the mechanism by which it uses host plant chemicals to make a larval defensive secretion have been the subject of intense study by research groups in Europe and the Nordic countries.
Description Phratora vitellinae adults range from 3.5–5.2 mm long. The opaque forewings (elytra) show longitudinal rows of clearly visible dots. Adults typically show metallic blue, green, or bronze colors. Adults show copper or purple colors at high elevations or in Arctic regions. One way to distinguish among adult Phratora beetles co-occurring on the same host plant is to gently squeeze the abdomen of females until the morphology of the genitalia can be observed from the ventral side. Phratora vitellinae females possess a wide smooth sclerotized plate running parallel to the posterior of the abdomen.
Eggs are typically laid in clutches of 8–16, arranged in rows on the underside of the host leaf. Like other Phratora species, eggs are partially covered with a crusty secretion. Eggs are about 0.8–1.0 mm long and 0.4–0.5mm wide. Larvae feed in groups in early instars (molts).
Phratora vitellinae beetles feeding on Populus may co-occur with two other Phratora species Phratora laticollis and Phratora atrovirens. The Brassy Willow Beetle is larger and more abundant than P. atrovirens and somewhat broader in body shape than P. laticollis. Phratora vitellinae populations on Salix purpurea sometimes co-occur with Phratora tibialis, which also has a thinner body than P. vitellinae.
Range Phratora vitellinae is a wide spread species in Eurasia. In Europe, it is found in Arctic regions and the Nordic countries, the United Kingdom, Germany to Spain, Serbia and Bosnia. It is also found in China and elsewhere in Asia. Populations occur at high elevations in parts of central Europe and China. Phratora vitellinae was introduced to Iceland in 2005 and is considered an invasive species there.
Host plants and habitats Phratora vitellinae adults feed and lay eggs on aspen and willow (Salix) trees, including Populus tremula, Salix borealis, Salix myrsinifolia (also known as Salix nigricans, and a closely related willow Salix hegetschweileri),Salix purpurea, Salix pentandra, Salix eleagnos, Salix euxina (syn. S. fragilis), Salix viminalis, Populus balsamifera, and Populus nigra.
They may be found on other hosts, including cultivars of Populus species, in plantations. Their larvae develop on the same host plants as adults. The Brassy willow beetle is typically found in moist habitats, which are where their host plants thrive. These include bogs, forests, hedge rows and creek or river banks.
Phratora vitellinae is exceptional within Phratora in that it consumes Populus and salicylate-rich willows, giving it a relatively broad host plant range. Its broad diet breadth appears to relate to its ability to sequester host plant salicin and related compounds to produce a larval defensive secretion that mostly consists of salicylaldehyde derived from the host plant.
Life history Urban (2006) described the life history of a population in the Czech Republic in detail, and his article contains a good review of older studies of P vitellinae natural history and host plant use. Like other Phratora species, P vitellinae adults overwinter under bark, within fissures of bark of trees found near summer host plants, or in leaf litter. In spring, after 8–10 months overwintering, adults disperse to host plants and consume new foliage for about a week before mating, and in another week they lay their eggs on the underside of basal leaves on basal shoots. They lay 200–500 eggs in small clutches for up to 8 weeks during the growing season.
After 8–14 days, hatchlings emerge from eggs and begin to feed on the host plant, often forming a row of feeding larvae. Larvae grow for about 8–20 days and undergo two molts before they reach the pupal stage. Before pupating, they migrate to the soil near host plants and make a pupal chamber. They remain in the pupal chamber for about eight days before emerging as new adults. In central Europe, this species can experience multiple generations per growing season (multivoltine), but it undergoes only one generation per summer in the Nordic countries or at high elevations.
Natural enemies Adults may be consumed by predatory insects or birds, and they may succumb to infection by the fungus Beauveria bassiana or nematodes. Eggs of P vitellinae are consumed by the syrphid fly Parasyrphus nigritarsis and probably are eaten by predaceous bugs and mites. Kanervo (1939) studied feeding behavior of ladybird beetles and found that some prefer leaf beetle prey over aphids. He found that Calvia quindecimguttata consumed P vitellinae eggs. Rank et al (1998) observed natural enemies of P vitellinae eggs and larvae in eastern Finland and found that the most common predators were P. nigritarsis, the bug Anthocorus nemorum, and a lacewing (Chrysopidae) larva. Rowell-Rahier (1984) observed anthocorid bugs, lacewing larvae, spiders, and a predacious sawfly Tenthredo olivacea consuming larvae at a locality in eastern France. The bug Rhacognathus punctatus consumes eggs, larvae, and adult beetles. Larvae are parasitized by Meigenia mutabilis, a tachinid fly The wasp Symmorphus bifasciatus feeds on larvae of Phratora species, including P. vitellinae. Some of these enemies appear to be attracted to leaf beetle secretions.
Taxonomy Classification of species within Phratora has been investigated by reconstructing a phylogenetic tree of evolutionary relationships among mostly European species, which allowed for a reconstruction of host plant preference within the genus. Phratora vitellinae is the most closely related species to the outgroup taxa, which are relatives within the Chrysomelini lineage within the subfamily Chrysomelinae. More recent studies have included mitochondrial sequences from additional North American species (cf. Figure 2 in Canty et al. 2019), and they are consistent with the position of P. vitellinae in the phylogeny of European species published by Köpf et al. (1998). Most Phratora species either specialize on willow or poplar, while P. vitellinae feeds on several host plants in both genera, and one interpretation of this pattern is that there is an evolutionary trend towards greater host plant specialization in Phratora. Another possibility is that the evolution of sequestration of host plant salicylates into the beetle's defensive secretion also allowed them to metabolize a variety of host plant chemicals more effectively and caused them to adopt a broader diet. A more comprehensive phylogeny that includes North American and Asian species might help distinguish among these hypotheses.
Larval secretion chemistry Phratora vitellinae sequesters host plant salicylates to make its larval defensive secretion. This is a unique derived trait in the genus Phratora. Most Phratora species possess the ancestral trait of synthesizing iridoid monoterpene larval defensive secretions themselves (autogeneously), independent of the secondary chemistry of the host plant. Although use of host plant compounds to make larval defensive secretions appears to be the evolutionarily advanced state of this trait, other Phratora species (e.g. P. laticollis) already possess precursor mechanisms to transport plant secondary compounds, that were evidently further modified in P. vitellinae to sequester those compounds.
Ecological experiments on host plant use As noted above, P. vitellinae is widespread and common and has an unusual mechanism for metabolizing host plant secondary compounds to make its own defensive secretion. Researchers have been interested in host plant chemistry and how it relates to the suitability of potential hosts for the diet of P. vitellinae for decades. They have also studied the effects of its host-derived defensive secretion on natural enemies, and these studies have often concentrated on generalist predators that are relatively easy to use in laboratory feeding trials. A summary of representative host plant and laboratory predator studies follows.
Rowell-Rahier (1984) published companion studies of field observations of P. vitellinae on different host plants in eastern France and laboratory tests of P. vitellinae feeding preferences. She found that salicylate-rich willows and poplars were favored over the salicylate-poor willows Salix caprea and Salix cinerea. Both of the salicylate poor species have dense hairs or trichomes on the undersides of their leaves, which might repel P. vitellinae. On the other hand, these hosts are favored and frequently used by Phratora vulgatissima, which suggests that the beetles can overcome the potential physical defense of leaf trichomes.
Tahvanainen et al. (1985) published a study of host plant preferences among native and introduced Finnish willows that varied in phenolglycoside chemistry for four species of leaf beetles occurring in Finland, including P. vitellinae. The native species included the salicylate-rich Salix myrsinifolia and Salix pentandra and the salicylate-poor Salix phylicifolia and Salix caprea and the introduced species included Salix cv. aquatica, Salix dasyclados, Salix triandra, and Salix viminalis. Overall, P. vitellinae preferred the willow species rich in salicylates over the other species, while the other leaf beetles tended to favor the salicylate-poor willows. The researchers considered leaf texture to be a less important trait for the beetle than the chemistry of the host leaves. They also noted that Salix pentandra was relatively unpalatable, possibly because it contained high levels of a phenolglycoside not found in the other willows.
Denno et al. (1990) hypothesized that the host plant use of P. vitellinae would be based on levels of salicylates in the leaves and that higher predation from natural enemies on salicylate-poor plants would generate a selection pressure favoring leaf beetle specialization on willows with higher salicylates. They evaluated preference and performance among three willows: Salix dasyclados (salicylate-rich, dense trichomes), Salix euxina (syn. S. fragilis), (salicylate-rich, sparse trichomes), and Salix viminalis (salicylate-poor, dense trichomes). They also evaluated the suitability of these host plants to another beetle Galerucella lineola that does not use host plant compounds to produce a larval defensive secretion. Their results showed that P. vitellinae preferred, performed, and survived better on S. euxina over the other two hosts, suggesting that host plant salicylates play a role in its host preference but also demonstrating that other factors might favor P. vitellinae avoidance of some salicylate-rich plants.