Convergent evolution of chemical defence in Galerucine larvae

Selection pressure by natural enemies on phytophagous insect larvae is intense and has frequently triggered the evolution of chemical defence as an effective counterstrategy. In the chrysomelid subfamily Galerucinae, glandular structures and defensive fluids have been described for the tribe Sermylini Wilcox, 1965. Previous morphological and ultrastructural studies raised doubts that these defensive devices in Sermylini can be traced back to a common origin. The taxonomy of the Galerucinae cannot clear these doubts because the phylogeny of this taxon is a matter of current debate. We therefore investigated the phylogeny of the Galerucinae based on approximately 1740 bp of the mitochondrial 12S and 16S rRNA and the nuclear elongation factor 1 alpha genes. Our data support the hypothesized close relationship between the subfamilies Galerucinae and Alticinae, yet, by contrast to other recent analyses, the two groups are mostly resolved as monophyletic sister groups or, in some analyses, with the Galerucinae nested paraphyletically within the Alticinae. Within the subfamily Galerucinae, only the tribe Galerucini formed a monophyletic taxon, except for one species, Cerochroa brachialis Stal, 1858. In none of our analyses were the Sermylini recovered as a monophyletic tribe. However, our data support monophyly of each of the three groups within the Sermylini that have morphologically distinguishable larval defensive openings. We conclude that the defensive structures in larvae of Sermylini have no common origin, but evolved independently. Our data suggest that the tremendous selection pressure by natural enemies led to the recurrent evolution of similar chemical defensive devices in Sermylini larvae.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 165–175.     

Influence of larvae of Gastrophysa viridula on the distribution of conspecific adults in the field

1. Previous laboratory bioassays indicate that exocrine glandular secretions of larvae of Gastrophysa viridula repel conspecific adults and deter them from feeding and oviposition. The present study was conducted to investigate the influence of larvae of G. viridula on conspecific adults in the field.
2. Within the G. viridula population studied, two generations were observed in a year. Occurrence of the different developmental stages overlapped temporally.
3. Some individual plants of Rumex obtusifolius , the host of the G. viridula population studied, grew so close to each other that they were considered as a plant group. When investigating the spatial distribution of larvae and adults within such plant groups, larvae were rarely found on plant groups on which adults were feeding.
4. A field experiment revealed that adults avoided plants of R. obtusifolius infested by conspecific larvae of the second and third instar. Adults still avoided these damaged plants when larvae had left them for pupation.
5. Gastrophysa viridula avoided oviposition on leaves and plants infested by conspecific larvae. Larvae of the second instar significantly deterred oviposition when present at a high density (33.3 larvae/dm²), whereas larvae of the first instar did not deter oviposition of conspecifics at either density tested. The oviposition deterring effect was also observed when just exocrine glandular secretion of larvae of the second instar was applied to the leaves in amounts equivalent to a density of 33.3 larvae/dm².
6. Availability of food ( R. obtusifolius ) largely exceeded its exploitation in each group of plants examined. This may be due to either the spatial separation of adults and larvae or the low population density observed on these plant groups.     

Effects of nutrition and soil warming on stemwood production in a boreal Norway spruce stand

The boreal forest is expected to experience the greatest warming of all forest biomes. The extent of the boreal forest, the large amount of carbon contained in the soil, and the expected climate warming, make the boreal forest a key biome to understand and represent correctly in global carbon models. It has been suggested that an increase in temperature could stimulate the release of CO₂ caused by an increased decomposition rate, more than biomass production, which could convert current carbon sinks into carbon sources. Most boreal forests are currently carbon sinks, but it is unclear for how long in the future the carbon sink capacity of the boreal forest is likely to be maintained. The impact of soil warming on stem volume growth was studied during 6 years, in irrigated (I) and irrigated‐fertilized (IL) stands of 40‐year‐old Norway spruce in Northern Sweden. From May to October heating cables were used to maintain the soil temperature on heated‐irrigated plots (Ih and ILh) 5 °C above that on unheated control plots (Ic and ILc). After six seasons' warming, stem volume production (m³ ha^?1 a^?1) was 115% higher on Ih than on unheated (Ic) plots, and on heated and irrigated‐fertilized plots (ILh) it was 57% higher than on unheated plots (ILc). The results indicate that in a future warmer climate, an increased availability of nitrogen, combined with a longer growing season, may increase biomass production substantially, on both low‐ and high‐fertility sites. It is, however, too early to decide whether the observed responses are transitory or long lasting. It is therefore crucial to gain a better understanding of the responses of boreal forest ecosystems to climate change, and to provide data to test and validate models used in predicting the impact of climate change.