The lifecycle of Agrilus biguttatus : the role of temperature in its development and distribution, and implications for Acute Oak Decline

he two spotted oak buprestid, Agrilus biguttatus Fabricus, is implicated in oak decline events across Europe, and is strongly linked to Acute Oak Decline in the U.K., although its role in the syndrome remains under investigation. In the U.K., the beetle is restricted to south and central England. The present study aimed to improve our understanding of the beetle's life history and thermal requirements, intending to explain its U.K. distribution, and to collect data for lifecycle modelling. Novel methods were developed to collect and culture the beetle in the laboratory, which enabled experiments to be carried out, providing data on the beetle's sex ratio, longevity and fecundity, and the development rates of eggs, larvae and pupae at constant temperatures. On average, females lived for 63 days and laid 82 eggs. Larvae developed through four instars. Sex ratio varied by site, with no overall trend apparent. The development rates of eggs, larvae and pupae (to adult emergence) had linear relationships with temperature, with lower developmental thresholds of 12.1, 11.9 and 15.1 °C, respectively. For each life stage, degree-day values were calculated. Beetles appeared to have an obligatory prepupal diapause at all temperatures studied, up to and including 25 °C. The implications of the developmental findings for the beetle's current distribution, as well as the possible effects of climate change, are discussed. The beetle appears to be thermally limited in the U.K. and, if so, its distribution, and perhaps that of Acute Oak Decline, may alter under climate change

Novel dendrochronological modelling demonstrates that decades of reduced stem growth predispose trees to Acute Oak Decline

Acute Oak Decline (AOD) is devastating thousands of native oaks, Quercus robur and Q. petraea, in the UK and continental Europe. The syndrome is principally caused by multiple, interacting bacterial species that degrade inner bark tissues. The level of host predisposition required prior to AOD infection is unknown. The two spotted oak buprestid Agrilus biguttatus is strongly associated with AOD, although its role remains unclear. To investigate the nature of predisposition in AOD, to explore the role of the beetle in the syndrome, and to examine growth trends after AOD onset, the stem growth of 243 trees with a range of severity of AOD symptoms was analysed at five sites in England. Novel mixed effects dendrochronological modelling methods were developed. The presence/absence of A. biguttatus exit holes in the trees was not specifically linked with reduced stem growth, nor was there evidence of further reductions in growth after the onset of AOD symptoms. Instead, trees with long-term AOD symptoms show significantly reduced growth compared to asymptomatic trees from as far back as the 1930s, following a period of widespread decline in English Oaks. These results suggest that a cohort of oak trees across Britain was permanently damaged in the 1930s and predisposed to develop AOD symptoms decades later. Additionally, correlations with climatic variables suggest that diseased trees are less able to take advantage of good growing conditions in the spring and autumn. This study sheds light on how historical episodes of stress may impact the future resilience of oaks to disturbance, and supports the use of dendrochronological modelling as a technique to study the underlying health status of oak tree populations, and to better understand tree decline episodes

Molecular phylogeny and historical biogeography of Apicotermitinae (Blattodea: Termitidae)

Soil-feeding termites are abundant in tropical regions and play an important role in soil bioturbation and in the organic matter cycle. The Apicotermitinae are arguably the most diverse lineage of soil-feeding termites, but they are also the most understudied, probably because many species are soldierless, which makes identification difficult. Although the backbone of the termite phylogenetic tree is now well-resolved, the relationships among representatives of Apicotermitinae are still largely unknown. Here, we present phylogenetic trees inferred from 113 mitochondrial genomes of Apicotermitinae representative of the group diversity. Our analyses confirm the monophyly of the Apicotermitinae and the basal position of soldiered taxa, within which two lineages of soldierless species are nested. We describe two new monotypic genera, whose phylogenetic position appeared of special interest: Koutabatermes gen. n., lies on a long branch among soldiered taxa, and Apolemotermes gen. n., is sister to Adaiphrotermes. We resolved, with high support, the position of Asian genera as sister group of a clade comprising the monophyletic neotropical Anoplotermes-group and the small African clade including Adaiphrotermes and Apolemotermes gen. n.. Our trees cast light on the intergeneric and interspecific relationships within Apicotermitinae and reveal the polyphyly of several genera, including Ruptitermes, Astalotermes and Anoplotermes. Biogeographic reconstructions revealed two dispersal events out of Africa, one to the Oriental realm and one to the Neotropical realm. Overall, the timing of Apicotermitinae diversification and dispersal, following the Eocene–Oligocene boundary, matches that found for other groups of Neoisoptera. Nomenclatural acts are registered in ZooBank: http://zoobank.org/urn:lsid:zoobank.org:pub:CA1A21B6-573E-4855-8C88-372453C922F7