Dormancy in laboratory-reared Asian longhorned beetles, Anoplophora glabripennis

Metabolic and transcriptomic characterization of summer and winter dormancy in the solitary bee, Osmia lignaria

The solitary bee Osmia lignaria is a native pollinator in North America with growing economic importance. The life cycle of O. lignaria provides a unique opportunity to compare the physiological and molecular mechanisms underlying two ecologically contrasting dormancies within the same species. O. lignaria prepupae become dormant during the summer to avoid high temperatures. Shortly after adult eclosion, they enter a second dormancy and overwinter as diapausing adults. To compare these two dormancies, we measured metabolic rates and gene expression across development as bees initiate, maintain, and terminate both prepupal (summer) and adult (overwintering) dormancies. We observed a moderate temperature-independent decrease in gas exchange during both the prepupal dormancy after cocoon spinning (45 %) and during adult diapause after eclosion (60 %). We sequenced and assembled a high-quality reference genome from a single haploid male bee with a contiguous n50 of 5.5 Mbp to facilitate our transcriptomic analysis. The transcriptomes of dormant prepupae and diapausing adults clustered into distinct groups more closely associated with life stage than dormancy status. Membrane transport, membrane-bound cellular components, oxidoreductase activity, glutathione metabolism, and transcription factor activity increased during adult diapause, relative to prepupal dormancy. Further, the transcriptomes of adults in diapause clustered into two groups, supporting multiple phases of diapause during winter. Late adult diapause was associated with gene expression profiles supporting increased insulin/IGF, juvenile hormone, and ecdysone signaling.

Parasitoid-induced changes in metabolic rate and feeding activity of the emerald ash borer (Coleoptera: Buprestidae): implications for biological control

Parasitoid-host interactions form the foundation of biological control strategies against many agriculture and forest insect pests. The emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), is a serious invasive pest of ash (Fraxinus spp.) trees in North America. Tetrastichus planipennisi (Hymenoptera: Eulophidae) is a gregarious, koinobiont endoparasitoid, attacking late (3rd to 4th) instars of EAB larvae, which feed in the live phloem of ash trunks or branches, making serpentine-like galleries filled with larval frass. In the present study, we tested the hypothesis that T. planipennisi regulates the host metabolism and feeding activity to optimize its offspring development and fitness. We first compared the respiration rate of parasitized and unparasitized host larvae at different times after parasitism, and then measured feeding activity of both parasitized and unparasitized host larvae inside their feeding galleries. Although parasitized host larvae increased metabolic rate and feeding activity in the first few days of parasitism, T. planipennisi parasitism induced an overall reduction of the metabolic rate and decrease in feeding activity of parasitized host larvae over their development period. In addition, there was a negative relationship between feeding activity of parasitized hosts and brood sizes of the parasitoid progeny-i.e., the more parasitoid progeny a host larva received, the less feeding activity the host had. These findings suggest that T. planipennisi has limited ability to optimize its offspring development and fitness through regulations of the host metabolism and feeding activity and its parasitism reduces feeding damage of parasitized EAB larvae to infested ash trees.

Molecular signatures of diapause in the Asian longhorned beetle: Gene expression

Most previous studies on gene expression during insect diapause do not address among-tissue variation in physiological processes. We measured transcriptomic changes during larval diapause in the Asian longhorned beetle, Anoplophora glabripennis (Coleoptera: Cerambycidae). We conducted RNA-seq on fat body, the supraesophageal ganglion, midgut, hindgut, and Malpighian tubules during pre-diapause, diapause maintenance, post-diapause quiescence, and post-diapause development. We observed a small, but consistent, proportion of genes within each gene expression profile that were shared among tissues, lending support for a core set of diapause-associated genes whose expression is tissue-independent. We evaluated the overarching hypotheses that diapause would be associated with cell cycle arrest, developmental arrest, and increased stress tolerance and found evidence of repressed TOR and insulin signaling, reduced cell cycle activity and increased capacity of stress response via heat shock protein expression and remodeling of the cytoskeleton. However, these processes varied among tissues, with the brain and fat body appearing to maintain higher levels of cellular activity during diapause than the midgut or Malpighian tubules. We also observed temperature-dependent changes in gene expression during diapause maintenance, particularly in genes related to the heat shock response and MAPK, insulin, and TOR signaling pathways. Additionally, we provide evidence for epigenetic reorganization during the diapause/post-diapause quiescence transition and expression of genes involved in post-translational modification, highlighting the need for investigations of the protein activity of these candidate genes and processes. We conclude that diapause development is coordinated via diverse tissue-specific gene expression profiles and that canonical diapause phenotypes vary among tissues.

Cold tolerance of laboratory-reared Asian longhorned beetles

Low winter temperatures in temperate climates can limit the success of non-native species. The Asian longhorned beetle, Anoplophora glabripennis, is an invasive wood-boring pest of hardwood trees in North America and Europe. Native A. glabripennis populations are spread across several climate zones in China and the Korean Peninsula and are likely to encounter low temperatures in at least some of this range. Understanding the lethal limits of the overwintering life stages of A. glabripennis is essential for accurately modeling the risk that invasive populations pose to non-native environments. In this study, we provide the first systematic characterization of the cold tolerance strategy and lower lethal limits of A. glabripennis eggs, larvae, and pupae. In diapausing larvae, the most common overwintering stage in this species, we measure hemolymph glycerol and osmolality and identify the effects of prolonged low temperature exposure. In developing pupae, we identify sublethal effects caused by low temperature exposure before freezing. Eggs and larvae were the most cold-tolerant life stages; eggs were freeze-avoidant with an average supercooling point of -25.8 °C and larvae were freeze tolerant with an LT₉₀ of -25 °C. Hemolymph osmolality of freeze-tolerant larvae, on average, increased to 811 mOsm during chilling. This increase was primarily driven by a concurrent, average increase of 232 mM hemolymph glycerol. Pupae died upon exposure to freezing temperatures, but accumulate strong sublethal effects prior to freezing, indicating that they are chill susceptible. Taken together, these data will be useful to inform species distribution modeling in A. glabripennis.