The amount of carbon stored in deadwood is equivalent to about 8% of global forest carbon stocks 1 . Deadwood decomposition is largely governed by climate [2][3][4][5] with decomposer groups, such as microbes and insects, contributing to variations in decomposition rates 2,6,7 . At the global scale, the contribution of insects to deadwood decomposition and carbon release remains poorly understood 7 . Here we present a field experiment of wood decomposition across 55 forest sites on six continents. We find that deadwood decomposition rates increase with temperature, with the strongest temperature effect at high precipitation levels. Precipitation affects decomposition rates negatively at low temperature and positively at high temperatures. As net effect, including direct consumption and indirect effects via interactions with microbes, insects accelerate decomposition in tropical forests (3.9% median mass loss per year).In temperate and boreal forests we find weak positive and negative effects with a median mass loss of 0.9% and -0.1% per year, respectively. Furthermore, we apply the experimentally derived decomposition function to a global map of deadwood carbon synthesised from empirical and remote sensing data. This allows for a first estimate of 10.9 ± 3.2 Pg yr -1 of carbon released from deadwood globally, with 93% originating from tropical forests. Globally, the net effect of insects accounts for a carbon flux of 3.2 ± 0.9 Pg yr -1 or 29% of the total carbon released from deadwood, which highlights the functional importance of insects for deadwood decomposition and the global carbon cycle.

Extreme climatic events, including droughts and heatwaves, can trigger outbreaks of woodboring beetles by compromising host defenses and creating habitat conducive for beetle development. As the frequency, intensity, and duration of droughts are likely to increase in the future, beetle outbreaks are expected to become more common. The combination of drought and beetle outbreaks has the potential to alter ecosystem structure, composition, and function. Our aim was to investigate a potential outbreak of the native Eucalyptus longhorned borer, Phoracantha semipunctata (P. semipunctata), following one of the most severe droughts on record in the Northern Jarrah Forest of Southwestern Australia. Beetle damage and tissue moisture were examined in trees ranging from healthy to recently killed. Additionally, beetle population levels were examined in adjacent forest areas exhibiting severe and minimal canopy dieback. Severely drought-affected forest was associated with an unprecedented outbreak of P. semipunctata, with densities 80 times higher than those observed in surrounding healthier forest. Trees recently killed by drought had significantly lower tissue moisture and higher feeding damage and infestation levels than those trees considered healthy or in the process of OPEN ACCESSForests 2015, 6 3869 dying. These results confirm the outbreak potential of P. semipunctata in its native Mediterranean-climate Eucalyptus forest under severe water stress, and indicate that continued drying will increase the likelihood of outbreaks.

The attack patterns, infestation success and larval development of woodborers within living trees are complex and are largely shaped by host tree characteristics. Following a severe drought in a native eucalypt forest where outbreak densities of a native Australian beetle, the eucalyptus longhorned borer (Phoracantha semipunctata), occurred, a tree dissection study was conducted in Australia. This involved felling 40 trees each of jarrah (Eucalyptus marginata) and marri (Corymbia calophylla) that were cut into 1-m sections and neonate larval galleries, larvae in pupal cells and adult borer emergence were measured and added to give total numbers per tree to determine the within-tree distribution and survival of P. semipunctata. There was a significant impact on larval survival in both species, in contrast, pupal survival remained high. Within-tree distribution of P. semipunctata was directional with borer emergence and incidence of larval galleries both negatively associated with tree section height above the ground and positively associated with section diameter and bark thickness, reaching a maximum towards the base of trees. High incidence and survival in lower thicker tree sections indicate a more conducive environment for larval development, in contrast to poor larval survival in smaller thinner sections at the top of trees. The dependence of larval survival on tree characteristics controlling the within-tree distribution of borer emergence is emphasized, and needs to be considered when estimating the spread of borer populations during outbreaks.

In southwestern Australia, outbreaks of cerambycids have been associated with infestation of drying and stressed trees. Felled timber provides an abundant habitat for a variety of subcortical beetles where their development can be modified by attack from natural enemies, including parasitoids. There are a variety of parasitoid wasps of cerambycids native to Australia, yet their effectiveness in controlling the larval performance of borers has been little studied. Approximately 700 Eucalyptus trees felled to realign a forest road provided an opportunity to determine how a mass of drying logs influences borer infestation rates and whether parasitoid wasps were effective in controlling borer survival. The native borer Coptocercus rubripes (Coleoptera: Cerambycidae) caused an extensive infestation of fallen trees at the clearfell site. These infestations were subsequently limited by the direct effect of parasitism from the native braconid wasps Callibracon limbatus and Syngaster lepidus emerging from a single hole in a white cocoon. The effects of parasitism showed inverse density dependency where high borer larval densities increased the chances of borer larval survival. The absence of parasitism in caged logs promoted high borer larval densities and greatly improved adult borer survival, and while this increased the demand for sapwood resources, it promoted early adult borer emergence. Control by native braconid wasps of high infestations of Co. rubripes borers attacking drying and stressed trees limits adult borer emergence and has the potential to partially regulate borer populations.

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