Forest pest damage is expected to increase with global change. Tree diversity could mitigate this impact, but unambiguous demonstration of the diversity–resistance relationship is lacking in semi-natural mature forests. We used a network of 208 forest plots sampled along two orthogonal gradients of increasing tree species richness and latitudes to assess total tree defoliation in Europe. We found a positive relationship between tree species richness and resistance to insect herbivores: overall damage to broadleaved species significantly decreased with the number of tree species in mature forests. This pattern of associational resistance was frequently observed across tree species and countries, irrespective of their climate. These findings confirm the greater potential of mixed forests to face future biotic disturbances in a changing world.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2015.1037

Tree diversity reduces pest damage in mature forests across Europe

The genus Castanea (family Fagaceae) is found in north temperate climates around the world, and is highly prized in many different cultures for its nutritious nuts and valuable timber. Selection for larger, better-tasting nuts has been ongoing in Asia and Europe for centuries. Early trade routes moved European chestnut trees (C. sativa) west of their native range (in the Caucasus mountains), and the Romans then moved them across their empire to provide support posts for grapevines, as well as for the nuts. Cultivar selection in Turkey, Italy, Spain, and Portugal has been extensive, and regional favorites developed. The many uses of the wood of American chestnut made this "all purpose" tree extremely valuable in its native range in North America. Nut production was important as a food source for rural families and many species of birds and animals. The other American species in the genus Castanea are classed as chinquapins, and may be divided into several or lumped as a single species. The small nuts from these trees and bushes serve primarily as mast for wildlife. Two serious diseases of chestnut trees changed the direction of chestnut research in the United States. Ink disease, caused by the root pathogen Phytophthora cinnamomi, was discovered to be the cause of widespread death of chestnuts and chinquapins in the southern United States, which had been observed since about 1850. This imported pathogen probably came into the southern United States before 1824. The second chestnut disaster was the introduction of chestnut blight disease, which was first found in the United States in 1904. The pathogen causing the lethal cankers is an Ascomycete now known as Cryphonectria parasitica. The longest continuing chestnut breeding program in the United States is in Connecticut.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-04-12-0350-FE

Chestnut Breeding in the United States for Disease and Insect Resistance.

. 1. Biological invasions of exotic species pose a major threat to native biodiversity. Invaders are known to have direct impacts on native species; however, less well studied are the indirect impacts mediated through the integration of invaders into trophically linked communities.

2. A survey of the chalcid wasp parasitoid community attacking the chestnut gallwasp Dryocosmus kuriphilus was carried out over a 5-year period at 26 sites in north-western Italy. More than 415 000 galls were collected and more than 10 000 parasitoid specimens emerged. Twenty-seven parasitoid species belonging to six families (Eurytomidae, Pteromalidae, Torymidae, Eupelmidae, Ormyridae, Eulophidae) were identified using morphological and molecular methods; seventeen are new records for the parasitoid community associated with D. kuriphilus in Italy. The morphospecies complexes Megastigmus dorsalis, Eupelmus urozonus, E. annulatus, and Eurytoma pistaciae were the dominant species; another six morphospecies were encountered regularly but at low frequency; and 13 species were recorded only occasionally. The attack rate of any individual parasitoid species was low, although the more common species appeared to be increasing their use of this novel host.

3. Biases observed in the sex ratios of parasitoids emerging from D. kuriphilus galls suggest that parasitoid individuals are making life-history decisions to take advantage of the high abundance of this host. Overall, these patterns imply that there is an ongoing transfer of parasitoids between oak and chestnut galls, and hence a high potential for this invading species to have a major impact on native oak gall communities via indirect competition mediated through shared parasitoids.

/pdf/chalcid-parasitoid-community-associated-with-the-invading-hjpa5f9jbn.pdf

Chalcid parasitoid community associated with the invading pest Dryocosmus kuriphilus in north-western Italy.

The impact of invasive herbivore species may be lower in more diverse plant communities due to mechanisms of associational resistance. According to the “resource concentration hypothesis” the amount and accessibility of host plants is reduced in diverse plant communities, thus limiting the exploitation of resources by consumers. In addition, the “natural enemy hypothesis” suggests that richer plant assemblages provide natural enemies with more complementary resources and habitats, thus promoting top down regulation of herbivores. We tested these two hypotheses by comparing crown damage by the invasive Asian chestnut gall wasp (Dryocosmus kuriphilus) on chestnut trees (Castanea sativa) in pure and mixed stands in Italy. We estimated the defoliation on 70 chestnut trees in 15 mature stands sampled in the same region along a gradient of tree species richness ranging from one species (chestnut monocultures) to four species (mixtures of chestnut and three broadleaved species). Chestnut defoliation was significantly lower in stands with higher tree diversity. Damage on individual chestnut trees decreased with increasing height of neighboring, heterospecific trees. These results suggest that conservation biological control method based on tree species mixtures might help to reduce the impact of the Asian chestnut gall.

/pdf/tree-diversity-limits-the-impact-of-an-invasive-forest-pest-564xemlenk.pdf

Tree Diversity Limits the Impact of an Invasive Forest Pest.

Native to China, the Asian chestnut gall wasp, Dryocosmus kuriphilus (ACGW), was first found outside its native range in Japan and the Korean peninsula in the mid-twentieth century. After appearing in North America in 1974, it was found in Europe a few decades later. Since then, the gall wasp has spread throughout the distribution of chestnut in Europe. The ACGW’s discovery in North America and Europe elicited numerous studies to understand its invasive potential in these areas and how to control its spread and impact on chestnut production. Although endemic parasitoids responded positively to D. kuriphilus with low parasitism rates, the most effective management tactic has been classical biological control via the introduction of the parasitoid Torymus sinensis from its native range in China. This review summarizes the history of introduction, spread, and current distribution of D. kuriphilus, and highlights one of the most successful cases of classical biological control against a forest pest.

The Asian chestnut gall wasp Dryocosmus kuriphilus: a global invader and a successful case of classical biological control

Dryocosmus kuriphilus (Hymenoptera: Cynipidae) is non-native to North America and induces formation of galls on petioles and leaves of all chestnut (Castanea spp., Fagales: Fagaceae). We investigated the interactions between the gall wasp D. kuriphilus, a native parasitoid, Ormyrus labotus (Hymenoptera: Ormyridae), and a non-native parasit- oid, Torymus sinensis (Hymenoptera: Torymidae). Galls were collected monthly from May to August and in January from four locations in the United States consisting of orchard-grown hybrid chestnuts (Hiram, OH and Meadowview, VA), suburban-grown orna- mental Chinese chestnuts (C. mollissima) (Broadview Heights, OH), or forest-grown American chestnuts (C. dentata) (Bowling Green, KY). Parasitoids were removed from galls and T. sinensis and O. labotus were identified using PCR-markers. The relative abundance of each parasitoid was compared in relation to collection date, habitat, presence of alternative hosts, and gall characteristics. T. sinensis was col- lected from each location and date, and was dominant in the orchard and suburban locations. However, relatively more O. labotus were collected within the forest, which had significant oak component and alternative cynipid hosts. O. labotus was only col- lected in spring and early summer, indicating the use of different summer and winter hosts. Observations suggest that in addition to parasitizing D. kuriphilus, O. labotus hyperparasitizes T. sinensis. T. sinensis has a longer ovipositor than O. labotus, and parasitized larger galls. This study improves our understanding of interactions between an invasive gall wasp, an intro- duced parasitoid, and native parasitoids, and illustrates novel relationships that may form as exotic species expand their geographic range.

https://entomology.ca.uky.edu/files/rkpdf/Cooper_Rieske%202011%20Biocontrol.pdf

A native and an introduced parasitoid utilize an exotic gall-maker host

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A native and an introduced parasitoid utilize an exotic gall-maker host