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Exotic Organisms Interact to Influence Persistence of a Native Species: Potential Interplay Between the Asian Chestnut Gall Wasp and Its Chestnut Hosts
Department of Entomology
The Asian chestnut gall wasp is an exotic invader that causes round, 8-15 mm diameter, greenish-red leaf and twig galls on all chestnut species, though differences in susceptibility differ among various chestnut species and varieties. The gall wasp was accidentally introduced into North America in 1974, and has become a serious pest of chestnut worldwide. Galls suppress shoot and twig growth, reduce tree vigor and wood production, and reduce fruiting and nut yield. Severe infestations can kill trees.
In the past American chestnut was a dominant component of hardwood forests of eastern North America. The accidental introduction of the chestnut blight fungus in 1904 virtually eliminated American chestnut from its former range. Today American chestnut persists as a blighted, non-flowering shrub that dies back when it reaches 2-3 m. In addition, Chinese chestnut are commonly used in ornamental and landscape plantings, and Chinese, Japanese, and European chestnut, and their hybrids, are cultivated for nut production, providing plentiful host material as the gall wasp expands its geographic range.
Extensive efforts at developing blight-resistant varieties have met with some success. Hybridization with blight-resistant Chinese chestnut, followed by repeated backcrossing, produce blight-resistant chestnut. These blight-resistant hybrids are appearing in restoration programs aimed at restoring American chestnut to the landscape, and in addition to other stressors, will be interacting with expanding Asian chestnut gall wasp populations. These interactions will undoubtedly affect chestnut production and American chestnut restoration efforts in eastern North America.
The overall goal of this research is to evaluate the geographic range expansion of D. kuriphilus in North America, the effects and ecological associations that have developed as D. kuriphilus extends its geographic range, and species-specific and varietal differences in Castanea susceptibility to the gall wasp, including qualitative and quantitative differences in source strength and signaling compounds.
The specific objectives are to
1) evaluate population characteristics of the Asian chestnut gall wasp, Dryocosmus kuriphilus, and its natural enemy recruitment, near the edges of its known geographic range in North America,
2) evaluate the extent to which its Castanea hosts influence gallmaker success via qualitative and quantitative differences in source strength, and
3) the extent to which its Castanea hosts influence qualitative and quantitative differences in plant signaling.
2010 Project Description
Non-native invasive species are gaining a foothold in forests of the US and Kentucky. We are monitoring forest plots to assess the ecological effects of the hemlock wooly adelgid, an exotic invasive herbivore that kills hemlocks and was first reported in Kentucky in 2006. We are assessing vegetative composition, forest structure, light penetration, soil characteristics, foliar characteristics, stream characteristics, and effects on aquatic organisms in hemlock stands as the adelgid expands it geographic range. Preliminary analysis of data suggests differences between hemlock dominated and deciduous dominated headwater stream water chemistry and benthic macroinvertebrate community composition.
In addition, we are working to gain a greater understanding of the ecological interactions associated with an exotic invasive species, the Asian chestnut gall wasp, as it expands its geographic range in North America. Expanding populations of the exotic gall wasp are recruiting native and introduced natural enemies in North America. Consequences of this recruitment are being investigated in consideration of ecological interactions associated with potential biological control of the gall wasp, which could play a role in commercial chestnut production and restoration efforts for the American chestnut.
We also evaluated the effects of stem galls caused by the gall wasp on the suitability of gall leaves to the gypsy moth, a generalist non-native foliage feeder, to assess the extent and relevance of gall defenses in important source leaves, and to evaluate developing relationships between a specialist and generalist herbivore.
Exotic species will undoubtedly alter ecological interactions within the forest community. We found four families of aquatic macroinvertebrates to be more abundant in hemlock dominated headwater streams. These families represent three functional feeding groups, including collectors, predators, and shredders. Each of these feeding guilds plays a role in the cycling of nutrients and energy throughout the stream continuum; changes in their abundance and composition could have far reaching consequences on fundamental ecosystem processes.
On gall wasp infested chestnut, relative growth rate of gypsy moth larvae was greater on gall leaves compared with normal leaves, indicating that, despite their importance, gall leaves may be more suitable to generalist insect herbivores, suggesting limitations to the extended phenotype of the gall wasp and altered host susceptibility to generalist herbivores. Our results improve our knowledge of host - Cynipid interactions, gall Source - sink relations, and Asian chestnut gall wasp community interactions.
Cooper, W.R. and L.K. Rieske. 2011. Chestnut species and jasmonic acid treatment influence development and community interactions of galls produced by Dryocosmus kuriphilus. Journal of Insect Science, In review.
Cooper, W.R. and L.K. Rieske. 2011. A native and introduced parasitoid utilize an exotic gall-maker host. Biological Control, In review.
Cooper, W.R. and L.K. Rieske. 2010. Gall structure affects ecological associations of Dryocosmus kuriphilus (Hymenoptera: Cynipidae). Environmental Entomology 39, 787-797.