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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.
2009 Project Description
Asian chestnut gall wasp galls are being collected from the leading edge of the infestation in eastern North America to evaluate natural enemy composition and recruitment. Galls are measured and dissected, morphological characteristics of inhabitants are recorded, specimens are extracted and stored for subsequent molecular analysis (pending). A Korean and a Japanese strain of Torymus sinensis, the primary biocontrol agent for the Asian chestnut gall wasp, are present in Asia, distinguishable through phenological differences in development. The strain released in the USA is unknown. Asian chestnut gall wasp collections are ongoing to monitor Torymus sinensis emergence, to evaluate which strain is present in the US. Collaborators in Pennsylvania, Ohio, and Maryland are contributing material.
We are evaluating selected plant signaling compounds for their potential roles in Asian chestnut gall wasp gall development, including fitness and defense. Selected compounds and associated inhibitors are applied exogenously to developing galls, including ethylene and its inhibitor (STS), and abscissic acid and its NDGA inhibitor. Gall weight, volume, and the number of fungal lesions are evaluated. The number of chambers, mortality, parasitism, and fungal infection are assessed. Attempts to artificially infest chestnut seedlings with the Asian chestnut gall wasp continue. American chestnut seed germinated and seedling development monitored. Attempts to infest seedlings with ACGW failed (similar to previous attempts by R. Cooper with F1 hybrids). Current focus is to manipulate European chestnut and attempt to artificially infest.
These project descriptions were disseminated to the scientific community via several presentations, including an invited seminar at the Virginia Tech Department of Entomology, and submitted presentations at the Entomological Society of America National Meeting in Indianapolis, IN, and the Southern Forest Insect Work Conference in Gulfport, MS. Lastly, a research summary was presented at the NE-1015 Regional meeting in Ocean Grove, NJ.
The numerically dominant parasitoid of the Asian chestnut gall wasp is Torymus sinensis, a hymenopteran introduced for gall wasp control in the mid 1970's. It is spreading with expanding gall wasp populations, and affords moderate population control.
Ormyrus labotus is a native hymenopteran that is the second most abundant parasitoid associate of the gall wasp. It is an associate of oak-galling cynipids, and appears to have expanded its host range to take advantage of the novel resource provided by the presence of the introduced Asian chestnut gall wasp.
Ethylene and its inhibitor affected several parameters on both American and F1 hybrids. Gallmaker fitness, as measured by gall weight and gall volume, was reduced by ethylene applications, as was the galls' source strength (number of leaves). The ethylene inhibitor compromised gallmaker defense, measured by the number of fungal lesions forming on the gall exterior. Exogenous applications of abscisic acid reduced the defense response of American chestnut, but weakly increased the defense response of the F1 hybrids. Abscisic acid and its inhibitor had other more moderate effects that were measurable only on the F1s, including effects on gallmaker fitness and source strength.
Cooper, W. R. and L.K. Rieske-Kinney. 2009. Sustainable Management of the Invasive Asian Chestnut Gall Wasp Interactions between native and introduced natural enemies. Nutshell, June.
Cooper, W.R. and L.K. Rieske. 2009. Woody stem galls interact with foliage to affect community associations. Environmental Entomology 38, 417-424.