Sirex Symposium

Ithaca, New York, USA
Monday, October 14, 2013 to Wednesday, October 16, 2013
Contact - Ann Hajek


Thanks to all participants of the first Sirex Symposium at beautiful Cornell Plantations! It was a great meeting with cutting-edge presentations, international attendance, and stimulating discussions. And a big thanks to the organizers -  Ann Hajek, Mark Whitmore, and Fred Stephen - for putting together such a stimulating and fun experience!



Abstracts:



1.   The introduction history of the Sirex woodwasp and its symbionts: current knowledge and tools to address knowledge gaps



Bernard Slippers1, Emilie Boissin1, Brett P Hurley2, Jeff Garnas2, Osmond X Mlonyeni1, Katrin Fitza1, Alisa Postma1, Sze Huei Yek1, Michael J Wingfield1



1Department of Genetics and 2Department of Zoology and Entomology, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria, South Africa



A number of studies over the past 15 years have attempted to reconstruct the introduction and invasion history of Sirex noctilio and Amylostereum areolatum, as well as their parasitoids used in biological control programs. These studies have been hampered by the poor collections of these organisms from native areas, as well as a lack of tools to study genetic diversity and relationships. Collections, analytical tools and associated data have steadily grown, but many questions remain regarding the global diversity and phylogeographic structuring of these organisms. It is clear that there has been a complex history of introduction, and reintroduction, of the woodwasp in many parts of the world, which contrasts with the limited number of parasitoid introductions. As a consequence there are widespread discrepancies in diversity between the parasites and their host, which is important to consider in biological control programs. The completion of the genome sequences of S. noctilio, A. areolatum and the parasitic nematode, Deladenus siricidicola, offers unprecedented opportunities to better characterize the diversity of these organisms and to better understand the patterns and consequences of co-evolution and rapid evolutionary changes in nonnative environments. Such insights promise to increase the biological understanding of these organisms and will likely offer novel avenues for control.



2.   Intercontinental comparison of the spread and impacts of Sirex noctilio



Flora Krivak-Tetley1, Matthew Ayres1, Andrew Liebhold2, Maria Lombardero3, Juan Corley4, Brett Hurley5



1Department of Biological Sciences, Dartmouth College, Hanover, NH, USA, 2USDA Forest Service Northern Research Station, Morgantown, WV, USA, 3Departamento de Prodúccion Vegetal, Universidad de Santiago, Lugo, Spain, 4Ecología de Insectos, INTA EEA Bariloche, Bariloche, Argentina, 5Department of Zoology and Entomology, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria, South Africa



Since Sirex noctilio Fabricius (Hymenoptera:Siricidae) was first trapped in the United States in September 2004, it has spread throughout New York and into Pennsylvania, Michigan, Vermont and Ontario, Canada. As invasion continues, the potential risk to pine stands in the southern and Lake states is substantial. Here, we outline research plans for a comparative study of S. noctilio in N. America with populations in Africa and South America (invasive) and Spain (native). Our objectives are (1) to develop an understanding of the factors controlling S. noctilio range expansion elsewhere such that we can predict future spread in North America; and (2) to examine patterns of abundance and growth to determine why this insect is more or less of a pest in different environments.



3.   Sirex noctilio and its associates in Ontario



            L.J. Haavik1, K.J. Dodds2, and J.D. Allison3



1Natural Sciences and Engineering Research Council of Canada, Great Lakes Forestry Centre, Sault Ste. Marie, Ontario, Canada; 2USDA Forest Service, Forest Health Protection, Durham, New Hampshire, USA; 3Canadian Forest Service, Great Lakes Forestry Centre, Sault Ste. Marie, Ontario, Canada



North American pine forests differ from many other areas where Sirex noctilio has been introduced. It is not known how S. noctilio will impact North America’s heterogeneous pine forests composed of mostly native pines, and rich community of subcortical insects, including other siricids. We report from several studies-in-progress on potential impacts and relationships between S. noctilio and the community of associates – including natural enemies and competitors – present in Ontario pine forests. Surveys indicate that S. noctilio activity is not related to the amount of susceptible host material (i.e., overtopped pines) available; we found no evidence that S. noctilio populations are host-limited. Conversely, parasitism by Ibalia spp. increased with site-level Sirex activity. Observational and manipulative studies suggest that associates may negatively impact S. noctilio populations.



4.   Phenology of Sirex nigricornis



            Jessica Hartshorn, Larry Galligan, & Fred Stephen



Department of Entomology, University of Arkansas, Fayetteville, AR 72701



Adult female Sirex nigricornis were collected over a four year period, 2009-2012, and their patterns of cumulative capture were analyzed using regression analysis. Adult flight occurred across the state of Arkansas from October through December. Cumulative capture curves exhibited both linear and non-linear patterns in different years and no significant differences in capture rates were detected among regions within any year. The timing of adult wasp captures differed significantly among years, which may suggest that climatic differences affected woodwasp development within trees, causing them to emerge at different times from year to year. Timing of flight can determine what resources are available for emerging adults. By comparing these data to phenological studies of S. noctilio, we are able to estimate if the invasive species will colonize pine hosts in the Southeast earlier in the year and, therefore, potentially have a competitive advantage over S. nigricornis.



5.   Spatial colonization patterns of Sirex noctilio at a pine plantation in the Adirondacks



Christopher Foelker & Melissa Fierke



State University of New York, College of Environmental Science and Forestry, Syracuse, NY



Spatial colonization patterns can be an important component of understanding an insect’s life history. Aggregation can play a key role in population biology by allowing insect herbivores to escape regulation by predators or overcome host defenses. Here, we analyze spatial colonization patterns of the European woodwasp, Sirex noctilio, at a pine plantation in New York State. Sirex noctilio is a recently-introduced invasive pest that has caused considerable economic loss in commercial pine plantation in the Southern Hemisphere and attacks and kills Pinus resinosa and P. syslvestris in North America. We used Kulldorff’s spatial scan statistic to assess whether tree mortality from S. noctilio was spatially aggregated more or less than expected given the a priori assumption of complete spatial randomness of events. We tested patterns of aggregation using four spatial scan windows sizes (5, 10, 20, and 30 m) to assess consistency in the model from a fine to coarse analysis. We found mortality attributed to S. noctilio was significantly clustered at each scanning window extent. However, patterns of mortality may not be attributed to the same underlying processes for each model.



6.   A volatile issue: kairomones in the host selection of Sirex noctilio



Katalin Böröczky1, Fred Hain1, Damon J. Crook2, and Coby Schal1



1Department of Entomology, North Carolina State University, Campus Box 7613, Raleigh, NC 27695-7613, USA, 2USDA, APHIS, PPQ, CPHST, 1398 W. Truck Rd., Buzzards Bay, MA 02542-1329, USA



What cues govern the host seeking behavior of Sirex noctilio females and from what distance? Do we have enough data to develop a Sirex specific trap?



Since the first report on Sirex noctilio in North America in 2005 significant efforts have been made to determine the most important factors in the interaction between the woodwasp and its North American pine hosts with an improved trapping method as the ultimate goal in mind. Terpenes emitted by the stem section of stressed pines are believed to attract ovipositing females. Though behavioral observation and development of laboratory bioassay are the first typical steps of classical chemical ecology research, relatively slow progress has been made in these areas due to challenges of rearing and the phototactic behavior of Sirex. A powerful method to obtain reproducible biological data with a relatively low number of insects in a laboratory setting is the electroantennogram technique combined with gas chromatography (GC-EAD). Here we describe the composition of the volatile emissions of four potential host species in the southeastern US and report on the components that can be detected by female Sirex noctilio. The results are discussed in context of electrophysiological and field trapping data available on Scots pine, a co-evolved host species, and of literature data from other continents the woodwasp had invaded. We hope to stimulate discussions about where future research should focus in order to answer the above questions.



7.   The importance of olfactory and visual cues in developing better monitoring tools for Sirex noctilio



Mark A. Sarvary1, Miriam F. Cooperband2, Ann E. Hajek1



1Department of Entomology, Cornell University, Ithaca, New York 14853-2601, 2USDA-APHIS-PPQ-CPHST, Otis Laboratory, 1398 W. Truck Rd. Buzzards Bay, MA 02542



To enhance Sirex noctilio monitoring, we evaluated three trap types, using an aggregation pheromone, a/b pinene attractant and light as baits. Both female and male wasps were studied in a walk-in wind tunnel using choice and no-choice tests. Traps baited with a lower pheromone concentration (0.1 mg) attracted both sexes, but the traps did not capture a large proportion of woodwasps. An increased pheromone concentration (1 mg) increased the success of captures in one of the three traps, and the tested traps were more successful at capturing females than males. Traps baited with a blank lure did not visually attract S. noctilio, but the addition of light to both clear and black intercept traps significantly increased the proportion of captured woodwasps. In the wind tunnel, lights were more attractive to the woodwasps than the widely used a/b pinene lure.



8.   Field testing a candidate male aggregation pheromone blend in South African plantations: Challenges and lessons learned



Jeff Garnas1,2, Miriam Cooperbrand3, Bernard Slippers1,4 and Brett Hurley1,2



1Forestry and Agricultural Biotechnology Institute (FABI), 2Department of Zoology and Entomology, University of Pretoria, 3Animal and Plant Health Inspection Service (APHIS), 4Department of Genetics, University of Pretoria



Efficient monitoring is critical to studying population dynamics and the success of any integrated pest management system. Kairomone lures comprising a blend of tree stress volatiles have been successful as an attractant for Sirex noctilio females where high-density populations occur (i.e., South Africa), but have been much less so in other parts of the wasp’s native and invasive range (i.e., Europe and North America). We tested synthetic pheromone blends based on male-produced compounds that elicited antennal responses and that were attractive to both males and females of S. noctilio in laboratory and wind tunnel assays. Field trials were performed during the flight season in 2010, 2011 and 2012 and factors hypothesized to influence trap-catch effectiveness, including major component concentration (0, 0.3, 3 and 30 mg per trap), presence versus absence of a minor component, trap type, trap height and pheromone lure type (rubber septa vs. FlexiluresTM) were examined. In all years, kairomone lures were highly effective at attracting female wasps. Male catch was universally low. Pheromone lures were not an effective attractant under any treatment combination and trap height (2 v. 8 m) likewise showed no effect. Black panel traps were by far the most effective when paired with kairomone lures.



9. Environmental conditions promoting mating in Sirex noctilio



Isis A.L. Caetano and Ann E. Hajek



Department of Entomology, Cornell University



This experiment was conducted during the summers of 2012 and 2013 to identify conditions associated with mating of Sirex noctilio. First trials were conducted in small containers with one male and one female in the laboratory and were often unsuccessful and insects were damaged. Next, larger cages made of netting with artificial lights and more males than females were used in the laboratory with low rates of mating. Finally, cages were placed in the shade outdoors with ten males and one female per trial, next to glass greenhouses. This combination of conditions was very successful for mating, especially on sunny, warm days.



10. The translocated peptide from Sirex noctilio venom that causes needle flagging in oviposited pines



J. Michael Bordeaux1 and Jeffrey F. D. Dean2



1Warnell School of Forestry and Natural Resources, 2University of Georgia, Athens, Georgia 30602 USA



During oviposition female Sirex noctilio woodwasps inject targeted pines with venom (acid) gland secretions that can cause rapid wilting (flagging) of needles in the tree crown. Early work demonstrated that introduction of the woodwasp venom was a critical component for establishing fungal infection and subsequent mortality of the trees, and it was presumed that the translocated factor responsible for needle wilt contributed to a depression of host defense responses that would otherwise limit infection. Using a qRT-PCR assay to follow the expression levels of pine genes that respond to the translocated factor in S. noctilio venom, we fractionated whole venom to isolate a small, heat-stable peptide. The purified peptide has a molecular mass of 1850 daltons as determined using MALDI-TOF and confirmed by electrospray ionization (ESI) mass spectrometry. Protein sequence determination using Edman degradation identified a core sequence of 11 amino acids harboring signatures for multiple post-translational modification events. The amino acid sequence for this peptide also matched uniquely to a highly abundant, but unannotated, transcript found in the transcriptome of the S. noctilio venom glands. Efforts are underway to validate function and test structural elements using synthetic peptides to replicate the effects of the peptide isolated from venom. The availability of a synthetic peptide capable of replicating the needle-wilt effect of S. noctilio venom will permit us to study signaling and molecular response pathways in pine that are targeted by this venom component. Improved understanding of how this system operates may inform breeding activities to develop pines that are more resistant to S. noctilio attack.



11. Chemical and Nutritional Ecology of Parasitoids of Siricid Wasps



K.J.K. Gandhi1, D. Robertson1, B.T. Sullivan2, J.R. Meeker3, W. Johnson3, and W.P. Shepherd2



1Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, Georgia, 2USDA Forest Service, Southern Research Station, Pineville, Louisiana, 3USDA Forest Service, Forest Health and Protection, Pineville, Louisiana



We are developing trapping technologies for hymenopteran parasitoids, especially Ibalia leucospoides ensiger (Norton) (Hymenoptera: Ibaliidae) that are known to be effective biocontrol agents of the exotic woodwasp, Sirex noctilio F., in other non-native habitats. Our research objectives are to: 1) assess olfactory attractants for parasitoids of native siricids that could be effectively used as baits in traps; and 2) determine effects of sugar feeding on parasitoids in the laboratory. We conducted experiments on the responses of I. l. ensiger to various olfactory cues from their hosts. Logs were cut from individual trees to which three treatments were randomly assigned: 1) sterile water drilled into log; 2) mucus taken from the venom gland of Sirex and drilled into log; and 3) fresh oviposition site of Sirex. SPME fibers were attached to each of the 4-5 locations on the log, and volatiles were collected every 2, 4, 8, and 16 days. Behavioral bioassays were used to gauge the individual wasp response to treatments. GC-EAD was used to assess chemicals present in the volatiles and responses of antennae of female I. l. ensiger to these compounds. Preliminary results from behavioral assays indicate that Sirex oviposition sites instead of venom gland and sterile water sites were highly attractive to the females of I. l. ensiger. Specifically, GC-trace from the oviposition site of Sirex showed that pine volatiles such as monoterpenes eluted first followed by oxygenated monoterpenes. Antennae of I. l. ensiger appeared to the responsive to both the kinds of terpenes. However, further work on synthetic compounds found in previous assays suggested that the antennae of I. l. ensiger were responsive to several oxygenated monoterpenes. We are currently synthesizing some of the oxygenated monoterpenes of interest to test them for attractiveness to I. l. ensiger under laboratory and field conditions. In a series of experiments, we also determined the adult survivorship of I. l. ensiger if provided with different types of sugars. Treatments included no water, sterile water, and sugars including glucose, sucrose, fructose, maltose, and lactose with several types of molarity (0.5, 1, and 2 M). Preliminary results indicate that greatest longevity was found for I. l. ensiger females when given monosaccharaides instead of disaccharides. Further, wasps provided with 1M sucrose had the greatest survivorship. There were no differences in survivorship of I. l. ensiger with and without water. Future work will be focus on determining the number of eggs per ovaries and their maturity-level to assess if sugar feeding had any effect on their reproductive potential. Overall, our research will assist in refining trapping methods for parasitoids of S. noctilio, and optimal feeding solutions to maintain their colonies under laboratory settings.



12. Adaptations for wood-feeding in the European Woodwasp, Sirex noctilio Fab. (Hymenoptera: Siricidae)



Brian M. Thompson and Daniel Gruner



Dept. of Entomology, University of Maryland, College Park, MD



Abstract- Nitrogen (N) is a limiting nutrient in most terrestrial ecosystems, but is exceedingly deficient in woody plant tissues (e.g. 0.03-0.1% in pine xylem). Feeding on wood is further deterred by ubiquitous lignocellulosic compounds. The European woodwasp, Sirex noctilio (Hymenoptera: Siricidae) is a prolific consumer of pine xylem. In this study, we explored adaptations to wood-feeding in Sirex noctilio including nutrient analyses of food inputs and symbiotic associations involved in ameliorating physical deterrents and nutrient deficiencies of wood. Plant material was the main source of essential sterols, but digestion of wood and N were primarily attributable to fungal and bacterial symbionts respectively. Morphological adaptations within Sirex noctilio facilitated symbiotic functions. The nature of coevolved adaptations for symbiosis and physical processing of wood may prove to be ubiquitous within the Siricidae and stand as major evolutionary steps to feeding on the wide-spread but recalcitrant food resource that is wood.



13. Amylostereum genetic diversity and Sirex associations in Europe and the US: Insights on sources of S. noctilio introductions and implications on control strategies



Louela A. Castrillo1, Ann E. Hajek1, Ryan M. Kepler1, Juan A. Pajares2, György Csóka3, and Iben M. Thomsen4



1Department of Entomology, Cornell University, Ithaca, NY 14853, USA, 2Sustainable Forest Research Management Research Institute, University of Valladolid, Palencia, Spain, 3 Hungarian Forest Research Institute, Sarvar, Hungary, 4Faculty of Life Science, University of Copenhagen, Copenhagen, Denmark



The invasive wood wasp Sirex noctilio is associated with a symbiotic white rot fungus, Amylostereum areolatum, which females inject into trees when they oviposit and serves as food source for immatures. In this study we examined the genetic diversity of A. areolatum isolated from S. noctilio and other woodwasps from Europe in comparison with samples from New York. Multilocus genotyping (ITS, mtssu, RPB2, tef1, and laccase genes) revealed multiple fungus genotypes in New York: one genotype represented one of two more widespread genotypes in Europe and the rest were unique, which suggested unrepresented source populations. These results concur with reports of multiple introductions of S. noctilio to North America. Our results also showed lack of fidelity between wasp hosts and Amylostereum species and suggest the need to test specificity and suitability of biological control nematodes Deladenus spp. against different genotypes of the Sirex-Amylostereum complex for effective wasp control.



14. A tangled food web between Deladenus, Amylostereum, and Sirex



E. Erin Morris1, Ann E. Hajek1, Elliott Zieman2, and David Williams3



1Department of Entomology, Cornell University, Ithaca, New York 14853-2601, USA. (eem62@cornell.edu),  2Department of Zoology, Southern Illinois University, Carbondale, IL 62901, USA, 3USDA, APHIS, CPHST, Buzzards Bay, MA 02542-1308, USA



Both Deladenus nematodes and Sirex woodwasps rely on the white rot fungus Amylostereum areolatum for continued survival. Multiple Sirex species occur in the United States, and these woodwasps carry different species and strains of Amylostereum fungus. We hypothesized that fungal strain would impact reproduction of the nematode. Evidence gathered by examining two species of Deladenus nematodes on various fungal isolates showed D. siricidicola, as well as a closely related native North American nematode, D. proximus, achieves significantly higher population numbers on some strains of the fungus than on others. Notably, D. proximus thrived on a strain of A. areolatum, previous assumed to be a non-host. We then focused on one possible reason for the differences in population number by hypothesizing that a role reversal occurs wherein fungal hyphae invade and kill nematode eggs, leading to lower populations of nematodes. D. siricidicola eggs were exposed to A. areolatum fungus to quantify the number of eggs lost to fungal invasion. Additional cryogenic scanning electron microscopy and fluorescence microscopy showed that fungal hyphae are able to make extensive assimilative hyphae in both eggs and adult female nematodes. We discuss implications of these findings in a biocontrol context.



15. Sirex nigricornis: Nematode abundance and infectivity; effects of insect associates on oviposition preference and performance



F. M. Stephen, A. Lynn-Miller, L. D. Galligan, and D. M Keeler



University of Arkansas, Fayetteville, AR



The ecology of native Deladenus nematodes and their relationship with native North American Sirex woodwasps is poorly understood. We initiated research to confirm the presence of Deladenus in S. nigricornis females and to compare parasitism rates among three pine-forested regions in Arkansas. In 2009 and 2010, Sirex nigricornis females were trapped, dissected and examined for Deladenus, which was found in woodwasps from all regions. Abundance of nematodes and parasitism rates varied, with a high in 2010 of 44% in the Ozark Mountains. Dissections revealed that percentages of S. nigricornis eggs infected by nematodes ranged from 0 to 100%. No difference was found in percentage of infected eggs within female S. nigricornis among regions or between years. In 2011we examined how Ips beetles and other pine associates affect oviposition preference and offspring performance of S. nigricornis. Females did not avoid drilling into bolts with other subcortical insects, but created fewer tunnels per drill site, and S. nigricornis offspring mortality was highest on bolts colonized by other insects.



16. Parasitism of Sirex noctilio by non-sterilizing Deladenus siricidicola in northeastern North America



Stefanie A Kroll1, E. Erin Morris1, and Ann E. Hajek1



1Department of Entomology, Cornell University, Ithaca, New York 14853-2601, USA. (eem62@cornell.edu)



The parasitic nematode Deladenus siricidicola has been extensively used for biological control of the invasive woodwasp Sirex noctilio in the Southern Hemisphere. The strain of D. siricidicola used for biological control sterilizes S. noctilio females, although non-sterilizing strains of this nematode also occur. A non-sterilizing (NS) strain is established in the most recent invasion of S. noctilio, in northeastern North America. This study analyzed the effects of the NS strain of D. siricidicola on invasive S. noctilio collected from New York and Pennsylvania. Nematode parasitism had both a direct negative effect on the number of eggs produced by adult female S. noctilio and an indirect negative effect, due to smaller size in nematode-parasitized females. S. noctilio with NS nematodes were found in 44.0% of trees and 26.9% of all individuals diagnosed, reaching 27.9 + 26.0 % parasitism (mean + S.D.) when averaged across sites. There was greater parasitism of female S. noctilio than males. We also compared parasitism by hymenopteran parasitoids to NS parasitism. Parasitism by nematodes averaged 31.9 + 35.4% per tree, while parasitism by hymenopteran parasitoids averaged 41.8 + 19.6%.NS D. siricidicola may be a less effective biological control agent than the sterilizing D. siricidicola or parasitic hymenopterans.



17. Response of Ibalia l. ensiger to volatiles of Amylostereum



Stephen Teale



Department of Environmental and Forest Biology, College of Environmental Science and Forestry, State University of New York, Syracuse, New York



Volatiles of A. areolatum are a potential cue for parasitoids because they are a reliable indicator of Sirex presence and life stage. Sirex parasitoid species exhibit searching behavior in response to Amylostereum odors. Ibalia leucospoides and Rhyssa persuasoria are most attracted to A. areolatum cultures that correspond in age with their preferred Sirex life stages. We are identifying the volatile components that mediate host finding by Ibalia leucospoides using laboratory and field behavioral assays, GC-EAD and GC-MS.



18. Biological control releases of Deladenus siricidicola



David W. Williams1, Ann E. Hajek2 and E. Erin Morris2



1USDA APHIS, CPHST Otis Lab, Buzzards Bay, Massachusetts 02542, 2Department of Entomology, Cornell University, Ithaca, New York 14853-2601



Deladenus siricidicola Kamona, the nematode strain presently mass-produced in Australia for biological control of Sirex noctilio, was investigated in the U.S. to evaluate overwintering, release methods, and efficacy against S. noctilio in North America. From 2006-2012 controlled releases were conducted each year with different numbers of trees and treatments. Kamona was mass produced in Massachusetts, trees infested by S. noctilio were located in New York and Pennsylvania, and during fall, nematodes were injected into holes punched into the felled trees. In late winter, 3 0.7 m long bolts were removed from each tree and Sirex and associated communities were reared under quarantine while the remainder of each tree was completely chipped. Levels of nematode parasitism were usually higher in releases than controls but parasitoids were often more abundant than nematode parasitism. Molecular work is being conducted to evaluate percentage parasitism by Kamona versus the non-sterilizing D. siricidicola already present.



19. Trace metals in the ovipositors of wood-boring Hymenoptera



 Karen Sime1 and Paul Tomascak2



1Department of Biological Sciences and 2Department of Earth Sciences, State University of New York at Oswego, Oswego, NY 13126



Metal-protein complexes can help strengthen parts of the insect cuticle subject to extreme wear and tear. High concentrations of Zn, Mn, and/or Ca have been reported in the mandibles of numerous taxa and in the ovipositors of hymenopterans that drill through wood. We report on a novel method of assessing trace metals that allows for quantitative comparisons between individuals and across taxa. Ovipositors of Sirex noctilio (Siricidae) were sectioned and dissolved. Analysis by inductively coupled plasma mass spectrometry revealed high concentrations of Zn in ovipositor tips. Manganese is present but in low abundance, comparable to Ni, Fe, and Cu. In addition, synchrotron X-ray fluorescence was used to map metal distribution. These scans indicated that Zn is concentrated in the outer surface of the cutting edges. Preliminary studies of rhyssine ichneumonids indicate enrichment of ovipositor tips with Mn and to a lesser extent Zn, with Ca more broadly distributed throughout the structure.