Research Article |
Corresponding author: Ann E. Hajek ( aeh4@cornell.edu ) Academic editor: Alain Roques
© 2017 Ann E. Hajek, Jacob C. Henry, Christopher R. Standley, Christopher J. Foelker.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Hajek AE, Henry JC, Standley CR, Foelker CJ (2017) Comparing functional traits and abundance of invasive versus native woodwasps. NeoBiota 36: 39-55. https://doi.org/10.3897/neobiota.36.14953
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Functional traits are useful for comparing the resource use of invasive and native species, with goals of identifying resource overlap to predict competitive interactions. The invasion of northeastern North America by the woodwasp Sirex noctilio has resulted in competition with the native congeneric Sirex nigricornis for suppressed and weakened pines. We compared sizes of adults, venom glands, fecundity, tree species use, voltinism and abundance of the invasive woodwasp S. noctilio with the native S. nigricornis in northeastern North American pines. Rearing adults from attacked pines showed that these species used the same tree species but S. noctilio were far more abundant, especially with increasing time since establishment. Adults of the invasive S. noctilio were larger than S. nigricornis, female S. noctilio had larger glands carrying phytotoxic venom in relation to body size, average-sized S. noctilio females carried more eggs, and S. noctilio developed faster than S. nigricornis. Sirex noctilio was the dominant woodwasp infesting suppressed pines in our study areas. We hypothesize that the future abundance of S. nigricornis could depend in part on the availability of wood for oviposition by this native that is not available or acceptable to the earlier-emerging S. noctilio.
invasive species, functional traits, Siricidae , wood borers
A major cause of global change is the pervasive introduction of alien species (
The nature and extent of ecological impacts of alien species depend on the functional ecology of these species in the context of the structure, diversity, and evolutionary experience of the recipient community (
Arthropods constitute the most diverse group of organisms (
The establishment of S. noctilio in eastern North America is the first time that this invasive has invaded an area hosting native Pinus and Sirex communities, including the congeneric native pine-specialist S. nigricornis which has never been considered a pest in North America. Sirex nigricornis is associated with one of two fungal symbionts: either the same fungal symbiont as S. noctilio (A. areolatum) or Amylostereum chailletii (
Red pines (Pinus resinosa) and scots pines (Pinus sylvestris) with resin beads characteristic of S. noctilio attack (
For studies comparing the abundance of Sirex species by tree species, Sirex were reared from 50 cm long bolts placed in horizontal cardboard rearing tubes. Bolts were spaced from tube sides using nails and were housed at ambient environmental conditions in a screened outdoor insectary. Glass emergence jars were fitted to the tube ends, oriented facing screened windows for a natural photoperiod, and emerging Sirex were collected daily.
To quantify the sizes of emerging adults, we followed
To compare sizes of S. noctilio adults with (n = 92) and without (n = 412) nematodes, we used randomly selected individuals emerging from barrels in 2012–2014. There were far fewer S. nigricornis emerging during this period and nematode infections were uncommon in S. nigricornis, so analyses included only S. nigricornis of both sexes without nematodes (n = 36).
To assess weights of venom glands, a total of 51 S. noctilio were randomly selected in 2014–2016 for dissection and 21 of these contained nematodes. Since few female S. nigricornis emerged from rearings from the northeast during 2014–2016, 30 S. nigricornis females from Arkansas and Louisiana that had been caught in panel traps were used for venom gland weights. We were concerned that these S. nigricornis from the southeast both had been flying before being trapped and were from a different geographic region. To test the accuracy of using southeastern S. nigricornis as replacements for northeastern S. nigricornis, we compared the relationship between numbers of eggs and body size for S. nigricornis females collected from traps in the southeastern vs. emerging from wood from the northeastern USA. Southeastern S. nigricornis carried the same number of eggs in relation to prothorax width as S. nigricornis emerging from wood in the northeastern US (t = -1.10; P = 0.2713), so we used venom gland weights from southeastern S. nigricornis for comparisons.
Use of P. resinosa versus P. sylvestris by S. nigricornis and S. noctilio was quantified using infested trees in northern New York State, with site information in
First and second year emergence by Sirex noctilio and Sirex nigricornis.
Location | 2 years of emergence | Total Sirex emerging | Emergence year 1 (%) | Emergence year 2 (%) | % trees with 2nd year emergence | |
---|---|---|---|---|---|---|
Sirex nigricornis | Warren County, NY | 2013–2014 | 38 | 44.7% | 55.3% | 50.0% |
Sirex noctilio | Warren County, NY | 2013–2014 | 15 | 73.3% | 26.7% | 25.0% |
Tioga County, PA | 2014–2015 | 1056 | 98.5% | 1.5% | 39.1% | |
Tioga County, PA | 2015–2016 | 709 | 89.1% | 10.9% | 42.9% |
On 4 June 2013, 12 Sirex-infested P. resinosa were collected from a plantation on River Road, Warren Co., New York (43°31'59.7"N 73°49'30.8"W). Trees were cut into 219 70 cm long bolts and woodwasps were reared as described above, with barrels checked every 1–2 days to collect individuals very soon after emergence; females were never found to be ovipositing when collected this soon after emergence. In November 2013, barrels were thoroughly checked for dead Sirex so that any first year emergers were not mistaken for second-year emergers. Barrels were stored in an unheated barn over the winter and were checked for emergence throughout the 2014 flight season. This procedure was repeated for a third year, through the 2014–2015 winter and 2015 flight season. As a continuation to this study, in Tioga Co., Pennsylvania, in spring 2014 we harvested 23 infested P. resinosa and, in spring 2015, 29 P. resinosa. Wood from trees cut in 2014 and 2015 was maintained and emergence was checked for two years.
Pines with resin beads indicative of S. noctilio attack were harvested in central and northern New York State and north-central Pennsylvania and Sirex were reared from them. Even within the same region, it was rare that the exact same site was sampled more than one year. Pinus resinosa were harvested from plantations in Tioga Co., Pennsylvania yearly, from 2011 and 2013–2015, with 20–30 trees harvested each year. Sirex were also reared from mixtures of P. resinosa and P. sylvestris from natural forests in northern New York State and from mature plantations in central New York State. In 2007 only P. sylvestris was sampled as described in
A general linear mixed model with year as a random effect was used to compare sizes of Sirex. To compare body size versus venom gland mass, the significance of difference between slopes was calculated (
Weights of venom glands increased with increasing body size (measured as prothorax width) (Fig.
Relationships between venom gland mass and body size, measured as pronotum width, for S. noctilio and S. nigricornis. Sirex noctilio data were analyzed by presence or absence of parasitism by nematodes (Deladenus siricidicola) while numbers of S. nigricornis parasitized by nematodes were too low for analysis. A Sirex noctilio females without parasitism by nematodes B S. noctilio females parasitized by nematodes, and C S. nigricornis without nematode parasitism.
For collections across 2012–2014, on average non-parasitized S. noctilio females (prothorax width: 3.90 + 0.83 mm) were larger than S. nigricornis females (2.46 + 0.35 mm) (t = 6.27; P < 0.0001) and non-parasitized S. noctilio males (2.92 + 0.83 mm) were larger than S. nigricornis males (2.38 + 0.56 mm) (t = 3.00; P = 0.0028). For male S. noctilio, nematode parasitism unexpectedly resulted in larger body sizes (t = -2.62; P = 0.0091) while this relationship was reversed for females (t = 2.34; P = 0.0195) (Fig.
Sirex densities by tree species did not differ between the two Sirex species (F1,52.6 = 0.01; P = 0.9395). The only main effect that was significant in the model was the comparison of densities of the two Sirex species (F1, 56 = 32.11; P < 0.0001; S. nigricornis density = 8.9 ± 2.6/m3, S. noctilio density = 44.3 ± 11.5/m3).
Nearly half of adult S. nigricornis emerged from wood during the first season in our rearings, over half emerged during year 2 (Table
Between 2010 and 2015, S. nigricornis densities were lower than densities of S. noctilio in central New York State and north central Pennsylvania, where most pines that were sampled had been purposefully planted (Table
Co-occurrence and densities of S. noctilio and S. nigricornis emerging from P. sylvestris and P. resinosa from New York and Pennsylvania, 2007-2015 (only including trees from which Sirex emerged).
Total infested trees sampled | Tree speciesa | Total trees with only S. noctilio | Total trees with only S. nigricornis | Total trees with both Sirex species | Mean S. noctilio/tree ± SE | Mean S. nigricornis/tree ± SE | P valuesb | |
---|---|---|---|---|---|---|---|---|
Central New York Statec | ||||||||
2007 | 6 | S | 3 (50%) | 0 (0%) | 3 (50%) | 218.8 ± 64.9 | 13.2 ± 10.4 | 0.0625 |
2010 | 9 | SR | 6 (67%) | 1 (11%) | 2 (22%) | 41.8 ± 9.8 | 1.6 ± 0.9 | 0.0117 * |
2011 | 19 | SR | 12 (63%) | 0 (0%) | 7 (37%) | 48.2 ± 14.9 | 5.5 ± 3.0 | <0.0001 * |
Northern New York Stated | ||||||||
2010 | 13 | SR | 3 (23%) | 3 (23%) | 7 (54%) | 13.4 ± 4.8 | 20.2 ± 6.5 | 0.5532 |
2011 | 8 | SR | 4 (50%) | 2 (25%) | 2 (25%) | 17.7 ± 5.7 | 3.7 ± 1.8 | 0.1641 |
North Central Pennsylvaniae | ||||||||
2011 | 19 | R | 15 (79%) | 0 (0%) | 4 (21%) | 38.4 ± 10.3 | 0.7 ± 0.4 | <0.0001 * |
2013 | 17 | R | 12 (71%) | 1 (6%) | 4 (24%) | 22.8 ± 6.0 | 1.6 ± 1.2 | 0.0001 * |
2014 | 26 | R | 22 (85%) | 0 (0%) | 4 (15%) | 52.5 ± 10.7 | 0.7 ± 0.5 | <0.0001 * |
2015 | 15 | R | 15 (100%) | 0 (0%) | 0 (0%) | 22.8 ± 7.4 | 0.0 ± 0.0 | <0.0001 * |
The overall body sizes of S. noctilio as well as the sizes of their venom glands were significantly larger than the bodies and venom glands of the native S. nigricornis. The venom glands of S. noctilio are also larger than those of seven other European siricids (
The densities of the two Sirex species emerging from P. resinosa and P. sylvestris did not differ by tree species. Studies have found a trend of P. sylvestris being colonized more frequently by S. noctilio compared with P. resinosa (
Sirex noctilio and S. nigricornis also differed significantly based on the percentages of the populations emerging from wood after 1 vs 2 years. In our studies, S. noctilio mainly emerged in year 1; from 1.5–27.7% of S. noctilio emerged the second year (Table
In northeastern North America S. noctilio is now the most abundant woodwasp attacking pines (
Sirex noctilio has a temporal advantage over S. nigricornis as many emerge 1–2 months before S. nigricornis, although there is overlap in emergence between these species in northeastern North America (
In this study, we investigated aspects of the biology and ecology of these now-sympatric native and invasive siricids toward predicting the impact S. noctilio might be having on this congeneric native species also utilizing suppressed pines (Suppl. material
We cannot definitively answer to what extent the presence of S. noctilio results in more or less habitat in which S. nigricornis can develop. Little is known of the biology and ecology of S. nigricornis but we know that it will oviposit and develop in pines already attacked by S. noctilio. Attacks by S. noctilio could create more suppressed trees acceptable to S. nigricornis by pre-injecting the symbiont plus venom and thereby disabling tree defenses and thus creating more habitat for S. nigricornis. However, abundance data suggest that few S. nigricornis emerge from trees that they co-inhabit with S. noctilio. As an alternative, since S. noctilio mostly flies and oviposits before S. nigricornis, perhaps this invasive attacks the best of the transient resource of weakened trees that could potentially be used by either species and S. noctilio thus uses most of this resource before S. nigricornis adult females would have emerged to oviposit. The extent that co-occurring S. nigricornis and S. noctilio will compete for recently dead trees remains to be determined. Further data documenting the ecology and naturally occurring densities of S. nigricornis in the southeastern US, where pine forests are extensive, will assist with predicting to what extent S. nigricornis niches and abundance will be altered when S. noctilio spreads into this region.
We thank Brad Regester, Bill Laubscher and Tim Marasco, Pennsylvania DCNR Bureau of Forestry for assistance with infested wood from north central Pennsylvania, Jim Meeker, Wood Johnson, Jessica Hartshorn and Fred Stephen for providing S. nigricornis from Louisiana and Arkansas, and
A. Association between health of pines and response by S. noctilio and S. nigricornis; B. Emergence of S. noctilio and S. nigricornis from pines collected from New York and Pennsylvania in spring 2012
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