sILPhIum GaLL wasPs: LIttLe-known PraIrIe

Silphium G a ll Wa sp s: L i t t l e- K now n P r a irie Sp eci a lis t s
Richard Henderson, Wisconsin Department of Natural Resources, Science Operation Center, 2801 Progress Road,
Madison, WI 53716-3339, [email protected]
scott b. sauer, 4302 Clover Ct., Madison, Wisconsin 53711
Abstract: The original prairies of the Midwest have nearly
vanished, and thus so have many of the insects linked to
prairie vegetation. In 1891, five species of gall-forming wasps,
Antistrophus spp. (Hymenoptera: Cynipidae), were described
from various plants in the genus Silphium (compassplant,
prairie dock, rosinweed, and cup-plant) growing in Illinois
prairies. These wasps are thought to live only in Silphium
spp., which are characteristic plants of our Midwest prairies.
Silphium spp. once dominated the landscape, but now they
are mostly limited to scattered remnants of original prairie
and prairie plantings. For one hundred years following the
description of these wasps, the scientific community paid
them very little attention. Then, starting in the early 1990s,
researchers began to take interest in the group, describing
additional species and investigating the wasps’ ecology and
response to fire. However, the distribution and conservation
status of Silphium gall wasps are still very much a mystery.
Recently, two species have been documented for the first
time in Wisconsin, and more species are suspected of being
present. Efforts to document the distribution and status of
these uncommon prairie specialists in Wisconsin and adjoining states are being made.
Key Words / Search Terms: insects, tallgrass prairie,
Antistrophus, Hymenoptera, Cynipidae, compassplant,
prairie dock, rosinweed, cup-plant, conservation
Discovery
Working in central Illinois in the late 1880s, as the last of
the tallgrass prairies were succumbing to the plow and cow,
Dr. Clarence P. Gillette, along with collaborators Mr. C.A.
Hart and Mr. J. Marten, discovered several species of small
(2-5 mm) gall-forming wasps associated with four plants of
the genus Silphium: S. laciniatum (compassplant), S. terebinthinaceum (prairie dock), S. integrifolium (rosinweed), and S.
perfoliatum (cup-plant). These were among the most dominant plants of the original eastern tallgrass prairies. Gillette and company made observations in the field and reared
adult wasps from plant tissue. From this work, Gillette published descriptions of five Cynipid (Hymenoptera) wasps of
the genus Antistrophus (Gillette 1891).
Antistrophus silphii
Gillette described this species as forming abrupt, large (2.54.5 cm) subglobular swellings (galls) at the tips of the stems
of rosinweed and cup-plant (Figure 1). Each gall supported
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Figure 1. Terminal stem gall on rosinweed (Silphium integrifolium).
Photo by R. Henderson.
many wasp larvae. Gillette concluded that adult specimens
(Figure 2) reared from both species of plants “were in every
way identical with those from galls of the other species.” He
found that the wasps overwintered as pupae in individual
cells within the pith of the galls, emerging as adults in May
or June (Figure 3).
Figure 2. Adult Antistrophus silphii gall wasp reared from a rosinweed
(Silphium integrifolium) terminal stem gall in Wisconsin. Photo by S.
Sauer.
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wasp larvae chambers
numerous than A. rufus. Dr. Gillette, along with Mr. Hart,
visited “fields” of compassplant and “found that the majority
of the stems were more or less infested with cynipidous larvae,
hundreds of which could, in some cases, be found in a single
stem.” They went on to investigate other Silphium species in
the area and “found similar larval cells abundant in S. perfoliatum, S. terebinthinaceum, and S. integrifolium.” However,
they did not rear specimens from these stems to see what species of Antistrophus wasps were present.
Figure 3. Cross-section of rosinweed terminal stem gall showing Antistrophus silphii larvae chambers. Photo by S. Sauer.
Antistrophus laciniatus
Gillette described this species as forming individual, small
(4-5 mm), egg-shaped galls occurring in clusters in the flower
disks of compassplant (Figure 4). Each gall supports one
wasp. The galls occupy the center of the flower disk among
the male (sterile) florets. The flowers appear unaffected by
the wasps, with the gall cluster becoming evident only as the
flower head ripens and falls apart, revealing the galls.
Figure 5. Cross-section of compassplant (Silphium laciniatum) stem
inhabited by Antistrophus wasps and Mordellid beetle larvae. Photo by
S. Sauer.
Antistrophus bicolor
Gillette did not offer much information about this species.
He described it from a single specimen from Normal, Illinois. Both the host plant and gall of the wasp were unknown
to him. Seventy years later, Weld (1959) reports the collection
date of Gillette’s type specimen as being July 6, 1884, and the
accompanying accession catalogue as stating “from Silphium
integrifolium.”
Figure 4. Antistrophus laciniatus galls from the flower disks of compassplant (Silphium laciniatum) in WI. Photo by R. Henderson.
Antistrophus rufus and A. minor
Gillette reared both of these species from flower stalk stems
of compassplant, but unlike the previous two species, there
was no evidence of gall formation. Instead, he found that the
wasps pupated in little cells within the pith just under the
outer wall of the stem (Figure 5). A. minor was smaller and less
Forgotten and Rediscovered
After Gillette’s work, there appears to have been nothing
published about these Antistrophus species for the next 35
years, except for some confusing name-changing and occasional misidentifications by taxonomists, and the passing
along (sometimes with errors) of Gillette’s original observations (Bassett 1900, Kieffer 1902, Beutenmuller 1910). For
example, Beutenmuller (1910) listed all four species of Silphium as hosts for A. rufus. This is likely an error, since his
only reference, Gillette (1891), confirmed A. rufus from only
compassplant. The Antitrophus wasp larvae Gillette found
in the stems of the other species were not reared out and
identified.
In 1912, Lewis H. Weld began to collect specimens and
make field observations of Silphium gall wasps in northeast Illinois (Weld 1926). He reaffirmed compass plant as
the host for A. laciniatus, A. rufus, and A. minor, and docu-
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mented A. minor as also using prairie dock. In 1915 and 1917,
Weld documented A. silphii forming terminal stem galls on
cup-plant in eastern Nebraska. This was the first published
record of any Silphium gall wasp being found outside of Illinois. Weld also presented information indicating that Gillette’s A. bicolor should be considered a synonym of an earlier
described species (Aulacidea harringtoni). Weld found the
plant genius Lactuca to be a host for this gall wasp. However, 30 years later, Weld (1959) treats A. bicolor as a distinct
species. In the Catalog of Hymenoptera in America North of
Mexico (Burks 1979), Antistrophus bicolor is listed as both a
good species in its own right and as a synonym of Aulacidea
harringtoni (same author citation for both listings!), with the
host listed as Lactuca spp. Burks (1979) listed Illinois as the
only location for A. bicolor, and, he states, “Said to produce
galls on Silphium integrifolium,” but he gave no reference
for this. Gillette (1891), the only Illinois source, provided no
information as to hosts or galls for A. bicolor. Weld (1959)
reported the type specimen as coming from S. integrifolium.
A. bicolor appears to be a poorly known species in need of
both taxonomic and ecological work.
Burks’s 1979 comments on A. rufus listed Kansas, in addition to Illinois, as a location for the species. This was the
first reported record outside of Illinois for this species, and
is presumably based on a museum specimen, but he provided
no reference. He listed compassplant as the only host for A.
rufus.
As far as we have been able to determine, it was not until
1991 that any new work on the ecology and habits of Silphium gall wasps was published since the works of Gillette
and Weld, 100 and 80 years earlier, respectively. Then the
activity of work on Silphium gall wasps picked up significantly and remains steady to the present. The “rediscovery”
of these wasps started in 1988, with work at the Konza Prairie
Research Natural Area in eastern Kansas, where Philip Fay
and others began to investigate the ecology of A. silphii on
rosinweed and its affect on that host plant (Fay and Hartnett
1991, Fay and Samenus 1993, Fay et al. 1996). Their research
appears to constitute the first and only published reports of
this gall wasp in Kansas. The authors reported its being very
common in the area, with over a third of rosinweed shoots
galled, and up to 30 larvae per gall (Fay and Hartnett 1991).
The authors also reported an unknown gall in the disk florets of rosinweed at Konza. They speculated that it might
be A. laciniatus (Fay et al. 1996), but did not rear out adult
specimens to confirm. It appears that A. laciniatus has yet
to be confirmed outside of Illinois.
In 1999, John Tooker and others began to investigate the
ecology and habits of internal stem Antistrophus wasps in
central Illinois, where the wasps were first discovered. They
looked at the endophytic insect communities inhabiting the
stems of compassplant and prairie dock (Tooker and Hanks
2004a). They found both A. rufus and A. minor to be common at all eight sites they surveyed, and present in both spe118
cies of Silphium (compassplant and prairie dock). However,
A. rufus was three times more abundant than A. minor. The
average density of internal galls per stem was 80 for compassplants and 62 for prairie dock. This appears to be the
first published documentation of A. rufus using prairie dock.
Tooker and Hanks also confirmed that A. rufus and A. minor
behave just as Gillette (1891) described for A. silphii, in that
the wasps overwinter as pupae in individual cells within the
pith of the galls, in this case within the pith of the stems, and
emerge as adults in May or June.
Tooker et al. (2004b) went on to study the genetic, morphological, and ecological differences of A. rufus populations
using compassplant, prairie dock, and cup-plant. They concluded that each species of plant has its own species of wasp
within what is now considered the A. rufus complex of species (an apparent example of co-evolution). The recognized
species are now A. rufus (restricted to compassplant), A. meganae (restricted to prairie dock), and A. jeanea (restricted
to cup-plant). This now makes six Antistrophus species confirmed as being restricted to prairie Silphium spp. But the
story is still unfolding.
Recent work, yet to be published, by Zhiwei Liu and colleagues at Eastern Illinois University is demonstrating that
the terminal stem gall wasps found on rosinweed and cupplant (A. silphii) may actually consist of two different, but
morphologically very similar, species, as was the case with
the A. rufus complex. This is based on host selection data
(Ginder and Liu 2010), and an ongoing project by Liu and
colleagues looking at genetic markers (Z. Liu, personnel
communication, 2010). If these putative species prove valid,
that will make seven species of Antistrophus tied to prairie Silphium spp. But this still may not be the end of the
story. Questions still remain about the host, ecology, and
taxonomic status of A. bicolor, and no one has documented
what Antistrophus species are in the stems of rosinweed, as
first noted by Gillette (1891). Might this be yet another new
species or simply A. bicolor? We have been unable to find
published documentation of internal stem galls using rosinweed since Gillette’s original observations. A complete list of
species along with listings of published host associations and
the states they have been reported in is provided in Table 1.
Associated Insects
The recent work on Silphium gall wasps has revealed a
whole community of endophytic (within a plant) insects associated with prairie Silphium spp., in some cases possibly
dependent upon them. Fay et al. (1996) observed an undescribed species of Eurytoma wasp regularly parasitizing A.
silphii wasps in rosinweed galls. Tooker and Hanks (2004b),
during a detailed study of endopyhtic insects of compassplant and prairie dock in Illinois, found eight species of parasitoid wasps attacking A. rufus and A. minor gall wasps, or in
some cases possibly attacking the parasitoid wasps attacking
the Antistrophus wasps. They presented evidence that one of
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Table 1. Antistrophus spp. (gall wasps) associated with prairie Silphium spp., along with documented host plant and location records.
SOURCE
WASP SPECIES HOST PLANT
Gillette 1891
Weld 1926 w& 1959
??2
A. bicolor
rosinweed?
IL (Normal)1
A. jeanae
cup-plant
IL
A. laciniatus
compass plant IL (Champaign)
A. meganae
prairie dock
IL
A. minor
compass plant
IL
A. minor
prairie dock
Winterringer
1961
Burks
1979
Fay et Tooker et
al. 1990
al. 2000
Liu
2010
IL in rosinweed3
IL
IL (Glenview)
IL
IL (Evanston)
IL
IL
IL (Evanston)
A. rufus
compass plant
IL
IL (Evanston)
A. silphii
rosinweed
IL (Champaign)
IA4
A. silphii
cup-plant
IL (Normal)
NE (Valley & Omaha) & IA4
Antistrophus sp.
rosinweed
IL
IL
IL & KS3
IL
KS
IL
IL & NE
KS, MO, & IL
IL
No rearing from stems or galls reported.
Taxonomy unclear; reared from Lactuca (1926); reports the accession catalogue of the type specimen as being from rosinweed (1959).
3 No source given, assumed to be from specimen labels or accession catalogues.
4 Unclear as to whether the reported IA collection was from cup-plant or rosinweed.
1
2
the parasitoids, Eurytoma lutea, may be a specialist on Antistrophus wasps. They also consistently found the larvae of a
mordellid (tumbling flower) beetle, Mordellistena aethiops,
burrowing through the stems of compassplant and prairie
dock (Figures 5 and 6), along with three species of parasitoid
wasps that appear to attack this beetle. They demonstrated
that M. aethiops larvae are omnivores, consistently eating
both plant tissue and wasp larvae (Tooker and Hanks 2004c),
which is unusual for the Mordellistena genus. Their larvae
are normally reported as stem-boring herbivores (Ford and
Jackman 1996).
Figure 6. Tumbling flower beetle, presumably Mordellistena aethiops,
emerging from a Silphium stem. Photo by S. Sauer.
Distribution, Status, and Future Work
Since 1996 we have had a collaborative project looking
into which insects species are associated with and dependent upon remnant prairies in Wisconsin and the Upper
Midwest. We have also focused on the status, distribution,
and response to management of these remnant-dependent
species. However, since up to 2,000 species likely fall into
this category, we have not been able to address all taxonomic
groups, and thus we had paid relatively little attention to gall
wasps. We were aware from the literature that there were
gall wasps associated with Silphium spp., but they had been
reported only from Illinois, Nebraska, and Kansas. There
were no specimens or records from Wisconsin. So we did
not spend time looking for them here until the fall of 2005,
when the lead author (Henderson) noticed a large terminal
stem gall on rosinweed in the Madison area and began to
wonder if this could be the work of A. silphii. He had seen
galls on rosinweed on rare occasions in the past, but had dismissed them as the likely product of generalist gall-formers.
This time, however, he collected the gall and waited to see
what emerged. Wasps emerged, and a year later we learned
that they were indeed A. silphii. So, in 2007, we began to
look informally for terminal stem galls on rosinweed. We
found them in a few more locations, but they were not at all
common, except at a site called Underwood Prairie, west of
Madison. Galls were common there, but still far below the
densities observed at Konza Prairie in Kansas, where 35% of
the rosinweed stems had galls (Fay and Harnett 1991). In
comparison, only 2% of the stems at Underwood Prairie had
galls. In 2007, we also collected a few stems of compassplant
to see what they might hold. They produced what has tentatively been identified as A. bicolor. If this proves true, this
will be not only the first documentation of this species in
the state, but the first documentation of its using a Silphium
species other than rosinweed.
Now knowing that at least some Silphium gall wasp species are in the state, we conducted an investigation to learn
more about their distribution and status here. In early April
of 2009, we visited some 40 sites in southern Wisconsin and
collected stems of compassplant, prairie dock, rosinweed,
and cup-plant, and looked for and collected terminal stem
galls on rosinweed and cup-plant. We also solicited people
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to be on the lookout for, and to send us, terminal stem galls
on Silphium spp. We also collected flower disk galls from
compassplant at a few sites. Anyone collecting compassplant
seed in Wisconsin knows that gall clusters in the center of
their flower disks are a common occurrence. But apparently
no one has tried to find out what is inside those galls. So we
collected them in anticipation of finding A. laciniatum. We
also made a few collections from a variety of Silphium spp. in
Iowa, since there does not appear to be any published documentation of Silphium gall wasps in Iowa either.
Identifications of the emerged materials have yet to be
confirmed, but there appear to be many Antistophus specimens, a variety of parasitoid wasps (Figures 7 and 8), and
many mordellid beetles, possibly Mordellistena aethiops
(Figure 6). If the identification of M. aethiops is confirmed,
this would be the first record of that beetle in Wisconsin.
We found terminal stem galls on rosinweed at six sites in
Figure 7. Parasitoid wasp (Eurytomid sp.) emerging from rosinweed terminal stem gall. Photo by S. Sauer.
Wisconsin and two in Iowa, and on cup-plant at four sites
in Wisconsin and two in Iowa. We found Antistrophus gall
wasps in the stems of compassplant at 14 of 18 sites in Wisconsin and 5 of 7 in Iowa; in prairie dock at 18 of 20 sites in
Wisconsin; in cup-plant at 3 of 4 sites in Wisconsin and 2 of
2 in Iowa; and in rosinweed at 9 of 11 sites in Wisconsin and
possibly 4 of 4 sites in Iowa. Lastly, we found what appear
to be A. laciniatum wasps at four sites in Wisconsin.
From what we have observed so far, A. silphii appears
to be the rarest of the Silphium gall wasps in the state of
Wisconsin, and may be worth consideration as a species
of greatest conservation need (SGCN). In the case of the
cup-plant gall, listing as state endangered or threatened
may be warranted. Only a third of the rosinweed populations we checked had terminal stem galls, and, with the
exception of the Underwood Prairie site, the gall numbers
were very low. Adding to this relative rarity is the fact
that, today, rosinweed is very limited in its occurrence on
the landscape compared to its great prevalence 200 years
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Parasitoid wasps
reared from
compassplant stems
Clalcidoidea sp. (reared from rosinweed gall)
Figure 8. Parasitoid wasps reared from Silphium galls and stems in WI.
Photos by S. Sauer.
ago, when prairie and savanna dominated the landscape
of southern and western Wisconsin. Galls on cup-plant
were extremely hard to find. After visiting 50 or more cupplant populations, and getting word out to people to send
us galls, we had only six galls from four sites to show for
it. We estimate that only 1% or 2% of cup-plant populations in Wisconsin support this wasp. The cup-plant gall
appears to be uncommon in Illinois as well. Winterringer
(1961) described it as rather uncommon in Illinois in the
1950s. Dr. Liu of Eastern Illinois University considers the
cup-plant gall to be much less common than the rosinweed
gall, and it should probably be considered endangered (Z.
Liu, personal communication, 2010).
We are currently working to get our material identified
by specialists, and are seeking funding to (1) conduct a
more thorough survey of Silphium gall wasps in the state,
(2) conduct research on the effects of fire on these wasps
and their associated endophytic insect communities, and
(3) investigate the distribution and status of these wasps
throughout their range.
This community of specialist insects shows that there is
still much to learn, and re-learn, about the tallgrass prairie
ecosystem, and that a great diversity of species is a part of,
and dependent upon, that ecosystem. Lastly, it is amazing
to us that such great diversity still exists, given that only
a small fraction of 1% of the original acreage of tallgrass
prairie has survived, and most in small isolated patches. It
gives one hope that recovery and restoration of the system
are still possible.
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