Ailanthus altissima (the stinking ash) is an invasive tree that has
spread to most states in the continental U.S. No formal study has yet
documented the Ailanthus pollination ecology. We observed the insects
visiting Ailanthus at four sites in a western suburb of Chicago over
several weeks during the summer. Numerous insects visited the flowers,
with flies and bees most common, though there was significant
heterogeneity among sites in the composition of the local pollinator
assemblage. Over half of the pollen carried on the legs of large bees
was similar to the Ailanthus morphotype pollen. These results suggest
generalist pollination mediated by geographically wide-spread insect
vectors, which would facilitate range expansion. Moreover, the reliance
on both bees and flies may help explain the species' preference for
disturbed sites and its ability to expand into northern latitudes. Ants
also were frequent floral visitors and potential symbionts, though they
are unlikely pollinators of this dioecious species.
The invasive tree Ailanthus altissima Swingle (stinking ash,
Tree-of-Heaven, Chinese sumac) is an increasing component of U.S.
forests and is a common weed tree in many cities where it is noted for
its capacity to thrive in the cracks in concrete and asphalt nearly as
readily as at a forest edge (Pan and Bassuk 1986). Its tenacity inspired
the book A Tree Grows in Brooklyn (Smith 1943), and later a movie and
musical by the same title. A deciduous member of the tropical family
Simaroubaceae (Quassia), A. altissima [Mill] Swingle was first imported
from China through England in 1784 by William Hamilton, a Philadelphia
gardener (Hu 1979). Additional introductions into the west coast are
thought to have occurred with Chinese immigrants during the 1800s.
Although other species of the genus exist, Ailanthus altissima
(hereafter, Ailanthus) has spread to most states in the United States
(USDA-NRCS 2008) by following human disturbances (Hu 1979, Huebner
2003), often along transportation right-of-ways (Burch and Zedaker
Ailanthus presents large pinnately compound leaves with
indeterminate growth, often yielding large swooping leaves that suggest
its tropical origin (Fig. 1). It incorporates many of the strategies
employed by invasive species including (a) early and profuse
reproduction (Feret 1973) wherein a single adult can produce a million
seed in a year (California Invasive Plant Council 2008), (b)
long-distance seed dispersal (Matlack 1987), (c) aggressive clonal
reproduction (Miller 2000), and (d) a reliance on high sunlight (Grime
and Jeffrey 1965). Moreover, (e) Ailanthus produces toxins that inhibit
plant growth and appear to render it unpalatable to many U.S. herbivores
(Heisey 1990a, b, De Feo et al. 2003). For a recent review of the
biology of Ailanthus altissima see Kowarik and Saumel (2007).
Much less studied are the interactions between Ailanthus and its
mutualists, such as pollinators. Some claim that A. altissima is
wind-pollinated (Ballero et al. 2003) but the strong fetid odor of its
flowers is thought to attract honey bees as well as beetles and other
insects (Hu 1979, Miller 1990). In fact, Ailanthus honey is made in some
quarters of Europe (Dalby 2000). There it is reported that bee keepers
with hives located near large Ailanthus stands produce a smokey,
greenish honey that some consider bitter and undesirable, though
pleasant once sufficiently aged. The flowers appear to fit a generalist
insect pollination syndrome as they are small and actinomorphic with
white to yellowish or greenish petals, presented in dense clusters of
racemose cymes (Fig. 1) in April through June and into July. The species
is dioecious, though hermaphrodites do exist (Gleason and Cronquist
1993). Flowers of both genders emit a strong fetid odor at times
compared to burnt peanut butter (California Invasive Plant Council 2008,
Global Invasive Species Database 2008), though some descriptions
emphasize the male flowers as particularly odoriferous.
Here, we report on the floral visitors of Ailanthus in the western
suburbs of Chicago. Information to date on Ailanthus pollination draws
largely on anecdotal records and casual observations. Our study provides
quantitative information on the putative pollinator assemblage of this
invasive species near its northernmost extent in the Midwestern U.S.
STUDY AREA & METHODS
We conducted the study in the Naperville area, a western suburb of
Chicago, Illinois, between the months of June and August 2006. The
western suburbs are a mosaic of predominantly urban-suburban habitat
intermingled with a large network of forest reserves. We selected four
roughly 1-hectare patches of forest that contained Ailanthus, with
distances between sites ranging from 0.4 to 3 km. Site 1 contained three
large (> 10 cm dbh) female Ailanthus residing in contiguous canopy
edge habitat adjacent to a parking lot and a major highway. Site 2 was
roughly two km away and contained a single, large, freestanding female
Ailanthus in a small park of both prairie (mostly non-native species)
and forest habitat. Sites 3 and 4 were another km away and only 0.4 km
apart in riparian edge habitat adjoining a major road (site 3) and a
parking lot (site 4). Both of these sites held several large Ailanthus,
(site 3, N = 7; site 4, N = 5), with both males and females present.
Note that population size estimates are only approximate since Ailanthus
can propagate clonally (Kowarik 1995) and so in several instances it was
unclear what was a ramet versus a genet.
Field observations and collections were made as follows. We marked
a single, large Ailanthus inflorescence per tree with string and made
insect observations and collections at these during a period of 45
minutes to an hour per site between 9 AM and 2 PM. Most time was
allotted per site to detailed observations and collections at a single
female focal tree, though we also surveyed and collected from other
marked inflorescences at each site every 10-15 minutes. Our goal in
conducting the study was to produce a list of candidate pollinators for
Ailanthus, and since an effective pollinator must visit both male and
female trees to move pollen between genders, one should be able to
sample either (or both) genders and capture the subset of visitors that
are the truly effective pollinators. Some have noted that the male
flowers emit a stronger odor than the female flowers (e.g., Global
Invasive Species Database 2008) and this might attract a larger
assemblage of floral visitors, though the male-specific visitors are of
less interest to our study. We visited all of the four sites every two
to three days over the course of two months with exceptions made for
rainy and windy weather (total hours of observation, 36). We rotated the
order in which we visited the sites so as not to conflate site effects
with temporal variation in insect activity.
Insects visiting Ailanthus flowers were collected with a net after
they were observed interacting directly with the flowers, rather than
simply being present in the vicinity or resting on the inflorescence. A
few exceptions to this rule were some of the smaller insects that were
captured via an aspirator, some directly from the flowers and some from
the net following a sweep. All specimens were stored in a freezer until
identification. Specimens were pinned, labeled, mounted, and identified
down to the family level of classification using two keys (Borror et al.
1989, Romoser and Stoffolano 1998). Visitation rates were calculated for
important taxonomic groups (flies, bees, etc.) and a Pearson chi-square
(SPSS program) was used to test for differences in visitor classes among
During identifications we found that several specimens,
particularly the larger bees, carried pollen loads on their legs, which
were assayed as follows. Pollen was removed from the leg of eleven bees
(family Apidae; ten bees from site 1, one bee from site 3) and placed,
separately, into microcentrifuge tubes containing Calberla's stain
(2 drops of saturated aqueous solution of basic fuchsin, 5 ml glycerol,
10 ml 95% ethanol, and 15 ml distilled water). These were vortexed and
examined under a compound microscope. As a control, we examined the
treated pollen from Ailanthus and used this morphotype to compare
against the pollen carried by the insects. For each sample we determined
the frequency of grains that were like and unlike the Ailanthus
morphotype with sample sizes ranging from 50 to 77 grains per bee. A
Pearson chi-square test was used (SPSS program) to determine if the
putative Ailanthus pollen load varied across bees. Note that we also
found pollen dusted on many of the other insects, including flies,
though it was unclear to what extent this was a byproduct of the net
capture process. As a result, these were not quantified.
Flies and bees were the most common insects visiting the Ailanthus
flowers. A total of 118 insects were collected representing five orders
and more than sixteen families (Table 1, Figure 2). The Dipterans
(flies) accounted for nearly half of the collections (48.3%) with
Hymenopterans second most abundant overall (32.2%), and roughly 2/3 of
these bees and the remaining 1/3 ants. Other orders were rare including
Hemiptera (true bugs, 10.2%), Coleoptera (beetles, 7.6%), and
Lepidoptera (butterflies, 1.7%).
The composition of the insect communities visiting the Ailanthus
flowers varied across the four sites, with bees dominant at one site and
flies dominant at two other sites. The single tree at site 2 received
very low visitation rates and so it was removed from the formal analysis
of site heterogeneity. At site 1, bees comprised a larger fraction of
the visitor pool (44.7%) than did flies (23.4%). In contrast, the
visitor pools at the other sites were dominated by flies while bees
represented only a minor component (site 3: 2.7% bees, 56.8% flies; site
4: 3.7% bees, 77.8% flies). A Pearson chi-square test showed that the
bee and fly numbers were significantly different when comparing site 1
with pooled data of the adjacent sites 3 and 4 ([chi square] = 32.78, 1
df, P < 0.001).
Morphotype analysis of the pollen carried by bees indicated that
bees were indeed actively collecting Ailanthus pollen, not simply
visiting the flowers for nectar or by happenstance. The large bees
visiting Ailanthus flowers (predominantly at site 1 with all female
Ailanthus) carried on their legs a large percentage (58.5%) of pollen
that was indistinguishable from Ailanthus pollen collected from a known
source (Table 2). The remaining pollen (41.5%) clearly derived from
several other unidentified species. Differences in the ratio of
Ailanthus versus non-Ailanthus pollen morphotypes were not significant
across bees ([chi square] = 13.26, P = 0.104). Allowing for the fact
that the pollen of some other species might appear morphologically
similar to that of Ailanthus, the 58.5% is an upper bound on our
estimate of the percent of Ailanthus pollen carried by these bees. But
assuming that these were indeed Ailanthus pollen grains, we have further
promoted the status of bees as effective pollinators of Ailanthus since
these large bees carrying Ailanthus pollen were collected visiting
female Ailanthus flowers, thereby establishing a male--female link.
These data support the notion that Ailanthus is pollinated by a
variety of generalist insects, most notably flies and bees. Although
these visitation data cannot prove effective pollination by these
insects, we have nevertheless circumscribed the list of candidates. This
list includes taxa recognized as likely pollinators in less formalized
studies of Ailanthus reproduction (e.g., bees noted by Dalby 2000).
Moreover our list includes many insect taxa known to be generalist
pollinators that associate with other invasive plant species (reviewed
by Richardson et al. 2000). Plants that maintain more obligate
associations with particular pollinators are more bound to the geography
of those pollinators and therefore less likely to become invasive. A
partial reliance on fly pollination (myophily) in a north-temperate zone
is reasonable given other studies of floral visitation that have shown
an increase in the importance of fly pollination with increasing
altitude and elevation (reviewed in Kearns 2001). The infestation of
Ailanthus in the Chicago area is near the northernmost extent of the
range of Ailanthus in the Midwestern U.S., where it reaches into
Wisconsin and Michigan but only sparingly with concentrations near
Chicago and in Milwaukee and Detroit (USDA--NRCS 2008).
Our list of floral visitors of Ailanthus is largely distinct at the
family level from the list of arthropods known to associate with
Ailanthus in China. This latter list reported by Siling (1997, cited and
described in Zheng et al. 2004) is comprised mostly of natural herbivore
pests, not necessarily floral visitors. Some of these associates may
serve to keep Ailanthus in check and prevent it from becoming locally
invasive. A total of 32 arthropod species are noted as associating with
Ailanthus in China: Lepidoptera (12 species of mostly moths and some
butterflies), Coleoptera (10 species of beetles), Homoptera and
Hemiptera (8 species of true bugs, cicadas, and planthoppers), and
Acariformes (2 species of mites). By contrast, these groups form only a
small part of our list of floral visitors, with only a single species of
butterfly along with a few beetles and true bugs present in our survey
of Ailanthus in the Chicago area. The bulk of the floral visitors in our
dataset (flies, bees and ants) were not mentioned in the China survey,
which is not surprising, geography aside, as the two studies appear to
have focused on different modes of insect-plant interactions.
As for flies, despite their generalist behavior and at times
inefficient transfer of pollen, flies often contribute to plant
reproduction and can rival bees as important pollinators in many
circumstances in part due to their sheer ubiquity (Motten 1986, Kearns
and Inouye 1994, Kearns 2001, Larson et al. 2001). There are two main
types of fly pollination, myophily and sapromyophily. Myophily entails
several fly groups that regularly visit flowers for nectar and pollen
and can be important pollinators. Families included here are the
Anthomyiidae, Bombyliidae, Calliphoridae, Muscidae, Syrphidae, and
Tachinidae (Kearns 2001); all but the Bombyliidae were detected at
Ailanthus flowers in this study.
The sapromyophilous group of flies, on the other hand, regularly
visits dead animals and dung, though they may also visit flowers that
emit similar fetid odors, as do the flowers of Ailanthus. If proximity
plays a role, the sapromyophilous flies should cross paths with
Ailanthus as both have a habit in urban settings of frequenting the
less-groomed portions of a city, areas in which it is not uncommon to
find garbage lying about (Aldrich, personal observation). In our study,
the sarcophagidae (flesh flies) were well-represented at each site.
Although their visits may have been largely cases of 'mistaken
identity', repeated albeit inefficient visitations may in the
balance have led to effective pollination, especially at sites 3 and 4
where bees were less common.
Several factors could explain the among-site heterogeneity of the
insect assemblages. First and foremost, the number of sites surveyed was
small and so we must be conservative in generalizing the pattern of
heterogeneity, though the heterogeneity that we did observe is readily
explained. Studies of fly visitation rates often show high spatial
heterogeneity in fly numbers and species by site (Herrera 1988, Kearns
and Inouye 1994). For example, in our study sites 3 and 4 were near a
stream which might have attracted a novel assemblage of insects
including more flies compared to the other sites, perhaps due to sewer
runoff, goose feces, and various other aquatic sources of carrion and
detritus. Another possible factor is the gender differences between
sites, as sites 3 and 4 included some male trees that conceivably could
have been more attractive to flies, although they would then be less
efficient pollinators if they failed to visit females.
The prevailing wisdom has been that bees are an important
pollinator of Ailanthus judging from a variety of anecdotal reports and
observations (e.g., Miller 1990, Dalby 2000). Our quantitative studies
of visitation rates and pollen morphotypes carried on bee legs support
this contention. The large bees that we captured visiting Ailanthus
flowers carried more Ailanthus morphotype pollen than pollen from other
sources (average for 11 bees, 58.5% Ailanthus morphotype pollen). These
bees also carried a very similar proportion of Ailanthus morphotype
pollen suggesting they were interacting with Ailanthus with some sort of
regularity. Moreover, since we captured bees at female trees we are able
to establish a male--female connection, further supporting the role of
bees as effective pollinators of Ailanthus, not just floral visitors.
Bees, like flies, are widespread and important generalist pollinators
that likely have facilitated the spread of Ailanthus. However, the
decline of bee and other pollinator populations in many areas of the
globe (Kearns et al. 1998, Biesmeijer et al. 2006) may lead to a rise in
the importance of less efficient yet ubiquitous pollinators such as
Several of the other insect groups visiting the Ailanthus flowers
in this study may contribute to pollination (e.g., butterflies,
Lepidoptera), though likely not all of these groups. Several of the
beetles (Coleoptera), true bugs (Hemiptera), and the ants (Hymenoptera,
Formicoidea) may have been casual or predatory visitors rather than
pollinators, and some of these groups are well-represented in the survey
of natural enemies of Ailanthus in China (Siling 1997, Zheng et al.
2004). Those unable to fly (ants) were further unlikely as pollinators
because Ailanthus is dioecious and the insect would have had to travel a
large surface distance to the next Ailanthus to affect pollination.
These less vagile floral visitors might prove more effective as
pollinators should gender determination prove to be more flexible or
certain modes of bisexuality more common in Ailanthus than is normally
Even though ants are unlikely pollinators of a dioecious Ailanthus,
ants may nevertheless be a symbiont. Ants commonly patrol the leaves of
Ailanthus, removing nectar from the flowers and from extra-floral
nectaries that occur at the leaf margins and stipules (Aldrich, personal
observation). Chemical analysis has shown that the extra-floral
nectaries of Ailanthus glandulosa produce sucrose, rhamnose, and several
amino acids, mostly serine, threonine, and proline (Bory and
Clair-Maczulajtys 1986). These resources may serve as a reward to the
ants for protection from herbivores, augmenting Ailanthus'
defenses, although ants might also deter potential pollinators as well.
The interaction is reminiscent of the symbiosis in the Central American
tropics between the bullhorn acacia (Acacia cornigera, Fabaceae) and an
ant (Pseudomyrmex ferruginea) that guards the plant against herbivores
and in return eats protein nodules from the leaf tips and nectar from
petiolar glands, and lives in the hollowed out Acacia thorns (Janzen
1983). Casual observations (Aldrich) of the Ailanthus--ant system in
West Lafayette, Indiana revealed that ants behave aggressively toward
other insects as they patrol the leaves, and will take up residence in
the hollow boles of large adults, in which heart rot is commonplace (Hu
These data demonstrate that bees, flies, and ants are frequent
visitors to the flowers of Ailanthus in the Chicago area. A dependence
on generalist pollinators would make colonization of new environments
predictably easier, and is a trait manifest by many invasive plants. We
have made a case for bees as effective pollinators, and proposed that
flies may be important as well although it is conceivable that fly
pollination might be less prevalent in lower latitudes. We also submit
that ants may be part of a broader generalist symbiosis that includes
floral rewards but whose influence may extend into protection from
herbivory, further augmenting the chemical defenses possessed by
Ailanthus. Such matters deserve further study.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
The authors thank C.R.H. Aldrich for field assistance, two
anonymous reviewers for comments, and the USDA Forest Service and
Benedictine University for support.
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Preston R. Aldrich (1), Anthony Brusa (1), Cheryl A. Heinz (1),
Gary K. Greer (2), and Cynthia Huebner (3)
(1) Benedictine University, Lisle, IL 60532
(2) Grand Valley State University, Allendale, MI 49401
(3) USDA Forest Service, Morgantown, WV 26505
Table 1. Insect visitation frequencies at Ailanthus altissima
flowers at four sites in suburban Naperville, IL between
June and August 2006.
Order Family Common name Site 1
Coleoptera Total beetles 3
Cantharidae soldier beetles 0
Cerambycidae longhorn beetles 0
Coccinellidae ladybugs 2
Diptera Total flies 11
Anthomyiidae flies 3
Asilidae flies 0
Calliphoridae flies 2
Helcomyzidae flies 0
Muscidae flies 0
Sarcophagidae flies 4
Syrphidae flies 0
Tachinidae flies 1
Other flies 0
Hemiptera Total true bugs 8
Hymenoptera Total bees / ants 25
Apoidea bees 21
Andrenidae bees 1
Apidae bees 1
Colletidae bees 2
Halictidae bees 2
Other bees 15
Formicoidea ants 4
Formicidae ants 4
Lepidoptera Total butterflies 0
Order Family Site 2 Site 3
Coleoptera Total 1 5
Cantharidae 1 0
Cerambycidae 0 1
Coccinellidae 0 2
Other 0 2
Diptera Total 4 21
Anthomyiidae 0 0
Asilidae 0 0
Calliphoridae 0 6
Helcomyzidae 0 1
Muscidae 0 3
Sarcophagidae 4 5
Syrphidae 0 5
Tachinidae 0 0
Other 0 1
Hemiptera Total 0 1
Hymenoptera Total 1 10
Apoidea 1 1
Andrenidae 0 1
Apidae 0 0
Colletidae 0 0
Halictidae 0 0
Other 1 0
Formicoidea 0 9
Formicidae 0 9
Lepidoptera Total 1 0
TOTAL 7 37
Order Family Site 4 Total
Coleoptera Total 0 9
Cantharidae 0 1
Cerambycidae 0 1
Coccinellidae 0 4
Other 0 3
Diptera Total 21 57
Anthomyiidae 1 4
Asilidae 1 1
Calliphoridae 0 8
Helcomyzidae 0 1
Muscidae 0 3
Sarcophagidae 3 16
Syrphidae 4 9
Tachinidae 0 1
Other 12 13
Hemiptera Total 3 12
Hymenoptera Total 2 38
Apoidea 1 24
Andrenidae 0 2
Apidae 0 1
Colletidae 0 2
Halictidae 0 2
Other 1 17
Formicoidea 1 14
Formicidae 1 14
Lepidoptera Total 1 2
TOTAL 27 118
* "Other" indicates specimens that were identified to order
but remained ambiguous at the familial level.
Table 2. Counts of pollen morphotypes collected from the
limbs of bees visiting Ailanthus flowers.
A-morph = Ailanthus-like pollen, N-morph = Non-Ailanthus pollen.
Specimen A-morph N-morph Total
AB007 37 26 63
AB008 33 22 55
AB009 29 27 56
AB031 33 29 62
AB042 30 20 50
AB043 32 20 52
AB053 35 16 51
AB056 50 27 77
AB063 23 31 54
AB077* 30 26 56
AB090 38 19 57
TOTAL 370 263 633
* Collected at site 3, all others collected at site 1.