Arboretum Images 10e
Wayne's Word Index Noteworthy Plants Trivia Lemnaceae Biology 101 Botany Scenic Wildflowers Trains Spiders & Insects Search
Palm Bamboo Agave Cactus Conifer1 Conifer2 Legume1 Legume2 Figs Trees1 Trees2 Trees3 Shrubs1 Shrubs2 Shrubs3 Natives
 Increase The Size Of Display On Your Monitor:    PCs Type Control (Ctrl) +     MACs Type Command (⌘) + 
   Sex Life Of Figs:     Part 1       Part 2       Cystoliths       Palomar Pollinators  
Fig Pollinators At Palomar College
© W.P. Armstrong Updated 3 January 2025

Pollination & Dispersal Of Australian Figs At Palomar College
Pollination By Fig Wasps & Dispersal By Birds
Fig Wasps & Viable Seed Production

On the Palomar College campus & Arboretum I discovered the symbiotic pollinator wasps for 3 species of Australian figs: Indian laurel fig (Ficus microcarpa), rustyleaf fig (F. rubiginosa), and Moreton Bay fig (F. macrophylla). A 4th species called banana fig (F. pleurocarpa) produced seeds but its pollinator (if any) is unknown. A 5th species (F. carica) also has its pollinator wasp on campus. It is included in Part 1 of above blue tab. Without their pollinator wasps, these fig species cannot produce viable (fertile) seeds. I recently noticed offspring from two Australian trees at the north end of campus, indicating wasp pollination and seed dispersal presumably by birds. Some fig seedlings were even in the crowns of large, Canary Island date palms. In tropical regions with abundant rainfall, fruit-eating bats & primates are also major agents of fig dispersal. This remarkable relationship between minute wasps and vertebrates is well-known in southern Florida, Hawaii and tropical countries. With 876 fig species worldwide, many with multiple symbiotic wasps (pollinator & nonpollinator), this is truly a remarkable and extraordinarily complex relationship between minute insects and their host trees. In fact, the DNA cladograms for figs and their wasps are absolutely mind-boggling!

Table Of Contents

    Subgenus Urostigma Section Conosycea

    1.   Indian Laurel Fig (Ficus microcarpa)

    Subgenus Urostigma Section Malvanthera

    2.   Rustyleaf Fig (Ficus rubiginosa)
    3.   Moreton Bay Fig (Ficus macrophylla)
    4.   Banana Fig (Ficus pleurocarpa)

Indian Laurel Fig (Ficus microcarpa)

An Indian laurel fig (Ficus microcarpa) with massive trunk near the music building complex at Palomar College. It is also a popular indoor container plant used for bonsai. Some of these remarkable growth variations may be attributed to cultivars, such as 'Golden Gate' grown primarily for bonsai.

Indian laurel fig (Ficus microcarpa) and its symbiotic wasp (Eupristina verticillata) from syconia on Palomar College campus. One wasp is squeezing through an exit tunnel cut by male wasps (white arrow).

Ficus microcarpa at Palomar College. Syconia of F. microcarpa contain male & female symbiotic fig wasps Eupristina verticillata. Inset: Minute amber-colored male wasp & black female wasp emerging from female flower inside the syconium. After mating, male wasp enlarges ostiole to enable her to exit syconium without damaging her wings. She then flies to another receptive F. microcarpa syconium. She carries pollen from male flowers in pouches on her body. This is good example of obligate mutualism where both members of the relationship depend on each other for survival.

The Pollinator Wasps For Ficus microcarpa) Are Really Minute!

The following online article shows many images of how wasp-pollinated, invasive Ficus microcarpa has spread in southern California, & why I decided not to plant this tree in my front yard!
Richard E. Riefner Jr. 2016. Ficus microcarpa (Moraceae) naturalized in southern California, U.S.A.: Linking plant, pollinator, and suitable microhabitats to document the invasion process. Invasion Of Ficus microcarpa in Southern California

Indian laurel fig (Ficus microcarpa) seedling growing from crack in granitic outcrop at north end of campus. It is under a large rustyleaf fig (F. rubiginosa). Ficus species are well-adapted to grow in shade with microscopic, calcareous structures called cystoliths in their leaves. The glistening cystoliths transfer light to photosynthetic, chloroplast-bearing cells.

Close-up view of one cystolith in above adaxial leaf image of Ficus microcarpa. It also shows central cellulose or siliceous stalk upon which calcium carbonate cystolith is formed. The stalk also supports cystolith within epidermal cell called a lithocyst. With minimal depth of field this is a difficult shot to get with limited viewing clarity through Sony W-300 pocket camera attached to eye piece of Olympus compound microscope. Using 10x objective (100x magnification) and extensive cropping, the total magnification exceeds 200x. The diameter of cystolith is 70 micrometers or 0.07 mm. This is about the average width of a human hair.

Rustyleaf Fig (Ficus rubiginosa)

The brown hairs (trichomes) on underside of rustyleaf fig leaves produce unique rust color of leaf surface; however, the color is variable depending on density of hairs. This species is easily distinguished from leaves of F. microcarpa.
We also have so-called "glabrous" rustyleaf fig (Ficus rubiginosa) in Palomar College Arboretum. In my opinion, the specimen I have observed in Arboretum is technically not glabrous: Minute hairs may be visible on underside of leaves (abaxial surface), especially on veins, with 10x magnification (hand lens or dissecting microscope). Some authors list it as var. glabrescens; however, World Flora Online lists var. glabresens as a synonum for this variable species.

It is also listed as Ficus rubiginosa f. glabrescens (F.M. Bailey) D.J. Dixon. The small f is a secondary taxon rank below variety. It denotes a special form of that species or variety.

Although underside (abaxial surface) of leaf appears glabrous to naked eye, the veins are covered with minute hairs (trichomes). The general term for plant part covered with trichomes is pubescent. The microscopic white dots are calcareous deposits caled cystolths within enlarged epidermal cells called lithocysts.

Rustyleaf fig seedling from nearby tree near campus greenhouse.

 Abaxial (lower) leaf surface & veins of this tentatively identified rustyleaf fig seedling in south Escondido have relatively few hairs (trichomes), but in my opinion, they are not truly glabrous. The clusters of white dots are microscopic cystoliths. Glabrous means smooth, glossy, and without hairs or bristles (i.e. lacking surface projections). I don't know if cystoliths affect definition of glabrousness.

Seedling discovered by Steve Disparti in south Escondido tentatively
identified as Ficus rubiginosa. It may have been introduced by birds.

Several exotic fig species, such as the commonly cultivated Benjamin fig (Ficus benjamina), Indian laurel fig (F. microcarpa), rustyleaf fig (F. rubiginosa), and Moreton Bay fig (F. macrophylla), often develop as hemiepiphytes (spending part of their life as epiphytes) with numerous aerial roots in their native habitats of Malaysia and tropical Australia. They sometimes germinate in the crowns of palms and crevices of buildings in moist climates, such as southern Florida. Due to the introduction of symbiotic wasp pollinators, some of these tropical figs in southern California, including Palomar College campus in San Diego County, produce viable seeds. They sometimes germinate in the crowns of palms; however, it is generally too arid for them to send aerial roots to the ground and develop into mature trees.

  Pollination Behavior In Different Wasp Genera  

A deceased female fig wasp (Pleistodontes imperialis) extracted from the ostiole of Ficus rubiginosa. Note the coxal comb at the base of front leg that it used to actively gather pollen from syconium and store it in ventral thoracic pocket (pollen pocket). Inset (left): SEM from Kjellberg, F., Jousselin, E., Bronstein, J.L., Patel, A., Yokoyama, J., and J.-Y. Rasplus. 2001. "Pollination Mode in Fig Wasps: The Predictive Power of Correlated Traits." Proceedings of the Royal Society of London 268: 1113-1121.

Epiphytic Strangler Fig On Canary Island Date Palm At Palomar College

An unusual sight on the campus of Palomar College in San Diego County, California: A rustyleaf fig (Ficus rubiginosa) that has developed from a seed in the leaf bases of a Canary Island date palm (Phoenix canariensis). If the climate was more humid, with frequent rain showers, this fig could probably send aerial roots to the ground and become a full-fledged strangler. In fact, aerial root resembling "botanical boa constrictor" appears in upper left of above image (red arrow).

The rustyleaf fig (Ficus rubiginosa) in distance produces numerous crops of seed-bearing syconia bearing symbiotic fig wasps (Pleistodontes imperialis). For birds feeding on the sweet syconia, it is a relatively short distance to the Canary Island date palms where fig seeds could be deposited with their feces.

How Date Palms Support Fig Seedlings In Dry Southern Calif. Climate

Ficus seeds can germinate in date palms because the environment within a date palm's fruit cluster provides the necessary conditions for germination, including moisture, warmth, and a suitable substrate for the seeds to root in; essentially, the date palm acts as a "nurse tree" offering a protected microclimate for the fig seeds to sprout, especially if the seeds happen to be deposited there by birds. In addition to a moist & sheltered enviroment, decaying organic matter in date palm fruit cluster also provides nutrient source for developing fig seeds.

Moreton Bay Fig (Ficus macrophylla)

Massive trunk and surface roots of a Moreton Bay fig (Ficus macrophylla) in the Palomar College Arboretum. In its native Australia it is a strangler fig.

Lower (abaxial) leaf surface of Ficus macrophylla in the Palomar College Arboretum. Like the rustyleaf fig (F. rubiginosa) there are numerous clusters of branched, microscopic brown hairs (trichomes) between the small veins (veinlets).

Syconium of Moreton Bay fig (Ficus macrophylla) in Palomar College Arboretum. Palomar College horticulturist Tony Rangel grew viable seeds from this tree, so I suspected that the pollinator wasp (Pleistodontes froggatti) must be present.

Unlike the similar P. imperialis of rustyleaf figs (F. rubiginosa), P. froggatti does not have a coxal comb at the base of its front legs. According to Cook, J.M. et al. (2004), the lack coxal combs indicates passive rather than active (purposive) pollination; however, they also state that "pollen pockets provide a good but imperfect index." In Table 1 of their article they state that pollination by P. froggatti is passive. Prior to my discovery of P. froggatti in the Moreton Bay fig at Palomar College (December 2011), the only fig wasps I have documented on campus are P. imperialis on rustyleaf figs and Eupristina verticillata on the Indian laurel fig (Ficus microcarpa).

Pleistodontes froggatti from Moreton Bay fig syconium in Palomar College Arboretum.

Banana Fig (Ficus pleurocarpa)

Seed germination of the banana fig (Ficus pleurocarpa) in the campus greenhouse area is perplexing because no wasp pollinators have been observed. There is no conclusive evidence that the banana fig on campus is pollinated by fig wasps. It was originally introduced without syconia, so it is doubtful that its native pollinator wasp species (Pleistodontes regalis and P. deuterus) were introduced. Its syconia appear to be packed with very hard-shelled "seeds" that sink in water. This is truly an enigma. If these "seeds" (technically drupelets) germinate, then how did they get pollinated? Were they pollinated by Pleistodontes imperialis from nearby rustyleaf fig (Ficus rubiginosa)?


Australian Figs At Palomar College
Pollinated By Symbiotic Fig Wasps

    Subgenus Urostigma Section Conosycea

    1.   Indian Laurel Fig (Ficus microcarpa)

    Subgenus Urostigma Section Malvanthera

    2.   Rustyleaf Fig (Ficus rubiginosa)
    3.   Moreton Bay Fig (Ficus macrophylla)
    4.   Banana Fig (Ficus pleurocarpa) ?

Host Sharing & Pollinator Sharing

Host Sharing: 2 or more pollinator wasp species per host.
Pollinator Sharing: 2 or more fig host species per pollinator.

Fig & Fig Wasp Pollination May Be Even More Complicated

When I began studying figs (Ficus) with horticulturist Steven Disparti over 30 years ago, I thought each species of fig had it own unique, symbiotic pollinator. This may still be true for some of the 876 species of Ficus; however, multiple pollinators and evidence from host & pollinator sharing data have complicated the fig & fig wasp story. The relationships undoubtedly follow kinships within subgenera & sections depicted in enormous Ficus DNA cladograms (evolutionary trees). There are 4 pollinated Australian fig species on campus in subgenus Urostigma and at least 3 verified species of Australian fig wasps: (12 possible interactions). If you exclude Section Conocycea & wasp Eupristina, and only consider the more closely related Section Malvanthera & 2 Pleistodontes wasp species on campus, there would be 7 possible interactions. Another complicating factor introduced into seedling diversity is possible hybridization between fig species pollinated by multiple wasp species each coming from at least 2 different host fig species.

Receptive Australian Ficus Species
F
I
G

W
A
S
P
Fig Wasps &
Fig Species
  F. microcarpa
Conocycea
  F. rubiginosa
Malvanthera
  F. macrophylla
Malvanthera
  F. pleurocarpa
Malvanthera
Eupristina
verticillata
(Conocycea)
Natural Host
Improbable
Improbable
Improbable
Pleistodontes
imperialis
(Malvanthera)
Improbable
Natural Host
?
?
Pleistodontes
froggatti
(Malvanthera)
Improbable
?
Natural Host
?
Table Showing Possible Fig Pollinations At Palomar College

Red: Improbable (Between Different Ficus Sections Conocycea & Malvanthera)
Green Squares Containing Question Mark ? (Unverified at Palomar College)

The pollination story for Ficus rubiginosa is even more complicated. According to C.T. Darwell, et al. (2014), the Pleistodontes imperialis wasps pollinating rustyleaf figs have diversified (evolved) into 5 distinct species across their entire host plant range along the east coast of Australia.

Clive T Darwell, Sarah al-Beidh, and James M Cook. 2014. "Molecular species delimitation of a symbiotic fig-pollinating wasp species complex reveals extreme deviation from reciprocal partner specificity. Darwell et al. BMC Evolutionary Biology 2014, 14:189

http://www.biomedcentral.com/1471-2148/14/189
The native pollinator of Ficus macrophylla is Pleistodontes froggatti; however, outside its native range, the syconia of F. macrophylla are also visited by P. imperialis, the pollinator for F. rubiginosa (J.M. Cook & D.W. Dunn, unpublished). In addition, DNA cladograms indicate that F. macrophylla and F. pleurocarpa are sister clades and are thus closely related species. Some interesting pollination hypotheses can be drawn from these documented observations. If the rustyleaf fig's pollinator (P. imperialis) is known to pollinate F. macrophylla, and if F. macrophylla is very closely related to F. pleurocarpa, could it also pollinate F. pleurocarpa?

As I stated in a Facebook note, the coevolution of nearly 900 species of figs, each with their symbiotic wasps, is the most amazing story of my biology career. It is difficult to summarize in a lecture because of its complexity. In fact, oversimplifications and gerneralizations by authors has contributed to errors in some publications.

As of 4 January 2025, how this fig was pollinated at Palomar College is an enigma.

In its native rainforests of northeastern Australia, Ficus pleurocarpa is pollinated by two species of fig wasps, Pleistodontes regalis and P. deuterus in the same syconium. This is a good example of host sharing where 2 species of fig wasps enter syconia of the same fig species. In fact, the assumption that fig species are usually pollinated by just one species of fig wasp was challenged by the discovery of species like the banana fig. We also have Pleistodontes froggatti in the Edwin & Frances Hunter Arboretum that pollinates the Moreton Bay fig (F. macrophylla) and P. imperialis that pollinates the rustyleaf fig (F. rubiginosa) on campus. As I stated above, outside its native range the syconia of F. macrophylla may also be visited by P. imperialis, the pollinator for F. rubiginosa. Without pollination there typically are no viable seeds in the syconium and no seedling trees.