Observations of Vespicochory of Trillium recurvatum diaspores by Vespula maculifrons in St. Louis County, Missouri

Review

Formica pallidefulva ants beginning to work on moving diaspores of Trillium recurvatum.

After observing Vespula vulgaris foraging for the diaspores of Trillium ovatum in Oregon, Jules (1996) coined the term vespicochory to describe the dispersal of seed by Vespid wasps, specifically members of the Vespinae – the Vespidae subfamily that is comprised of yellowjackets and hornets. Few other descriptions have been recorded on observations of Vespinae acting as secondary seed dispersers in myrmecochorous plants. In controlled experiments, V. maculifrons has been reported to disperse T. cuneatum, T. undulatum and T. catesbaei in North Carolina and South Carolina (Zettler et al., 2001) and T. discolor in South Carolina (Bale et al., 2003). To my knowledge, this is the first recorded observation of the dispersal of T. recurvatum by Vespid wasps.

Before I begin describing my observations, I will first review and discuss the potential implications of Vespinae dispersal as an alternative to myrmecochory in trilliums. In most descriptions, myrmecochory has been described as a mutualist symbiose, meaning that both plant and ant species benefit from the relationship. Ants benefit by gaining the lipid and protein rich eliasome of the diaspore to feed their young while the plants benefit by having their seed dispersed from the parent plant and gain potential benefits in overwintering and germination environments. Similarly, for vespicochory to be considered as an important seed dispersal syndrome, we should consider the benefits to both sides and compare the role these wasps play to that of their ant cousins.

A Formica subsericea ant carrying a Trillium recurvatum diaspore.

Yellowjackets, in addition to seizing seed directly from ants, removed more seeds (40%) from index cards than did each of three ant species observed (8 – 28%) (Bale et al., 2003). Zettler et al. (2001) measured dispersal distances by V. maculifrons and found an average distance of 1.4 m compared to a mean of 0.98 m in global cases of myrmecochory (Gómez and Espadaler, 1998). This difference in dispersal distance alone is significant; however, 53% of the seeds removed by V. maculifrons in this study were moved beyond 20 m – the furthest extent of their measuring capabilities, and were unrecovered, indicating a much higher than calculated average dispersal distance.

A single foraging Vespula maculifrons finds the fruit.

In addition to dispersal distance, another important thing when considering the benefits to the plant in a particular dispersal syndrome, is what is done with the diaspore once removed. The mandibles of Vespids are considerably larger and assumedly much more powerful than those of the ant species involved in dispersing trillium seed throughout their range. It is therefore a possibility that the seed could face catastrophic damage from the foraging wasp. Of the original seed recovered by Zettler et al. (2001), 95.7% had the eliasomes removed. Of these, 17% of the seeds had scarification near where the eliasome was attached but no seeds showed visible signs of embryo damage. The ultimate use of the eliasome was unknown in Zettler et al. (2001); however, Jules (1996) observed yellowjackets taking diaspores directly into their nest where, presumably, they were fed to developing young. Vespids typically nest underground and waste (i.e. seed portions of diaspores) are deposited below the nest. As mentioned by Zettler, “…we do not know how seed burial in these nests might affect seed germination and seedling emergence.” In cases where eliasomes are removed and seed are dropped randomly on the ground, it would be interesting to know how these seed fare in comparison to those buried within the nest. Further study is needed to determine the fate of seeds moved by these wasps when compared to myrmecochory.

Observations

The ants try to keep their prize but the yellowjackets are too strong and fast and easily claim most of the diaspores.

The following observations and accompanying photographs were conducted at August G. Beckemeier Conservation Area in St. Louis County, Missouri. On August 5, 2021 at ~ 18:00 hrs., I collected a ripe fruit of T. recurvatum and placed it with about 25% of the seed exposed within 20 cm of a nest of Formica pallidefulva ants. My goal was to observe and photograph the ants carrying away the diaspores. The ants found the fruit within minutes and quickly began moving the loosely separated diaspores. After approximately 10 minutes the first V. maculifrons found the fruit and quickly left with a diaspore. It returned alone five times with gaps ranging between approximately one and three minutes. After the fifth visit, two to four wasps were at the scene at any given time, each working to free seeds from the fruit until all seeds were removed. I found that the wasps were able to pull the diaspores free from the fruit capsule matrix much easier than the ants. The ants tried, at times, to defend the fruit and the wasps did give them a wide berth. When two or more wasps were on the fruit at one time, however, the ants were ineffectual in their defense.

A Vespula maculifrons seizing a Trillium recurvatum dioaspore.

I was not setup to make accurate counts or to try and make seed dispersal distance measurements. It appeared the wasps moved at least 75% of the seed while F. pallidefulva moved the remaining into their nest. I believe this discrepancy was primarily due to the ability of the wasps to excise the diaspores from the fruit capsule matrix more quickly and easily than the ants. I watched one wasp perched on a short sapling approximately 1.5 m from the fruit. It removed the eliasome, letting the seed fall to the leaf litter below and then left carrying the eliasome with it.

Photography Details

The ants try to keep their prize but the yellowjackets are too strong and fast and easily claim most of the diaspores.

These images were taken using a full-sized sensor digital camera and a 180mm macro lens with a 1.4x teleconverter and 30 mm extension tube stacked between the lens and camera body. This combination of equipment provides quite a long focusing distance, ensuring the photographer does not disturb the subjects. An off-the-body external speedlight “flash” was used at varying levels of power to obtain the extra light needed. Most of these images were taken at f/16, 1/100 sec. and ISO-640 and were taken handheld while using a fallen log for additional support.

Conclusion

This was an anecdotal observation of a single occurrence of vespicochory. This is a subject that warrants further investigation. Could vespicochory be just as or even more important in the dispersal and emergence of some “myrmecochorous” plants as myrmecochory? It would be interesting to know more about the frequency and dynamics of this unique seed dispersal mechanism.

References

  • Bale, M.T., J. A. Zettler, B.A. Robinson, T. P. Spira, & C.R. Allen. 2003. Yellow jackets may be an underestimated component of ant-seed mutualism. Southeastern Naturalist 2(4):609-614.
  • Gómez, C. & X. Espader. 1998. Myrmecochorous dispersal distances: a world survey. Journal of Biogeography 25:573-580.
  • Jules, E.S. 1996. Yellow jackets (Vespula vulgaris) as a second seed disperser for the myrmecochorous plant, Trillium ovatum. American Midland Naturalist 135(2):367-369.
  • Zettler, J.A., T.P. Spira, C.R. Allen. 2001. Yellow jackets (Vespula spp.) disperse trillium (spp.) seeds in eastern North America. American Midland Naturalist 146(2):444-446.

Myrmecochory – Dicentra cucullaria (Dutchman’s breeches)

In continuing my work from last year, this year I was able to capture a few Aphaenogaster rudis moving the diaspores of Dicentra cucullaria (Dutchman’s breeches). Although this was the best year I’ve ever seen for D. cucullaria, getting everything to work just right in order to photograph this process was difficult. I was often short on the time needed to do this. Also, the cool temps we had this spring made it a bit difficult to find the foraging ants, even when the supply of diaspores I had at my disposal were ample.

-OZB

Myrmecochory – Seed dispersing ants!

An Aphaenogaster rudis ant shown in the act of myrmecochory – here dispersing the seed of the forest understory forb, Sanguinaria canadensis (bloodroot).

Myrmecochory is a term that comes from Greek, created from “myrmeco” – of or pertaining to ants, and “chory” – plant dispersal. It is one of approximately seven plant “dispersal syndromes” classified by ecologists, is found in approximately 5% of the angiosperms and occurs in numerous ecosystems around the world.

Showing the extreme relative strength of the ants, this Aphaenogaster rudis is moving a diaspore that must be several times its own weight.

Mutualism is thought to be the basis for this dispersal syndrome. Although this is not necessarily crystal clear, the ants are attracted to the eliasome – the fleshy structure attached to the seed that is a rich source of lipids, amino acids and other nutrients. The ants typically will move the diaspore (eliasome + seed) back to their nests. Dispersal distances vary, but are generally not great – most often 2 meters or less. However, for small forbs this distance is often adequate for moving these propagules outside the range of competition of the parent plant.

Two Aphaenogaster rudis ants attempting to move this Sanguinaria canadensis (bloodroot) diaspore. This was not seen very often and shortly after this image was taken, one ant gave up its pursuit.

Distance dispersal is not the only selective advantage that plants gain from this mutualistic relationship. When the ants have moved the seeds to their nests, they remove the eliasome to feed their young and typically dispose of the seeds in their midden heaps or eject them from the nest. Seeds that are moved to midden heaps or other such locations benefit in multiple ways. First, they are placed in microenvironments that are conducive for germination and early growth. They are protected from heat of fire that could destroy the seeds and benefit from not being accessible to birds and other seed predators. This is referred to as ‘directed dispersal.’ Some studies have shown that the removal of the eliasome may promote germination, similar to the process of seed being removed from their fleshy fruit as it is passed through the gut of a vertebrate.

I rarely had to wait more than 15 minutes before the first Aphaenogaster rudis foraging scout found the pile of diaspores I placed on the ground. Mere minutes after that it was advertised across the colony and other workers showed up to carry the spoils back to their nest.

Their is typically no specialization of particular ants dispersing a particular plant species, with almost any ant species being ready to take advantage of a free meal. The possible exception being that larger diaspores must be dispersed by larger ant species.

Using one of nature’s great predators to disperse your seeds can be risky business. As seen here, the testa (seed coat) of this Sanguinaria canadensis (bloodroot) as well as most myrmecochorous plants is hard and smooth to avoid the bite that ants can deliver.

When a Camponotus pennsylvanicus ant finds a diaspore, the photographer must act quick. They don’t need much time to haul it away!

My hope was to photograph myrmecochory across a variety of species this year. I was fortunate to find success with Sanguinaria canadensis but had no luck in my attempts with Dicentra cucullaria (dutchman’s breeches). I tried hard for trillium species as well but discovered the plants I was waiting for mature fruits for weeks were being harvested most likely by SNR staff. I will be trying for these again in the future and hope to photograph prairie species as well.

A freshly fallen Stylophorum diphyllum (celandine poppy) fruit with diaspores waiting for the ants to disperse them. Note the different testa pattern and eliasome structure compared to Sanguinaria canadensis.

The fruits of Stylophorum diphyllum (celandine poppy), I discovered, had a much smaller window of ripening. I had to check at least every two days or  I would miss the opportunity of a large fruit full of diaspores.

The ubiquitous Aphaenogaster rudis is a key disperser of Stylophorum diphyllum (celandine poppy).

As with many mutualistic relationships, cheaters are known in myrmecochory. Too small to properly move and disperse a diaspore of this size, this Nylanderia faisonensis is seen eating the eliasome on the spot. This was not a very common observation and it is doubtful that this would ultimately hurt the plant species.

See below for my attempts at filming myrmecochory. This was definitely challenging. I had troubles predicting the ants’ behavior, especially while under the bright, continuous lighting needed for high-magnification photography such as this. Something else to try and improve upon next year.

I’d like to thank James Trager for his assistance with ant species identification.

-OZB