Pollinators of Physalis

The following is a guest post by Julie Davis, a first year PhD student in the Cornell University Department of Entomology

Spring is finally yawning and stretching before it takes off full-force here in Ithaca, NY. Hyacinths, cherry blossoms, cilia, crocuses, red maples, daffodils and forsythias have bloomed, and my neighborhood is full of mason, honey and queen bumble bees. In preparation for a field season where I will grow several varieties of Physalis philadelphica, or tomatillo, at the Cayuga Nature Center, I have been thinking a lot about the pollinators that visit plants in the Physalis genus, and the quality of pollination services they provide.

Entomologists and ecologists categorize insects in many ways, including by their feeding habits. One characteristic of interest to us is the degree to which an insect specializes on its food source, which refers to the diversity or breadth of species it eats. The most extreme form of a specialist feeder is one that only eats one type of food. Monarch butterflies, whose caterpillars only eat milkweed plants, exemplify dietary specialization. On the other end of the spectrum are generalist feeders such as Japanese beetles, which feed on many plant species.

Generalist pollinators that visit Physalis include bumble, honey, and halictid (or sweat) bees. From left to right: Augoclora pura, a sweat bee that can be found in central NY. Photo: bugguide.com, A bumblebee on a tomatillo flower. Photo: Harmony Valley Farm,Apis Mellifera, the familiar honey bee. Photo: Ask a Biologist

Specialization interests pollination biologists for several reasons. Ecologists don’t fully understand why pollinators would specialize on a single flower species. It is possible that specializing on a food source can be beneficial if it reduces the number of other species with which the bee species has to compete for food, or if it improves the bee’s ability to digest or find its food, but these hypotheses require more investigation^1,2,3. On the other hand, it can be risky to specialize because if your flower species goes extinct (or flowers one year at a different time than your foraging period), you are out of luck! Generalist pollinators, on the other hand, insure themselves against fluctuations in species-specific resource availability by feeding on several flower species. However, this also means they compete more for food with other generalist feeders.

Specialist bees include Colletes latitarsis and Colletes willistoni both referred to as plasterer bees because they line their nests with an organic plastic secreted by adult female glands; an Andrenid bee Perdita halictoides; and a halictid bee Lasioglossum pectinatum. There is record of all these species in central New York, except Perdita halictoides, although our region is located within its range. From left to right: Colletes latitarsis, photo: Discover Life. A Colletid nesting aggregation, photo: J. Davis. Perdita halictoides, photo: Bug Guide. Lasioglossum pentinatum, photo: Discover Life.

From the plant’s perspective, being attractive to a single pollinator species may result in enhanced pollination. This is because specialist pollinators can have fine-tuned behavior that allow them to deposit more pollen, and more heterospecific (pollen from the same plant species) onto a flower, possibly resulting in higher seed or fruit set for the plant. However, this strategy can be risky because if the specialist pollinator goes extinct, the plant will not receive pollination services that help it set seed, which is really plants’ mission in life! So, many plants use a generalist strategy, making themselves attractive to many different species of pollinator. This can allow them to enjoy high rates of visitation. The trade-off is that they may not receive as high-quality pollination services per bee visit because the pollinators visiting them may not have fine-tuned behavior, and may carry pollen from other species. Some Physalis species, including tomatillo, are self-incompatible, or dependent on pollinators to produce fruit. Like many plants, Physalis species are visited by both specialist and generalist pollinators⁴.

It is important to note two things: First, it seems that while many specialist bees preferentially feed on a species or genus of plant, it is not uncommon for them to also visit other species. For example, at least one Physalis specialist that emerges in the spring before Physalis blooms, relies on early-flowering species. Second, it is rare for both a plant and pollinator to reciprocally specialize on each other. Rather, pollinators that provide specialized services to a plant (ie, that plant relies heavily on that pollinator to set seed) typically visit many other flowering plants. Likewise, pollinators typically specialize on plants that rely on many different pollinator species to set seed. There are a few famous examples of reciprocal specialization, including the fig-fig wasp and yucca-yucca moth mutualisms, but these appear to be the exception rather than the rule.

Ecologists, as well as farmers care about pollinators because they ultimately allow plants to set fruit. From the agricultural perspective, fruit set enabled by pollinators is important because humans often harvest the fruit for food! Fewer pollinators can lead to lower yield and quality of fruit. From the evolutionary ecology and plant conservation perspective, pollination is important because it majorly contributes to a plant’s fitness and therefore its ability to persist throughout generations.

Ongoing projects on tomatillo-insect interactions

 This summer, I will quantify the local community of insects, including pollinators associated with tomatillo in central NY. I am especially excited to compare the pollination efficiency of these different bee species. Are specialist pollinators of Physalis better pollinators than generalists? Do they transfer more pollen to the plant, or help the plant set larger, more uniformly-shaped fruits? I’ll also study whether different varieties of tomatillo are more resistant to herbivores such as the destructive three-lined potato beetle pictured below. Check back through the season to find out!

Lema daturaphila the three-lined potato beetle. Photo: discoverlife.org

Glossary

Fitness – the ability of an organism to reach reproductive age and produce offspring. Typically, biologists consider individuals producing more offspring as having higher biological fitness.

Foraging period – refers to the time during which a bee is actively searching for food. Changes in weather patterns may disrupt timing of bee emergence in spring, or flower blooming. If these changes happen inconsistently between bees and flowers, a mismatch between bee foraging period and spring bloom could result in less food for the bee and reduced pollination for the plant.

References

1. Fowler, J. 2016. Specialist Bees of the Northeast: Host Plants and Habitat Conservation. Northeastern Naturalist 23:305–320.
2. Praz, C. J., A. Müller, and S. Dorn. 2008. Host recognition in a pollen-specialist bee: evidence for a genetic basis. Apidologie 39:547–557.
3. Minckley R.L., Roulston T.H. (2006) Incidental mutualisms and pollen specialization among bees, in: Waser N.M., Ollerton J. (Eds.), Plant-pollinator interactions: from specialization to generalization, The University of Chicago Press, Chicago and London, pp. 69–98.
4. Roulston T.. 2005A comparison of specialist and generalist pollinators on a shared host, Physalis longifolia ssp. subglabrata., Entomological Society of America Abstract 2005, Section A: Systematics, morphology and evolution. http://esa.confex.com/esa/2005/techprogram/paper_22200.htm.

Pictures

https://bugguide.net/node/view/1096356

https://www.discoverlife.org/mp/20q?search=Lema+daturaphila

 http://harmonyvalleyfarm.blogspot.com/2014/08/featured-vegetable-this-week-tomatillos.html

https://askabiologist.asu.edu/explore/honey-bees

https://www.discoverlife.org/20/q?search=Colletes+latitarsis

https://www.discoverlife.org/mp/20m?w=720&r=0.05&e=-69.00000&n=37.50000&z=0&kind=Perdita+halictoides&la=37.5&lo=-69?289,136).

https://bugguide.net/node/view/1245916/bgpage

http://www.virginia.edu/blandy/blandy_web/arboretum/photo_db_folder/photos/photos_600/Lasioglossum_pectinatum.jpg

https://www.discoverlife.org/mp/20p?res=md&see=I_SD13597;I_SD13598;I_SD13819;I_SD13820;I_SD13821;I_SD15226;I_SD15227;I_SD15228;&start=/mp/20q%3Fsearch%3DLasioglossum%2Bpectinatum