Extrafloral Nectaries

Last weekend I participated in the Berkshire BioBlitz, a 24-hour event in which people gathered on Mt. Greylock in Adams, Massachusetts to see how many different species of animals, plants, and fungi they could find.  Since there are generally plenty of people who know about plants and the more charismatic animals at these sorts of events (and there was a great crew of mycologists at this one, too), I decided to focus on the less conspicuous insects.  Now, a “real” entomologist would have collected specimens–and there were in fact people there collecting bees, moths, arachnids, and dragonflies (if they couldn’t be identified in the field)–but my personal preference is to collect photographs instead.  True, many insects cannot be identified to species from photographs, but a surprising number can, thanks to the generous help of the experts and very knowledgeable amateurs on http://www.BugGuide.net.  Plus, to me the point of a BioBlitz is to get people excited about the diversity of what’s out there, and I feel I can do that better with easily sharable photographs of living insects in their natural habitats than with a box of dead, discolored specimens.  I also doubt anyone really cares what species of dark-winged fungus gnat I found.

I barely left the parking lot, and I ended up with 160 or so different insects, springtails, and arachnids.  There were some that I was able to identify to species on my own; the rest I submitted to BugGuide for help, and you can see them here.  (The first several rows of photos with a white background were taken at night at a moth sheet put up by lepidopterist Mark Mello.)  So far five of them have turned out to be new species for BugGuide.  I always feel a sense of accomplishment when I’m able to find something that Tom Murray, a fellow Massachusetts resident who has contributed over 24,000 photos to BugGuide, hasn’t photographed yet.

Anyway, the point of all this is that by poking around and looking for bugs doing their thing, rather than surveying for them by setting traps or blindly sweeping vegetation, I was able to see some neat behaviors.  An example is the aforementioned dark-winged fungus gnat (Sciaridae; ID thanks to John Carr), which caught my eye because of the odd way it was straddling the petiole of a choke cherry (Prunus virginiana).

A dark-winged fungus gnat (Sciaridae; ~4.3 mm) on a choke cherry petiole.

Notice the little wartlike thing behind the gnat in this photo.  This is what the botanical manuals blandly refer to as a “gland,” without (in my experience) ever mentioning what its purpose is.  It’s an extrafloral nectary!  By which I mean, it is a nectar-producing structure not associated with a flower.  The nectar in flowers is meant to entice bees and other pollinators to visit them, whereas extrafloral nectaries attract ants, which defend the plant against herbivorous insects. I had first heard about these as one of the many bizarre plant-insect symbioses that occur in the tropics, and only recently learned that it is a common phenomenon in temperate North America as well.

Tilman (1978)* studied the relationship among the extrafloral nectaries on black cherry (P. serotina), the ant Formica obscuripes, and eastern tent caterpillars (Malacosoma americanum), and suggested that the timing of nectar production has evolved to coincide with the emergence of tent caterpillars, when the caterpillars are still small enough for the ants to prey on them.  In addition to Prunus species, he mentions that extrafloral nectaries can also be found on Populus (poplar), Crataegus (hawthorn), Sambucus (elderberry), Viburnum, Rosa (rose), Acacia, Quercus (oak), Syringa (lilac), and Ailanthus (tree of heaven).  To that list I can add what James C. Trager told me in a discussion on BugGuide last year: “Extra-floral nectaries occur on a wide variety of our native plants, especially, but by no means exclusively, in legumes [Fabaceae, the pea family], Rosaceae [the rose family, which includes cherry, hawthorn, and rose, among many other things], Bignoniaceae [the family that includes trumpet creeper and catalpa], Passiflora [passion flower], and composites [Asteraceae, the aster family].

Just as flowers attract many insects that don’t necessary help with pollination, it seems that extrafloral nectaries attract some insects that won’t provide the plant with pest control services.  I watched for several minutes as this fungus gnat buried its face in the nectaries, taking occasional breaks to walk back to the leaf blade and rest, only to return once again, too fixated on sipping the sweet nectar to be at all concerned about my presence.

Immediately before I took these photos, I took a series of shots of a sawfly feeding on the same extrafloral nectaries.  I somehow didn’t notice what it was doing when I took the photos, and the shots that show it actually feeding are a bit out of focus, but here is one that shows it just as it is departing from the nectaries:

So, is the sawfly a freeloader like the dark-winged fungus gnat, or might it help the plant in some way as the ants do?  I would have thought the former, but I saw something else at the BioBlitz that made me wonder.  To be continued!

* Tilman, David. 1978. Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation to Susceptibility of Caterpillars to Ant Predation. Ecology 59:686–692.