Vol. 02, No. 09.c | Calling in predators

After the hard frost, the sensitive ferns have utterly and completely given up at standing tall! Their wilting is but another sign that fall is most certainly upon us. And while my timing to talk about leaves might be a bit late in the season, but late than never, eh? More here on the defensive role of leaves. Oh, and there’s a new Single Acorn episode out now where Glenn and I talk about how animals use stools as tools: listen now. And I’ve been putting photos of leaves in each newsletter, so pop quiz: which leaf is in the image below? 

~ Teage

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Coping with Tiny Predators and Parasites

In the last newsletter we looked at how leaves can be modified into thorny projections to avert the ravenous gaze of big hungry herbivores. One of the problems with this type of solution to predators is that it’s geared towards big mammal-sized problems. Another problem is that modifying leaves in too many directions can stretch a plant pretty thin, essentially becoming a generalist organ that trades in effectiveness for breadth. A few clever plants (about 1% of all plants) have circumvented this problem by outsourcing their defense to specialists.

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Contracting out defense

Looking at leaves with a hand lens (or just looking through binoculars), reveals a tiny world constructed on the scale of insects. When I first looked at the petiole (stem) of a pin cherry leaf (see above), I was surprised and then confused by the pairs of knobs running its length. I thought, perhaps, that they were small galls formed by insects. But they were always in growing pairs. I went home and did a little research on these strange growths and discovered they were extrafloral nectaries, or EFNs.

Since finding these first EFNs on pin cherry, I’ve looked for these on every other tree I’ve encountered, and it was with delight that I found these on peaches, other cherries, plums, aspens, and cottonwoods. So what are these EFNs? As their name implies, they’re structures outside of the flower that also produce nectar. Nectar-rich flowers are produced by plants that grow in areas with an excess of sunlight and can “waste” solar energy by producing enough nectar to entice pollinators to a flower. It’s the same with EFNs, and species in both Prunus (cherries, peaches, plums) and Populus (the cottonwoods and aspens) tend to be extremely shade-intolerant, and have a glut of sugars to spend on creating EFNs.

The EFN itself is a small gland (they have evolved independently in many plant lineages and can have many different shapes) that concentrate the products of photosynthesis in small dispensaries. The sugar secretions attract beneficial insects that fend off or parasitize leaf herbivores. This, of course, comes at a cost, as the sugars used in their construction could better be used for growth in reproduction. As a result, frugal trees, like cherries, will withhold the production of EFNs in the absence of predators (source).

The Defenders: Ants
The beneficial insects come in two flavors – insects that defend the leaf (like ants) and insects that parasitize leaf herbivores (like wasps). Both categories of beneficial insects are attracted to the nectaries and lap up the sweet nectar. The ants roam up and down the leaves in search of the sweet glands. If potential leaf herbivores visit the leaf, the ants will vigorously and aggressively fend off the predator. Under high pressure from leaf herbivores, cherries will produce more EFNs, therefore attracting more ants.

The Parasites: Wasps
Plants can also attract potential parasites of leaf herbivores to do the same task. The larvae and pupae of gypsy moths, for example, are generalist herbivores, feeding on a variety of trees. When they’re on leaves with EFNs, they are significantly more likely to be parasitized. Parasitism both dissuades the parasite and also reduces its efficiency at exploiting its host. Similarly, the pear slug (which is a sawfly larvae and not a slug) is a nasty pest found on many different Prunus species. The more eggs laid on a leaf, the more EFNs the leaf develops to attract parasitic wasps that help control the pear slug population (source)

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Without defense

Below are some examples of galls that form on leaves. The structure of the galls is produced by the leaf itself, and any tissue produced by the leaf is energy the plant is not spending on growth and reproduction.

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New Podcast Episode – Stools as Tools!

Sure everybody poops, but not everyone poops on their legs to stay cool, eats poop to make themselves more attractive to possible mates, builds homes out of poop, or lures in unsuspecting prey by looking and smelling like prey. Learn more about Stools as Tools as Glenn and I talk about the clever uses of scat in the latest Single Acorn Podcast episode: