June 21, 2018
Pollinator Week Part III: Teamwork, cheating, and checks-and-balances
As I mentioned in our last post highlighting pollinators, both the plant and the pollinator do what they do because it benefits them, not out of altruism for the other. Sometimes the arrangement works out to be mutually beneficial, but sometimes the arrangement is more nefarious. There are also some really cool examples of checks and balances to keep the arrangement mutually beneficial.
I’ll use alfalfa leafcutter bees as an example of a mutually beneficial pollinator. Alfalfa flower anthers (the part with the pollen) grow under tension, and the shape of the flower creates a sort of “tripwire.” When a pollinator triggers the tripwire, the anther springs out of the flower and punches it in the face, dusting it with pollen in the process. Generalist pollinators like honey bees are repelled by the face punches, but the specialized alfalfa leafcutter bee charges straight in and gets pummeled by the anthers, gathering up a load of pollen to feed her larvae in the process. Leafcutter bees are sloppy pollinators; rather than storing pollen in tidy baskets on their legs, they’ll brush it onto a frizzy mop of fuzz on their bellies. That belly fuzz drops much more pollen than a leg pollen basket, so a single leafcutter bee, working just as hard as a honey bee, can get far more pollinating done.
The most straightforward way that a pollinator can cheat the system is by nectar larceny. Some pollinators have adaptations that enable them to extract nectar from a flower without actually transferring pollen. Two examples that immediately come to mind are carpenter bees, whose powerful jaws can bore holes straight into hard wood, and sphinx moths, whose proboscides can be longer than the rest of their bodies. Some plants produce deep flowers that force a pollinator to really shove its face inside to reach the nectar, ensuring that it is covered by pollen as it explores the flower. However, a flower is far easier to bore into than wood, and carpenter bees will sometimes tear into the base of a flower and drain the nectar without going anywhere near the pollen. Sphinx moths, with their incredibly long proboscides, are often the target pollinator for such deep flowers, and different flowers produce nectar spurs of different length to suit different sphinx moths. However, a sphinx moth adapted for the deepest flowers can easily cheat shallower flowers, drinking the nectar from the flower’s depths while hovering well away from the pollen.
There are lots of examples of plants “betraying” pollinators. Carnivorous plants such as sundews, pitcher plants, and venus flytraps each set their traps with very similar lures as flowers to draw in insect prey, producing nectar, attractive smells, and in some cases attractive colors to lure insects to their doom. Other plant cheaters are a little more benign. The spider orchid produces the female sex pheromone of a type of solitary wasp. Male wasps follow the scent and attempt to mate with the flower, even though it looks nothing like a female wasp, and transfer pollen in the process. Many of these male wasps eventually learn to avoid the flowers, but they can pollinate several flowers before realizing they are being duped.
Some of the most tightly coevolved insect-plant pollination mutualisms involve figs and fig wasps and yuccas and yucca moths. These pairs certainly are not identical, but the same general pattern follows each. The plant is wholly dependent on its specialized pollinators for pollination, and the pollinators are wholly dependent on the plant for their entire lives. Flowers are produced in clusters, and these appear to be the unique cases of pollinators seemingly pollinating intentionally rather than accidentally as they gather rewards. As the female pollinator works on pollinating flowers, she’ll lay eggs on some of them, and her larvae will devour the resulting seeds and fruit. This is where the checks and balances come in; if the pollinator lays eggs in too many flowers, the plant will abort the entire flower cluster, killing both its flowers and the entire brood of the overly greedy pollinator. If a pollinator lays too few eggs, she fails to live up to her reproductive potential and will eventually be outcompeted by more aggressive egg layers; if she lays too many eggs, the plant will execute her entire brood. Similarly, if a plant is too quick to abort flowers, it will wipe out its pollinators and most of its flowers; if the plant lets the pollinator lay too many eggs, then it loses reproductive potential because the pollinators are eating up its seeds. This coevolutionary system of checks and balances constantly course corrects, but seems to remain stable if the pollinator can lay a moderate number of eggs. As an interesting aside, the fig flowers are contained within a fig, and female fig wasps die in the fig that they pollinated. Therefore, almost all figs contain fig wasp carcasses.
Pollinating is a dangerous job. Most flowers will be visited by several pollinators before they stop producing nectar and go to seed, and a viable flower can also be a set trap. While many ambush hunting insects, such as mantises and assassin bugs, make use of these ready-made traps, two specific types particularly focus on hunting pollinators. Ambush bugs, a small and stocky variety of assassin bug, have yellow-and-black patterning that blends in well on a goldenrod flowerhead. They sit perfectly still until a bee, fly, or butterfly strays too near, then seize it with their mantis-like front legs. Crab spiders take a more sinister approach. They are typically white or yellow, changing color to blend in with their floral background… to our eyes, anyway. They reflect ultraviolet light, which is visible to insects and which flowers use to guide pollinators straight to the pollen and nectar. While waiting for prey, the crab spider poses itself just right so that its own UV reflections look like the nectar guides on a flower, and they direct pollinators directly to the spider’s fangs.
Photo credit: Kirk Anderson Ph.D., showing a nectar thief (top) and pollinator (bottom) of the western prairie fringed orchid.
