Notes from Windward: #69

Harvesting Duckweed and Yellow Jackets

addressing an unexpected consequence


     Duckweed is a sustainability technology that's key to our goal of feeding people in circumstances involving limited water and space. If you found yourself without food and were looking at going hungry until you were able to harvest something, then duckweed would be the way to go. It's a fast-growing, nutritious plant that taste-tests better than alfalfa sprouts or watercress.

the cascading duckweed troughs

     One reason why it's such an efficient plant is that it doesn't expend energy to grow a stem structure--it just floats on the water soaking up sunshine. Three small rootlets gather nutrients from the duck-water, and three small leaves convert that into more duckweed. The roots don't even have to gather water since the leaves lay flat on the water and can adsorb water directly. I'm also guessing that this enables duckweed to take up carbon dioxide at an accelerated rate since the solubility of carbon dioxide in water is seventy times greater than its solubility in air.

     We purchased our initial seeding of duckweed from The Wet Spot a tropical fish store in Portland for a buck or two. There are lots of different varieties of duckweed, some with leaves half an inch across, but the variety they carry is smaller with leaves only about an eighth of an inch across. Research indicates that the smaller varieties are more palatable to fish and humans; if we were growing duckweed to feed to our ruminants, we'd probably want to work with a larger variety.

     At Windward, we're very sensitive to the First Law of Ecology which holds that you can't change just one thing. Any material change in a system will trigger changes in other parts of the system. Our efforts to grow duckweed this year have provided a fresh example of how that works.

a quart of duckweed ready to feed to the gold fish

     The predatory wasp known as a Yellow Jacket is a part of Windward's ecology. During the spring, Yellow Jackets hunt for any sort of meat that they can chew up and take back to the nest. They provide the developing larva with meat in exchange for which the larva provide the workers with sugar. Later in the summer, when there are no more larva in the nest to provide them with sugar, the adult workers hustle to find their own sugar sources such as rotting fruit. Since the food humans like to eat is often rich in sugars, Yellow Jackets can quickly ruin a late summer picnic.

     One natural factor that limits the growth of Yellow Jackets at Windward is the extreme dryness of late summer and early fall. Without an adequate source of water, the nests and the larva they hold would dry up and die. This summer, we unintentionally provided the Yellow Jackets with an ideal solution to their problem with the result that we're seeing a bloom of Yellow Jackets that's forcing us to respond. A sting or two a summer is tolerable, but a number of us have been stung multiple times this summer to the point where there are concerns about triggering allergic responses.

     It took a while to figure out what was the factor that triggered the Yellow Jacket bloom--the duckweed was responsible. There are lots of water sources at Windward but it appears that Yellow Jackets have a difficult time drinking directly from the water surface. I don't know if it's a problem with their being able to see the surface of the water accurately enough to be able to land on it, or if their feet aren't large enough for them to ride on the surface tension of the water like many other insects do.

      Whatever the reason, a wall to wall covering of duckweed provides a verdant landing surface that enables Yellow Jackets to land, drink their fill and fly away. Now, having figured out the cause, the next challenge was to come up with a solution that meshed with our sustainability practices. One of the guidelines we work with is to note up front that there is no such thing as waste--there's just undeployed resources. Often the challenge lies mostly in figuring out effective and efficient ways to gather those resources so that they can be repurposed in some productive manner.

     We let our breeding ducks out of the Duck Palace during the day so that they can gobble up the bugs and grass they need to round out their diet. When they discovered that we had incorporated two watering troughs into the Duckponics setup to grow duckweed, they jumped in and promptly ate up all the duckweed. <sigh>

     To protect the duckweed, we cut scraps of cattle panels to fit the troughs and covered them with chicken wire. That kept the ducks out and let the duckweed thrive. Duckweed doubles in mass every couple of days so we soon had lots of duckweed that we could harvest and feed to our fish, but we also found that we had more Yellow Jackets than we'd seen in over a decade.

     What to do?

     We don't use insecticides at Windward for a variety of reasons, so poisoning them was out. I'm delighted to report that we've figured out a way to "harvest" the surplus Yellow Jackets thereby converting them into a high-protein food supplement for our chickens.

     We keep guinea hens because yellow jackets are one of their favorite foods. When they find a ground nest, they'll stand outside the nest and eat the emerging wasps until the nest is no more. They're very good at what they do, but this is another case where balance is key.

     Because of predation two winters ago, our guinea flock is undersized given the area they need to patrol. We currently only have four adult birds. One of them, "Hoppy" lost a foot to frostbite last winter and isn't able to range very far. Guineas are very monogamous, so Hoppy and her mate stay close in while the other pair range farther afield. We also have some eight guinea chicks that Opalyn's been able to hatch this year, but they're not big enough to turn loose quite yet.

      The covers that we made for the duckweed troughs have a notch cut in them to allow for the pipes that bring in water from the duckponics system. The Yellow Jackets can fly through the horizontal chicken wire, but it takes some effort. They prefer to enter through the notch cut through the cover. By positioning the intake from a Shop Vac near the cover's opening, the vacuum sucks up the yellow jackets as they try to pass through. This arrangement enables us to harvest between thirty and fifty wasps a minute during the heat of the day when they're desperate for water.

using a Shop Vac to harvest
Yellow Jackets from a duckweed trough

      From there it is a simple matter to place the Shop Vac into one of our backup freezers over night to kill the wasps without contaminating them by spraying them with an insecticide. The next morning the birds in the Chick Plex are treated to "bug-cicles" for breakfast.

     Vacuuming up the wasps isn't a way to wipe them out, which is good because as insect predators, they play an important role in keeping insects in check--this is just a way to work back towards a balanced ecology without triggering more unexpected consequences.

     As least that's "The Plan." At each step of the journey, nature seems to always have another surprise or two in store just to keep the game interesting.

Notes From Windward - Index - Vol. 69