Teaching the Trees

Black Locust

67 I am drunk . . . drunk with the smell of locust blossoms.The long, dull winter is finally over, the grass is finally green, and the irises and black locust trees (Robinia pseudoacacia) are blooming. I want to do nothing but feast my eyes on the colorful irises and breathe in the sweet smell of the locust flowers all day long. The bumblebees gathering nectar from the blossoms cannot stay away either. Of course, their efforts ensure food for the young bees that are developing in underground nests, while my lack of effort ensures that my “to-do” list will be even longer tomorrow. But still I am pinned to this place in space and time, surrounded by black locust trees dangling their white, honey-scented clusters of flowers. black locust flowers produce a lot of nectar. A local beekeeper keeps a couple of hives of honeybees on my farm Black Locust 68 to take advantage of this bounty. In exchange for hive space he occasionally gifts me with a quart of his honey. So I, too, get to fuel my body on the nectar from the locust blossoms. The locusts are so generous with their nectar that they even provide some outside the flowers—in little ducts near the leaves—for insects such as ants and ladybugs. Evolutionary biologists believe that these “extrafloral nectaries” must have evolved because plants with the extra ducts attracted beneficial insects that enhanced the plants’ survival and allowed them to produce more offspring.1 Why would having resident ants be an advantage for a tree? One reason might be that they are good protectors. All plants are plagued by insects that want to eat them. But plants cannot move away from insect swarms or clean themselves like animals can. Instead the plants must depend on their own leaf chemistry and on the services of the beneficial insects they are able to attract.Ants help keep the trees clean by attacking and eating any insect eggs and larvae they find. This cleaning service reduces the number of insects eating the leaves. A few kinds of insects, though, are safe from the ants’ attacks; in fact, the ants protect them, guarding them and milking them like a herd of miniature cows. Tiny sucking insects like aphids and treehoppers get their nourishment by sticking their needlelike mouthparts into plant vessels carrying the sweet sap. They remove nitrogen and some of the sugars from the sap for their own use, but the rest of the water and sugars in the sap move through the insect’s body and come out the other end as a sticky substance called honeydew, a polite name for aphid urine. If you have ever parked your car under a leafy tree on a warm summer day, you might have returned to find it sprinkled with “sap.” 69 70 That “sap” is not from the tree directly but is the liquid that has passed through the bodies of sucking insects. Ants protect the aphids for their own benefit rather than attacking them for the benefit of the tree. When an ant strokes an aphid with its antennae, the “milking” action stimulates the release of honeydew. Ants drink the honeydew , a good source of water and nutrients, directly from the surface of the insect “cows.” If it is a year of few aphids, however, the ants will not starve if they are on a plant that provides nectar through extrafloral nectaries. In large numbers aphids are harmful to trees because they remove sugars the trees have produced to meet their own energy needs.Other beneficial insects,such as ladybugs, eat aphid eggs and larvae and help to keep their numbers in check. The ladybugs will also drink from the extrafloral nectaries, which may keep the ladybugs loyal to a tree in the absence of aphids. When sucking insects do appear, the beetles are present to feed on them. when taking a boat up the river or a car up the road, I see locust trees here and there. Certainly the locust is not a dominant tree in any of the area’s forests, but they always look “at home.” The locust trees seem such a natural part of the landscape here that for a long time I assumed that they were native to the area; when I checked a range map, though, I learned that they...