This is one of the many articles contributed by the Prairie Monk for publication in the WEFT Revue. It appears on this blog as part of an ongoing project to archive Monk's writings and make them more accessible to the community.
An observant visitor to our nearby urban prairie noted ants climbing an evening primrose plant. They were headed for the flower head of the plant where there was a lot of aphid action. The small, plump, almost transparent green insects, with their discarded exoskeletons and sugary exudates, are easy to recognize. The seed head was obviously having a difficult time as a result of the aphid infestation. The meristematic, or rapidly growing parts of the plants, such as the young seed pods, are the most attractive to the aphids. These young seed pods were not filling out in the same healthy manner as the lower pods that had escaped the aphid onslaught. Aphids have sucking mouth parts that remove nutritional sap from the plant and spread disease.
The ants were present because they utilize sugary substances exuded by anal circi that project from the abdomen of the aphid. The ants can encourage the production of these sugary materials by stroking the anal circi. Some of the sugary material spills onto the surrounding leaves to create a recognizably shiny, sticky surface that has a reflectance very different from that on uninfected parts of the plant. The ants gather their booty and head home, down the main stem of the plant.
At the same time one has to wonder about the presence of certain wasps and lady bugs. The lady bugs at least have the intent of eating the aphids. Other insects, just like humans, have an addiction to the sugary materials.
Why would this small drama on the prairie be of interest? It is but one of a million such interactions that are common to any ecosystem. In the first place it is a matter of curiosity, and we should all be curious. So ten "brownie points" to the observer who noted the line of ants moving up and down the stem of the plant. We also should know what is happening around us in case what we observe has an impact on the understanding of the prairies we care about. Then there is the added concern as to what these creatures might do in the commercial world of agriculture or gardening, if given the chance. Should we indeed be avoiding growing this plant?
Let us establish a few facts about the plants and creatures involved.
Firstly, the evening primrose is a prairie plant, but is also a little on the weedy side. We don't encourage it a lot. We don't discourage it either, but it is easy to see why a farmer might not want it near to a commercial crop.
Interestingly the plant is a nocturnal bloomer, pollinated by a sphinx moth, so when I give flashlight tours of the urban prairie at night, it is often blooming. The dry seeds of the evening primrose are also very attractive to the seed-eating birds like goldfinches, which, in season, visit our prairie. We have another favorite use for the primrose which has to do with its capacity to grow quickly. If we plant a prairie, which we know will take a long time to reach maturity and which may be subject to possible mowing and herbiciding pressures, we will sometimes plant a perimeter of evening primrose to create a recognizable natural barrier to delineate the site.
On the farmer's side, the presence of weedy patches of primrose are a target for elimination, for the farmer knows that aphids can infest this plant and possibly transmit diseases to crops. That is not an unfounded assumption; this insect can not only do damage to a single plant but it can also increase and disperse quickly. In the first place, the aphid is tremendously fecund. Its populations can seemingly appear overnight. This is helped by the fact that aphids can reproduce at least some generations without sexual interaction. Worst of all, as far as the dispersal of disease is concerned, there are winged generations that can fly to other places. The farmer's talent is to eliminate the plant and/or apply insecticides at the right moment so that population increase and dispersion can be reduced. It should be remembered that the challenge is exacerbated by large crops of weeds that are encouraged by soft and fertile farm soils, and on the prairie we don't have that situation.
Another interesting thing about aphids is that they leave detritus on the leaf. The aphid does not reproduce through an egg larva pupa adult life cycle, but rather through a series of instars, or stages—each of which is a facsimile of the last instar (only a little larger and more sophisticated)—until the adult instar is reached. This leaves exoskeletons, which have been exfoliated by growing instars that are detectable on the leaf surface.
Should we keep our evening primroses? I think so, because the primrose has a niche and a role to play; it has a place in our native ecosystem. The strength we have is that of biodiversity, so that when a disease attacks a prairie, the overall prairie is not devastated—just a small part of it. The farmland monoculture of a single plant crop is, on the other hand, very vulnerable to an invasion that can take the whole crop out or reduce its yield. Biodiversity is also important for farming if we are to take advantage of crop rotations and other agricultural practices that encourage a range of crops, rather than specialization.
Come visit our prairie sometime and watch its creatures grow.
—The WEFT Revue, Vol. 10 Issue 5: Sept.-Oct. 2002, p.6.