When we consider the natural relationship between all living things, we realize the foundational importance of the insect world. In one square yard of soil and plants in the average Pacific Northwest garden, there can be approximately 2,000 different types of insects functioning as part of a highly complex web of life. From a purely ecological point of view, there is no such thing as a “pest”.
However, when the leaves and buds of our rose bushes are twisted and deformed or our ripening cabbage has layers of aphid stuffing between the leaves, we are inclined to interrupt this natural progression of things.
There are more than 4,000 different aphid species in the world. Each separate species is specific to certain host plants. Aphids are soft-bodied, sucking insects that are an important food crop for many beneficial insects. It is good to have some aphids in the garden, and they are easily controlled when the number get too high or they take up residence where you do not want them.
Aphids have the most amazing life cycle. You gotta love these guys (gals)…
Eggs are laid in autumn and hatch in the spring with an all female generation. These females are born wingless and pregnant and give birth to live young without the benefit of fertilization by males. This continues as they pile on top of each other, increasing in number. They feed by piercing the plant tissues, siphoning the juices and excreting excess sap called “honeydew”. When they get too crowded for that plant, they give birth to a generation of winged females. These winged aphids then fly off to find another host plant and continue to produce many more generations of wingless, pregnant females. This goes on throughout the summer. When the temperatures begin to drop toward autumn, true males are born. The males and females mate and a new crop of eggs are laid to overwinter and the cycle begins again in spring. Entomologists estimate that if all the descendants of a single aphid lived and reproduced, there would be 5 billion by the end of summer. That’s a lot of honeydew!
Usually there are enough predator insects in the garden to control the aphid population and keep things in balance. Avoiding toxic sprays will encourage the presence of lady beetles, green lacewings, hover flies, damsel bugs and more beneficial insects. If the predators haven’t gotten the aphids that are bugging you, a firm blast of water from the garden hose should take care of it. Once the aphids are knocked off the plant, they cannot get back up there (remember they are wingless) and will die. If the problem is out of control or the plant too large to deal with manually, a spray of insecticidal soap or a pyrethrins based insecticide will cuff them back to a manageable level. The important thing is to target the aphids without creating a toxic environment for the predator insects, birds, bats, honey bees, children and other humans.
Aphids on Kohlrabi Leaf by Downtowngal (Own work) [CC-BY-SA-3.0, via Wikimedia Commons