Here I thought that the Anise Swallowtail and its eggs made up the extent of bug life on fennel. So wrong! This morning I saw that the fennel in the park was the battlefield for two ancient antagonists familiar to any home gardener: aphids and ladybugs. Of the two, the ladybug is the more familiar. The aphid is by far the more amazing. Read on.
The ladybug in the photo has no spots. Lady beetles (Hippodamia convergens) can have any number of spots from zero to 13, but the ones without spots get eaten by birds more often because some poisonous bugs have them and the birds are being careful. The convergens in their scientific name comes from the two white streaks on the thorax that converge toward the rear. They spend the summer among plants wherever aphids may be found, which is just about everywhere. The larvae attack aphids by sucking out their insides. Adult beetles tend to chew up and eat the whole aphid. Adults have to eat a quantity of aphids or similar small bugs in order to lay eggs. The female lays the eggs on vegetation, and may lay two or more generations of eggs in a season, depending on her condition and the environment. While feeding, the adults are usually solitary, but in winter they fly to higher elevations like the Sierra Nevada and gather in huge swarms that cover tree trunks and fallen logs, and mate. There, commercial collectors vacuum them up, refrigerate them, and then sell them in spring by weight to growers and gardeners who want them as biological controls for aphids. However, within a day or two the bugnapped beetles generally fly away; they stay and do their intended work only when confined in a greenhouse. In the wild, they do an effective job reducing the aphid population. They can eat hundreds of aphids in a day. However, no matter how voracious the beetles may be, they don’t stand a chance against the aphids’ powers of reproduction.
At one point in the original Jurassic Park movie, someone expresses confidence that the dinosaurs won’t reproduce because they’re all female. “Nature will find a way,” says a scientist. They ought to make an Aphid Park movie — they’re much more scary than dinosaurs. Here, nature has found a way. Female aphids don’t bother mating, laying eggs, and all that. They need no contact with males, and there aren’t any. The females give live birth to about 50 or more female offspring, and those offspring are already pregnant with other females when born. Scientists call this telescopic reproduction. When they grow up, in a matter of days, each offspring in turn gives live birth to about 50 or more additional females who are also already pregnant, and so on and on. In this way, one female under ideal conditions could produce billions of descendants. Later on in the season, depending on food supply and climate, some of those females may grow wings and fly to other spots, and lay eggs like ordinary insects do. Those eggs may overwinter, and when they hatch, some of them will be males. These males may lack wings or even mouthparts to eat with. All they have is genitals, and they mate. That’s the end of their usefulness. The mated females lay eggs that develop into females that give live birth to more pregnant females, without further male assistance, and so on. In some conditions, the aphids skip the mating and egg laying altogether and just reproduce asexually, generation after generation.
When predators descend on them, aphids exude alarm pheromones to warn the others. The aphids may kick the attacker, excrete a waxy liquid to gum up the attacker’s mouth, drop off the plant, and/or give birth to offspring with wings that fly away to safer spots. Aphids also have friends. Some species of ants protect the aphids against predators, including ladybugs. The ants cultivate the aphids like farmers keep dairy cows. They stroke the ladybugs with their antennas and suck the honeydew that the aphids excrete from the anus. (You can see several of the honeydew bubbles in the photo above.) Before I could follow up on this particular aphid colony, which did not yet have ants, Parks management knocked down that fennel bush. But in a neighboring bush I found another aphid colony, this one with green aphids (they come in colors), and a chain of ants busy tending them. I have to apologize for the image quality here, the wind was blowing and drew the stalk in and out of focus:
The ant inside the V is feeding on the honeydew the aphids excrete. When under ant hegemony, aphids issue their honeydew slowly so that the ant can get it all, instead of ejecting it in a big droplet away from the body as in the photo at the top of this post, where the aphids are sovereign. You can also see the two spikes at the back of each aphid, called cornicles, through which they exude the defensive wax if attacked. The aphids do not consider the ants attackers. These are probably Carpenter ants (Camponotus spp.) which are known aphid farmers, but there are other ant species that cultivate aphids and I could be way off. A great deal of research has gone into the ant-aphid relationship.
Some ants carry aphid eggs into their nests over the winter, keep them warm, and put them back on plants in the spring. Ant queens starting a new colony take an aphid egg with them.
Aphids also rely on bacteria friends. They host Buchnera aphidicola bacteria in specialized cells, and have swapped genes with them. The bacteria share the aphids’ meal of plant sap and convert part of it into amino acids that the aphid requires but cannot get otherwise. The bacteria are transmitted from mother to daughter at birth. This symbiosis began more than 160 million years ago.
There are thousands of known aphid species. Some of them cause heavy damage to crops, in part by sucking the plant sap, but more importantly by spreading deadly plant viruses. Because of this, a considerable body of scientific knowledge has grown up about them. I’ve had my battles with aphids in gardens, but I never drilled deeper into the subject until I saw these fat and messy little parasites on the fennel in the park. I wish the spotless ladybug “bon appetit.”
An afterthought. The Lady beetle gets its Western common name from the resemblance of its red body to medieval paintings of the Virgin Mary (“Our Lady”), traditionally clothed in red. But beetles reproduce sexually, by cross-gender carnal copulation, not at all like the biblical mother. The aphids, not the beetles, are the true Virgin Mary bugs: females never touched by males give live birth, again and again.