Aphids, how many is too many?

                                        Microsiphum aphids on rose bush

I read somewhere (I wish I could find that reference now) that the aphid biomass in an acre of meadows can be equivalent to that of an elephant. Perhaps that is a gross exaggeration, but, even if it was comparable to the mass of a cow or a dog, that is a lot of aphids. Let us face the facts: aphids are here in our gardens in substantial numbers, whether we see them or not. I am sure that I have eaten my share of them on leaves of lettuce, and you have done so too.

Is that a bad thing? Probably not, as long as there are only a few per plant. They don’t cause serious damage and they form part of the food chain, nourishing numerous kinds of wildlife, small and large. Aphid populations can be kept in check by the combined action of all those eaters most of the time. It is a beautiful system when it works well, but sometimes it gets disrupted.

Ecologists explain repeatedly that trouble often arises when pesticides wipe out not just the aphids, but their predators. Predator populations take longer to recover, allowing newly arrived aphids to explode in numbers before balance is reestablished.

I have been looking for these aphid controls in my garden. There is a whole assortment. Usually ladybugs take most of the credit, but others are just as effective or even more so, although less visible: parasitic wasps, flower flies and lacewings, among others. Some small birds, such as nuthatches and chickadees have been seen eating aphid eggs in winter.

Syrphid fly larva feeding on aphids

Syrphid flies, also called flower flies or hover flies interest me the most because they provide another important service in the garden: they are pollinators. The adult flies spend a lot of time hovering over flowers. They land on them to feed on pollen and nectar and to meet partners and mate. Their two common names refer to these behaviors. They look very much like bees, giving rise to amusing mistakes of which I have collected a few illustrations: a book named “Bees of the World”, a business card for a shop called “The Bees Knees” and an article on bees on the French newspaper, Le Figaro. In all these cases the image is not that of a bee but of an impersonator, a fly. If you know of any other examples, please let me know, so I can add them to my list.

The larvae of some of them, those called Syrphini, feed on aphids. The females are very good at spotting incipient populations of aphids and laying their eggs nearby. When the maggots emerge from the eggs they go right to work and can dispose of dozens of aphids in the ten days or so that it takes to complete their growth.

To learn more about their life cycle I collected some, kept them well supplied with aphids and saw them grow to full size, become pupae and finally emerge as adults. I took pictures and videos of the whole thing. The videos may be gross but fascinating. A maggot blindly thrashes around until it finds an aphid, pierces its skin and proceeds to suck it dry, discarding the shriveled husk. The whole process takes only a few minutes. It resumes its search right away. No wonder they can be so good at controlling aphid populations!

Another form of trouble occurs when new species of aphids are introduced into an ecosystem. This doesn't happen intentionally, but aphids easily travel with their host plants and may be able to adapt to related plants found in their new home.

Oleander aphids on milkweed

Such is the case of the oleander aphid. It has adapted to plants of the genus Asclepias, the milkweeds, and it can wreak havoc on them. It multiplies in large numbers, free from its habitual restrains. A few species parasitize of predate on oleander aphids, such as some parasitic wasps and lacewings and ladybugs, but all them together are not enough to keep the populations of oleander aphids under control.