Whose Lunch Is It, Anyway?

It isn't hard to spot the aphids in this picture:

Aphids feasting on a maple leaf.

But, we aren't the only ones checking them out:

The aphids are spotted. So is the lady beetle.

(Two-spotted Lady Beetle, aka Adalia bipuncta).

In a matter of minutes, the lady beetle devours half of the aphids.

If the lady beetle is lucky, it will keep on eating aphids. There are plenty more on other leaves in the tree. Of course, it is not alone. There are other lady beetles around, as well. There are also eggs and larvae, promising new swarms to come. They aren't exactly free to take over the tree, however. Other predators are lurking about: several varieties of wasps, sparrows, and chickadees. The black-capped chickadees (Parus atricapillus) seem especially fond of the little beetles, and will probably show up more frequently as the population of insects rises. One chickadee can eat hundreds of insects in a single day, while supplementing its diet with seeds and berries.

Last year, the populations of lady beetles on the maple tree in front of my house were sustained throughout the summer. With just several chickadees coming to visit, and a hive of paper wasps (Polistes dominulus) nearby, many thousands of insects must have lived and died in the tree. Needless to say, the large maple tree was almost constantly covered with beetles, remnants of eggs,  half-eaten aphids, and drips of honeydew, left by the aphids. This year may be no different. (I'm rather thrilled to see there's a chance!)

Why must the chickadees eat so many insects? Or why must the lady beetles eat so many aphids? (If you compare the last two pictures above, taken 9 minutes apart, you can see the beetle ate more than one a minute.) Even the hawk who occasionally swings by to lunch on a chickadee can't eat just one. As you move higher up in the food chain, consumption becomes less efficient.

The maple tree only converts about 1% of the sunlight that hits its leaves into food. In turn, the aphids turn about 10% of the food they eat into body mass, which is consumed by the lady beetle. The lady beetle converts about 10% of the aphids it eats; the chickadee converts about 10% of the lady beetles and seeds it eats into flesh, and so on. Now, don't forget, we like some of that food too... like good old fashioned maple syrup! Our food is used with about the same efficiency as the other animals. Imagine how much energy goes into producing a portion of maple syrup, versus the amount of energy that goes into producing a bird.That makes a pretty significant difference in the amount of energy we put to good use when eating pancakes versus eating fried chicken.

All images in this post were created by the author.