Beautiful Native Plants

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Thursday, June 15, 2017

Insects are a Weighty Matter

Aphids on a pear tree
One of many out of sight colonies,
amounting to several pounds of insects per tree
© Beatriz Moisset
If all the insects in an acre of land chose to sit on you, they would crush you. Is that possible? How many insects are there in an acre and how much do they weigh?

To get a better perspective on the importance of insects, I set myself the task of finding out how many insects there are in any ordinary garden or field. An entomologist/gardener, Frank E. Lutz did just that more than 70 years ago.

For four years Lutz collected and counted every species of insects in his garden and published his results in a fascinating little book, A Lot of Insects. The total number of species was 1,402. You would think that his garden was crawling with pests, but you would be wrong. The carnivores (both predators and parasites) kept the herbivores in check preserving the balance of nature. As a result, his garden won several awards, in part thanks to all those insects.

This doesn’t tell us the total number of insects. Some species may have been represented by just one or a handful of specimens, others by thousands. For instance, you don’t find just one ant or one aphid but entire colonies. Some entomologists have tried to come up with estimates. Larry Pedigo and Marlin Rice tell us in their textbook, Entomology and PestManagement, that there are 400 millions insects per acre.

Any number bigger than a million is hard to grasp. Let us look at insects in a different way. Their total weight, also called biomass, may tell us something significant. What is the biomass of all the insects in an acre of land? How does it compare to the biomass of vertebrates in the same field? Gilbert Waldbauer says in his charming book What Good are Bugs? that in a field in Georgia, grasshoppers outweigh seven times the total biomass of 10 sparrows and 10 mice (7/10 lb.) that may occupy that same acre. When we include all insects, not just grasshoppers, Pedigo and Rice inform us that the average insect biomass per acre in the United States is 400 pounds. The biomass of birds, mice, salamanders and all other vertebrates in the same area may be a mere 40 pounds. Any vertebrate that feeds on insects may rejoice at the abundance of food.

Such amount of insect flesh per acre is hard to believe. Where are all the insects that account for so much weight? Insects frequently go unnoticed for several reasons. Most are tiny, 1/16 in. or less, even such minuscule ones add up to a fair amount of mass because they are so numerous. Others are well hidden. In addition to the ones we see flying around, scampering on the ground, or visiting a flower, myriad insects and their larvae are buried in the soil, inside tree trunks, on the underside of leaves, or high in the foliage.

An assortment of insects were hidden
under a piece of loose bark of one of these logs
© Beatriz Moisset
A shiny metallic Augochlora pura bee sleeping under the bark
© Beatriz Moisset
Even the ones that don’t take pains in hiding often remain unseen by most of us. Last spring I spotted some lady bird beetles on the tree trunks of some pear trees. This alerted me to the possibility of aphids nearby. Each flower cluster was home to a colony of green aphids. I would have never seen them if I hadn’t searched for them. The trees appeared healthy and the flowers, lovely. Nevertheless, I suspect that each tree held a few pounds of aphids.
The sheer numbers and bulk of insects give us some idea of the magnitude of their role in any ecosystem. It is good to remember that most of them are not pests: They provide food for birds and other vertebrates, they recycle organic matter, they pollinate flowering plants, and they keep other insects in check.
Ants and termites contribute heavily to the total insect biomass
© Beatriz Moisset

© 2014, Beatriz Moisset. First published in Beautiful Wildlife Garden, 2014