Garden Q&A – Insect-Friendly Gardens
CNPS Garden Q&A with Bob Allen
I’ve been reading about the insect apocalypse and declining monarch butterfly populations. Why are native insects important, and how can I support them in my garden?
Native plants serve as food and shelter for many species of native insects. Whether in the wild or in the garden, they exhibit a variety of fascinating interactions. We can boil them down to either one- or two-sided interactions.
One-sided interactions benefit the individual insects that feed on the plant, such as on sap, leaves, buds, and/or other parts. Since most herbivorous insects are host-specific — they feed only on one or a few related plant species — just having these native plants in the garden promotes survival of their dependent native insects.
- Larvae of the achemon sphinx moth, Eumorpha achemon, feed on leaves of native grapes, both Vitis girdiana and V. californica. Every year, the plants in my yard are inhabited by these caterpillars, which are surprisingly difficult to locate.
- When manzanitas (Arctostaphylos sp.), bloom in winter-spring, they are host to numerous species of native butterflies, flies, and bees, such as the red admiral butterfly (Vanessa atalanta). Every garden needs at least a handful of manzanitas.
Two-sided or mutually beneficial interactions are activities that provide benefit to both insect and plant.
Certainly the most widely discussed two-way interaction is pollination.
As the insect gathers nectar and/or pollen, pollen sticks to its body, sometimes pow-dered all over like snow. Even the most careful insects inadvertently drop some of the pollen they so assiduously collect. Pollen that lands on a receptive stigma, either within the same plant/flower or between the flowers of different individual plants, successfully becomes an agent of pollination. Pollination, when successful, leads to fertilization, seed production, fruit growth, and seedling production.
If insects were 100 percent successful at gathering and retaining the pollen they encounter, there would be no polli-nation and therefore no reproduction of insect-pollinated plants. As Charles R. Darwin said, “Natural selection will not produce absolute perfection.” (On the Origin of Species, Chapter VI.) Fortunately for plants, he was correct; pollinating insects are sloppy.
A garden full of locally native plants provides food and shelter for native insects.
Adult insects use the pollen and nectar they collect to feed themselves and/or feed their young. Thus, pollen-collecting and successful pollination are both mutually beneficial interactions. Insect-pollinated native plants require insects for their own reproduction.
How can I support native insects in my garden?
We can support these native insects by using locally native plants, eliminating pesticides, reducing garden disturbance, controlling our use of mulch, and changing human notions about native insects.
- A garden full of locally native plants provides food and shelter for native insects.
- A hard look at pesticides reveals that nearly all are used against non-native insects that feed on non-native garden plants. A native garden, then, has no need for pesticides. The insects that show up to feed on those native plants belong there.
- Soil cover (mulch) is often overused. Sure, it helps keep weeds down, but it also covers the bare soil required for nesting by many species of native bees, egg-laying by grasshoppers, and soil pupation by many species of moths and beetles. Try to keep at least some areas of undisturbed bare soil to allow these insects to survive.
We must change the all-too-common human perception that having insects on our plants is a bad thing — it’s not. Enjoy the insects!
Robert L. “Bob” or “BugBob” Allen has studied insects and plants since early childhood in San Juan Capistrano, California. He is now an adjunct professor of biology at Santiago Canyon College and Orange Coast College, a research associate in entomology at the Natural History Museum of Los Angeles County, a research associate at the Rancho Santa Ana Botanic Garden, a horticultural consultant, and author of Wildflowers of Orange County and the Santa Ana Mountains.