Conservation Biocontrol, or The Love of Insects

by Janet Richards

My interest in biocontrol started with slugs. I was looking for a way to attract ground beetles and other slug predators to my garden. What I learned is that control is less about pitting one species against another, and more about creating and sustaining the arena where they play out their struggle. It takes an ecosystem.

Using animal predators to control pests is called biocontrol. You can buy predators, for example, lady bugs, hoping they stick around in your garden to eat aphids. However, introduced insects tend to disperse rapidly and leave your garden entirely. They are often from non-local populations, which only adds to their inability to live in their new home.

Attracting native predators of pests to your garden is called conservation biocontrol. It involves providing habitat (forage and nesting sites) for native beneficial insects so that they live in your garden and eat your pests. Conservation biocontrol can involve attracting other animals, but it is most often about attracting insects.

In my experience, insects are not widely popular, even though less than 1% of identified insects are what we humans consider pests. Lately, their role in pollination has made bees more popular, and we plant flowers in our gardens to attract them. But planting a lavender garden and walking away is not enough. Bees need not only pollen, but year-round habitat. Native bees nest either in the ground or in cavities, so need warm, bare soil, fallen logs, dead trees, stumps and spongy plant stems.

And what about other invertebrates? Take butterflies and moths, for example. They do a bit of pollinating but are not the pollination superstars that bees are. However, in their larval stages, they are important food to birds. In North America, some 96% of land birds feed insects to their young (mostly caterpillars and adult moths). Adult birds make hundreds of feeding trips to the nest each day, and so need lots of caterpillars. As adults, moths and butterflies need nectar, but they also require a place to lay their eggs. As it turns out, it’s the native trees, shrubs and grasses that provide these nesting sites, but are in shorter and shorter supply as we clear land and re-place the landscape with lawns and introduced flowers, shrubs and trees. The sad news is that the number of insects has been reduced 45% globally since 1974. It is possible that many of us haven’t even noticed. Causes for this loss include pesticide use and a warming climate, but also very important is the reduction of habitat.

Every resource I read ad-vised increasing habitat by planting native plants. Recent research presented in Nature’s Best Hope (Douglas W. Tallamy, 2019) shows that native plants can support a larger variety of insects. For example, the common reed (Phragmites australis) has replaced large areas of wetland in the Eastern U.S. It is now considered naturalized because it has been in North America for nearly 500 years. A study from 2002 showed that this species of reed supports 170 species of insects in Europe where it is native. In North America, it supports only five species.

In the Pacific Northwest, red alder supports about 227 species of moths and butterflies, native willows support 339, and North American goldenrod, 59. Next time you see caterpillars in an alder tree, instead of thinking that you must kill the insects to save the tree, consider that you could be doing more harm than good. The alder provides habitat for butterflies and moths in their larval stage, which provides food for birds, allowing the birds to survive. In almost every case, the alder tree will be fine.

Native plants support more insect species than introduced plants because insects and plants co-evolved over tens of thousands of years. Most plants evolved to taste bad so that they are not eaten. Milkweed, for example, contains cardiac glycosides (it even sounds like it tastes bad) and also has a milky latex sap that glues the mouthparts shut of any insect that tries to eat it – except monarch caterpillars. These larval-stage butterflies can block the flow of the sap to the milkweed leaves by the way that they eat. The specialization between monarchs and milkweed is well-known, and the disappearance of milkweed from our landscape has imperiled the monarch’s survival. This specialization is not exceptional. Ninety percent of plant-eating insects are specialized.

I have seen many non-native plants covered with bees, which used to make me think, Eureka! But now, I wonder if perhaps those bees are generalists in their foraging. I learned that many species of bees are specialists. They rely on certain native plants for forage because of the plant’s bloom time, flower shape or color, and pollen morphology. If I want to help prevent these bee species from disappearing, I need to plant natives.

Tallamy describes keystone plants, that is, native plants that support the greatest number of insect species. The National Wildlife Federation’s Native Plant Finder website lists plants in your zip code ordered by how many moth and butterfly species they attract. Audubon also has a website recommending wildlife-friendly plants according to your zip code (website addresses given at the end of this article).

Now I want to attract any and all types of insects to my garden. They pollinate 87.5% of the plants and 90% of flowering plants globally, provide pest control, perform decomposition and release nutrients. In other words, they sustain plants, and plants sustain animals. Because of my dive into conservation biocontrol, my plan is to try to introduce more and more native

plants—especially keystone species—into my garden and yard, while keeping the friendly introduced plants. And, if I’m lucky, some slug-eating insects will set up house.

Resources:

The Nation Wildlife Federation’s Native Plant Finder website, www.nwf.org/nativeplantfinder.

Audubon Native Plants Database, https://www.audubon.org/native-plants.

Tallamy, Douglas, Nature’s Best Hope, 2019

Eierman, Kim, The Pollinator Victory Garden, 2020

WSU Extension publication, Beneficial Insects, Spiders, and Mites in Your Garden: Who They Are and How to Get Them to Stay, 2014, pubs.extension.wsu.edu/beneficial-insects-spiders-and-mites-in-your-garden-who-they-are-and-how-to-get-them-to-stay-home-garden-series

Xerces Society publication, Habitat Planning for Beneficial Insects – Guidelines for Conservation Biological Control, 2016, xerces.org/publications/guidelines/habitat-planning-for-beneficial-insects