In a recent column, we described the first report of the Royal Horticultural Society’s “Plants for Bugs” study, which focused on Pollinators. In this column, we’ll look at the second report of this study, “Gardens as Habitats for Plant-Dwelling Invertebrates” (August 2017), which is about insects and invertebrates that live in our gardens, and the plants they love.

Let’s first review the related taxonomic issues.

Gardeners should appreciate the difference between “bugs” and “insects.” The title of the RHS study uses “bugs” to refer informally to a large and diverse group of invertebrates. To be precise, true bugs are members of the order Hemiptera, which is within the class Insecta. There are some 50,000 to 80,000 species of true bugs, all of which have sucking mouthparts. Examples of true bugs include cicadas, aphids, planthoppers, leafhoppers, and shield bugs. By the way, none of the true bugs are pollinators.  

The Insecta is a class within the phylum Arthropoda, which includes 6 to 10 million species, all of which are hexapod (six-legged) invertebrates.

To put the true bugs in perspective, they represent only a tiny fraction of the Insecta.

Returning now to the RHS study, the first report addressed garden insects that are Pollinators, which include bees, butterflies, and a few other insects.

The second report addresses other significant categories of garden insects, which include these groups:

• Herbivores. Invertebrates that feed on living plants, using chewing mouthparts (e.g., caterpillars) or sucking mouthparts (aphids are a familiar example).  
• Predators. These are invertebrates that eat other invertebrates. These include lacewings, beetles, e.g., ladybirds, some true bugs, spiders, and parasitoid wasps, which kill their hosts if you have a problem with them at home better check out wasp nest removal.
• Omnivores. These invertebrates feed on both plants and other invertebrates. This group includes the harvestman (a spider relative), earwigs, and aphids.
• Detritivores. Invertebrates that feed on decomposing organic matter. Examples include springtails, woodlice, and some beetles.

These groups, along with the Pollinators featured in the first report of the RHS study, constitute a vital component of the garden’s balanced ecosystem. In addition to breaking down dead plant material, these plant-dwelling invertebrates provide food for other wildlife, including birds, reptiles, and small mammals.

In their four-year Plants for Bugs study, the RHS scientists used suction samplers to collect about 18,000 of plant-dwelling invertebrates. Their samples included 18% Herbivores, 3% Omnivores, 18% Predators, and 61% Detritivores. They also collected about 4,700 uncategorized invertebrates.

They recorded these collections of invertebrates separately from plants that were native to three different areas: the United Kingdom (called ”natives”), other areas in the Northern Hemisphere (“near natives”, and Southern Hemisphere (“exotics”).

As they found with Pollinators, the scientists found that the native plants supported the largest numbers of the plant-dwelling invertebrates. By comparison with the native plants, the near-native plants supported about 10% fewer invertebrates, and the exotic plants supported 20% fewer.

Overall, these observations indicated that native plants are most important in supporting these groups of invertebrates, and the near-native and exotic plants also provide effective support at somewhat reduced levels.

The study concluded that gardens should emphasize native plants, but could include near-native and exotic plants as well. The most important consideration was to develop a dense planting scheme so that the garden could support all kinds of plant-dwelling invertebrates as part of a balanced ecosystem.

Exotic plants are important to include in the garden for Pollinators because Southern Hemisphere plants often bloom during months when Northern Hemisphere plants are dormant, and thus provide Pollinators with food sources for a longer period of the year.

Although this study was conducted in England, its findings could apply reasonably also to gardens of the Monterey Bay area. With that interpretation, we would treat California native plants as the “natives,” plants from the Mediterranean climate areas as “near natives,” and any other plants as “exotics.”

These two studies support the usual assessment that native plants are most supportive of local plant-dwelling invertebrates, while showing that near-native and exotic plants also provide effective supports for the garden’s ecosystem.

These findings might apply as well to local birds, reptiles and mammals, but demonstrating those relationships would require another study. For example, while berry-producing shrubs provide natural food for birds, separate counts of bird visits to native, near-native, and exotic berry-producing shrubs might yield interesting results.

That would be a very challenging assignment!