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The garden is located at the intersection of Wildcat Canyon Road and South Park Drive in Tilden Regional Park near Berkeley, CA


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February 2015

Plant-Insect Interactions
—and Some Fascinating Chemistry

California bay laurel (Umbellularia californica)

Have you ever wondered why some plant leaves or flowers are easily attacked by insects (like roses devoured by aphids or cucumber beetles), while other plants are pretty much left alone, like leaves of bay trees or rosemary? Some plants have evolved potent defenses to deter insects from eating them, and then again some insects have developed ways to ingest plant defensive substances and use them to keep away their own predators. People working with plants frequently observe, invite, or have to contend with plant-insect interactions. If we want to attract bees and butterflies to our gardens, we plant colorful, fragrant plants that reward the insects by providing pollen and nectar for them. We may not realize that all these plant-insect connections involve amazing chemistry.

Plants, as “autotrophs,” can synthesize the basic sugars through photosynthesis and can further create from them attractive scents and colorful pigments that call out to pollinators. Plants can also produce defensive smells and toxins that keep predating insects away. As for the insect perspective, most insects obtain their required nutrients from plant sources, namely the basic sugars, amino acids, and vitamins. Carnivorous insects get the basic nutrients from animal sources that had earlier ingested plant foods (like pesky mosquitoes sucking human blood). Honey bees and butterflies find nutritious sugars and proteins in pollen or nectar from flowers, and aphids obtain them in tender shoots and leaves. Insects and other animals are “heterotrophs” and cannot synthesize the basic metabolites in their own systems. As insects are usually short-lived, much shorter-lived than plants in general, they can produce generation after generation in large numbers and can rapidly evolve to adapt to the chemistry of the plants they visit.

After these general introductory thoughts, I would like to focus on some specific plant-insect interactions from our Berkeley/East Bay area that have greatly fascinated me. We’ll begin with bright-yellow aphids (Aphis nerii) feeding on milkweed plants as observed at the Regional Parks Botanic Garden in Tilden Park and then go on to strikingly-colored black-and-orange caterpillars of the pipevine swallowtail butterfly (Battus philenor) feeding on pipevine leaves, also spotted at the Regional Parks Botanic Garden. We’ll conclude our insect-plant case studies with the mysterious charcoal beetles (Melanophila consputa) that could be observed en masse right after the fall 2013 wildfire on Mt. Diablo.

Showy milkweed (Asceplias speciosa) with milkweed aphids (Aphis nerii)

Many hundreds of different species of aphids feed on tender plant parts. While some aphid species, like the pea aphid (Acyrthosyphon pisum), are quite indiscriminate plant eaters, other types of aphids are more selective in their choices of food plants. An example of the latter is the bright-yellow oleander aphid (Aphis nerii), also known as milkweed aphid, seen in the photo feeding in masses on a showy milkweed (Asclepias speciosa). This type of aphid obtains nutrients, but also defensive substances, from plants of the milkweed family (Asclepiadaceae). It also infests plants of the dogbane family (Apocynaceae), like oleanders (Nerium oleander, introduced from the Mediterranean). Milkweeds and oleanders contain cardiac glycosides that are plant steroids, i.e., organic defensive substances that contain a steroid structure. The general name of “cardiac glycosides” indicates that they affect the heart muscle of animals (and humans) ingesting them. The yellow milkweed aphids (note the warning color!) ingest the toxins with the plant saps and can sequester them. When a predator, like a spider, disturbs the aphids, they exude the toxins with the sticky honeydew, and predators back off.

If you are intrigued by aphids, here is a book to learn more about them:
Blackman RL, Eastop VF. 2000. Aphids on the World's Crops: An Identification and Information Guide. Wiley, New York.

 Caterpillars of pipevine swallowtail butterfly  (Battus philenor) feeding on California Pipevine (Aristolochia californica)

At the right time, usually during the month of May or early June, we can observe bright-colored black-and-orange caterpillars of the pipevine swallowtail butterfly (Battus philenor) feeding on pipevine leaves (Aristolochia californica) as in the photo. Their feeding is so voracious that, with eyes closed, we can hear the rasping sound of the caterpillars eating the leaves. The caterpillars not only obtain their basic nutrients from the pipevine plants, but also toxic aristolochic acid, an alkaloid found in plants of the Aristolochiaceae, like Aristolochia species. The caterpillars of Battus philenor can ingest and sequester the alkaloids and are not affected by the poisons. But the caterpillars as well as the emerging butterflies are toxic to predators. Female pipevine swallowtail butterflies pass the toxins along to their eggs, which thus are also protected (and have a red-orange color!). The caterpillars feed on pipevine leaves and on immature fruits. Mature leaves have higher concentrations of the bitter alkaloids than young leaves and are unpalatable. Therefore caterpillars feed only for a short time on a single leaf and then move on. Eventually most leaves end up damaged, but few are eaten completely, and the plant lives on. The emerging pipevine butterflies have a very different diet, sipping nectar from many types of flowers.

Here is a good website on the pipevine swallowtail butterfly:
Art Shapiro’s Butterfly Site: Battus philenor

Charcoal beetles (Melanophila consputa)

As our last example of plant-insect interactions and their chemistry, I would like to describe charcoal beetles (Melanophila consputa), a type of beetle that emerged in great masses shortly after the Morgan Fire on Mt. Diablo in September 2013. Researchers who visited shortly after the big wildfire excitedly told me about these black beetles (see photo). They crawled over people’s limbs and bit where there was exposed skin. Soon after, the beetles disappeared. Where to? And what attracted them in the first place? And what did they eat? Captivated by these beetles I consulted with a firefighter from Plumas County, and he became excited about these beetles, telling me how they appear in large numbers in a hot wildfire and crawl up firefighters’ protective gear—and bite wherever they find an uncovered body part. They appear and are equally quickly gone, and reappear at another wildfire place. When at the fire site, the insects feed on wooden debris like tree stumps. The charred wood is palatable to them, as the hot fire has destroyed many of the chemical plant defenses, like tannins, resins, or some of the toxins. The beetles mate and lay their eggs in the burnt-out stumps, where their emerging larvae have protection and find their basic nutrients from the freshly-killed stumps. Then they crawl into burned trees ready for the next fire, which may be as far away as more than 100 miles. While researching the life habits of these fascinating beetles, I learned about their incredibly high sensitivity towards infrared (IR) radiation emitted by fires. The beetles’ transient, fire-loving behavior has made studying them quite challenging.
If you would like to learn more about these fascinating beetles, here are two good articles:

Hamilton, Joan. Fire-Chasing Beetles Make an Appearance. Bay Nature. Published online January 8, 2014. 

There are countless examples of plant-insect interactions where one or several specific chemical compounds, either made by plants or modified by insects, have key roles, be it to attract pollinators, to repel insects, to eat insects for improved plant nutrition (carnivorous plants!), to provide protection to insects, or to help insects find mates. Many plant-insect interactions, with their specific chemistry, are of commercial interest to us. Honey bees pollinate our blossoming orchard trees. Silkworms (which are caterpillars of the silk moth) specifically feed on mulberry leaves. On the challenging side, termites eat our woodwork as they have the correct enzymes to digest wood, and aphids infest our agricultural crops in masses. All these cases involve more or less specific chemical compounds that either encourage or discourage the interactions.

Carnivorous sundew (Drosera rotundifolia)
— Greti Séquin
Greti is working on a book, The Chemistry of Plant-Insect Interactions: Plants, Bugs, and Molecules, planned for publication by RSC, Cambridge, UK, in 2016
Upcoming Classes
A click here will take you to a full description of the class as well as the class registration form
Wednesdays, Jan 21-May 13, 10 am–3 pm 
Native Plant Habitats: A New Class Series Class Full
Saturday, March 28, 10 am–4 pm 
Botanizing California: Oat Hill Mine Road Class Full
Saturday, April 11, 10 am–3 pm Botanizing California: Hite Cove in the Foothills near Yosemite
Class Full
Saturday, April 25, 9:30 am–3:30 pm Native Botanicals: Color Studies in the Garden
Space Available
Sunday, April 26, 9:30 am–3:30 pm Native Botanicals: Watercolors in the Garden
Space Available
Friday-Saturday, May 1–2 Botanizing California: Table Mountain and Feather Falls from Oroville in the Northern Sierra Foothills
Class is Full
Saturday, May 9, 10 am–12 pm Gardens All Abuzz: Partnering with Pollinators
Space Available
Saturday, June 6, 10 am–4 pm On-the-Trail Nature Journaling Space Available
Illustration/Photo Credits
©Greti Séquin
Milkweed aphids, pipevine swallowtail butterfly caterpillars
©Joan Hamilton Charcoal beetle
Noah Elhardt Carnivorous sundew (Drosera rotundifolia)
John Rusk California bay laurel (Umbellularia californica)