I recently attended a Lunch Bunch at Mercer on ‘Insect and Plant Interactions’, if you have a chance to hear that talk I strongly recommend it.
In your garden is an evolutionary arms race that has gone on since plants and insects first appeared on earth. Sometimes there are truces, sometimes one or both will adapt, and sometimes it’s all out war.
Plants have developed many defense mechanisms to protect themselves from herbivore insects ( about half of all insects ) including chemical toxins, physical barriers, trickery, but sometimes resort to a symbiotic relationship with the insect.
Some plants can send warnings to other plants when they are under attack by insects. These warnings are volatile organic compounds the leaves release into the air. Nearby plants sense the compounds and begin to ramp up their toxin production. This saves the plant from spending energy making toxins when they are not needed. Trees attacked by pine bark beetles will do this, legumes are also know to release warning chemicals when under attack.
Sometimes plants release chemicals that attract beneficial insects for pollination or to attack herbivores. Some plants create chemicals that make them undigestable to insects. Other plants release compounds to repel insects ( deet ). Some common beans create toxins that when eaten by caterpillars will prevent proper butterfly development. Nicotine is a toxin to ward off tobacco pests.
Physical barriers take the form of thorns ( roses ), hairs ( lamb’s ear ), thick walled leaves ( cactus), and grit on the leaves (bamboo). Plants that are wounded may release antibacterial chemicals and cell strengtheners to wounded areas.
Trickery by plants is more common than you’d expect. The passionflora vine has little nodules at the base of the leaves. These nodules look like butterfly eggs. When butterflies mistake these for other butterfly eggs, they move on and look for a less crowded place to lay their eggs.
Some plants have gone proactive and eat the insects. ( Venus flytraps, pitcher plants ). These plants either trap by trickery or close when a trigger hair is touched and capture, dissolve and eat the insect.
Waterlilies sacrifice a bee for each pollination. In the first stage the lily holds water in the cup. The bee arrives to drink but drowns because the plant has put surfactant into the water. The pollen that the bee has carried from previous flowers is released to fertilize the murderer. The next day the flower opens, dries and produces its own pollen for the next bee to collect and carry off.
Other plants are slightly less proactive predators. Pipevines attract flies who climb into the flower and are trapped in the bulb at the bottom by hairs that face in along the tube. After the fly created a ruckus getting covered in pollen the flower relaxes in the morning to let the fly escape and pollinate the next pipevine flower.
There are many symbiotic relationships, flowers have colors in visible and infrared light as well as scents and shapes to attract bees. Some like bluebonnets change color to announce whether there is nectar remaining.
Others give off heat and or less pleasant scents to attract beetles and flies. ( Sago, stapeliads, aroids )
Some plants provide shelter for ants who in turn protect the plant from other predators and feed the plant. Some ant species will even strip bare competing foliage plants.
Many species of plants and insects have developed a one to one relationship, wipe out the insect and you wipe out the plant species. ( yucca, Senita cactus/moth )
Insects continue to evolve ways around plant defenses including neutralizing or becoming resistant to plant toxins. For instance monarch butterfly caterpillars feed on toxic milkweed, black swallow tail butterflies on pipevines. Sometimes these insects use the ingested toxin to become toxic to their own predators.
Clever carnivorous insects will hang out on a plant, wait for the plants predator insect and have it for dinner. Some like ranching ants will milk and ranch the aphids that feed on a plant.
Sometimes only one or a few insects can eat a given plant, providing no food for competing insect species. (monarchs and milkweed ). Others like locusts, adapt to eat anything and everything in their path. Yet other insects can evolve to learn to eat the parts of the plants that do not contain a toxin.
All of which makes the garden a fascinating place.
More information for the curious:
Pollinators can drive flower evolution
Ecological Genomics of Plant-Insect Interactions
Plant insect interactions: An evolutionary arms race