I spotted another article today about the way plants protect themselves from insects. The bottom line of the article is that plants detect the digestive juices of insects that are chewing on them and respond by sending out chemical scents that attract beneficial, predatory insects. These predators then eat the pest insects and thus protect the plants.
That makes a lot of sense to me. I have to spray pesticides inside my greenhouse to give the hibiscus relief from various chewing and sucking insect pests. However, when I plant the same hibiscus in my garden they rarely need my help with anything except thrips. We are surrounded by lots of landscaping and natural wild fields so there are plenty of beneficial insects that can be attracted by the calls of the hibiscus for help. Perhaps they don't react to Western Flower Thrips the same way because the thrips are not eating the leaves or sucking juices, instead eating pollen and invading flower buds.
Excerpt From Science Daily:
"The best way to protect our crops is to figure out how they protect themselves.
What we're studying is how plants are using these chemicals to help to defend themselves," Schmelz said. These chemicals call to other bugs for help. Plants can't feel a caterpillar munching on it, but it recognizes the chemicals from the digestive juices from the caterpillar's mouth. Sensing something is wrong, the plant releases odors to attract other bugs to get rid of the problem. "A tiny bite from a very small caterpillar will start this process going," Schmelz explains.
Researchers hope to learn more about the chemical process plants use to protect themselves. This information may help lower the use of pesticides.
"If we can have a plant that is producing a stronger defensive reaction once it's chewed on, that pest may no longer be a problem. We might not need pesticides," Schmelz said.
Not only can bugs detect the odors plants emit for protection, but many farmers notice when army worms are in their cornfield. They smell very sweet. And plants react to each attack differently, emitting different odors to attract different bugs to help defend them."
The whole article is here: http://www.sciencedaily.com/videos/2007/0906-plants_under_attack.htm