So plant reproductive success and growth is heavily affected by coevolutionary relationships with pollinators. I write about this all the time, as it is one of my main areas of interest. But pollination is not the only way in which insects interact with plants -- and the flowers aren't the only part that can end up attracting insect attention.
While I was wandering around last week, I managed to get a few photos of some members of the Orthoptera (an order of insects that includes grasshoppers, locusts, katydids, etc). These are usually plant predators.
Take, for example, this funky-looking guy:
|Katydid nymph on Lotus corniculata - notice the holes in the two petals in front|
Similarly, I also managed to get a photo of this guy:
So you might look at the grasshoppers and katydids, and at other herbivores (eg aphids, deer) and think to yourself: well, that's a stacked match-up. The plants can't get away.
And you'd be right that the plant doesn't have running away as an option (although some do grow in places or times that are less accessible to herbivores). But plants have other defense mechanisms against herbivory that are pretty awesome.
Some plants develop permanent or constant physical defenses such as thorns, which make them unpleasant or difficult to consume. Or they produce poisons that deter predation.
And then there are a whole lot of complex chemical things that plants can do to deter herbivory. Some plants, when they start getting eaten, will produce volatile organic chemicals which will in some way deter the herbivore: some will be directly unpleasant to the herbivore, some will prevent herbivore eggs from developing correctly and hatching (thereby reducing their numbers and the damage they can do), and others will even attract the predators of the herbivores (basically recruiting other insects as enforcers).
So although it looks like an uneven matchup, plants are not nearly as helpless as they might seem.
Today's material primarily sourced from the wikipedia page on the topic of plant defense against herbivory and from my lecture notes. I've really only just scratched the surface here, so if you're interested I would highly recommend reading the wiki and some of the sources linked on that page.