Rice University researchers have shown that Arabidopsis plants use their circadian clocks to ensure that their peak toxicity coincides with the feeding behavior of their insect nemesis, Trichoplusia ni, a.k.a the cabbage looper.
Circadian clocks control the internal rhythms that living things use to organize their day. If you always get drowsy at 2 pm and hungry at 6 pm, you can thank your circadian clock. Other creatures also exhibit different behaviors at different times of day. For example, cabbage loopers feed most actively in the late afternoon. Arabidopsis secretes plant hormones that regulate the production of toxic chemicals so that they peak at exactly that time.
The researchers grew Arabidopsis under varying conditions. In some cases, the plants were subjected to the same twelve hour light/dark cycles as the caterpillars. In other cases, the plants were twelve hours out of phase with the herbivores. The plants that were on the same cycle as the insects showed significantly less predation than those that thought it was day when the insects thought it was night. When this same set of experiments was done with plants that had defective circadian clocks, there was no difference between in phase and out of phase samples.
You can watch an explanation below. As an aside, I was particularly impressed by the camouflage of these caterpillars.