According to this article in the San Francisco Chronicle, the EPA has found a clear level of concentration of the pesticide imidacloprid in which things start to go badly for the local honey bee population: "If nectar brought back to the hive from worker bees had more than 25parts per billion of the chemical, 'there's a significant effect,' namely fewer bees, less honey and 'a less robust hive,' said Jim Jones, EPA's assistant administrator for chemical safety and pollution prevention. But if the nectar chemical level was below 25 parts per billion, it was as if there were no imidacloprid at all, with no ill effects, Jones said. It was a clear line of harm or no harm, he said."

The level of chemical also interacted with type of crop. For instance, concentrations that     were harmful in cotton and citrus fruits were not harmful for corn. And treating seeds with the chemical didn’t seem to harm bees.

This is a wonderful example of dose effects: it's not the substance but how much of the substance that counts. And the relation between dose and effects is not linear. The question is not: "is this toxic" but: "at what dose does this become toxic?" Often what is harmful at high doses is benign or even beneficial at low doses. Something to keep in mind whether we're talking about pesticides or sweeteners in tea.