Almost all insecticides are toxic to bees. Very few of them are specific enough in terms of how they work that they could kill the target insects without also being toxic to bees. That's not the important consideration, though. The thing you should consider is: how likely are you to kill bees (and butterflies and whatever other insects you'd prefer to not kill) if you apply a given pesticide in particular way. That's a far more complex question that requires some understanding of how the chemical moves through the environment and the biology of the insects in question.
Even Bayer, the manufacturer of Merit, freely admits that imidacloprid is highly toxic to bees if they are exposed to it. Their contention is that bees don't get exposed to it because they'd have to eat the leaves of treated plants for that to happen, which they generally don't do. However, researchers started testing the nectar and pollen of plants (which bees do eat) treated with imidacloprid and found it to be present at levels they believe to be dangerous. The big problem with systemic pesticides like imidacloprid is that they are typically applied in the spring and persist in the tissues of the plant throughout the growing season. So, if it's a plant that has flowers that are attractive to bees, the bees are going to suffer. On the other hand, treating plants that don't have bee attracting flowers, turf grass for example, obviously creates far fewer problems, at least for the bees.
If I broadcast sprayed my whole yard with carbaryl during the peak of iris bloom, it would be a catastrophe for the bees. I obviously don't do that, and spraying tiny amounts onto pods after flowering is done presents a minimal threat, simply because there is an extremely small possibility of bees actually being exposed to it. They don't chew on iris seed pods.
It's all about how and when the pesticide is applied. With systemic pesticides, you don't get that option. It's essentially like spraying your plants every day.
