Once you're making your robots fight insect/animal pests, it's probably more efficient to attract natural predators. They're like highly-optimized, self-maintaining, self-constructing robots already.

This is the awful irony of a lot of these farm robots. Like the pollination robots, for example. Maybe I'm just being shortsighted, but it seems like we already have pollination robots, called bees. Native bees don't even require a hive or beekeeper.

But, apparently, it would appear "not using neonicotinoids" is a lot harder than "designing & manufacturing millions & millions of tiny robot bees"

Bees are not always efficient pollinators and they can also be most costly.

More costly than building thousands of tiny robots or pollinating plants by hand?

You need to consider the costs of all these robots. Farmers would in razor thin margins.

how fast and of what size must such a robot be so that it becomes cost efficient?

There are existing prototypes of laser systems for killing fast moving insects such as mosquitos. Imagine a device on a fence pole covering the surrounding acre.

There are plenty of slow moving pests, weeds and the like as well.

Where do you think that canned and baking fruit comes from? It's grown just the same.

Presumably, canned fruit could involve less organic waste as the canner can use ugly fruit that consumers may reject for long enough that it spoils.

I don't know enough to guess whether this is more energy 'efficient', given that we need glass or metal containers, in this case.

Yes, but there's probably less loss & spoilage in the supply chain for canned/frozen/dried/etc.

Same pest/weed control, though, and that's what we were talking about.