What's attacking one Lupin but not its neighbor?

Question

I have two Lupin plants next to each other in my front yard:

They are both Russell hybrids, both grown from seed and planted there at the same time in Spring 2009. The one on the left is healthy, but recent growth on the one on the right is stunted and deformed. The leaves are smaller, with narrower leaflets, and there are large gaps between individual flowers on the spikes.

I don't see any sign of insects on the affected plant (or on nearby plants, which include roses, lavender, Lithodora and Dianthus).

If it's any clue, I had several other Lupins near that spot that didn't grow this year, but that may be because it's been a colder than usual Spring here (Pacific NW, USA).

Is this curable? If not, should I dig out the affected plant? Can I replace it -- and the ones that didn't grow this year -- with more Lupins, or should I plant something else?

Answer 1

As it turns out, the answer was as Mike Perry suggested in the comments on the question: wait and see. One year after I posted this question, the previously "bad" Lupin is growing and flowering strongly, while the "good" Lupin is a lot smaller than last year, but otherwise looks healthy:

Answer 2

Lupins are notoriously susceptible to slugs and snails, which never go further than they have to from their retreat. My guess would be that somewhere near the lupin that is being eaten a snail lurks beneath a flowerpot or a brick.

Answer 3

I have a very similar issue with Lupins, 2 plants bought at the same time that have previously flowered well in the same bed are being stripped of the flowers, just leaving a damaged stem, I have no issues with slugs or snails, the plants leaves are good, however one plant is affected and the other is not, the damage is being caused by Sparrows seeking bugs from the young flowers, I have been spraying bug spray and there are no signs of bugs, however the Sparrows still look for them and cause damage to the young flower, once fully flowered they seem to leave them alone, it is only the young flowers, most of the buds are knocked off by the birds early in the morning, one day it's fine and the next they are stripped and the fallen buds are just sitting on the leaves below. How I stop the Sparrows doing this... I don't know, and why they seem to only strip one plant and not the other is a mystery too, maybe one gives off a slightly different scent or maybe one sits in the flight path of the Sparrows so it's the one they land on first, then when they find no bugs they don't bother with the other plant??