Climate: Hot and sunny through Summer (a little bit north of Toronto). Beans usually thrive. Soil is amended triple mix. Two beans are in pots, others are in a raised bed which is at least 40ft away from the pots. All bean plants have flowered and I have harvested a few beans (less than a dozen).
Something is eating the immature beans. I’m left with lots of tiny stalks where beans should be. The leaves are all in great shape, no evidence of bug infestation. No sign of slugs or snails. This hasn’t happened before and I’m wondering what could be causing this.
Thank you for asking the Toronto Master Gardeners this very interesting question about the mysterious bandit that is eating your Scarlet Runner beans when they are still immature. This has been quite a difficult question to answer since the most common diseases or insects of the runner bean have been excluded by you saying that: “The leaves are all in great shape, there is no evidence of bug infestation, and no sign of slugs or snails”.
Under these circumstances, I suspect that the culprit may be mice or earwigs. To check for mice, look for their little black droppings close to your plants. To check for earwigs, go outside at night with a flashlight, and you should be able to see them scurrying away from the light. However, squirrels or birds can also eat immature runner beans, but not that often.
For more information on Scarlet Runner Beans:
wikipedia on Scarlet Runner Beans
For more information on earwigs:
University of California, earwigs