Last year I learned something really valuable from
@Artorius , & I've since read more about this phenomenon.
Much to my surprise I learned that there is a specific pest which attacks bean seeds and it is all over the world, including the U.S. and Canada. It's called
Delia platura. I think it's fair to say we all have this bug flying around our gardens, and most of us don't know we have it (I didn't until Artorius shared the initial info with me) mostly because you usually don't see them and they need certain conditions to be a problem. By the time they do damage you notice, the flies have matured and are long gone. When a bean emerges and has cotyledon leaves but what is called in bean agronomy a 'bald head' ---->
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(it also goes by the name 'snakehead') a bean seed fly has probably visited. I assumed when I saw something like the above picture it was just bad seed, genetic rejects, but realistically it often isn't. Ontario is a big bean growing province, with a great deal of resources in regards to this topic (that's where the photo is from) and between that and talking with one of the bean scientists in their employ I learned a lot. Yes, it is true that cracked seeds will emerge looking like this or seed that has been seriously injured in some way - but most of the time this is mechanical injury from machinery harvesting/planting. Given that most of us are collecting and sorting our beans by hand it is not likely that we will cause much serious damage to our beans. I've planted perfect, fresh bean seeds from the year prior and seen these bald heads. It's also true that planting too deep, or in very hard crusting soil can cause it, as well as bean seeds with very low moisture content (below 16%) but again this has more to do with mechanical planting and the damage a machine is able to do to a drier seed, not so much the seeds of home gardeners.
The flies are active in moist weather and especially cool, moist weather. They plant imperceptibly tiny eggs in the soil next to a planted bean seed and the newly born larvae eats just enough to impair the bean seedlings. development. I started 90% of my beans this year inside in pots to avoid them, considering network beans are precious and every one counts. BUT I still had some damage in the form of bald heads. Why? Because the flies will still lay eggs next to an emerging or sprouting bean at any stage of it's growth while undergound. I saw the smooth green neck of the germinating beans just beneath the surface in the pots and put them out during the day, for sun (which I felt I had to do) and the flies still got a few of them. So even starting indoors is not a guarantee unless you wait until the bean has emerged fully from underground and is fully upright.
The good news is these flies usually get a bean here and there, but not many. It's the odd seed that doesn't seem to germinate or the bald headed ones, or when you see a discoloured brown cotyledon that tells they've been there. I mention all this because many of us here are network growers and it's something to think about when you have a small quantity of precious ones you don't want to risk losing. These flies have probably always affected a percentage of bean seeds in any given garden, depending on the years conditions, but it was insignificant enough to not really notice.
They can also cause hollow stemmed beans even in the fall when there are wet seasons, causing poor development in the bean pods. If you pull the bean stems apart in the fall and there is a rust colour, or they seem not solid inside that is damage from this fly as well. The odd time you can catch a tiny maggot in a bean cotyledon, but that is only if you go digging in the dirt for your seedlings and get lucky with the timing. Most times they'll be gone by the time you notice anything, and this I think is why this pest is so unknown. It also explains why beautiful, fresh seed can come up with bald heads.
