I've had this happen the last couple of years to individual lily stems that were part of larger patches that were mostly unaffected. In every case I found that a borer had gotten into the stem anywhere from about 6" to 18" up from the ground. In fact, the very bottom of the stem, below the borer hole, stayed green and healthy and kept its leaves; above the hole, the plant got limp and yellow, and eventually turned brown (if it didn't break or if I didn't cut it off first). A couple of the stems I cut off at the point where I found the borer hole, and I split the cut stem from the bottom up. Eventually I came to a grub about 3/4" long eating its way up the pith.

I asked one of my horticulturist friends about this, he says there are several different borers that will get into the fleshy stems of a wide range of perennials, including lilies. They generally spend the first part of their lives eating leaves of grass and weeds, then when the weather starts to get hot, they look for a fleshy stem to bore into to complete their life cycles.

To control them, the main thing is to not have lots of overgrown grass/weeds in the vicinity (in my case I had a neighbor who was not very good about keeping his back yard under control, and I only had problems with lilies planted along his fence while lilies in other parts of the garden had no borers). This is not foolproof, but the best start. My friend also suggested Marathon used Mid-May (for me, in zone 7) as a systemic; unfortunately the critter has to munch a little bit on your plant to be killed, but it should die before much damage occurs and the stem should survive it. His other suggestion was to get a topical pesticide spray labeled for ornamentals and use according to package directions over a period of a month during the time when the grubs are migrating (again, for me in zone 7, he said probably end of May to end of June, which fits with my observations from last year), spraying just the bottom two feet or so of the lily stems. I am using Sevin, started about 10 days ago and will have done a total of three applications at two week intervals when I stop. I chose Sevin because it has high toxicity, persists on the surface of leaves or in soil only about 3 to 4 days (outdoors, exposed to sun and rain), and persists in the tissues of the leaves for about 12 days. I also used it only very locally on the lily stems - not the thing I would use over broad areas in my garden for general control due to its high and general toxicity to both beneficial and damaging insects.

The bad news is that by the time I noticed the change in the lily stem (looking fine one day, looking peaked the next), it was too late to save the stem from the borer damage. The good news is that the lily bulb itself seemed able to survive the one-year interruption, and I don't think I've lost any lilies from it so far.

- Steve