Q. A friend of mine is having trouble with his Century plants, Agave and Cactus turning yellow. These plants are in a rock yard with hand watering, they are 2 years old. Last year he had something chewing a cactus and eventually lost the plant. This year he has had no chewing. I hope you can help us without pictures.
A. Two things come to mind when agave is yellowing this time of year...watering too often or agave weevil. Because this is the time that we see agave weevil damage below ground showing itself to our eyes above ground I tend to think of agave weevil. Particularly if it is century plant and this time of year.
Treatment to prevent damage by this insect has to be done earlier in the season before they really get going on damaging the roots.
I hope these links help.