Question
For the past twoyears my potted geraniums (zonal) get lovely blooms for a week or two then a worm of some type begins eating the blossoms before they have a change to fully bloom.  I have sprayed with a brand name spray for pests, but it doesn't seem to get rid of the caterpillar type worm.  I live in Southern California, inland.

Answer
Bacillus thuringeinsis, called BT, is a pathogen that parayzes budworms, which is what I suspect you are troubled with. It is called a geranium budworm, or sometimes called a tobacco budworm. BT is either sprayed or dusted on the plants.
While it presents few risks to humans there is a caution that it may cause skin, or eye irritation, or an allergic skin reaction.
Once the BT enters the worm, it causes them to stop feeding, and they starve to death. Because BT is inactivated by heat, and light, applications shoud be scheduled in the afternoon, or evening, or on overcast days. It's also important that it be applied in the stage when they are just beginning to eat. At a more mature stage, they can go without food for several days and may pass up eating the leaves coated with BT.
You may not find BT products in local garden centers. It is most often available through mail-order suppliers and organic gardening retail sources.
Remove dried-up buds and flowers that may harbor the worms. BT will kill budworms if it can be dusted or sprayed on plants before the worms enter the buds. You can also spray plants with an insecticide containing pyrethrin, or carbaryl, but this too will be effective only when applied immediately after the eggs hatch. Once they enter the buds, the worms will be protected from the spray.
Another such worm that could be the cause of your problems is the corn earworm. The treatment for it, is the same as it is, for the geranium budworm.

bakerplanter