|Dangling Sunflowers Caused by Sunflower Headclipping Weevil|
It's not vandals or strong winds that broke the broken stem of your sunflowers. It may be the work of the sunflower headclipping weevil (Haplorhynchites aeneus).
This ¼ inch long shiny black weevil attacks other members of the aster family including purple coneflowers, cup and compass plants. It feeds on the stems just below the developing flower or on the leaf stems. The partially severed stems remain attached dangling from the plant. The adults can often be found within the flower feeding on the pollen, mating and laying eggs.
The eggs hatch when the flower hits the ground. Here the immature stage of the weevil, called grubs, feed and spend the winter. They pupate and emerge as adults next year.
Carefully remove dangling flowers and drop them into a can of soapy water to kill adult weevils. Dispose of these and any infested flowers on the ground to reduce future infestations.
A bit more information: It is believed that only the females do the stem clipping. This energy intense behavior may discourage other flower-feeding insects from moving in to compete with the headclipping larvae.