Hi Shirley –
This is a tricky situation where any pesticidal spray is going to affect both the milkweed weevils and also the monarch butterflies. I think you were on the right path with the pheromones, but there might be some other, better options. I’ve found some articles that suggest gypsy moth traps have been shown to capture a few milkweed stem weevils, however it seems like the weevils were probably there by chance due to the high amount of traps they applied in the study (most pheromone lures are specific to a pest). A more effective control (in terms of cost and eradicating the weevil) would be to apply nematodes directly to the larva in the stems. Mix up a solution of nematodes with water, pull some of it up with garden syringe, and inject a bit into each entrance hole on the stems. Along with this, another preventative option would be to create a barrier between the plant and the ground, such as a plastic mulch. Since the weevils drop from the plant to pupate in the ground, this would block them from being able to do that.
Hope it helps!
Any insecticide will kill the Monarch butterfly. The only way to kill milkweed pests like the milkweed bug and the long horned beetle without harming the butterfly is to just knock them off into a jar of undiluted dish soap. It kills them almost instantly and it is easy. It also works for the earwigs and Japanese beetles. You will never get rid of all unwanted insects but it does cut down on their numbers and the damage they do.
Bugs don’t really bite, so another solution to the milkweed bug problem is to pull them off the plant and drop them into a bucket of soapy water or drop them to the ground and squish them with your foot.
Thanks for sharing this helpful information.