I have six coneflowers in my butterfly garden. As the older blossoms are turning brown and starting to fall off (I assume going to seed), there are small worms under the buds. Are those worms invading my flowers or are they simply helping the garden out? A majority of the blossoms falling off have these worms under the them. If they are invading the flowers, how do I get rid of them without poisoning the plants, which would affect the caterpillars? C.H., Wayland
The sunflower moth is one of the most damaging pests of sunflowers and coneflowers, and we have seen its presence in our area. Adult sunflower moths are nocturnal, gray to tan in color, about 2/3 inch long, and rest with wings clasped tightly to the body. Flowers in the early stages of bloom are favored for egg laying at the base of the florets. The moths can complete a generation every 30 days, so several overlapping hatchings can occur. From hatching to full maturity is about 15 to 19 days. The half-inch caterpillar has light brown and white stripes along its length. They begin feeding on the pollen and florets, later boring into the head. The first indicator of trouble is a tangled mat of webbing and caterpillar frass that looks like fine sawdust. The top of the seed head looks deformed and bumpy, with the individual seeds pushed up and away from the flower base. The injury caused by the larval feeding provides infection sites for rhizopus head rot, which is what causes the blossoms to deform, turn brown and fall off.
Remove a coneflower head and cut into it to see if larvae are or were present. Take the sample to a knowledgeable nursery or the MSU Extension for positive identification. Begin with removal and disposal of infected flower heads and daily monitor newly emerging buds for fresh larvae activity. Handpick the newly hatched larvae from the coneflower heads. Many of the larvae pupate within the flower heads. However, others descend to the ground on silken threads to pupate in crevices or under leaf litter and ground debris. The key here is to remove any compromised seed heads before the larvae can pupate. You interrupt the repetition of the life cycle.
Secondly, cultivate and clean the area at the base of your coneflowers to a depth of 2 to 3 inches to prevent any overwintering. Sunflower moths do not attack coneflowers until the first flowers open since they are attracted to pollen and scent. So there is no point in scouting for moths until the flowers begin to open. Then it must be done frequently because migratory moths can appear in large numbers virtually overnight. Scouting should be conducted about an hour after sunset when moth activity begins to peak by using a flashlight. There are also pheromone traps that attract and capture male moths.
A preventive insecticide that contains organophosphate materials is best applied as blooms begin to open and has a somewhat greater residual activity than pyrethroids. The insecticide must be applied to the flower face to be effective. Controlling the moth larvae raises concerns about impacting pollinators such as bees. Applications should be made in early morning or late evening when pollinators are not flying. Evening is preferable if you have healthy bee activity, as this will allow some dissipation of material overnight before the bees are active again. Pyrethroids tend to be safer for bees because of their repellency. Dust and wettable powders tend to be more toxic formulations for bees than solutions or emulsions.
In short, it is preferable to trap the moths, handpick the larvae and sanitize the soil base before using an insecticide that could potentially harm an already stressed population of pollinators such as bees.