Lesson learned: Sunflower bud moths in the Boswell Garden

By Connie Mehmel
WSU Chelan/Douglas County Master Gardener

Sunflower stem cut open to show bud moth caterpillar frass.
Sunflower stem cut open show bud moth caterpillar frass and feeding. – Provided photo/Connie Mehmel
Connie Mehmel
Connie Mehmel – WSU Extension Chelan/Douglas County Master Gardener – Provided photo

Sunflowers are wonderful garden plants. They are easy to grow and offer a wide variety. Huge Mammoth sunflowers can grow 12 feet tall with 12-inch-wide flower heads heavy with seeds that can be roasted for delicious snacks. You can also grow dwarf varieties less than 12 inches tall with 5-inch wide pollenless blooms for cut flowers or bouquets.

The classic sunflower is bright yellow in color, but you can find varieties with flowers of white, lemon, orange, red or even green. A beautiful Teddy Bear sunflower won the Best of Show flower at the Chelan County Fair in 2021, so Master Gardeners decided to feature it in the Boswell Garden at the Chelan County Expo Center for 2022.

Things were going well … then came our first encounter with sunflower bud moths.

The sunflower bud moth (Suleima helianthana) is found in North America from Mexico to Canada. Adults are pale gray moths about 7 millimeters (five-sixteenths of an inch) long with two dark bands across the wings. The first generation emerges in late May or early June and lays eggs on the tips of immature sunflower plants or at the bases of the flowers of mature plants.

When eggs hatch, the caterpillars typically bore into the stem and feed on the pith. The boring holes can be recognized by lumps of black frass exuding from them. Feeding on the pith generally causes only minor damage. But sometimes caterpillars will also feed on the fleshy parts of the flower heads, causing the flowers to fade prematurely. A second generation of moths emerges in August, but this generation usually does not cause damage.

We have grown a variety of sunflowers in the Boswell Garden for a number of years without seeing damage from bud moths, but last year we had many deformed flowers.

What was different in 2022? It may have been our timing. We wanted our Teddy Bear sunflowers to get an early start, so we started the seeds in pots indoors under lights. The plants were out in the garden with well-developed flower buds by the time moths emerged in early June. Research sponsored by the National Sunflower Association suggests that early planting may increase the likelihood of damage to flowers. If the plant has fully developed buds, the moths will lay eggs on them, and caterpillars will feed in the flower heads. If there are no fully developed buds, the eggs will be laid on the stem.

Last fall, we removed all of the old sunflower stems and disposed of them, since they are overwintering sites for next year’s bud moths. We will keep timing in mind when we plant sunflowers this spring.