My last post was devoted to the diversity of insects that find sunflowers (Helianthus spp.) irresistible thanks to the plant's extrafloral nectaries that provide nourishment for a host of wasps, bees, and other insects. Today, let's look at insects that feed on sunflower buds, leaves, stems, and roots. In stands of native sunflowers, these phytophagous (plant-eating) insects are a natural part of the ecosystem; but where commercial sunflower is cultivated for seeds and oil, those species can be pests.
Some of the most conspicuous sunflower feeders are beetles. The longhorned beetle Dectes texanus is damaging to sunflower in the larval stage. The female beetle lays her eggs in leaf petioles (the short stalk that attaches the leaf to the stem). The larva that hatches from each egg feeds inside the petiole, then moves down the inside of the main stem, eventually reaching the base of the plant. There, it girdles the inside of the stem and moves below this belt of death to insulate itself for the winter. It packs its own fibrous poop around itself and pupates. An adult beetle emerges the following summer.
Mecas pergrata is another longhorned stem- and root-borer that exploits many plants in the sunflower family (Asteraceae).
Sunflower Beetle, Zygogramma exclamationis, is a leaf beetle that feeds on sunflower as an adult and a larva. The adults emerge from hibernation in late spring or early summer, coinciding with the sprouting of sunflower seedlings. The beetles feed on the young leaves. The beetles feed during the day, but their larval offspring feed at night, gathering in small groups among the bracts of flower buds in daylight. There is one generation per year, with adults emerging from the pupa stage in the soil in late summer. They feed briefly before returning to the soil to overwinter.
The Palestriped Flea Beetle, Systena blanda, is another kind of leaf beetle (family Chrysomelidae), and very small. This species has a wide range of host plants, many of them crops, including sunflower. The adult beetles overwinter, emerging in late spring and doing the most damage to the leaves of young sunflowers. They leave lace-like patterns of injury in their wake. The role of the larval stage in sunflowers is unknown, and perhaps they feed on a different plant.
Weevils, family Curculionidae, are beetles, too, and a whole suite of species is associated with sunflowers. The Sunflower Root Weevil, Baris strenua, feeds on the roots as a larva, and on the leaves as an adult beetle. The adults gnaw holes in the foliage in morning and late afternoon; but they move to the roots near the soil surface to create callous tissue into which the female deposits roughly three eggs at a time. The feeding activity of the larvae that hatch usually results in wilting of the plant due to dehydration. By autumn, each larva has created a soil capsule in which it will pupate. An adult beetle emerges the following year.
Sunflower Stem Weevil, Cylindrocopturus adspersus, can be seen on the stems of sunflower plants, but they bear a strong resemblance to plant debris and are easily overlooked. Eggs are laid in the stem, and the larvae that hatch bore downward, reaching maturity at about the time they near the base of the plant. They hollow out chambers in the pith in which they will pupate the following year, usually in June.
The Red Sunflower Seed Weevil, Smicronyx fulvus, is covered in rust-colored scales that rub off as the insect ages. The adults occur in late June and early July, feeding mostly on buds, then pollen once the flowers open. Eggs are laid internally in developing seeds, from the edge of the flower disc inward. Each seed usually feeds one larva, which consumes about one-third of the seed before exiting through a hole it chews, and plummeting to the ground and burrowing beneath the surface. Pupation occurs in the soil the following June or July.
Gray Sunflower Seed Weevil, Smicronyx sordidus, follows a similar life cycle as the Red Sunflower Seed Weevil, except that females deposit eggs externally on developing seeds while the flower is bud is still closed. Feeding by the larva results in an enlarged seed, clearly protruding above surrounding, unaffected seeds.
The Sunflower Head-clipping Weevil, Haplorhynchites aeneus, belongs to the family Attelabidae rather than Curculionidae. Adults of this species emerge in mid-summer, females feeding on pollen and nectar. Each female prepares for egg-laying by gnawing a perferation around the circumference of the sunflower stem, just below the flower head. She then deposits a single egg in the head. This eventually causes the head to fall off, and her larval offspring feeds in the head, eventually exiting into the soil to pupate.
Black Sunflower Stem Weevil, Apion occidentale, is a member of the family Brentidae, or "primitive weevils." Adult beetles first appear in late spring or early summer, and feed on leaves and stems. Larvae feed internally on the pith of stems and the leaf petioles. Pupation occurs within the plant, adult beetles chewing their way to freedom in late July and August. Again the feed on foliage and stems but eventually move to the flower bracts by the end of summer. From there they enter the soil to overwinter.
Moths are another group of insects with many sunflower specialists. The Banded Sunflower Moth, Cochylis hospes, is a member of the leafroller moth family Tortricidae. The adult moths start showing up in mid-summer, but spend the day mostly away from sunflower plants. Females gravitate to the plants at twilight, laying eggs on the outside of bracts on the sunflower head. The caterpillars that hatch move onto the flower disk where they feed on seeds at all stages of maturity. Each larva eats five to seven seeds before leaving the plant for the soil where they spin a cocoon in which to pupate and overwinter. This moth is a certifiable pest to commercial sunflower growers.
Another tortricid moth is Suleima baracana, the caterpillar of which bores in stems of the Common Sunflower (Helianthus annuus). Look closely for it on the upper surface of leaves, and do not dismiss what you think is a bird turd. This moth looks exactly like the waste of a goldfinch, and appears at about the same time as that avian animal.
Sunflower Moth, Homoeosoma electella, is a pyralid moth (family Pyralidae). The adult females flock to sunflower heads that are just beginning to open, and lay roughly 30 eggs per day on the heads. Young caterpillars feed on pollen and florets, but by the third instar (an instar is the interval between molts) they are tunneling into seeds. They also spin silk webbing over the flower head that becomes littered with caterpillar poop (frass). Mature larvae that have finished feeding then descend the plant to the ground where they spin silk cocoons and spend the winter before pupating in spring.
Several species of cutworms (family Noctuidae) and other moths also affect sunflowers. Even the Painted Lady butterfly may feed on sunflowers as a caterpillar, though they are usually found on thistles.
Flies, specifically true fruit flies in the family Tephritidae, make up the last contingent of sunflower consumers. The Sunflower Receptacle Maggot, Gymnocarena diffusa, is a pale, attractive insect with patterned wings. They feed on the extrafloral nectaries. Females begin laying eggs in mid-summer between the second and fourth layers of bracts on the sunflower head. The maggots that hatch bore into the head where they feed. When finished, they usually chew a hole in the head and drop to the ground where they dig more than six inches deep before pupating. Some larvae may pupate within the sunflower head.
The Sunflower Seed Maggot, Neotephritis finalis, first appears around the fourth of July as an adult fly. The female lays her eggs around the corollas of partially-opened florets in the flower disk. The larvae feed within the undeveloped ovaries of the flowers, thereby reducing seed set. Two generations of flies are produced each season. The first generation passes the pupa stage in the flower head; the second generation overwinters in the pupa stage in the soil.
The diversity of insects associated with sunflowers gives you some idea of what most all plants are up against in terms of insect enemies and affiliates. Each part of the plant is a likely target for at least one insect species. We know collectively little about the insects hosted by plants that are of no economic value, so much has yet to be learned. Better get to work, my friends!
Sources: Knodel, Janet J., Laurence D. Charlet, and John Gavloski. 2015. "Integrated Pest Management of Sunflower Insect Pests in the Northern Great Plains," North Dakota State University Extension Service, publication E1457. 20 pp.
"Insects," National Sunflower Association.
"Facts & Information on Sunflower Pests," Kansas State University Department of Entomology.