Last summer I came across a downy woodpecker hammering away at the stalk of a sunflower. I wondered what it was going after, and so I split open a stalk lengthwise to find the center of the stem hollowed out and several small larvae squirming through the debris left behind. A quick internet search later and I was learning about sunflower stem weevils, specifically Cylindrocopturus adspersus, which seems to be the species getting the most attention online and the stem-dwelling weevil that commercial sunflower growers seem most concerned about.
However, the range of sunflower stem weevil doesn’t appear to extend into Idaho, and so this is not likely to be the larvae I was seeing. There are other weevil species whose larvae can be found inside the stems of sunflowers (The sunflower I was observing was Helianthus annuus. I wasn’t specific about naming a particular species because it is my understanding that these weevils can be found on a variety of different Helianthus species., such as the cocklebur weevil (which is found in Idaho), but since larvae can be difficult to identify, I’ll wait to confirm the identity until I hear from an expert, find an adult weevil, and/or raise the larvae in captivity and see what it turns into. If and when that happens, I’ll be sure to update you. Until then, I present to you the life cycle of a sunflower stem weevil, which is still quite interesting, even if it’s not the species I found inside my sunflower stalks. And to be clear, the sunflower I observed was Helianthus annuus; however, the weevils I refer to in this post can be found on a number of different Helianthus species and related genera.
Sunflower stem weevils are in the family Curculionidae, which is the snout and bark beetle family. There are tens of thousands of species of weevils, a handful of which interact with sunflowers (plants in the genus Helianthus). Some weevil species eat the seeds, others eat the leaves, some are root feeders, while others are stem feeders. Depending on the life stage of a particular weevil species, it may consume multiple parts of a sunflower. Another interesting weevil is the sunflower headclipping weevil, which you can read about at The Prairie Ecologist.
Adult sunflower stem weevils are about 3/16 inch (4-5 mm) long and somewhat egg or oval shaped. They are grayish-brown with white spots. Their eyes, antennae, and snout are black, and their snout is short, curved, and held beneath the head. As adults, they can be found on sunflowers and sunflower relatives eating the leaves. However, they are not easily found. Their size, for one, makes them difficult to see, and they also move to the opposite sides of leaves and stems when disturbed, sometimes dropping to the ground as a threat approaches. You can see images of them on BugGuide.
The larvae of sunflower stem weevils are about a quarter of a inch long and creamy white with a small, brown head capsule. They feed in the vascular tissue of sunflower stalks during the summer. In the fall, they migrate to the base of the stalks and create chambers in the woody tissue of the stalks and root crowns for overwintering.
Sunflower stem weevils have a single generation per year. After overwintering as larvae in the base of last year’s sunflowers, they pupate and emerge as adults in late spring or early summer. They find young sunflower plants and begin feeding on the leaves. After about 2-4 weeks, the weevils mate and lay eggs just beneath the epidermis of sunflower stems, usually in the stalk just below the cotyledon leaves. The eggs hatch a short time later and begin feeding in the stem until it’s time to overwinter.
The damage caused by sunflower stem weevils is generally only a problem on sunflower farms, and only when weevils are found in high enough numbers to cause significant yield losses. Damage to leaves by the adults isn’t usually a concern. On the other hand, as the larvae tunnel through the stem, they can cause the plant to lodge (i.e. fall over prematurely), which is a problem particularly when the plants are machine harvested. Sunflower stem weevils can also introduce and help spread a fungus that causes black stem rot.
Read More About Sunflower Stem Weevil and Other Insect Pests of Sunflowers:
- Colorado State University: Sunflower Stem Weevil Management
- Pacific Northwest Pest Management Handbooks: Sunflower-Stem Weevil
- Kansas State University: Sunflower Insects
- Texas A&M: Managing Insect Pests of Texas Sunflowers
I appreciate your careful, scientific explanations. Perhaps I missed it, but I do not see the scientific name of the sunflower your weevil article is about. I take it that you are referring to Helianthus annuus.
Thank you! Great question. The sunflower I was observing was Helianthus annuus. I wasn’t specific about naming a particular species because it is my understanding that these weevils can be found on a variety of different Helianthus species.
Thanks for your reply. I have learned that in order to avoid confusion I am always on the safest ground when I use the scientific name (in addition to or instead of the common name) . In the weevil case there are so many people who use the common name, “Sunflower”, referring only to Helianthus annuus. I point out to these people that there are about 35,000 plants that are “Sunflowers”, so we need to come up with a much more specific common name for the annual sunflower. I think it would be good to update your posting pointing out just what you said to me, “The sunflower I was observing was Helianthus annuus. I wasn’t specific about naming a particular species because it is my understanding that these weevils can be found on a variety of different Helianthus species”.
Here is the URL for my website of 22 years in case you would ever like to delve into a cool issue about Rocky Mountain flora: http://www.swcolordowildflowers.com .
Thanks for the suggestion. I’ll be sure to make that edit on the post. And thanks also for sharing a link to your website. It looks amazing. I will definitely be spending some time on there.
This will sound silly but I have always liked weevils, I think is the name. Maybe the song by The Presidents of the United States of America “Boll Weevil” has something to do with it. This is a lovely and informative post, thank you. Also Al I am afraid your link is not working, it may be missing an ‘a’. I am very interested in Rocky Mountain Flora, I will attempt to find your site another way. Thanks to both of you for the knowledge!
Thanks for your comment Claire! Great to hear that you found the post informative. I really like weevils too!!
Hi Claire. Yes, you are correct that I did not spell the name of my website correctly!! It should be, http://www.swcoloradowildflowers.com . I hope you enjoy browsing it. Let me know if you have any questions or comments. Thanks.