We held our OSU Buckeye Environmental Horticulture Team strategic planning meeting this week at Mohican Lodge and Conference Center. A hike in Mohican State Forest revealed that Yellow Poplar Weevils (Odontopus calceatus, family Curculionidae) are producing noticeable damage on their namesake host in that part of the state.
This native weevil is found throughout much of the eastern U.S. matching the native range of its namesake host, yellow poplar (Liriodendron tulipifera) (a.k.a. tulip poplar or tuliptree). Weevils may also be found on magnolias (Magnolia spp.) and sassafras (Sassafras albidum).
Although yellow poplar weevil is the common name approved by the Entomological Society of America, the weevil is sometimes called the sassafras weevil, magnolia weevil, magnolia leafminer, and tuliptree leafminer. It’s difficult to explain the weevil’s odd host range using the classic taxonomy of its hosts.
Tuliptrees and magnolias belong to the family Magnoliaceae in the order Magnoliales, while sassafras belongs to the family Lauraceae in the order Laurales. However, all three are now grouped in the clade Magnoliids based on phylogenetic patterns. Still, the three hosts seem like strange bedfellows.
The small (2/16" long), oval-shaped yellow poplar weevils range in color from black to brownish-black to reddish-brown and have deeply grooved wing covers (elytra). Although they are good flyers, the weevils often elect to fold their legs to "play dead" when disturbed; a defense strategy that is common among weevils. In some people's eyes, yellow poplar weevils resemble ticks, which may generate calls to Extension offices concerning "flying ticks" during outbreak years. Of course, ticks can't fly.
Weevils are beetles with their chewing mouthparts located at the end of their snouts (rostrum). Yellow poplar weevils damage leaves in two ways. Overwintered adults may chew holes in leaves that are unfurling from buds. The small holes become large, ragged holes as the leaves expand.
As the leaves expand further, the adults consume the lower epidermis and leaf mesophyll leaving behind the upper epidermis. Eventually, the thin epidermis dries out, turns brown, and is shed from the leaf to produce bean-shaped holes. Adults may also feed on flowers.
Although the yellow poplar weevil has one generation per season, adults feed on leaves twice during the growing season. The so-called “spring generation” of adults spent the winter in protected sites such as in the duff beneath trees. They emerge in the spring to feed, mate and lay eggs in leaf midribs.
The yellow poplar weevil eggs hatch into white, grub-like larvae that feed as leafminers between the upper and lower leaf epidermis. The leafmining larvae may feed singly or in groups of up to 19 larvae to produce large blotch mines. Larval frass (excrement) is extruded as dark filaments that look like hair. All of the leafmines that I opened contained larvae that had not yet completed their development.
The yellow poplar weevil larval blotch mines are most often found on yellow poplar and magnolia. I have never observed leafmines on sassafras.
The so-called "summer generation" of adults arises from the leafmining larvae. Typically, there are a greater number of adults in the summer generation compared to the spring generation meaning most of the leaf damage occurs in mid-summer.
Although larval leafmines are noticeable, the most serious damage is caused by adults, particularly the summer generation. Numerous feeding holes can cause leaves to wilt, turn brown, and die making heavily infested trees look “burned.” Indeed, severe adult damage is sometimes mistaken for drought damage.
The summer generation adults drop from their hosts in mid-summer, usually sometime in July in Ohio, and crawl into the duff. They undergo a period of heat-related dormancy (aestivation) and then fall into a deep dormancy (diapause) for the winter.
Yellow poplar weevil populations usually rise and fall dramatically with 2 - 3 consecutive years of "outbreak" population densities followed by several years with almost no weevils observed. However, the collective damage produced by the adults and larvae during outbreaks years is seldom severe enough to cause significant harm to the overall health of affected trees.
On a side note, yellow poplars and magnolias may also host entirely different types of leafminers belonging to the moth family Gracillariidae. Silvery, snaking leafmines are the handiwork of the Magnolia Serpentine Leafmining Caterpillar (Phyllocnistis magnoliella) and the Tuliptree Serpentine Leafmining Caterpillar (P. liriodendronella) on their namesake hosts.