On Thursday, I came across the handiwork of the Solitary Oak Leafminer (Cameraria hamadryadella, family Gracillariidae) on a swamp white oak (Quercus bicolor) in southwest Ohio. The leafmines were very evident; however, the population was not large enough to cause serious damage.
I was struck by the similar appearance and behavior between the solitary oak leafminer and the closely related Aesculus Leafmining Moth (C. aesculisella, family Gracillariidae) on buckeye and horsechestnut (Aesculus spp.). I posted an Alert earlier this season on the Aesculus leafminer: https://bygl.osu.edu/node/2653
Both the solitary oak leafminer and the Aesculus leafming moth are called “microlepidoptera.” This refers to the moths' diminutive size rather than serving as an official taxonomic designation.
The solitary oak leafminer is so named because each leafmine houses a single caterpillar. The leafmines vary in their appearance from circular to circumscribed blotches to blotches with multiple lobes.
Unlike other leafminers that consume the entire leaf mesophyll, causing the upper and lower epidermis to delaminate, both the solitary oak and Aesculus leafminers appear to only partially consume leaf mesophyll. The caterpillars feed on leaf mesophyll just beneath the upper epidermis, which becomes a paper-thin protective cover. In fact, as shown in the images above, you can clearly see the oak leafminer caterpillars through the veneer.
The leafmines aren’t evident when viewed from below. Of course, another way to spot the caterpillar, or otherwise assess development, is to use sunlight as a backlight.
The solitary oak leafminer is native to North America. The iNaturalist map below shows reported observations demonstrating that the leafminer has a wide geographical distribution.
The number of generations in Ohio is unclear. The literature indicates the moths may have multiple generations per season. My observations suggest that leafmines develop twice during the growing season in Ohio.
It’s reported that the moths spend the winter as diapausing caterpillars in their leafmines in fallen leaves, and pupation occurs in the spring. If the caterpillars experience obligate diapause, they are genetically “pre-programmed” to enter the physiological stasis. This means development is synchronized when adults emerge in the spring.
Female moths glue individual eggs onto upper leaf surfaces. The resulting caterpillars initiate the first full generation. I believe we are nearing the end of caterpillar development for this generation because I observed late-instar caterpillars and pupae this past Thursday.
This suggests we will see another round of moths and subsequent larval leafmines in southwest Ohio this season. I suspect it will be the late instar caterpillars in the upcoming leafmines that will eventually spend the winter diapausing inside their leafmines in fallen leaves.
Interestingly, some leafmines acquire a bizarrely mottled, almost ornamental appearance, which heralds pupation. I could find no references describing what is responsible for the dramatic color change, but I suspect it may be the result of fungal degradation of frass. The function of the mottled appearance is unclear.
The leafminer is most commonly found on members of the white oak group, particularly white oak (Q. alba), swamp white oak (Q. bicolor), and bur oak (Q. macrocarpa). It’s less frequently found on oaks in the red oak group.
I’ve never observed population densities that would threaten the overall health of infested oaks. Although the solitary oak leafminer has a history of periodic outbreaks in its southern range, I’ve never observed serious damage in Ohio. Indeed, I consider the leafminer more of an oddity than a serious pest.
As noted above, the caterpillars spend the winter in their leafmines in fallen leaves. Consequently, raking and destroying fallen leaves in autumn should be an effective management option.
