My job at Holden Forests & Gardens as the propagator is to produce and take care of baby plants.  That often involves going out into the gardens and natural areas to collect propagules, such as seed and cuttings.  While out in the gardens, I often encounter interesting interactions between plants and other organisms.  Of particular interest to me are micro herbivores, especially gall-forming and leaf mining insects.  Occasionally, these interactions are interesting enough to warrant spending some break time to come back for a closer look.

Last February, I was out in the June Room searching for a particular Pieris which needed repropagation.  In my efforts, I ran into a rhododendron with loads of conspicuous chew-marks on its leaves.  The arrangement of these markings suggested to me that the leaves were being mined – eaten from the inside by some as-yet-unknown insect.  I took a couple of photos and carried on my way.

I later sought an expert’s opinion on the photos I had taken.  Charley Eiseman is the foremost authority in North American leaf mining insects, and he often lends his expertise to hobbyists across the country for identification.  He suggested that what I found mining that rhododendron was a species of moth called Phyllonorycter rhododendrella.

P. rhododendrella was first reared and described by a trailblazing female scientist from Ohio, Annette F. Braun in 1935, from mined leaves collected in Tennessee from Rhododendron minus.  The moth was not seen again until the early 21^st century, this time as adults at the Arnold Arboretum in Massachusetts.  Finding it in Ohio last year added a third state to its known distribution, and last August, this species was found in the wild again in North Carolina.

I returned to the mined rhododendron in May in the hopes of collecting occupied mines to rear adults.  Because this species is so poorly known, pinned specimens are ideal for confirming this range extension.  I grabbed a few apparently undamaged mined leaves.  Examining a few other nearby rhodies, I found that this moth was only mining small-leaved rhododendrons, and only those which have at least some Rhododendron minus* in their pedigree.  My theory is, this moth is a specialist on R. minus in its native range.  The relative rarity and inaccessibility of R. minus across its range may help explain why this moth is so infrequently found.

I spent some time examining available published images of R. minus, hoping to find overlooked evidence of P. rhododendrella outside its known range.  I looked at hundreds of photos, and found only seven with leaf damage consistent with that caused by moths in the genus Phyllonorycter.  All were geolocated in or near the Atlanta metro area.  In each of these, there were only one or two mines visible per plant – a far cry from the dozens upon dozens I found on that rhododendron at Holden Arboretum in February.

Rhododendron minus is native to the southern Appalachians – southern Kentucky down to southern Alabama.  If this moth feeds exclusively on R. minus (which is absolutely just a hunch of mine, and in no way tested or proven), how, then, did it make its way to Northeast Ohio?  Although it cannot be said for certain, there are a few possibilities that stand out in my mind.

Holden Forests & Gardens is made up of three campuses, and one of these is the David G. Leach Research Station.  David Leach, the original proprietor of this station, is known worldwide for his work with the genus Rhododendron, including numerous collecting trips to the Appalachians targeting this genus.  It would not be a stretch of the imagination to suspect that the moths were brought back north incidentally during one of these trips on live plant material, and managed to establish a population in Madison, Ohio.  From there, the jump to Kirtland is relatively easy, especially with the moving back and forth of plant material between the two sites that continues to this day.

Alternatively, perhaps a different plant collector was responsible for transferring the insects north, this time to the Arnold Arboretum.  The Arnold Arboretum and Holden Arboretum have similarly engaged in frequent transferal of plant material from one to the other, from seeds to cuttings to live plants.

Perhaps the bugs were moved north directly to Holden Arboretum from the mountains, or maybe from the Appalachians to a nursery somewhere in Lake County and then to the Arboretum.  There are numerous possibilities.

Buried in the excitement of this discovery is a cautionary tale.  A plant pest, poorly known and understood, and rather inconspicuous in its home range, was accidentally brought somewhere outside its home range.  There, it was able to establish a breeding population and balloon in numbers on new hosts, more susceptible to it than its original host plant.  This story has been repeated over and over in this country, often with wide-ranging negative effects to native ecosystems.  Emerald ash borer, chestnut blight, spongy moth, Japanese beetle, and so on are all innocuous, even minor, parts of their home ecosystems.  Here, they have wreaked havoc.  Thankfully, the subject of this blog is not so menacing.

This isn’t the first time pests have been moved around through horticulture trade, and it won’t be the last, despite the best efforts of the state and local agricultural authorities, and well-meaning nursery folk. It likely isn’t even the only time non-native pests have been inadvertently introduced to the Arboretum’s grounds via plant material.  In my time at HF&G, I’ve found around a dozen plant pests and pathogens that have never before been seen in the state of Ohio.  While most of these likely represent overlooked native species, there’s at least one or two that appear to be non-native pests, accidentally brought to Ohio by human activity.

Fortunately, there’s something that can be done about this problem, at least from within the nursery trade.  Fastidious examination of incoming and outgoing material, and removal of any pests or diseases, is a must.  Positive working relationships with state and federal agricultural authorities has been beneficial to Holden Arboretum’s nursery.  Whenever we have a mystery pop up on our plants, they are happy to help us solve it.

For those outside the nursery trade, there are still ways to help.  Go on hikes!  Take pictures.  Ask questions.  Connect with other interested parties.  The more eyes are out there on nature, the better.  iNaturalist and Bugguide are great resources for connecting with experts and knowledgeable hobbyists.  Show them things you find, ask them questions.  Most folks are more than happy to help, to encourage curiosity.  Of course, most things anyone encounters will likely be well-established and well-known, if not native.  However, every now and then, someone somewhere will find a mystery worth investigating.

*I am following the broad treatment found in Flora of North America for this species.  Other treatments will split R. minus into two or three species – minus, carolinianum, and chapmanii.

Photo credit

Adult shot in Massachusetts © Hans Holbrook https://creativecommons.org/licenses/by-nc/4.0/