With its rapid, towering growth and showy floral display in the late spring, the Tulip Poplar (Liriodendron tulipifera) is a tree that stands out in the piedmont of south central Pennsylvania. It is one of the largest and most successful trees in the temperate deciduous forests of eastern North America, prized for its value as a lumber species in addition to the beauty and ecosystem services it provides. But this year, the mighty Tulip Poplar has stood out to me for reasons other than its size and beauty.

In natural areas that I frequent for hiking, and even public parks with native plant conservation landscaping, Tulip Poplar is a fairly common component of the overstory. In the depths of late winter this March, I happened to pause under one of these trees, absentmindedly staring up at its branches and dreaming of the impending explosion of green, yellow, and orange it would bring with the arrival of springtime. My daydreams were interrupted when I noticed a series of small black specks that covered the low-hanging branches of the tree, covering almost entirely the surface of the bark. Like any curious modern-day nature lover, I took photos on my phone to submit to iNaturalist for some help identifying what, if anything, these might be.

Small black specks covering small branches of a tulip poplar. (Photo credit: Dan Stevenson, Alliance for the Chesapeake Bay)

I was quickly informed that the peculiar black specks I observed were actually overwintering Tulip Tree Scale (Toumeyella liriodendri), one of the largest native soft scale insects in eastern North America. While its primary host is the Tulip Poplar, it can also be found on native or cultivated Magnolia trees such as the commonly used Star Magnolia (Magnolia stellata) and Saucer Magnolia (Magnolia x soulangeana), linden and basswood trees (Tilia spp.), hickories (Carya), and walnuts (Juglans). The insect appears on the branches of the tree as a warty, waxy covering and uses its piercing-sucking mouthparts to pull out nutrient-rich juices from beneath the bark. Tulip Tree Scale produces one generation per year, with females giving birth to nymphs called crawlers in late summer-early fall that overwinter in sheltered locations on the tree. The overwintering crawlers begin feeding again in the spring as the sap flow begins regularly within the newly awakened trees.

Photo credit: Gerald J. Lenhard, Louisiana State University, Bugwood.org)

The prolific consumption of plant juices by the Tulip Tree Scale produces copious amounts of sticky honeydew (i.e. scale excrement) that leaves a characteristic “lacquer-like” shine to surrounding leaves and attracts the likes of ants, wasps, and flies in search of a sweet treat. This honeydew is in fact desirable to the point that ants are known to defend colonies of Tulip Tree Scale in order to maintain the production of the sugary substance, a similar strategy to that of “aphid farming” by ants commonly seen on milkweed and other garden plants (Farmer ants and their aphid herds). The ants are so voracious in their appetite for honeydew that they can actively thwart the efforts of natural predators and parasitoids of the scale, leading to prolonged and more severe infestations of the insect.

As I rambled around the Susquehanna Riverlands through this spring and summer, I could not help but notice that the Tulip Tree Scale was really hitting some trees hard! What was first a glossy shine to the leaves gave way to the crusty black covering of sooty mold, a group of various fungi that use the honeydew as their primary food source. Sooty mold does not infect plants and is not directly harmful to its hosts, but the thick black covering it produces can be so thorough that it could potentially impede the plant’s ability to photosynthesize and cause premature leaf drop. I knew through my research that the sooty mold was not necessarily damaging the tulip trees or their understory counterparts, but it sure made a messy-looking scene in the eyes of the beholder! As the year progressed, the proliferation of the Tulip Tree Scale and its associates became more and more pronounced, but never quite deadly to its host tree. The high temperatures and humidity through summer seemed to support a huge growing year for plants, pests, and pathogens alike, but as we enter into the fall our weather has shifted to milder temperatures and drier conditions that seem to have slowed down biological processes all around.

Below are images of trees from a local park that I frequent in late August with several infested tulip trees, the understory of which are still mostly covered in the black sooty mold:

Photo credit: Dan Stevenson, Alliance for the Chesapeake Bay

The Tulip Tree Scale is rarely fatal to well-established trees but is known to weaken younger individuals by removing significant amounts of plant sap and reducing the energy reserves within the tree. Premature leaf drop and yellowing as well as twig/branch dieback, may occur with heavy infestations. If other plant stresses such as compacted soils and prolonged drought are present, the combination of these factors can lead to serious damage or mortality for the tree. This especially rang true for the Tulip Trees that I see frequently, as last fall we had a fairly significant drought for well over a month. As with many other plant pests, the best overall management strategy is to minimize external plant stresses where possible by addressing soil conditions and providing soil moisture in extended dry periods. The scale can be mechanically removed from trees by using a standard dish scrubber early on in its life cycle if time and helping hands are not a limiting factor. There are also natural predators of the Tulip Tree Scale that, given time and the correct conditions, can help manage, if not eliminate their populations, most notably the Signate Lady Beetle (Hyperaspis signata), whose larvae will make a meal out of the Tulip Tree’s enemies.

Because the emergence period for Tulip Tree Scale crawlers is fairly limited, emerging right around this time of year, pesticide applications to manage their populations can be very challenging to coordinate and execute properly. A certified arborist should be consulted for options if cultural or mechanical management fails to address the problem.

I will be keeping a close eye on my friends, the Tulip Poplars, in the years to come to monitor how they respond to Tulip Tree Scale damage and observe how changing environmental conditions might affect the growth of this pest. Here’s to hoping that with time and the right conditions, they will live happily for many years to come!