Cool temperatures during the past few weeks hastened the annual departure of leaves from our deciduous trees. With the loss of leaves many of the aphids, caterpillars, and beetles we visited in Bug of the Week have departed as well. However, on a small tuliptree not far from my house, I spied what appeared to be black paint on the trunk. Now, tree bark is supposed to be brown, tan, or grey in most cases, but not black. I suspected the devious work of a real sucker, the tuliptree scale. A close inspection confirmed my suspicions and revealed an encrustation of dozens of scales on every branch.
Tuliptree scale belongs to a group of insects known as soft scales, so named for the soft texture of their outer skin. Compared to most insects, these creatures move very little. The adult females hunker down on twigs and branches and engage in the business of producing young. Unlike many insects that lay eggs as part of their life cycle, tuliptree scales give live birth to small black babies called crawlers. Due to the prodigious reproductive potential of these scales, each female may give birth to hundreds of young, in autumn there really is a sucker born every day. Crawlers move away from their mom, find their own little piece of bark, and settle in for the winter. Dozens of these small black crawlers can occupy a square inch of bark on a heavily infested branch at this time of year.
However, cold temperatures and ravages of hungry predators take a toll on the young scales. When spring returns, only a few will survive to carry on the lineage. As the tuliptree awakens in spring and produces leaves to resume growth, the young scales also become active. The flow of nutrients in the tree provides food that supports growth and development of the scales. Like the aphids we met on May 1, 2006 in "An aphid is born", scale insects have sucking mouthparts that are inserted into the plant. Much the same way you sip a drink through a straw, they withdraw nutrients from the vascular system of the tree with their proboscis. Excess fluids taken in while feeding are excreted by the scale in the form of sticky liquid called honeydew. Honeydew falls on sidewalks, cars, and leaves beneath the tree.
Where scales are abundant, the honeydew may feel like a gentle shower to someone standing below. When honeydew falls on bark and leaves, it serves as a substrate for the growth of a fungus called sooty mold. While this fungus does not cause disease in the tree, it does stain the bark and leaves deep brown or black. Honeydew is a prized food of many other insects such as wasps and ants. Even butterflies forage on the sugary honeydew. By mid-summer the scale insects have grown plump and are about the size of a peppercorn. Most tuliptree scales are protected by an entourage of feisty ants capable of warding off hungry predators in search of a scaly treat. Heavy infestations of tuliptree scales can reduce the vigor of trees by robbing the products of photosynthesis needed for growth. If only a few scales are on a tree, they may be eliminated by pruning the infested branch or simply by crushing the scales with your fingers. In some cases it may be necessary to treat a tree with insecticides to prevent branches from being killed by large numbers of scales. While tuliptree scale attacks primarily tuliptree and magnolias in our area, other trees may be attacked by other soft scales. So, if you see a tree with dark brown or black bark, inspect the branches and see if these small suckers are to blame.
"Managing Insects and Mites of Woody Plants: An IPM Approach" by John Davidson and Micheal Raupp was used as a reference for this episode. For more information on tuliptree scale, please visit the following web site.