A Brief Description of a Common Pest:
If “Lecanium” is a new word for you, consider yourself lucky, or at worst, blissfully ignorant. If you have experienced species of this genus in your landscape you may know how devastating, unsightly, and generally… uncomfortable this insect can be. If you have ever stood under a tree covered in a scale population which is actively feeding and digesting you will know why the word “uncomfortable” came to mind. In this installment I will briefly describe Lecanium scale and its life cycle. In the next installment, I will share an unusual finding from last season, and stand out on a limb to make a forecast for this season.
Lecanium is a genus in the family Coccidae. Within this genus there are many different species of scale insects, but fortunately they are very similar in their appearance, feeding habits, and life cycle. This helps arborists identify the insect when populations reach a threatening level on a plant. Because the life cycle is similar from one species to another (with very few exceptions), treatment intervention is relatively straightforward. For the purpose of this article, and for discussion among arborists, we lump these scale species into a generic type we call lecanium scale.
Lecanium scale starts life as an egg. In central Ohio egg hatch usually takes place in June, when Washington hawthorn is blooming. (This event is a phenological indicator for lecanium scale hatch. For more on that topic, read my article here). The first nymphal form (called an instar) is the most mobile state within the lifecycle of the insect. Because of this we call this form a crawler. Crawlers will move out onto the leaf of a plant or tree and affix themselves adjacent to the veins of the leaf where they will feed through the summer. Sometime in late summer these crawlers turn into the second-instar form, and they move back onto the twigs to overwinter. Here one must pause for a moment of silence and ponder the question: How do these nymphs know that their host’s leaves are going to fall off for the winter? Do they really “know” at all? Why do they do what they do?
Female lecanium scale
Moving on, we find these second-instar nymphs waking up with the rest of the plant and insect world the following spring. They begin to feed on the twigs now, and are no longer mobile. A waxy covering begins to form over their bodies as they grow. Most of the lecanium scales will have a domed appearance, some flatter than others, some with different colors, but easily recognizable as lecanium scales once you know what to look for. When the adult female matures, she lays her clutch of eggs underneath her “shell” and dies soon after. The eggs hatch soon and the cycle continues.
Now some comments to finish up the picture:
Many plant hosts are targeted by this insect, including oak, hickory, honeylocust, crabapple, cherry, pear.
There is only one generation per year, which greatly simplifies the treatment process.
The crawler stage is the most vulnerable to treatment and is the one we try to target for control.
The most damaging stage is the second-instar nymph which feeds voraciously on plant sap in order to grow into an adult and lay eggs.
During this stage, digested sap is excreted as honeydew, a sweet substance that coats leaves, sidewalks, cars, and yes, people if you stand under the tree for too long. This honeydew is fed upon by sooty mold, turning the surfaces black.
Large populations can weaken a plant, causing dieback, stress, and even plant death due to the amount of sap extracted during feeding. Sooty mold interferes with photosynthesis, further stressing plants.
How new scale populations arrive onto formerly uninfested trees seems to be a bit of a mystery.
Lecanium scales are grouped with sucking insects (as opposed to chewing insects). They insert a proboscis into plant tissue to feed on sugars and other nutrients in the sap.
So much for the entomology lesson. Congratulations to you stalwart readers who have made it this far! You now have a good base of knowledge that will help you understand how I came to certain conclusions about the 2020 population of this pest, and what this might mean for 2021. I hope you can meet me here once more next week for the final installment!
>> (2/15/21 Update: Read Part 2!) <<
Your friendly neighborhood arborist,
José Fernández | Regional Manager, Russell Tree Experts
José became an ISA Certified Arborist® in 2004, and a Board-Certified Master Arborist® in 2015. Currently he is enrolled at The Ohio State University pursuing a Master’s Degree in Plant Health Management. José likes working around trees because he is still filled with wonder every time he walks in the woods. José has worked at Russell Tree Experts since 2012.
[Some technical notes retrieved, and some fact-checking facilitated by W. T. Johnson and H. H. Lyon, Insects That Feed On Trees and Shrubs, Second Edition, Cornell University, 1991.]