By Brandon Gallagher Watson
Scale insects are important pests of shade trees, and, as many practitioners will tell you, they can be challenging to manage. Their diversity, cosmopolitan dining tastes, and staggering reproductive rates are partly why they have a reputation as being difficult, but by understanding a few key points about scales, you can set the expectations and offer these services with confidence.
The sheer scale of it all
With 8,000 recognized species, scales are found pretty much everywhere. Most make their living as plant parasites, and can be found on thousands of different plant species. The scale insects are also very diverse in their appearance. Some are quite large — up to 1/2 inch in diameter — while others are barely visible without magnification. They can have complicated and varied lifecycles, with at least nine different types of sexual, hermaphroditic, and even parthenogenic reproduction strategies that would take you all day to Google and still never fully understand. A single female can lay upwards of 4,600 eggs per brood and have two to three generations in a year, so, suffice to say, however they get it done, it seems to be working.
The males and females can look very different from each other and the nymphs can look very different from the adults. They can be shiny, spotted, waxy, dull, showy, drab, smooth or sticky. Commonly, they cover themselves with a protective layer that gives them the appearance of being fish or reptile scales on twigs and leaves, which gave rise to their common name, scale insects.
There are currently 48 families of scale insects classified in the superfamily Coccoidea, but arborists are mostly concerned with just two of these families. The Coccidae, which we refer to as soft scales, and the Diaspididae, which are the hard, or armored scales. There are other frequently seen scale cousins, such as mealy bugs and cottony cushion scales, which are not in either of these families, but most practitioners just lump them in with the soft scales as their management is similar. Proper management of scale pests starts with understanding a little of their biology and their differences. This is because the optimal control method varies greatly depending upon which scale you are dealing with.
Don’t call ‘em soft
Soft scales, while not considered easy to manage, are relatively easier to control than the armored scales — although even that generalization doesn’t hold up across all soft scale species. Depending on where in the country you live, soft scales may also be referred to as wax scales or tortoise scales, and despite the soft scale name, many have hard, crunchy shells. There are important shade tree pests in the soft scale family that can feed on several different host trees, including calico scale, which can be found on several trees such as maples, honeylocust, dogwoods and fruit trees. Other scale pests can be specific to just a single species of tree, or, like magnolia scales, they can be specific to a whole genus of tree species.
They’re soft, sticky sweet
An easy way to identify soft scales from armored scales in the field is honeydew. When soft scales settle in to feed on a plant, they poke their tube-shaped mouthparts into the phloem stream to drink the sugary-sweet sap. The high pressure of the sap stream forces more sap into the insect than they can drink so most passes right through the other side as honeydew. The sticky goo will collect on leaves and twigs — often resulting in a black fungus known as sooty mold that is well known to gardeners. While honeydew can be an objection to many plant owners, there are many insects that benefit from this free food source, so it’s all a matter of perspective.
Hard and tough
Armored scales are even more widely varied than the soft scales, as nearly one-third of all scale species worldwide are considered hard scales. Tree care professionals will be familiar with prominent hard scale mainstays such as white peach scale, euonymus scale, false oleander scale, tea scale, oystershell scale, and pine needle scale, to name a few. No matter where in the world you practice tree care, there is likely an armored scale on a tree in your neighborhood.
Their “armored” scale moniker is a fitting name for this group, as nearly all of them produce some sort of protective covering over themselves once they settle in to feed. Although this likely developed as an adaption against predation from birds, lizards, and other insects, it has also served the armored scales well against modern management practices. The hard shell makes it difficult for many traditional pesticides to be effective.
Armored scales do not secrete honeydew like the soft scales do. Researchers think this is due to having more agile tongues than their softer cousins. Rather than plunging right into the high-pressure stream, an armored scale’s mouthpart can weave in between cells and feed on the freely available and sugary-rich fluid that floats there. Exactly why the armored scales evolved this extracellular feeding strategy remains a mystery, but judging by the diversity and distribution of the hard scales, again – it seems to be working.
To treat or to not treat?
While it’s true nearly all scale insects make their living as plant parasites, do all scales require some sort of management? It depends on the tree and that tree’s management goals. Light or infrequent scale infestations often have little impact on an otherwise healthy tree, and the rub-the-branch-with-a-glove-on method may be the solution for management on a small trees. For heavier infestations, or when dealing with mature trees, other management actions may be required.
Horticultural oils have been the first line of defense in the war on scales for more than 100 years. Horticultural oil is somewhat unique in the insecticide toolbox as it kills the insects by suffocating them rather than affecting the nervous system as many others do. As mentioned, the protective covering over armored scales and even some soft scales makes it very difficult for chemical treatments to penetrate. The insect still has to exchange gases, however, so it has little tubes that go through the cover, called spiracles, through which it breathes. Hort oil clogs these tubes, kills the insect, and this works on both soft and hard scales.
Hort oils, unfortunately, are not without their drawbacks. High temperatures and low temperatures will cause severe leaf damage, so most are applied when there are no leaves on the tree. This limits the treatment window and even the winter months can be a phytotoxicity risk for conifers if the temps are not optimal. There are some oils that may be applied in summer, but be cautious of tree species that are still considered sensitive.
Looking down to protect up
The challenges of spray treatment timings, the associated mess, and the inconsistency of coverage lead many practitioners to look at soil-applied treatments. This is where the important distinction between how hard and soft scales feed inside the plant comes in. This also helps explain why their management strategies are different.
The soft scales, which plunge their mouths right into the vascular system, traditionally respond very well to systemic treatments such as imidacloprid. This has not been the case with the hard scales, adding to their difficult-to-control reputation.
During the past few years, researchers have observed imidacloprid’s relative, dinotefuran, providing a high level of control for armored scales — even when soil-applied. Scientists believe the reason for this is related to dinotefuran’s higher degree of solubility. This allows it to move into that extracellular fluid the hard scales tap into where other treatments could not reach. Whatever the case, it serves as good news to arborists who use these tools to protect their client’s trees, as the more tools in the toolbox, the better.
Tip the scales
Scales will remain a mind-boggling diverse group that will continue to present trees and arborists with problems for, well, pretty much forever. Arm yourself with a little knowledge of the key ones in your area, be conscious of the hard scale /soft scale difference, get to know the treatment options, and you can be diagnosing and offering these services with ease.
Brandon Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).