Scale Insects of Cut Woody Stems: Biology and Control Options

Many cut flower growers have developed markets for cut stems of woody plants. This is a good strong market to enter as demand appears to be healthy. Production is a little like growing in an in-ground nursery, but instead of harvesting the whole plant, you cut off the stems and sell them for either their flowers, stems, color or shape, or in some cases for the beautiful bracts that form.

Many shrubs and trees are susceptible to a “low-key insect group” called the scale insects. They tend to blend in on the plant and go unnoticed until they have built up to damaging levels. Control at high infestations levels is very difficult. In 2012 I published an article on one of the nastiest of the armored scales, called Japanese maple scale.

Over the last five years I have seen a steady increase in damage caused by several species of soft and armored scales. This is a timely lesson for you to learn to recognize and deal with this pest group before they build to uncontrollable levels.

To prevent losses in your business you need to understand the pest you are dealing with before you attempt a control strategy.

Scales, as a group, are one of the most significant arthropods in terms of their detrimental impact on an extremely wide range of woody plants. There are more than 8,000 described species in seven families. They were previously classified in the order Homoptera, but are now listed in the order Hemiptera, suborder Sternorrhyncha, and superfamily Coccoidae.

Fortunately for cut flower growers, only two major scale families cause 95% of the plant loss in temperate regions. The two families we will discuss in this article are the armored scales in the family Diaspididae, and the soft and wax scales in the family Coccidae.

It’s often difficult for the untrained eye to detect scale because they don’t look like a typical insect. They tend to blend in with the plant tissue or overwinter in buds. They are small, sometimes flattened or raised into shapes that look like part of the plant, often with various colors that blend in with the bark or leaves. This ability to blend is part of their survival mechanism.

Scale Biology

Scales show high degrees of sexual dimorphism: the two sexes of the same species exhibit different physical characteristics not related to their sexual organs. Males, which are not produced in all scale species, actually look like insects in the sexual mature stage of development, possessing one pair of wings, as opposed to the usual two paired wings on most insects. (Other species of insects with just one pair of wings are in the order Diptera, which contains flies and mosquitoes.) The males are present for a mere 12 to 24 hours, with the sole purpose of finding a female with which to mate.

Females of most scales are immobile in the mature stage, and retain the immature external morphology even when sexually mature. They produce a pheromone that attracts the male. Upon finding a female mate, the male uses its antennae to drum on top of the female. If the female finds the beat alluring, mating will occur.

For scale species with males, the males will mate with a couple of females and die shortly afterward, having depleted their energy reserves. The female lives up to several months as her eggs continue to develop. This length of survival depends on the species and the time of the year. If she is an overwintering species, she will live until the next spring, at which time she will lay eggs that will hatch the following fall. Some species have one generation per year while others have multiple generations per year. A scale in a southern climate may have more generations than the same species damaging plants in a northern location. Scale insects in tropical areas can have multiple and often overlapping generations year round.

Several of the scales called soft scales do not produce males, and the females reproduce without mating. This is called parthenogenesis.

Soft scales and wax scales have their waxy covers integrated into their bodies. Most soft scales in temperate climates overwinter as immature females. Female soft scales tend to be rather flattened in profile and oval in their perimeter shape in winter. In spring or early summer, their bodies will swell as eggs develop. This causes the waxy cover to swell up in raised profile. Soft scales feed on the phloem of the plant, extracting plant juices. They feed heavily as eggs develop, often excreting large quantities of honeydew, a sticky sap that covers anything underneath the plant. This honeydew can be used for detection of a soft-scale problem on a plant.

Some armored scales overwinter as immature, but some overwinter as pregnant females ready to give birth in spring or summer. Eggs develop under the armored scale without the very obvious swelling that occurs in soft scales. Armored scales in general lay upward of 100 to 300 eggs per female.

For both armored and soft scales, the stage that follows is called the crawler stage. The first instars (stages) of most species emerge from the egg with functional legs. They immediately crawl around in search of a favorable spot to feed. In some species they delay settling down, but no longer than 12 to 24 hours. Sometimes they are blown away by wind onto another host plant, where they may establish a separate colony.

Identifying Scale

You can often distinguish an armored scale from a soft scale by trying to pry off the cover. With armored scale, the outer cover is hard and if you pop that cover off it comes completely free. Many armored scale female bodies, once the cover is removed, are yellow, orange or purple in color. The body and cover of soft scale insects are integrated and the cover does not come free easily, or at all, without destroying the insect.

The first two instars of male armored scales look just like the females. Males stop feeding after the second instar and excrete a substance out of their back called a carneae, made out of white wax. This carneae provides protection for the developing body of the insect. They continue to grow through a third, fourth, and fifth instar before emerging as an adult, with a more typical insect appearance.

At the third instar stage, the female forms a cover. The shape of the cover can be round, oval, pear-shaped or oyster shell-like. She continues to feed through all the instars, with a piercing, sucking mouth part inserted into the plant. At maturity, the female does not have a typical insect appearance.

Most arborists identify scale by its female cover and the plant species it is found on. Work with the Extension entomology specialist in your state to help identify difficult-to-recognize scales. It is critical to correctly identify the species to determine when the crawler stage is active. The crawler stage is often the best stage to control. Learning to identify the various scales may take some time, but realize that by properly identifying the scale you can develop the best control strategy.

Control

As mentioned above, the most effective control of scales is achieved during the crawler stage. This is the period when the young insects have the least protection. Crawlers are often oval, have antennae and legs, and look like typical insects, but are very small. Depending on the specific scale, they have different crawler periods, from May through fall along the Mid-Atlantic United States. Some have one generation of crawlers per year, while others have two and some three.

While the scales are in the crawler stage they are very susceptible to insecticides. This is when most scale predators and parasites are active. With the number of predators and parasites attracted to the crawlers, it is important to select control materials that have minimal impact on beneficials. Several different materials have been used over the years to control scale. Pyrethroids such as bifenthrin and permethrin products are inexpensive and effective. Unfortunately, many of these are broad spectrum and kill beneficials. The preferred control for scales is an insect growth regulator, or IGR. IGRs impact the early instars; death isn’t immediate so it won’t act like a pyrethroid with a quick kill.

IGRs disrupt the development of the exoskeleton. Hormones in their body tell the insect to form a new layer of skin and shed the old one. The synthetic growth regulator mimics natural hormones. Upon entering the next instar, the hormones go a bit crazy and the insect can’t shed its old skin, resulting in death. It takes a few days for them to die, but it is effective in killing them. The benefit of using growth regulators is that they have less impact than many broad spectrum pesticides on the predator and parasite insects in their adult stage. They affect only immature insects.

A couple of growth regulators are available such as Talus and Distance. Each has a slightly different mode of action, which is good because we need different classes of materials, but are both still insect growth regulators. In late fall, horticultural oil may be used to control soft scales. A 2 to 3 percent oil applied when temperatures are above 50 degrees Fahrenheit is effective. Insects obtain their supply of air through openings along the side of their body called spiracles. Horticultural oils work by coating the small sensory hairs, called cilia, in the spiracles, preventing the cilia from moving air into the insect’s body. Oils are effective and have been refined over the past 30 years to produce fewer plant-damaging residues. The horticultural oils work best when applied to low- to medium-sized scale populations. With larger populations, especially of armored scales, the scale covers are clustered, making contact with the horticulture oil difficult.

Research

The University of Maryland Extension Service received a grant from the Maryland Nursery and Landscape Association for 2014 to 2017. Our goal was to identify major scale insects and to develop a monitoring program that would enable us to detect and alert growers and arborists to the crawler period. We hired students and technicians to scout and sample plant material, monitoring the different scales and their life stages, and correlate that with degree-day information and plant phenology. From this information we developed IPM Alerts and Scale Fact Sheets. Arborists can visit our website at www.Extension.umd.edu/IPM. Go to the fact sheet section to download the PDFs. Each Friday we publish a weekly IPM Alert that lets growers know what insects and diseases are active, and when specific scale species are in the susceptible crawler stage.

Insects respond to temperature, not the calendar. Using degree-days instead of actual dates takes into account yearly temperature fluctuations to help pinpoint the crawler stage. The weekly IPM Alert reports also contain information on degree-days accumulated in the different areas of Maryland. Degree-days are obtained using a base of 50 degrees Fahrenheit. Insect development is controlled by temperature, and we use degree-day accumulation to predict when crawler stages of the various scales will occur. This provides better information to the arborist for timing sprays. For example, let’s say the first generation is known to be present at 402 degree-days. When you reach 350 or 400 degree-days, you should start looking at your plants for presence of crawlers.

A helpful scouting method is to use the 3M brand blue painter’s tape or black electrical tape. Wrap it around a branch sticky side out and monitor for crawlers as they move onto the tape and get stuck. This will provide you with more timely local information. Another method of estimating the crawler stage is based on plant phenology, a correlation with the time particular plants are in full bloom. For example, if Chinese lilac, Syringa chinensis, is in full bloom at the time the crawler stage of the scale is present, you can focus on that plant material as you scout the neighborhood.

Prunicola Scale

Prunicola scale is found mainly on plants in the genus Prunus. In Maryland, prunicola is found on cherry laurel, a common landscape plant, but it is also increasingly seen on cherry trees, rhododendrons, and boxwoods. The female scale has a round cover compared to the bright white elongated cover of the male.

The crawlers are bright yellow with the typical six legs, antennae, and ability to move around common to most insects. Crawlers do not have wings and therefore don’t fly. Only the males will develop wings; the females never fly. Crawlers are small and lightweight; a certain number will settle close to where they emerged, on the same plant. Other crawlers are a bit like hitchhikers, they catch a ride on birds’ feet or on larger insects.

They have three generations per year, but may go unnoticed until the end of the season when the males’ bright white covers are very noticeable. The first generation begins in the spring, usually May, followed by a midsummer generation and then again late in the season. By the end of the season, the population builds to such a high level that they cover the stems and twig dieback becomes obvious.

Lecanium Scale

Lecanium scale is a soft scale that overwinters in its second instar. Soft scales retain their legs during all life stages so they are capable of moving if needed, but are usually not mobile after initially settling in the first instar. As leaves begin to drop, the insects move down toward the stems or trunk until the following spring.

The East Coast has seen a huge rise in these populations. Lecanium scale feeds on the growing tips of plants. To obtain a particular protein, these insects need to take in a large quantity of plant sap and water. In the process, large amounts of sugar also enter the insect, which they excrete as honeydew.

Larger numbers of lecanium are found in urban areas with higher temperatures, as compared to wooded sites. Higher temperatures cause the scales to appear earlier in the season, before beneficials reached their peak population, with fewer natural predators and parasites available to suppress them.

Indian Wax Scale

Introduced from India, this has become established on approximately 122 different species of landscape plants. They are commonly found on evergreen and deciduous holly, pyracantha and hemlock, and in Baltimore County have been found on Japanese maple.

This insect looks like a blob of white wax stuck onto a plant. It often goes unnoticed until the foliage drops—once the stem is visible, the white bodies of the scale become obvious to the layperson. Since they aren’t readily identified as insects, they have been mistaken for a fungus. Upon closer examination, a little horn is visible, protruding from the wax covering.

The crawler stage for these is between 854-985 degree-days, with one generation per year. In most years this falls between late June and early July. Horticulture oils are not effective for Indian wax scale because the oils will not stick to their waxy coating. Systemic insecticides or neonicotinoids are effective, as are the IGRs. We are presently testing a new systemic insecticide called Altus for soft scale control and should have results by early 2018.

Juniper Scale and Minute Cypress Scale

These are both armored scales, and with slightly different crawler periods, are difficult to distinguish in the field, though treatment for both species is similar. The Leyland cypress has been the major host for the minute cypress scale in Maryland. Green giant cypress is gaining popularity and although we haven’t seen minute cypress scale on these yet, I suspect that as the population of green giant cypress increases we will begin finding minute cypress scale on them.

Both have one generation per year. You should begin scouting for crawlers at 530 degree-days. An IGR is my first choice for control. Horticulture oils can be used to suppress the population.

Japanese Maple Scale

The Japanese maple scale is dangerous. It is the nastiest scale we have dealt with, and most prevalent in nurseries and the landscape in the last decade. In a normal season, the first crawler stage is six to seven weeks long. This is very long compared to the typical two to three weeks for most scales. Because they produce crawlers over a longer period, they have a greater survival rate, and there are two generations per year. The second generation can have crawler periods lasting seven or eight weeks.

Chemical controls may break down before the crawler period ends; in some cases, two applications might be necessary. Most standard pesticides are not effective against this pest. We were not successful with pyrethroids, neonicotinoids or most systemics. Control was achieved when the plants were treated with insect growth regulators. In our trials, we found that the IGR Distance does a good job. In heavy populations, it may take up to two years of application to bring the scale under control.

Part of our research grant from the Maryland Nursery Association was used to monitor these insects for two years. The first generation of Japanese maple scale begins crawling at 806 degree-days and ends at 1,144. The second generation starts at 2,200 degree-days and ends at 3,037. The insect finishes crawling and overwinters in its second instar on the bark in late fall.

In 2017 we began working with Bayer Company and Brian Kunkel of the University of Delaware Extension Service, to evaluate new systemic low-risk pesticides for scale. We should have more information on the efficacy of these materials in 2018.

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor rank-principal agent) in IPM and entomology with the University of Maryland Extension based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]