Why Scale Can Spread To Other Plants (And What To Do)

Scale is one of the most mysterious and challenging to identify of all the pests that can plague your garden. These common pests of leafy trees and shrubs are so small the most experienced gardeners can miss them. If you have noticed the scale on one of your precious plants, you may wonder if and when it will spread.

Scale insects spread to other plants in search of food and to reproduce. Adult scale insects are immobile, but the young scale can wander to another plant before feeding. You can prevent and control scale by isolating affected plants, pruning foliage, and applying neem oil.

Removing scale can be tricky due to its unique life cycle. However, preventing scale from spreading is the best opportunity to limit damage from this insect. Keep reading to learn why scale spreads and what you can do about it!

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How Does Scale Spread To Other Plants?

Given the opportunity, scale insects will spread to other plants. Over 120 species of scale insects exist in the United States, each with different host plants and reproductive strategies.

Despite this diversity, it is helpful to understand the average life cycle of a scale insect to understand why they can spread.

Scales are tiny insects that damage plants by inserting their long straw-like mouths into a plant, destroying the plant tissue, and sucking out the juices. Most of the scale’s life is spent immobile and attached to the same location on a plant.

Scale Can Affect Trees

Generally, two types of scale infect trees and shrubs, soft-scale species and armored-scale species. Armored scales are covered by a hard protective exoskeleton that makes the scale hard to remove and impervious to insecticides. 

Conversely, soft scales secrete a waxy coating that protects their body while they feast on the plant. Soft scales vary in size, but their wax coating makes them look like fish scales. 

Soft-scale insects secrete a sticky substance called honeydew that may make a plant’s leaves look shiny and damp. Ants may be attracted to this sweet substance, so the first indication of a problem with scale insects may be seeing more ants on your plants than usual.

So, if there’s honeydew on a nearby tree, scale will GLADLY move over there.

What’s Honeydew?

Honeydew can sometimes encourage the growth of sooty mold, a fungus that can be harmful and unsightly.

If you notice a powdery substance on the surface of your plant vegetation, be on the lookout for scale insects.

Scale Loves to Spread To Neighboring Plants

Scale loves to travel between plants. Scale most often travels to other plants during the crawler stage in their life cycle. Some species of these insects will crawl to nearby vegetation, while others travel by the wind or attach to birds’ feet. 

Crawlers are newly hatched young in the nymph stage of their life cycle. This stage is the only stage of their life where they can walk to infest nearby vegetation.

Though tiny, young crawlers can travel to nearby plants, especially if there is overlapping vegetation.

Some crawlers are blown from one tree canopy to another by strong winds.

Scale Insects Don’t Usually Travel Between Distant Plants

Imagine traveling long distances in search of food before you’ve ever eaten. This hungry voyage is the job of a newly hatched scale insect.

At the crawling stage, scales are so small they look like a speck of dust on the plant’s surface and are very difficult to spot, even by the most well-trained eye.

According to Clemson University, scale enters the crawling stage when the plant has a new flush of fresh vegetation, usually in the spring.

Scale in their crawling stage are so tiny that they cannot make it very far before they need to hunker down and eat. Therefore, scales do not travel long distances to reach new plants but prefer to stick close to the original host plant.

Why Does Scale Spread To Other Plants?

white scale insects.
White Scale Insects of the Superfamily Coccoidea.

There are three main reasons why scale insects may spread to other plants:

  • In search of food.
  • For reproduction.
  • When they are done with a plant.

Continue reading for details on each reason why scale insects spread.

Scale Spread To Increase Their Food Supply

A female scale insect can lay up to 1,000 eggs at one time according to the NC State Extension, resulting in an explosion of nymphs in the crawling stage that will need to travel in search of food. If all of these offspring stayed to feast on the same plant, they would quickly run out of food.

Instead, scale insects move to another area of the same plant or a different nearby plant that overlaps with the original host plant.

Luckily, most scale insects are designed to feed on a specific type of plant or a group of plants within the same family. Therefore, the scale will have to find the right type of plant before it can begin to feed.

Scale Spread To Reproduce

Females do most of the damage done to plants by scale insects.

This phenomenon is because male scales do not have mouth parts and do not feed on plants at all. Instead, they have one singular goal; reproduction.

Adult male scale insects are small flying insects that look like giant gnats. They will fly in search of a new female to mate with. This male will travel to new locations in search of a suitable female feeding on the correct type of plant.

Male scale insects only choose plants that already have feeding adult females, so identifying the seldom-seen male scale is not necessary to control the scale spread.

Scale Spread When They Are Done With A Plant

Imagine spending time enjoying your garden when you notice a plant’s leaves have become mottled and wilted.

It is rare for scale infestations to get so out of control that they injure an entire pant, but it could happen under the right conditions.

In this case, the scale nymphs in the crawling stage will have no choice but to travel to a new healthy plant to find sustenance. When an infestation reaches this level, it is best to remove and destroy the plant in the hopes of destroying the scale insects at all stages of their life cycle.

How Does Scale Spread To Other Plants?

Human intervention is the most common way new-scale species spread to new areas. Scale extends to other plants in the following ways:

  • Crawling between plants.
  • Attaching to old plant pots.
  • Blown by the wind.
  • Transfer from animals.
  • Traveling in oil soil.

Don’t let a scale infestation get out of control! Continue reading to learn more about how scale spreads to other plants.

Scale Insects Can Crawl Between Nearby Plants

Scale insects can travel from the host plant to a new plant during the crawling stage of their life cycle. However, this is the only time in a female’s life that it is mobile.

During this time, a newly hatched nymph will slowly crawl in search of a new location to eat. Then, a hungry nymph will travel to another section of the host plant or find a new plant entirely.

Scale crawlers most commonly travel to other plants when the vegetation overlaps, making it easy to crawl from one leaf to the next. 

Scale Insects Are Blown to Nearby Plants by Wind

Scale insects in the crawling stage are smaller than the head of a pin and very lightweight. When the scale has infected the canopy of a tree, the nymph can become dislodged by a strong breeze and blown to a nearby tree. 

If the tree is the correct type of host for the species of scale, it will find a place to begin feeding. The life cycle of the scale will continue on this new tree until more of the tree is infected. 

Scale Insects Can Travel on The Feet of Birds

Though the evidence is limited, researchers have seen scale insect spread by attaching to the legs and feet of animals in the environment. For example, as a bird visits an infected tree, tiny scale nymphs attach themselves to the bird and hitch a ride to a fresh tree.

This type of travel will only occur when scale insects are mobile in the crawling stage. 

Scale Insects Can Hitch A Ride In Old Pots

Scale insects that have infected a tree or shrub in a nursery pot may cling to the ridges of the pot and lie in wait for a new plant. 

Transplanting during the scales crawling stage creates the most opportunity for transfer, as this is when the insects are active enough to move around.

Old Soil Can Sometimes Carry Scale

Reusing the same soil after a scale infestation is a sure way to continue the infestation. Scales in different life cycle stages may remain in the ground for a few days after removing a plant. 

Continue reading to learn how to disinfect soil to limit scale spread. 

How Do You Prevent Scale From Moving To Other Plants?

close up of scale Insect on plant

Now that you understand how and why scale moves from one plant to another. Stop scale in its tracks by implementing a few simple measures to reduce the chance of scale spreading. 

First, the best way to control scale insects is to identify what kind of scale you have to find the most appropriate control method. Scale identification tools, such as the book Armored Scale Insect Pests of Trees and Shrubs, can help you narrow down the specific type of scale.

If you follow these simple recommendations, your garden will remain unharmed by the occasional scale insect.

Prevent scale from moving to other plants by:

  • Isolating affected plants.
  • Giving plants breathing room.
  • Prune or treat affected plants.
  • Wash pots before reusing.
  • Inspect old soil before reusing.
  • Avoid insecticides.

Now as a quick tangent, I highly recommend taking a look at our guide on the best ways to get rid of scale if you’d like to get them off your plants after you’ve got them isolated.

Isolate Affected Plants

The best way to prevent scale from moving from one plant to the next is to isolate infected plants. The next time you go to the nursery, inspect the plant for signs of scale insects.

If you find scale on your plant, isolate the plant in a location far from your other plants. While scale can spread from plant to plant, it cannot travel far. A distance of five feet or more is sufficient to stop the spread.

Even if you cannot initially see a scale on a plant, consider isolating and monitoring the plant for two weeks before planting it in your garden, just to be sure. 

Give Plants Some Breathing Room

Since scale cannot travel long distances, one of the easiest ways to prevent scale from spreading is to give your plants enough space to grow without touching or overlapping.

This strategy will make it difficult for the scale in the crawling stage to move from one plant to another. 

Consult the planting recommendations that came with your tree or shrub. Note the eventual mature height and width of the plant and plant them at that distance or greater. This simple step will go a long way in stopping the spread of scale insects.

Prune Affected Plants

If an infestation of scale is contained in one area, you may be able to control the spread by pruning the infected portions off of the host plant. 

Inspect the plant closely to identify where the scale is located. Then, with a clean pruning tool or scissors, cut away the affected portion in its entirety.

Take the pruned vegetation and dispose of it far away from susceptible plants. Remember, scale insects do travel, but not for long distances. So dispose of infected plant material a few meters away from the healthy plants.

Controlling the scale by pruning allows you to catch the adult female scales before their eggs hatch, stopping the scale infestation in its tracts!

Treat The Plant If Necessary

If your scale infestation covers a large portion of the plant, treating the scale may be your only option.

Good quality horticultural oil, such as Monterey Horticultural Oil Concentrate, or neem oil, such as Harris Neem Oil Spray, are good choices for controlling scale. These oils work by getting rid of scale during every life stage.

Houseplants with minor scale infestations are treated by removing the insects with a cotton swab, or soft toothbrush dipped in soapy water.

Avoid Insecticides When Treating for Scale

Once you’ve identified your plants are infected with scale, it may be your first reaction to reach for a traditional insecticide. But, unfortunately, in the case of scale, an insecticide may do more harm than good.

First, scales are resistant to insecticides because of their unique life cycle. Adult scale insects have a protective coating of wax that covers their bodies. Armored-scale insects have a tough exoskeleton that is impervious to chemical control.

Second, powerful insecticides could limit the number of beneficial insects in your garden. For example, hoverflies and parasitic wasps help control scale by destroying the adults. If you use a potent insecticide, you may not have enough of these helpful insects to control the harmful insects.

If you’re noticing that you have wasps near your plants, take a peak at our article on attracting wasps to your yard as this can be an indicator of scale!

And remember, if you do use an insecticide, try neem oil.

Attract Beneficial Insects Instead

In most cases, scale is controlled with environmental interventions, like attracting more beneficial insects to your garden.

Beneficial insects love the flowers of common culinary herbs like dill, cilantro, and fennel. Interplanting these herbs among your woody plants will help keep scale and other pests at bay.

Wash Pots Before Reusing

Even if you don’t have a pest infestation, it is always a best practice to wash pots before planting a new plant. This method ensures that none of the potential pests from the previous plant transfer onto a new plant. 

Cleanse old pots with hot water and dishy soap, and let them dry out in the sun before putting in a new plant.

That’s a wrap!

Scale infestations may seem like serious business, but in reality, the chances of scale infestation taking out an entire plant are relatively low. In most cases, scale insects are managed well by any garden habitat’s natural and biological factors. 

Scale insects will travel from plant to plant during the crawling stage of their life cycle. Limit the spread of scale by isolating affected plants, cleaning old pots before reusing, and pruning affected leaves and branches.

Follow these suggestions, and scale will remain a simple and minor nuisance in your garden.

References 

Frank, S. D., & Just, M. G. (2020). Can cities activate sleeper species and predict future forest pests? A case study of scale insects. Insects, 11(3), 142.

Hodek, I., & Honěk, A. (2009). Scale insects, mealybugs, whiteflies and psyllids (Hemiptera, Sternorrhyncha) as prey of ladybirds. Biological control, 51(2), 232-243

Miller, D. R. (2005). Selected scale insect groups (Hemiptera: Coccoidea) in the southern region of the United States. Florida Entomologist, 88(4), 482-501.

Miller, D. R., & Davidson, J. A. (2005). Armored scale insect pests of trees and shrubs (Hemiptera: Diaspididae). Cornell University Press.

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