Scale insects on houseplants can be a major headache. There are more than 8,000 different species, all within the superfamily Coccoidea. Around 1,000 scale species can be found feeding on plants in North America. They are extremely diverse in appearance, behavior, lifecycle, and reproductive strategies.
Scale bugs are difficult to control due to the protective covering females create which protects them from insecticides. They reproduce quickly and have overlapping generations adding to management difficulties. There are four common scales on indoor plants which can be identified rather easily. The best control method for scale bugs is prevention.
What Are Scale Insects?
Scale insects are tiny, soft bodied, plant feeding insects in the order Hemiptera. Hemipterans feed with piercing, sucking mouthparts which resemble a needle. They stick their mouthparts into their food source and feed on the liquids.
As scale insects feed, stealing nutrients from plants, plant growth slows. Heavy infestations lead to leaf and stem dieback. These parasites, when left unmanaged, can kill plants.
The Lifecycle Of Scale Insects
The lifecycle of scale insects is complicated and can vary significantly by species. Here we provide a basic description of the typical lifecycle.
When an egg hatches, the immature scale emerges and is known as a crawler. Crawlers travel to newer plant material, insert their mouthparts, and begin to feed and grow. Females start to excrete material to create a protective covering.
Most females will never move again. They lay their eggs under their protective covering. Males resemble a winged aphid and fly to females and mate. Males of some species won’t have wings and will crawl to females.
There are some scales that can reproduce without fertilization!
Types of Scale Insects
Based on the type of covering they create, scales are divided into two main groups; armored and soft. A third type of scale, which are not true scales, are mealybugs.
Armored scales are not attached to the covering they create while soft scales are. Soft scales consume so much plant liquid they produce honeydew while armored scales don’t. Honeydew is sugary and can lead to the growth of black sooty mold which causes further plant damage.
The different types of scale insects you will most commonly encounter on houseplants are the brown soft scale (Coccus hesperidum), hemispherical scale (Saissetia coffeae), fern scale (Pinnaspis aspidistrae), and mealybugs.
Mealybugs (family Pseudococcidae) account for more than 25% of species in the superfamily Coccoidea. They differ from other scales in that they will remain mobile as adults, though they typically don’t move very far.
How To Identify Scale Insects
The best characteristics for scale identification are the shape and color of their protective covering and the plant species they are feeding on. Many scales only feed on specific plants which narrows down which species you’re dealing with. You can easily identify the four common houseplant scales.
The brown soft scale feeds on hundreds of different plants. The protective covering of adults is only slightly convex and has a yellowish-brown to green color, often with brown mottling.
Hemispherical scales, another soft scale, also feed on many different plants. Their protective coverings are more convex and shiny brown compared to the brown soft scale.
The fern scale is an armored scale and feeds primarily on ferns but also on several other houseplants. The female covering is brown and oyster shell shaped.
Mealybugs are larger than true scales and have a white cottony-looking wax on their pink bodies. They can slowly move around on plants.
How To Get Rid Of Scale Insects
First, quarantine infested plants from healthy plants to reduce spread. Repot plants if possible to remove infested soil. Prune any heavily infested stems and leaves. Destroy pruned material immediately.
Management options, especially the use of insecticide sprays, work best on crawlers. Remove as many adults as you can by gently scraping them off or wiping them off with a cotton ball soaked with 70% rubbing alcohol.
You can also spray the plant with a 1 to 1 mix of rubbing alcohol and water. Use cation with rubbing alcohol on delicate plants.
Use organic insecticides such as neem oil or insecticidal soap. Synthetic insecticides to use include pyrethrins, imidacloprid, malathion, and carbaryl.
Make sure any insecticide used is labeled for indoor use and follow the directions exactly. Scale treatments should be repeated every 1 to 2 weeks, for at least 6 to 8 weeks, for complete control.
How To Prevent Scale Insects
The most important prevention methods are using sterilized soil and carefully inspecting new plants for scales. You should quarantine new houseplants to ensure no scales are present. Fresh cut flowers from the store can harbor scales. Keep them away from houseplants.
Keeping plants indoors year round will prevent scales. During the crawling stage, scale bugs can be transported by wind, birds, pets, and on your clothes. If you place plants outside during summer, you can use tape traps (double sided tape wrapped around stems) to monitor for any new scale infestations.
Frequently Asked Questions
Scale bugs may seem like they just magically appear. However, there are several ways you can unknowingly infest plants with these little critters including: using infested potting soil, using a dirty pot, bringing infested plants (including cut flowers) inside, or putting your plants outside in summer. They will also hitch a ride on the wind, birds, or your clothes.
Adult scales are very hard to get rid of on indoor plants because their protective covering limits insecticide use. Imidacloprid, pyrethrins, and neem oil may be useful in certain situations. Adults can also be physically removed by gently scraping them off or pruning off heavily infested leaves or stems.
Scale insects can spread to other plants very easily. After hatching, they enter their crawling phase and will seek out plant tissue to feed on. However, scales are tiny insects and they don’t have wings. Therefore, they typically can’t move too far without the aid of humans transporting infested plant material, soil, or gardening tools.
Scale insects are not harmful to humans physically, but they are common pests of several agricultural crops costing farmers millions in crop losses each year. They can easily spread to and damage ornamental plants, impacting both nurseries and homeowners.
Since adult scales don’t move, and are concealed beneath their protective covering, it is difficult to tell if they are alive. The best method to see if scales are dead is to gently squish them and see if there is still liquid inside. Dead scales will be dry and shriveled up under their protective cover.
 Gullan, P.J. and Cook, L.G., (2007). Phylogeny and higher classification of the scale insects (Hemiptera: Sternorrhyncha: Coccoidea). Zootaxa, 1668(1), pp.413-425.
 Kabashima, J. N., & Dreistadt, S. H. (2014). Scales: integrated pest management for home gardeners and landscape professionals. University of California, Agriculture and Natural Resources, Statewide Integrated Management Program, Pest Notes, Publication 7408, Davis, CA.
 Koranga, R., Maurya, R.P., Dubey, V.K. and Dobhal, P., (2021). Mealybug, An Emerging Pest and its Biological Control. Vigyan Varta, 2(8), pp.9-12
 Day, E.R., (2015). Scale insects. Virginia Cooperative Extension: https://vtechworks.lib.vt.edu/bitstream/handle/10919/75449/2808-1012.pdf?sequence=1.
 Johnson, P.J., (2009). Scale insects on orchids. Tillgänglig: http://www.staugorchidsociety.org/PDF/ScaleInsectsonOrchids.pdf.