Brown soft scale, Coccus hesperidum
Order: Hemiptera
Family: Coccidae
Description
The brown soft scale is the most important scale insect that occurs on indoor grown plants in Colorado. When full-grown it is about 4-5 mm long (1/8 inch) and normally has an oval body form, although shape can vary depending on the site of the plant where it develops. The body is flat, appearing only slightly domed when viewed from the side. Color ranges from yellow-green to yellow-brown and scales may be mottled with brown spots. Older scales usually become darker brown.
Quick Facts
- Brown soft scale is a common insect pest of indoor and greenhouse plants, including fig trees, schefflera, English ivy and citrus.
- Adult brown soft scales are yellow to brown oval and flattened, they can be found on leaves stems and petioles.
- When feeding, brown soft scales excrete a shiny, sticky fluid called honeydew that may cover leaves and facilitate the growth of sooty mold.
- Insecticidal soaps or horticultural oils can control brown soft scale but are only effective when applied repeatedly.
Brown soft scales are small flattened oval insects that are common pests of greenhouse and house plants. Scales decrease plant vigor by removing phloem and excreting sticky honeydew. Image credit: Lesley Ingram, Bugwood.org
A pool of honeydew on a citrus leaf. The honeydew was ejected horizontally about one inch from the scale insect, which can be seen on the adjacent stem. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org.
Adult brown soft scale. Image credit: John A. Davidson, Univ. Md, College Park, Bugwood.org.
Various life stages of brown soft scale and honeydew on Ficus. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org.
Life history and habits
All scale insects produce waxy secretions through specialized glands. The armor of soft scales remains attached to the body of the insect and provides protection from chemical controls and natural enemies. When mature, the female produces eggs that hatch within her body, producing very tiny (0.4 mm) first stage scales known as crawlers. Crawlers may live for a few days under the protective cover of the mother scale, but later move to colonize other areas of the plant. Stems of plants are usually favored sites where brown soft scales will settle, but they also can occur on leaves. The crawler period is the most mobile stage of the insect and is the stage that can infest new plants. Crawlers can be moved and carried while handling infested plants and may be able to be dispersed short distances through wind.
Once adult females begin to reproduce, they do so continuously, so that a few crawlers will be produced daily over 3-4 weeks or more, after which the female dies. On indoor plants multiple generations will be produced each year. Because the scales are laying eggs over an extended period, generations overlap and there will be no distinct annual peaks in egg production and new crawlers.
Injury
Brown soft scales injure plants while feeding with their piercing-sucking mouthparts. These are used to reach the sugar-rich fluids present in the phloem of the plant. Sizable colonies can remove large quantities of plant fluids and can cause wilting and decrease plant vigor, but they seldom kill their host. However, heavy infestations can result in reduced flower and fruit production. As they feed, the scale insects excrete excess water and sugars, producing honeydew, a sticky, shiny fluid. Large amounts of sticky honeydew can cover leaves and create serious nuisance problems. In Colorado, brown soft scale is only known to occur on indoor grown plants but occurs year-round on many kinds of landscape plants in areas within the United States that have milder winters.
Monitoring
Early detection of this insect can help inform decisions related to management. Although the insect can be difficult to detect since they are small and blend in well with the plant, the honeydew they produce is easy to spot and can be a great diagnostic tool. The honeydew may pool next to the insect or be ejected and land on leaves or other plant structures below the insect. Plants should be inspected every couple of weeks for the presence of honeydew. These inspections can also help determine whether an insecticide application was effective since dead scale insects often remain attached to the host plant. It is important to note that honeydew can also be produced by other pests such as aphids and mealybugs.
Biological control
In outdoor settings there are numerous natural enemies that can limit densities of brown soft scale. Most important are several types of small parasitoid wasps that develop within the body of the scale. The developing wasp kills the scale insect, and the adult emerges by cutting a hole in the back of the host’s body. In some instances, generalist predators such as lady beetles, green lacewings, and predatory flies can help reduce infestations on outdoor plants. However, it is worth noting that the accumulation of honeydew can reduce the efficacy of these natural enemies, and it is recommended that plants periodically be sprayed and washed to remove honeydew. Presently there are no reliably effective natural enemies available to control brown soft scale indoors.
One species of entomopathogenic fungus, Lecanicillium lecanii (Ascomycota: Hypocreales), is an important pathogen of scale insects. The conidia of entomopathogenic fungi infect a host by penetrating the insect integument, and research has shown that the strain L. lecanii No. 3.4505 is highly infective against the brown soft scale. Additional research is needed to determine whether this pathogen can be incorporated with integrated pest management programs for brown soft scale. Some important considerations include the optimum conidia concentration, formulation, and whether field conditions can support the infection process enough to adequately reduce populations of brown soft scale.
Chemical control
There are several insecticides that can be used to control brown soft scale. Spray applications of diluted insecticidal soaps can be an effective means of control when applied to exposed crawlers. However, thorough coverage is necessary to kill the insect, and several repeated applications will be necessary to kill crawlers as they continue emerging. In addition, mineral oils and neem oils are marketed for insect and mite control. These oils are diluted with water to a concentration of 1-2% before spraying directly on the pest. As with insecticidal soaps, multiple applications of horticultural oils will be needed. Horticultural oils are safe to use on edible plants.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Isabel Queiroz, A. 2020. An Early Bioinvasion in the Azores. Global Circulation and Local Dynamics (1840s–1860s) in Response to the Brown Soft-Scale Coccus hesperidum. Humanities. 9(78): 1-14. Available https://mdpi-res.com/humanities/humanities-09-00078/article_deploy/humanities-09-00078-v2.pdf?version=1597112480
Weimin, L., X. Yingpin, X. Jiaoliang, Z. Yanfeng, and Z. Xiaomin. 2010. Ultrastructural and cytochemical characterization of brown soft scale Coccus hesperidum (Hemiptera: Coccidae) infected by the Lecanicillium lecanii (Ascomycota: Hypocreales). Micron. 42: 71-79.