By James A. Bethke, for Let’s Talk Plants! May 2022.
Soft scale insects can be quite a pest in the landscape and garden in southern California, especially if the population grows through the summer months. In my travels as a UCANR advisor, I have witnessed many cases of soft scale infestations that have blackened sidewalks, trees and other landscape plants due to heavy honeydew deposition and subsequent black sooty mold, including some in my own yard.
Soft scale insects are one of those piercing sucking insects that I have talked about in past articles (see previous article, “What is a Piercing Sucking Insect?”). They use their thin mouth parts like a straw to pierce and feed on the vascular system of the plant. Soft scales are typically oval, dome shaped and come in many colors and morphological designs. Female adults can harbor hundreds of eggs in or under their shell with some laying eggs and some bearing live young. Some species like the cottony cushion scale produce a cottony ovisac that can contain hundreds of eggs. Newly hatched young are the mobile crawler stage that are in search of a suitable part of the plant in which to feed. Some soft scales are sessile, but others can pull their mouth parts and move.
What makes the soft scales so impactful is that they feed principally on the phloem of the plant, which is the food distributor of the vascular system. The phloem carries lots of sugars, but soft scales, like other phloem feeding insects, are not as interested in the sugars as they are in the other phloem contents. Therefore, they tend to excrete most of the sugars which drop down onto anything below feeding sites leaving a sticky spot. The sticky spots become infested with mold spores that are endemic, and finally produce black sooty mold (see previous article, “Sooty Mold”).
Along with honeydew deposition, a heavy infestation of soft scale insects can cause chlorosis, distorted foliage, growing tip distortion, twig dieback, or defoliation. In my yard, the wax scale (Figure 1.) came to us on an infested dwarf citrus tree we purchased at a local nursery. They have not been a real problem on the citrus, but they were transported to other plants in the yard, likely via wind, birds or other insects. They proliferated like crazy on our ornamental passion vine, Passiflora spp. Because the vine covers our arbor, we have experienced lots of sooty mold on the plants (Figure 2.) and on the concrete pad below and some leaf chlorosis and defoliation.
Unfortunately, because the soft scales produce honeydew, ants (Figure 3.) tend to feed on the honeydew and protect the soft scales. Therefore, ant control will significantly improve control of the scales mostly because once the ants are gone, the natural enemies of the scale insects (parasitic wasps, ladybugs, lacewings, etc.) will knock the population down. Following ant control, scale insects can be easily controlled using horticultural oils and soaps, especially if the applications target the crawler stage. Crawler stages emerge in synchrony, so watching for crawler stage emergence is key. Wrapping a small piece of double-sided sticky tape around branches or stems will capture the emerging crawlers and indicate when to treat.