Scale and Mealybugs

Background

Scale insects and mealybugs are all members of the same order: Homoptera — like aphids. Though it isn’t how most growers think about them, mealybugs are technically a type of scale insect. To most growers, they’re either dealing with “scales” or “mealies.”

Scale species are broken down into several families, of which three matter most: Coccidae (soft scales such as brown soft scale and cottony maple scale), Diaspididae (armored scales such as San Jose scale and pine needle scale), and Pseudococcidae (mealybugs, including the long-tailed mealybug and citrus mealybug).

How They Become a Problem

Collectively, there are thousands of scale insects. Of the families mentioned, hundreds have destructive behaviors affecting fruit trees and crops worldwide. Two of the most famous — the cottony cushion scale (Icerya purchasi, family Magarodidae) and the cassava mealybug (family Pseudococcidae) — are landmark biological-control success stories, where a single imported predator saved an entire industry.

Scales like the euonymus scale are largely sedentary as adults — they crawl as “crawlers” in the first instar, then settle and feed in place for the rest of their lives. Many growers first detect them only after sticky honeydew or sooty mold appears on the foliage below the infestation. By that point, populations are often already advanced.

Mealybugs are more mobile and tend to congregate in leaf axils, stem crotches and root zones — including underground on the roots themselves (root mealybugs). They’re routinely introduced on incoming plant material, which is why quarantine and inspection of new plants is the single highest-leverage control practice.

Most scale and mealybug infestations are imported on new plants. Inspect every shipment, isolate suspect material, and you’ll prevent more outbreaks than any spray can fix.

A Serious Pest

Scales can cause considerable economic damage. Like aphids, they have a proboscis or stylet — a straw-like mouthpart they insert into the phloem to draw sap. This feeding often goes unnoticed until the plant or tree weakens and dies, by which point it may be past self-repair. It’s therefore imperative to spot the pests and take corrective action before the plant shows outward signs.

The mere presence of scales makes a plant unsalable. Soft scales generally appear as small bumps on the bark with sticky honeydew; armored scales look like specks with no honeydew and a hard waxy cover; mealybugs appear as fluffy white insects with cottony egg masses (except root mealybugs, which look like their above-ground counterparts but live on the roots).

Some Advice on Scale Insects and Mealybugs

One aesthetics problem — if a soft-scale or mealybug population becomes large enough — is the formation of black sooty mold, which grows on the sugary, ant-attracting honeydew these pests produce. Honeydew attracts sugar-feeding ants, which may, in defense of their food source, protect the scales from biocontrol agents.

If a scale population grows large enough to support ants, non-chemical control becomes much harder. Armored scales do not produce honeydew, so this isn’t a factor with them — but you also lose honeydew as a scouting cue, so armored-scale infestations are easier to miss.

Scouting Scale Bugs on Plants

Scout incoming plants before adding them to your collection. Look for: three to six-millimeter bumps on bark with shiny, sticky honeydew (soft scales); one to three-millimeter specks with no honeydew (armored scales — note both soft and armored scales have hard outer coverings, so don’t be confused by the names); and three to seven-millimeter white insects with shiny honeydew and cottony egg masses (mealybugs, in most cases).

For root mealybugs, slip the plant from its pot and look for white, fluffy clusters on the root ball. Tap-test the pot — the abnormal lightness of a root-mealybug-infested plant is a giveaway.

Crawler stage is your control window

Newly hatched first-instar “crawlers” are the only mobile, unprotected scale stage. Time horticultural-oil sprays and parasitoid releases to coincide with the crawler emergence — before they settle and grow their waxy or armored covers.

Some Solutions for Scale & Mealybug Control

Several commercially available scale and mealybug biocontrols are stocked here. The parasitoids are fairly selective, so accurate species identification is essential. If you’ve identified the pest correctly, chosen the matching biocontrol, and introduced it under the right conditions, you can achieve very long-term control.

Predators tend to be opportunistic and act faster across a wider pest range, but they don’t persist as long, so they’ll usually require multiple maintenance releases. The classic mealybug predator is Cryptolaemus montrouzieri — the “mealybug destroyer” ladybird — whose larvae mimic mealybug nymphs in appearance.

Horticultural oils, neem oil and insecticidal soaps can also work — especially against crawlers — with care taken to avoid harming beneficials. Where ants are running interference, address the ant population first (boric-acid baits or barrier products) and your biocontrols will work properly.

Scale and mealybug control is a matching game — ID the species, match the parasitoid, and time the release to the crawler stage.