Description of the Pest
Hot temperatures during crawler development in the spring typically limit population growth. Therefore, scales are rarely found in orchards and are only occasional pests. Citricola scale, black scale, brown soft scale, and European fruit lecanium are all soft scales found on pomegranate.
Citricola scale can be confused with brown soft scale, but coloring and the number of generations distinguish them. Citricola scale adults are grey, while brown soft scale adults are yellow to brown. Citricola scale has one to two generations per year and so at any time during the year most individuals on leaves will be near the same stage. Brown soft scale has multiple generations per year and so all stages can be present at the same time.
Crawlers of the citricola scale appear in June and July. They settle primarily on the underside of leaves, with a few on fruit. Young scales are flat and almost translucent. On fruit, they grow quickly, maturing in late July and August and producing a partial second generation of crawlers. On leaves, they grow slowly over the course of the summer and fall, molting only twice. A few may mature on the leaves, but most turn a mottled dark brown color and migrate to twigs or under the bark of the trunk in November. They complete their life cycle the following spring, reaching maturity on twigs in April or May and laying eggs. In citrus, each female may produce 1,000 to 5,000 eggs, but the number of eggs laid in pomegranates is not known.
Black scale adult females are about 0.2 inch (5 mm) in diameter and dark brown or black, with a prominent H-shaped ridge on the back. In San Joaquin Valley orchards, immature black scales overwinter under the bark on trunks and reach maturity there around mid-May. They reproduce without mating and lay 1,000 to 2,000 pink eggs over a period of two months. Eggs are laid under the scale's cover and have been observed in May and June.
After about a week, the tiny yellow to orange crawlers hatch and move around for a day or two before settling to feed on the trunk under the bark. Immature scales retain barely visible legs through two molt cycles and are able to move slowly. After the second molt, young scales migrate to twigs or fruit. There they grow rapidly, becoming nearly circular, with leathery dark mottled gray covers and the developing H-shaped ridge. Once egg laying starts, the covers become harder and darker and the H-shaped ridge often disappears.
The second generation matures and produces eggs from mid-July through August or later. After hatching, crawlers settle and are partially grown before winter.
Brown Soft Scale
Brown soft scale looks similar to citricola scale. The coloring and number of generations distinguish them apart. Brown soft scale adults are yellow to brown, while citricola scale is gray. Brown soft scale has multiple generations so that all stages may be present at the same time versus citricola scale, which only has one or two generations per year.
The seasonal history of brown soft scale has not been studied on pomegranates. On citrus there are three to five overlapping generations a year, with numbers usually highest from midsummer to early fall, developing on twigs, leaves, and fruit.
By early August scales are found on fruit, where they reach maturity and begin laying eggs. Female brown soft scales are somewhat flattened, yellow or dark brown, and lay a few eggs at a time. Eggs hatch almost immediately and crawlers start to feed. Young scales move around until they are about half grown. They have mottled, yellowish, rounded covers. The immature scales molt twice and reach maturity on leaves, twigs, or fruit.
European Fruit Lecanium
This scale has two generations a year. It overwinters as a nymph on twigs and small branches. In spring it grows rapidly, becomes convex, and forms a shiny brown cover about 0.25 inch (6 mm) in diameter with several ridges along the back. In late spring females lay many eggs, which fill the entire space beneath their cover, and die after egg production.
Newly hatched nymphs, or crawlers, emerge from beneath the scale cover from late May through June and settle mostly on the underside of leaves. In late July through early August they migrate to the fruit.
|Scale||Immature||Adult||Number of generations|
|Citricola scale||Mottled yellow brown||Somewhat flattened, grey||One to two (all the same development stage)|
|Black scale||Mottled dark grey to black with H-shaped ridge present or developing||Bulbous, black, may have H-shaped ridge||Two (all the same development stage)|
|Brown soft scale||Rounded, mottled yellow or brown||Somewhat flattened yellow or dark brown, smaller than citricola scale||Several (different development stages present at the same time)|
|European fruit lecanium||Brown||Domed, shiny brown with several ridges along the back||Two (all the same development stage)|
The most important economic damage from all four soft scales is the light-colored spot left when a scale is removed from a fruit, where the scale has blocked sunlight and prevented fruit coloring. If a fruit has more than one or two such spots, the fruit may be downgraded in quality.
Multiple scales on a fruit have been observed more frequently with brown soft scale than the other three species. Citricola scale stays on leaves once settled, and a 20 to 30% leaf infestation translates to only a 2 to 4% fruit infestation, usually with only one scale per fruit. Black scale has an affinity for twigs, and typically settles on the stem just above a fruit. However, black scale can settle on fruit and will sometimes reach economically damaging levels. The scales that reach maturity on fruit in August and September will produce crawlers, but the new scales will not grow large enough to be noticed at harvest.
The harmful honeydew and sooty mold associated with these soft scales in citrus and olives has not been observed on pomegranates. On pomegranates, late second instars of all four species on twigs and fruit typically produce a discrete pile of sugar, rather than a diffuse scattering of honeydew. The sugar does not grow sooty mold and is easily brushed off during harvest and packing. The extreme densities of younger stages on the leaves, often observed in citrus and olives, have not been seen on pomegranate. Ants may also help by cleaning honeydew off of leaves and fruit. Heavy honeydew and sooty mold in pomegranates is typically caused by aphids, whiteflies, or cherry leafhoppers.
Hot weather and natural enemies keep soft scales below economically significant levels in most years in most orchards. In olives, pruning to keep trees open usually keeps black scale numbers low. This is probably true in pomegranates also, and for the other scale species.
If pesticide applications are necessary, the most effective timing for these applications is June through July, when young stages are exposed on leaves and fruit, before they grow large enough to be harmful. In August, potential scale damage becomes more evident (monitoring of young scale is difficult) and can be managed using pesticide applications.
Soft scale natural enemies have not been identified in pomegranates, but they presumably are some of the same species that attack soft scale in other crops. Parasites play a leading role, and some parasites attack more than one species. The primary parasites include Metaphycus and Coccophagus.
On pomegranates, parasitism close to 100% has been observed in September for brown soft scale on fruit, where numbers are reaching economic levels. If pesticide applications are avoided, scale numbers the following year are very low. Parasitism of citricola scale on fruit is also common, but little or no parasitism of citricola on leaves has been observed. Parasitism of black scale and European fruit lecanium has been observed, but the parasites have not been identified. In other crops, lecanium parasites are common but do not result in control of European fruit lecanuim.
Specialized scale-feeding lady beetles include Chilocorus, Hyperaspis, and Rhyzobius species, and along the south coast, the steelblue lady beetle (Halmus chalybeus). Lady beetles can easily be overlooked because many are tiny, colored and shaped like scales, or (as small larvae) feed beneath the scale body.
Lacewings, predaceous bugs, and predatory mites are among the other invertebrates that at least occasionally feed on scales.
Controlling ants that interfere with parasites and predators will improve natural enemy activity, but an ant-free orchard is not necessary for adequate biological control.
Pruning to keep trees open increases parasitism and reduces black scale numbers in olives. This practice could also reduce numbers of black scale and potentially the other soft scales in pomegranates.
Organically Acceptable Methods
Biological and cultural control and botanical insecticide sprays are acceptable for use in organically managed orchards.
Monitoring and Treatment Decisions
Soft scale populations will vary considerably within each orchard, so it is important to map presence and abundance in different areas. Numbers are typically low in spring, and the scales are difficult to find on the twigs and under bark. All four species produce many crawlers in June through July, and if numbers are high, the young stages will be easy to find on leaves. Sticky tape traps can also be applied by wrapping double-sided sticky tape around small branches to catch moving crawlers. Traps should be placed when crawlers are expected and replaced weekly during this time. Secure the used trap between clear plastic and paper so you can transport it and more easily assess crawler numbers.
It is important to distinguish between the nymphs of citricola scale, black scale, brown soft scale, and European fruit lecanium, because they have different economic thresholds. Use the table to identify adults, and with practice, nymphs. The history of the orchard or the presence of old and empty scale covers may provide clues to identification.
- Do not apply insecticides for citricola scale. The presence of citricola nymphs on 20 to 30% of leaves may be tolerable. They can produce honeydew on which sooty mold grows, but they are not likely to infest fruit.
- Soft brown and black scales seem to have a greater affinity for fruit, and lower numbers may call for pesticide applications, especially if biological control has been disrupted. Past history of the orchard and careful monitoring can determine if a pesticide application is needed.
- The presence of multiple scales on a significant number of fruit at any time beginning in June will result in downgrading at harvest if no pesticide is applied. However, without careful monitoring, harmful numbers may only become evident in August, and may require a pesticide application at that time.
Buprofezin applied to the younger scale stages in June and July is the most effective treatment. Methomyl is effective on soft scales but is also harsh on parasites. Certain botanical insecticides may also help, but they have not been adequately tested.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|MODE-OF-ACTION GROUP NUMBER1: 16|
|COMMENTS: Apply at peak of crawler hatch. Good coverage is essential.|
|(Sivanto Prime)||10.5–14 fl oz||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 4D|
|(Admire Pro)||7–14 fl oz||12||0|
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: Do not apply prebloom (during bud elongation; March–April), during bloom (May–August), or when bees are actively foraging. Do not apply when fruit is present (June–October). Apply systemic imidacloprid via chemigation.|
|(Lannate SP)||1 lb||48||14|
|MODE-OF-ACTION GROUP NUMBER1: 1A|
|COMMENTS: Disruptive to natural enemies of mealybugs, caterpillars, soft scales, aphids, and other pests. Use of this pesticide may result in outbreaks of these pests. Methomyl is also toxic to bees and should not be applied when bees are actively foraging.|
|(PureSpray Green)#||See label||4||0|
|MODE OF ACTION: Contact including smothering and barrier effects.|
|F.||ROSEMARY OIL/PEPPERMINT OIL|
|(Ecotrol EC)#||2–6 pt||0||0|
|MODE OF ACTION: Contact including smothering and barrier effects.|
|COMMENTS: Volumes up to 100, 150, and 200 gallons/acre, use 4, 5, and 6 pints/acre respectively.|
|MODE OF ACTION: — (a botanical insecticide)|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers ("un"=unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).|
|#||Acceptable for use on organically grown produce.|