Aonidiella aurantii (red scale)
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PicturesTop of page
|Title||Adults and nymphs on citrus fruit|
|Copyright||M.A. van den Berg|
|Adults and nymphs on citrus fruit||M.A. van den Berg|
|Title||Old adults and females on citrus fruit|
|Copyright||M.A. van den Berg|
|Old adults and females on citrus fruit||M.A. van den Berg|
|Title||Old adults and females on citrus fruit (close up)|
|Copyright||M.A. van den Berg|
|Old adults and females on citrus fruit (close up)||M.A. van den Berg|
|Title||Comperiella bifasciata (parasitoid)|
|Caption||Adult female of the introduced parasitoid, Comperiella bifasciata, ovipositing in the grey adult female stage of red scale.|
|Copyright||M.A. van den Berg|
|Comperiella bifasciata (parasitoid)||Adult female of the introduced parasitoid, Comperiella bifasciata, ovipositing in the grey adult female stage of red scale.||M.A. van den Berg|
|Title||Chilocorus cacti (predator)|
|Caption||Chilocorus cacti, a predator of red scale, on citrus fruit.|
|Copyright||M.A. van den Berg|
|Chilocorus cacti (predator)||Chilocorus cacti, a predator of red scale, on citrus fruit.||M.A. van den Berg|
|Title||Chilocorus cacti adult|
|Caption||Chilocorus cacti adult, a predator of red scale.|
|Copyright||M.A. van den Berg|
|Chilocorus cacti adult||Chilocorus cacti adult, a predator of red scale.||M.A. van den Berg|
|Title||Chilocorus cacti larva|
|Caption||Fully-grown larva of Chilocorus cacti, predator of red scale.|
|Copyright||M.A. van den Berg|
|Chilocorus cacti larva||Fully-grown larva of Chilocorus cacti, predator of red scale.||M.A. van den Berg|
|Title||Chilocorus nigritus (predator)|
|Caption||Fully-grown larvae of Chilocorus nigritus, predator of red scale, A. aurantii.|
|Copyright||M.A. van den Berg|
|Chilocorus nigritus (predator)||Fully-grown larvae of Chilocorus nigritus, predator of red scale, A. aurantii.||M.A. van den Berg|
|Title||Rhyzobius lophanthae (predator)|
|Caption||Larva of an introduced Australian ladybird beetle, Rhyzobius lophanthae, feeding on red scale.|
|Copyright||M.A. van den Berg|
|Rhyzobius lophanthae (predator)||Larva of an introduced Australian ladybird beetle, Rhyzobius lophanthae, feeding on red scale.||M.A. van den Berg|
|Title||Aphytis melinus (parasitoid)|
|Caption||Two pupae of A. melinus below the scale cover of a grey adult stage of A. aurantii.|
|Copyright||M.A. van den Berg|
|Aphytis melinus (parasitoid)||Two pupae of A. melinus below the scale cover of a grey adult stage of A. aurantii.||M.A. van den Berg|
IdentityTop of page
Preferred Scientific Name
- Aonidiella aurantii (Maskell)
Preferred Common Name
Other Scientific Names
- Aonidia aurantii (Maskell)
- Aonidiella citri (Comstock)
- Aonidiella gennadi McKenzie
- Aspidiotus aurantii Maskell
- Aspidiotus citri Comstock
- Aspidiotus coccineus Gennadius
- Chrysomphalus aurantii (Maskell)
- Chrysomphalus citri (Comstock)
International Common Names
- English: California red scale; citrus red scale; orange scale; scale, California red
- Spanish: aonidiella dell'arancio; cochinilla roja australiana; cochinilla roja de los agrios; escama roja de California; escama roja de los agrios
- French: chermes rouge; cochenille rouge de l'oranger; pou rouge de Californie
- Portuguese: cochonilha vermelha dos citros; escama vermelha
Local Common Names
- Germany: Rote Orangen-Schildlaus; Schildlaus, Kalifornische rote; Schildlaus, Rote Orangen-; Schildlaus, Rote Zitrus-
- Israel: haknima haaduma
- Italy: cocciniglia rossa degli agrumi
- Japan: aka-marukaigaramusi
- South Africa: rooidopluis
- Turkey: kirmizi kosnil
- AONDAU (Aonidiella aurantii)
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Uniramia
- Class: Insecta
- Order: Hemiptera
- Suborder: Sternorrhyncha
- Superfamily: Coccoidea
- Family: Diaspididae
- Genus: Aonidiella
- Species: Aonidiella aurantii
Notes on Taxonomy and NomenclatureTop of page
Armoured scale insects belong to the family Diaspididae of the division Sternorrhyncha and the superfamily Coccoidea. The family Diaspididae currently consists of about 2200 described species and 390 genera.
Females of the tribe Aspidiotini are characterized in possessing one-barred ducts (generally long and slender), unilobed pygidial lobes, well-developed, glanduliferous plates, a circular to pyriform body shape and a reduced number of glandular organs (notably the perispiracular pores) on prepygidial segments (Ben-Dov, 1990c).
DescriptionTop of page
Adult female (Diaspidiae)
In all species of the Diaspididae, in the female the head is closely fused with two or all thoracic segments, almost without indication of the comprising segments. On the abdomen, however, the separation between segments is distinct and can be interpreted by following the fixed marginal setae, the marginal constrictions and the intersegmental lines. The diaspidid female possesses eight or nine segments (Ben-Dov, 1990b).
The general outline is conspicuously distinct in some taxa of the Diaspididae, to the extent that it is a reliable indication for a generic placement of species. The reniform shape characterizes the genera Aonidiella and Africonidia (Ben-Dov, 1990b) although one species of Aspidiotus is also reniform (Williams and Watson, 1988).
The differences in shape of the ducts (secretory organs), their frequency and distribution provide some of the major characters for distinguishing between taxa at all levels.
According to Ben-Dov (1990b), the ventral or dorsal aspects of the pygidial segments of some Diaspididae are reticulated in several species with characteristic scars, which are helpful in determination of the taxa. The most conspicuous example was demonstrated by McKenzie (1937), who employed such structures to separate the extremely similar species Aonidiella citrina and A. aurantii.
The following description of A. aurantii is largely based on Bedford (1998):
The female gives birth to active young crawlers. After a period of rest they emerge from under the female's scale and search for a suitable spot on which to settle permanently. Ebeling (1959) states that the majority emerge before 12.00 h and settle within 6 hours. Crawlers will settle on older branches and leaves, but they are strongly phototactic and tend to move outwards onto the new growth and green fruit. Crawlers settle mostly on the upper surface of leaves, especially alongside the midrib or the larger veins. On very young fruit they usually settle in the depressions of the oil glands, but mature fruits can be completely encrusted with scales of all sizes.
The crawler settles and secretes a white, cottony covering; this is known as the whitecap stage. Whitecaps are very conspicuous when a new generation of females start producing. This stage is considerably prolonged by cold weather and the cap may grow out into a long, white thread.
The body of the first instar becomes circular and now secretes a thin, waxy covering which is formed by the pygidium as the insect revolves. This is the nipple stage, with the raised secretion of the whitecap in the centre. The enlargement of the waxy covering continues until the female is fully grown or until the male is ready to pupate, except for the intervening moults.
Towards the end of the first instar the insect prepares to moult. During the moulting period the body is distended and watery while the integument is hard and brittle and is sealed onto the scale covering. The cast dorsal exuviae remains incorporated with the waxy covering. The cast ventral skin is very frail. The second-instar female moults in the same way. Dickson (1952) gives a detailed description of the construction of the scale covering and an analysis of the covering.
Legs are no longer present. Just after the first moult, the two sexes cannot be distinguished from the dorsal aspect because the new fringe of wax is circular. This very small stage is normally still too small for oviposition by Aphytis africanus [Rhopalosiphum maidis]. The sexes can, however, be distinguished from the ventral aspect of the scale covering; the underside of the first exuviae of the female appears shiny and polished while that of the male is dull (Bedford and Cilliers, 1994). A few days later, the male starts forming an elongated covering which is then easily differentiated from the thin circular female covering. Both male and female can now be parasitized by A. africanus and other Aphytis species. The male moults into a prepupa while the female moults into the grey adult stage.
Grey adult stage of female
The female and its waxy covering enlarge considerably during this stage of the adult. The rim of wax is very thin and soft with a distinct grey colour. The outline of the initially pear-shaped female body shows through it. The adult male fertilizes the female during the beginning of this stage. It is therefore considered incorrect to refer to the entire third stage (third instar of some authors) as the virgin female stage. The grey adult stage terminates when the body of the female and the waxy covering reach their final size and become fused together. This third stage is conveniently divided into three sizes: (a) small, 0-1/3 full size; (b) medium, over 1/3 up to 2/3 full size; (c) large, over 2/3 to full size (based on width of the waxy skirt).
Full-grown or mature adult female (A- and A+)
When fully-grown, the body becomes distended, hardened and completely sealed to the wax covering and the dorsal exuviae, as well as adhering to the thin delicate ventral female exuviae. This stage is referred to as the A- stage or gravid female before crawlers are produced, and the A+ stage or reproducing female as soon as the crawlers are produced ('A' indicating the fully-grown adult stage sealed to the scale covering).
Reproducing adult female (A+)
The scale covering of the fully-grown adult female is circular, flattened, about 1.5-2 mm in diameter with two circular exuviae forming a harder and darker central disc. The average size varies according to the host plant and the part of the plant attacked (Ebeling, 1959). The scale is reddish in colour as a result of the body of the female showing through the scale covering. The body is crescent-shaped, with the lateral margins extending on either side of the pygidium. A small space just behind the pygidium forms a brood chamber in which the crawlers rest before emerging from beneath the edge of the scale covering. A small, whitish pellet can be found on the floor of the brood chamber of females which have been producing young for some time, larger and more conspicuous pellets being found under the older females. This pellet is formed by the egg shells discarded by the ovoviviparous female after the birth of each crawler, the eggs having hatched in the body. This confirms the observation by Nel (1933) that the dry, discarded amnion coverings can often be found.
Red scale is not parthogenetic and the females must be fertilized. Unfertilized females remain in the grey stage. Fertilization usually occurs soon after the female has moulted for the second time (Parry-Jones, 1936). The sex ratio of females to males varies from 1:1 to 2.6:1 according to the different seasons and different workers (Parry-Jones, 1936; Ebeling, 1959). Stofberg (1937) gives a ratio of 1 female to 3 males.
Development of the male
The male becomes elongate after the first moult and two prominent pairs of purplish eyes appear. The final covering is 1-1.3 mm and half as broad, oval in shape and reddish-brown. This instar is followed by a prepupal stage with short wing pads, and then by the pupa in which the various appendages are larger and more clearly defined in their sheaths. The adult males are frail two-winged insects which emerge from beneath their scale coverings. They can sometimes be seen flying among the leaves on heavily infested trees at about sunset. Detailed descriptions of the male stages are given by Nel (1933), Quayle (1941) and Ebeling (1959).
A description with colour photographs of all the life stages and the stages parasitized is given by Flanders et al. (1995).
DistributionTop of page
A. aurantii is potentially a severe pest of citrus in California (USA), South Africa, Australia, New Zealand, Mexico, Chile, Argentina, Brazil, Israel and islands of the eastern Mediterranean (Ebeling, 1959). It also occurs in many other countries (Quayle, 1941). It is thought to be indigenous to South-East Asia. Red scale occurs as a pest on citrus throughout South Africa and it also occurs further north in Africa (Bedford, 1998). Davidson and Miller (1990) summarize the distribution of the red scale as occurring in the tropics and in greenhouses in cold areas.
A record of A. aurantii in Colombia (IIE, 1996) published in previous editions of the Compendium was based on a report by Toro (1930). More recent reports (Kondo, 2001; Ramos Portilla and Serna Cardona, 2004) indicate that A. aurantii is not present in Colombia and is in fact considered a quarantine pest in that country. The status of the record for Columbia has therefore been changed to 'Absent, unreliable record'.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Country||Distribution||Last Reported||Origin||First Reported||Invasive||References||Notes|
|Bangladesh||Present||APPPC, 1987; IIE, 1996|
|British Indian Ocean Territory||Present||IIE, 1996|
|China||Present||Rose, 1990; IIE, 1996|
|-Hong Kong||Present||IIE, 1996|
|-Andhra Pradesh||Present||IIE, 1996|
|-Himachal Pradesh||Present||IIE, 1996|
|-Indian Punjab||Present||IIE, 1996|
|-Madhya Pradesh||Present||IIE, 1996|
|-Tamil Nadu||Present||IIE, 1996|
|-Uttar Pradesh||Present||IIE, 1996|
|-West Bengal||Present||IIE, 1996|
|-Irian Jaya||Present||IIE, 1996|
|Iran||Present||Rose, 1990; IIE, 1996|
|Israel||Present||Ebeling, 1959; Rose, 1990; IIE, 1996|
|Japan||Absent, intercepted only||IIE, 1996|
|Kuwait||Absent, intercepted only||IIE, 1996|
|-Peninsular Malaysia||Present||IIE, 1996|
|Pakistan||Present||Rose, 1990; IIE, 1996|
|Saudi Arabia||Present||IIE, 1996|
|Sri Lanka||Present||IIE, 1996|
|Syria||Present||Rose, 1990; IIE, 1996|
|Turkey||Present||Rose, 1990; IIE, 1996|
|Congo Democratic Republic||Present||IIE, 1996|
|Egypt||Present||Rose, 1990; IIE, 1996|
|Libya||Present||Abd-Rabou & Amin, 2004|
|Morocco||Present||Rose, 1990; IIE, 1996|
|Saint Helena||Present||IIE, 1996|
|South Africa||Present||Ebeling, 1959; Rose, 1990; Bedford, 1990; IIE, 1996|
|-Canary Islands||Present||IIE, 1996|
|Tanzania||Present||Bohlen, 1973; IIE, 1996|
|Bermuda||Absent, intercepted only||IIE, 1996|
|Mexico||Present||Rose, 1990; IIE, 1996|
|-California||Present||Ebeling, 1959; Rose, 1990; IIE, 1996|
|-Florida||Present||Rose, 1990; IIE, 1996|
|-Texas||Present||Rose, 1990; IIE, 1996|
CENTRAL AMERICA AND CARIBBEAN
|Antigua and Barbuda||Present||Ebeling, 1959; Rose, 1990; IIE, 1996|
|Costa Rica||Present||Ministerio de Agricultura y Ganadería Servicio Fitosanitario del Estad|
|Puerto Rico||Present||IIE, 1996|
|Saint Lucia||Present||IIE, 1996|
|Saint Vincent and the Grenadines||Present||IIE, 1996|
|Trinidad and Tobago||Present||IIE, 1996|
|Brazil||Present||Ebeling, 1959; Rose, 1990; IIE, 1996|
|-Rio de Janeiro||Present||IIE, 1996|
|-Rio Grande do Norte||Present||IIE, 1996|
|-Rio Grande do Sul||Present||IIE, 1996|
|-Santa Catarina||Present||IIE, 1996|
|-Sao Paulo||Present||IIE, 1996|
|Chile||Present||Ebeling, 1959; Rose, 1990; IIE, 1996|
|Colombia||Absent, unreliable record||Ramos & Portilla Serna Cardona, 2004; Kondo, 2001; IIE, 1996|
|Paraguay||Present||Rose, 1990; IIE, 1996|
|Uruguay||Present||Rose, 1990; IIE, 1996|
|Cyprus||Present||Sisman & Ülgentürk, 2010; IIE, 1996|
|France||Restricted distribution||IIE, 1996|
|Italy||Present||Rose, 1990; IIE, 1996|
|Australia||Widespread||Ebeling, 1959; Rose, 1990|
|-Australian Northern Territory||Present||IIE, 1996|
|-New South Wales||Present||IIE, 1996|
|-South Australia||Present||IIE, 1996|
|-Western Australia||Present||IIE, 1996|
|Cook Islands||Present||IIE, 1996|
|French Polynesia||Present||IIE, 1996|
|New Caledonia||Present||IIE, 1996|
|New Zealand||Present||1878||Ebeling, 1959; APPPC, 1987; Rose, 1990; IIE, 1996|
|Papua New Guinea||Present||APPPC, 1987; IIE, 1996|
|Solomon Islands||Present||APPPC, 1987; IIE, 1996|
|Wallis and Futuna Islands||Present||IIE, 1996|
Risk of IntroductionTop of page
According to Burger and Ulenberg (1990), A. aurantii is listed on the quarantine lists of many countries.
Hosts/Species AffectedTop of page
A. aurantii occurs on numerous host plants throughout the world but is mainly known as an important pest on citrus. Lists of food plants which include weeds are given by Quayle (1941). In South Africa, it has been recorded on 201 host plants including fruit trees, cultivated shrubs and plants indigenous to South Africa (Munro and Fouché, 1936).
The severity of red scale attack varies on different citrus cultivars, the descending order of susceptibility being lemons, grapefruit, navels, mid-season and Valencia oranges (these two the same) and finally naartjies. All cultivars can become heavily infested, especially if ants or insecticides disturb the natural balance sufficiently (Bedford, 1998).
Growth StagesTop of page
Fruiting stage, Post-harvest, Vegetative growing stage
SymptomsTop of page
Reddish scale insects that are circular, flattened and about 1.5-2 mm in diameter, may be present on the stems, leaves and fruit of citrus and other host plants.
Citrus leaves as well as the leaves of some other host plants show characteristic yellow spots around each reproducing female. This may be followed by leaf drop and defoliation, accompanied by the dying back of twigs and eventually large branches (Bedford, 1998). Maturing citrus fruit can become completely encrusted with all stages of red scale. Such fruit start to dry out and fall off the tree (Bedford, 1998).
The entire trunk can become heavily infested with red scale, especially in the case of young citrus trees. This often leads to a severe set back of the tree, branches and even the tree dying (Bedford, 1998).
Symptoms ListTop of page
|abnormal leaf fall|
Biology and EcologyTop of page
Detailed descriptions of the biology of A. aurantii are given by Quayle (1941), Bodenheimer (1951) and Ebeling (1959), as well as by Parry-Jones (1936) in Zimbabwe. A short account is given by Stofberg (1937) and Wentzel (1970) in South Africa. For further information on life stages, see section on Morphology.
The close correlation between temperature and the development of each stage of red scale is discussed by Parry-Jones (1936) and Stofberg (1937). The period from crawler to crawler for red scale females on leaves under outdoor conditions at Nelspruit, South Africa, (Stofberg, 1937) varies from a minimum of 61 days in mid-summer to a maximum of 138 days in winter. The development of both sexes is slower on the leaves than on the fruit while the females on leaves also produce fewer crawlers (Parry-Jones, 1936).
On fruit at Mazoe, Zimbabwe, (Parry-Jones, 1936), the average duration of the first and second instars and the adult stage up to the commencement of reproduction by the females varies from a minimum of 13, 10 and 32 days, respectively, in early summer to a maximum of 24, 24 and 70 days, respectively, in winter. The total developmental period of the female varies from 55 to 118 days. That of the male varies from 26 to 76 days. The actual moulting period for each of the two moults (second moult for female only) lasts from 3 to 4 days according to Nel (1933), the life cycle of the insect takes longer under field conditions in California, USA (Nel, 1933; Quayle, 1941). The average production per female on fruit at Mazoe (Parry-Jones, 1936) varies from 66 crawlers over a period of 48 days in winter to 143 crawlers over a period of 33 days in late summer. During the winter-spring period, only 37 crawlers per female are produced in a production period of 19 days. The highest rate of production is almost seven crawlers per day per female in summer, but only one crawler per day in winter.
Although all stages of red scale are present throughout the year in South Africa, without any dormant stage, detailed life tables show that there is a distinct sequence of four field generations in parts of South Africa (Wentzel, 1965; Bedford, 1968). Population counts can be based on the three peaks of mature females which occur on the fruit (Bedford, 1971; Bedford et al., 1992). Stofberg (1937) refers to four generations, with sometimes a partial fifth. Parry-Jones (1936) calculated five generations for shade conditions and up to seven in the sun, but Bedford (1998) found the same number of field generations throughout the tree, despite the different rate of growth on leaves and fruit. Grout et al. (1989) monitored male red scale caught on sticky traps with a female pheromone in six localities in South Africa. They found that males exhibited a two- or three-cohort population structure on citrus and resulted in four to six generations per year on orange trees, and five to seven generations per year on lemon trees. Barely two or three field generations can occur in southern California, USA (Ebeling, 1959) according to locality and climatic conditions. According to Davidson and Miller (1990), A. aurantii has two to three generations in California, six generations in Argentina and four generations in Cyprus.
Once the crop is harvested, the scale is confined to the wood and the leaves. Scales which survive the winter mature in early summer and the crawlers infest new growth and the young green fruit. In the southern hemisphere, fully-grown females usually reach a peak in either January or March and then decline due to the activity of natural enemies. A rapid increase can continue from March to June-July if insecticides or ants have disturbed the natural balance during the period from September to March, or even during the previous season. Population trends of red scale are quite different in biocontrol orchards compared with those under temephos (Bedford, 1971, 1998).
Dispersion and Resident Populations
Red scale crawlers, like other scales, can be distributed by wind, flying insects and birds (Ebeling, 1950). Experience in South Africa indicates that, apart from the dispersion of crawlers in new plantings containing a possible percentage of scale-free trees from the nursery, every established citrus tree is permanently infested with a resident population of red scale. Any increase in the population on individual trees is a result of the interaction between climatic factors, natural enemies, the presence of ants or dust, and the application of different insecticides, fungicides and trace elements, as well as the application of sprays like pyrethroids on adjoining crops (Bedford, 1998).
Notes on Natural EnemiesTop of page
Compere (1961) has given a comprehensive account of the worldwide search for natural enemies of red scale which was started by Californian entomologists. A historical account is given of attempts to control the pest biologically from 1879 to 1948. He includes his own observations on the biocontrol of red scale in South Africa. Extensive work on all aspects of the biological control of red scale has been carried out by Flanders (1951), DeBach and White (1960), DeBach (1962, 1964) and others in California, USA. DeBach (1962) reviewed the biological control of red scale on citrus around the world and concluded that there is little question that large acreages (of citrus) were being needlessly treated for control of A. aurantii in various countries, but since then many countries have initiated IPM programmes (Bedford, 1998). However, Aphytis melinus played a major role in successful biological conrol of red scale on citrus in California and elsewhere (Rosen and DeBach, 1990). Another parasitoid, Comperiella bifasciata has been introduced into Africa, Australia, Europe and North and South America (Noyes, 1990) where it helps to control the red scale.
The one indigenous natural enemy of overwhelming importance as an effective controlling agent of red scale in South Africa is Aphytis africanus [Rhopalosiphum maidis] (Bedford, 1998). The predatory mite, Hemisarcoptes malus attacked more than 70% of red scale crawlers at one stage in an orchard in Israel. This reduction dropped later to around 20% (Gerson et al., 1990).
ImpactTop of page
A. aurantii has been the most notorious pest on citrus in many citrus producing countries in the world. The literature on this pest is very extensive, most of the research work having been conducted in California, USA. Excessive populations can build up very rapidly and, as the saliva injected during feeding is exceptionally toxic to the plant tissues, considerable damage can result, including the shedding of leaves and fruit, the dying back of even large branches and the death of young trees (Bedford, 1998). This pest has often been very difficult to control. Oil sprays and fumigation with hydrocyanic acid gas were the recognized methods of control for many years. During recent years there has been an increasing effort to maintain red scale under biological control especially in California, Australia, Israel and South Africa (Bedford, 1998).
According to Outspan International, no more than six scale insects of 1 mm diameter or larger (of any species) are allowed per average-sized orange for Export Grade (count 88), or 10 insects per grapefruit (count 40). For lemons and soft citrus, the figures are five and four scale insects on counts 113 and calibre 3, respectively, which are allowed per average-sized fruit for Export Grade. As a population density of only 50 females per 100 fruit results in culls of nearly 1% (1% of crop rejection for export in the packhouse), any increase above this level soon entails commercial losses. When red scale infestations are exceptionally heavy, cull can reach 100% at a scale density of 7700 per 100 fruit (Bedford, 1971).
In cases of severe infestations, developing scales form prominent pits on young fruit which are still evident when the fruit matures. Another culling factor results from dead scales which are still attached to the fruit because corrective sprays were not applied when necessary. Severe infestations can also reduce the final crop and the following year's crop by defoliation and dieback (Bedford, 1998).
Damage to the Tree
Heavy infestations of red scale are very toxic to the leaves, twigs, branches and fruit of citrus trees, most probably due to the action of the salivary juice. The red scale feeds by inserting its mouthparts deep into the plant tissue and sucking the parenchyma cell sap (Bedford, 1998).
As a severe infestation increases, the leaves show characteristic yellow spots around each reproducing female, followed by leaf drop and defoliation, accompanied by the dying back of twigs and eventually large branches. Maturing fruit can become completely encrusted with all stages of red scale and start to dry out and fall off. Even the entire trunk can become heavily infested, especially in the case of young citrus trees. Newly planted trees may be severely set back, or even killed if the branches die back. Furthermore, neglected heavy infestations lower the production of an orchard (Bedford, 1998).
Detection and InspectionTop of page
Direct inspections can be made of the leaves, branches and fruit of plants. Look for reddish scale insects that are circular, flattened and about 1.5-2 mm in diameter (the adult females). Between these there are usually many immature scale insects which may be very small and white (first instar) and others which are greyish to red and are larger than the first instar and smaller than the adult red scale.
Synthetic sex pheromones are also available that attract the males to specific traps. The presence of red scale in an orchard can be determined by hanging traps with pheromones in the orchard. The pheromones come in small caps which are placed in a tent trap and hang about 2 m above the ground. At least three traps should be used in small orchards but one per 4 to 8 ha is sufficient in larger orchards. Careful inspection twice a week is recommended and pheromone caps should be replaced every 6 weeks. However, the sex pheromones of red scale may act as kairomones by attracting parasitic wasps and this effect should be considered (Gieselmann and Rice, 1990).
Moffitt and Baritelle (1990) mention that in the San Joaquin Valley, California, USA, citrus growers are advised to control infestations of A. aurantii by spraying before the number of second-flight males caught in pheromone traps placed 1/ha reaches 13,000.
Males of the red scale are also attracted to white and yellow colours and traps can be hung out in orchards. This method is, however, not specific for this species.
Similarities to Other Species/ConditionsTop of page
According to Ben-Dov (1990b), the ventral or dorsal aspects of the pygidial segments are reticulated in several species with characteristic scars, which are helpful in determination of the taxa of the Diaspididae. McKenzie (1937) used these structures to separate the extremely similar species Aonidiella citrina and A. aurantii. Microscopic examination of slide-mounted adult females is required for authoritative identification to species. A key to separate economically important species of Aonidiella in the tropics is given by Williams and Watson (1988). McKenzie (1938) provides a key to separate all the species of Aonidiella.
Prevention and ControlTop of page
From 1895 to 1950, hydrogen cyanide (HCN) fumigation, augmented when necessary with an oil spray, was the general method of controlling red scale in many countries.
The corrective spray treatments which were recommended by Georgala (1967), were intended to be applied when the crop is actually threatened by scale in mid- and late-summer. These sprays were often essential when one or more preventive treatments gave insufficient control. Bedford (1979) used a single corrective spray of dimethoate plus mineral oil in difficult orchards to assist in the transition to complete biological control of red scale. The use of oil reduces the detrimental effects of organophosphates on parasitoids (Searle, 1964).
No less than 20 early and late preventive or corrective treatments are now used for the control of red scale. The grower is advised to aim for an IPM programme for the other pests and to resort only to an annual oil spray to control red scale, or still better to maintain it under complete biological control without applying any sprays for this pest (Bedford, 1998).
When red scale is controlled chemically, it is usually essential to wet all parts of the tree, including the main framework, very thoroughly, irrespective of the type of spray machine used. Oil sprays have the advantage of causing the minimum disruption of the natural enemies if compared with the organophosphates previously used, but great care should be exercised with the application and the soil should not be at all dry. Oil sprays applied to bearing trees are safest in mid-summer (Bedford, 1998).
The following is a summary of an abstract made by Bedford (1998) regarding the control of A. aurantii:
The fumigation of citrus trees at night (Smit, 1937; Quayle, 1941) is a complicated and laborious process, entailing the covering of each tree with a gastight canvas sail.
The use of sprays such as temephos to control thrips and psylla caused outbreaks of other pests which were controlled in turn by other modern insecticides which only aggravated the situation and increased pest repercussions and spiralled the cost of pest control (Simmonds, 1960; Bedford, 1968, 1969). Pesticide usage reached an all-time peak from about 1960-1976, when no less than 11 previously minor pests in South Africa required chemical control (Bedford, 1978).
To make matters worse, red scale in parts of South Africa was shown by Georgala (1975) to have become resistant to organophosphate pesticides. Growers were now faced with a serious predicament. Because of this resistance, even as many as three preventative sprays were sometimes necessary to control red scale and even this was sometimes inadequate. Damage to trees and crop losses were phenomenal, and some growers almost faced bankruptcy. Tons of fruit were unexportable because they were too heavily infested with red scale (Bedford, 1998).
Control of ants: the key to biological control of red scale
Certain species of ants are major pests of citrus and should be controlled because they cause coincident infestations of red scale (DeBach et al., 1951; Steyn, 1954a, b; Annecke, 1958; Compere, 1961; Brettell, 1962; Bedford, 1968). These ants all follow regular trails into the trees and continually patrol up and down the branches to obtain honeydew, especially from soft scale insects, mealybugs, the Australian bug (Icerya purchasi) and aphids. Their restless activity disturbs and reduces the parasitoids and predators so that not only the honeydew-producing insects increase rapidly and provide more food for ants, as well as causing sooty mould, but the red scale en route along the twigs and branches increases quite out of proportion to its normal low level in the absence of ants. Ants can thus be responsible for making red scale a major pest, when its correct status is that of a minor pest in orchards in which ants are controlled and only integrated sprays are applied (Bedford, 1998).
Compere (1961) has given a comprehensive account of the worldwide search for natural enemies of red scale which was started by Californian entomologists. A historical account is given of attempts to control the pest biologically from 1879 to 1948. He includes his own observations on the biocontrol of red scale in South Africa. Extensive work on all aspects of the biological control of red scale has been done by Flanders (1951), DeBach and White (1960), DeBach (1962, 1964) and others in California. DeBach (1962) reviewed the biological control of red scale on citrus around the world and concluded that there is little question that large acreages (of citrus) were being needlessly treated for control of A. aurantii in various countries, but since then many countries have initiated IPM programmes.
It should be stressed that the control of ants is of fundamental importance in the biocontrol of A. aurantii (Bedford, 1998).
Integrated Pest Management
Transition to biological control of red scale
It should be emphasized that an outbreak of red scale sometimes takes place when sprays are withdrawn so that it is essential to monitor the pest to determine whether late preventative or corrective sprays are necessary (Bedford, 1998).
Transition on non-bearing trees (new plantings)
According to Bedford (1998), infestations of red scale in new plantings can be higher than on older trees, for red scale tends to diminish on citrus trees as they get older. Larger populations of scales should therefore be tolerant on non-bearing young trees, but the infestation should not be allowed to build up to a population that will damage the trees. Ants should also be properly controlled every year throughout the summer months. A few orchards may require spot-spraying or a general corrective spray before the red scale subsides permanently, and remains below the level of economic damage as a result of biological control.
Non-bearing trees may be spot-sprayed for red scale early in spring, or if necessary from mid-summer to autumn, especially during the first season. A tree-to-tree survey will show up single trees that should be treated. However, if more than 20% of the trees are badly infested, it is better to spray the entire orchard. Once the whole farm is under integrated control, it is not usually necessary to spray any additional new plantings for red scale (Bedford, 1998).
Transition on bearing trees
The sudden withdrawal of organophosphates previously used to control red scale can be a costly process if badly managed. It becomes most important to monitor the red scale population in particular, so that early preventive or corrective sprays can be applied in individual orchards as and when required, especially during the first year of transition to biocontrol (Bedford, 1998).
When in transition to an integrated control programme, in cases where the whole farm is still sprayed preventively for red scale, it is advisable to start with just one or two orchards, preferably old Valencias or mid-seasons. Two red scale surveys should be carried out, one commencing in mid-summer and the other 2 months later. Only healthy, full-grown, mature red scale females are counted and the count is used to determine the degree of infestation and to assess whether a corrective spray is necessary. The threshold for the 1% cull level is 50 red scales per 100 fruit. Up to 150 can be allowed in new plantings (Bedford, 1998). An alternative method is to use sticky traps which catch the males (attracted by the sex pheromone of the female scale) (Grout and Richards, 1989), but these require considerable experience to interpret.
A corrective spray may be necessary where more than 25% of the fruit is infested with one or more healthy adult red scale females, unless parasitism of the second instars and the grey adult female stage is very good. Only in cases where the natural balance has been badly disturbed by previous sprays is a corrective spray necessary during the first season. Red scale appears earlier on the fruit in orchards under biological control, but most of it disappears before the fruit is picked. With experience, the degree of parasitism can be judged visually, otherwise it should be determined under the microscope. Parasitism by Aphytis spp. of 20% or more of the susceptible scale population, including adult males and females which have escaped parasitism, is usually sufficient, provided the scale population is not too high (Bedford, 1998).
Transition in orchards with resistant red scale
The only solution to the problem of red scale becoming resistant to organophosphates is to allow the natural enemies of red scale to build up again in the orchards as quickly as possible by including mineral oil in the programme, with or without organophosphates, and by methodically reducing the numbers of preventive sprays for red scale (using a sequence, for example, from two to one to no sprays per annum). Only selective sprays, as recommended for integrated control, should be used for those pests for which chemical control is essential (Bedford, 1998).
The temporary use of organophosphates plus oil or double oil sprays for red scale should be discontinued as soon as possible. Red scale is then brought under complete biological control. Alternatively, control can be aided by a single spray of narrow range mineral oil in early summer, when the oil is used with certain fungicides for the control of black spot, control can then be increased from 0.5 to 1%. Only in abnormal seasons may it be necessary to revert temporarily to the use of an organophosphate with oil, or oil alone, as an early or late preventive spray. Many growers use only narrow range mineral oil sprays applied in late winter, spring and early summer to halt resistant red scale (Bedford, 1998).
Mineral Oil Sprays
Riehl (1983) reported that N-R 415-type oil in dilute aqueous mixture gave economic control of A. aurantii and no reduction of yield of navel oranges and grapefruit and with NR 440-type oil on lemon.
A high pressure unit for rinsing citrus fruit in the packhouse has been developed to remove armoured scale insects such as A. aurantii. For this purpose, a water jet at a pressure of 28 bar is used (Bedford, 1990).
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- = Present, no further details
- = Evidence of pathogen
- = Widespread
- = Last reported
- = Localised
- = Presence unconfirmed
- = Confined and subject to quarantine
- = See regional map for distribution within the country
- = Occasional or few reports