Chrysomphalus aonidum (Black Scale or Florida Red Scale)
C. aonidum is a polyphagous species with a preference for citrus, particularly navel and Valencia oranges and grapefruits (Bedford, 1989). It has been recorded from hosts in 77 plant families, including crops, ornamentals, palms and forestry trees (Borchsenius, 1966).
In the laboratory, C. aonidum can be mass-reared for biological control purposes on Citrullus spp. (melons or watermelons) (Bedford, 1989) or on potato tubers or pumpkins (Li and Liao, 1990). In tests, Ceballos and Hernandez (1986) found mass-rearing most successful on Cucurbita pepo
Affected Plant Stages: Flowering stage, fruiting stage, post-harvest, and vegetative growing stage.
Affected Plant Parts: Leaves, stems, and fruits/pods.
Host List
Primary hosts
Citrus, Citrus aurantiifolia (lime), Citrus limon (lemon), Citrus maxima (pummelo), Citrus sinensis (navel orange), Citrus x paradisi (grapefruit)
Secondary hosts
Asparagus officinalis (asparagus), Camellia sinensis (tea), Carica papaya (papaw), Cinnamomum verum (cinnamon), Cocos nucifera (coconut), Dracaena, Gossypium (cotton), Lauraceae, Malus pumila (apple), Mangifera indica (mango), Musa (banana), Musa x paradisiaca (plantain), Palmae (plants of the palm family), Phoenix dactylifera (date-palm), Pinus (pines)
Biology and Ecology
Reproduction in C. aonidum is sexual; no evidence of parthenogenesis has been recorded. The sex ratio in C. aonidum has been found to be biased in favour of females (Nur, 1990). Each adult female lays about 50-150 oval eggs under the scale over a period of 1-8 weeks, depending on the part of the plant infested (those on the leaves being less fecund than those infesting fruits) (Rose and DeBach, 1978). Eggs hatch under the scale and the first-instar nymphs or crawlers walk about to find a suitable feeding site before settling to a sessile lifestyle. The second-instar nymphs are the main feeding stage in both sexes. Development to adult takes 7-16 weeks according to temperature. In California, USA, C. aonidum has up to six generations per year (Gill, 1997). In countries with a cold winter, such as Taiwan, there may be three distinct generations per year (Su, 1983), while in tropical conditions (and heated greenhouses) breeding is continuous and generations are asynchronous.
C. aonidum has a preference for humid environments and cannot tolerate freezing temperatures. It tends to prefer the lower and central parts of mature citrus trees and rarely infests green wood (Rose and DeBach, 1978). Male stages are rather more tolerant of lower humidity than females, so male scales are more often found on the upper surface of the leaf while females congregate on lower leaf surfaces (Bedford, 1989). Like other diaspidid scale insects, C. aonidum suffers increased mortality in heavy rain and reaches high population levels during dry weather.
The first-instar stage is the sole dispersal stage. Each crawler walks to an exposed position on the plant, from which air currents may carry it as much as several tens of kilometres away (Greathead, 1990). Passing animals or people can also can carry the crawlers over great distances. Movement of infested planting material or produce is the main way in which C. aonidum has been introduced to other countries.
This species has 2n=8 chromosomes (Nur, 1990).
Symptoms
C. aonidum is a leaf-infesting species, but in high-density infestations it may spread to fruits, stems and trunks, and may cause premature leaf and fruit drop and stem dieback. The scales appear as circular dark spots.
An infestation appears as dark-purple to reddish-brown or black spots with paler margins, on both surfaces of shaded leaves of the host plant. Heavy infestations cause yellowing of the leaves, followed by defoliation of part or all of the host. C. aonidum prefers shade and is therefore most common in the lower part of the canopy.
Descriptors: Leaves: honeydew or sooty mould; abnormal leaf fall; abnormal colours; necrotic areas. Stems: dieback; external feeding. Fruits/pods: discoloration; external feeding; premature drop; lesions: black or brown.
Morphology
Adult and intermediate-stage females form flat to moderately convex, circular scales up to 2 mm in diameter, each with a slightly raised, sub-central point which is sometimes pale. If the scale is lifted off with the point of a needle, the insect beneath is yellow and up to 1.7 mm long. Male scales are slightly paler than female scales, and are elongate-oval and half the size. The adult male is 0.7 mm long and has a single pair of wings, two pairs of simple eyes, no mouthparts and very long genitalia. First-instar nymphs are 0.3 mm long and have legs but soon settle to form circular white scales (the whitecap stage) up to 0.4 mm across. These become incorporated into the scales of subsequent stages, forming the paler subcentral point.
Detection and Inspection Methods
Examine plants closely for dark spots on the leaves, especially in shady parts of the plant. Good light conditions are essential and in poor light, a powerful flashlight is helpful. A large hand lens may assist in recognition of small, immature stages.
Control
Regulatory Control
Pre-export inspection should be sufficient. Leaves, particularly lower surfaces, and fruits should be examined.
Chemical Control
Numerous citrus-producing countries, such as Israel and South Africa, have controlled C. aonidum and other scale insect pests of citrus with insecticides (particularly organophosphates). However, the long-term use of pesticides is expensive, can be environmentally damaging and leads to outbreaks of other pests due to elimination of their natural controls and development of pesticide resistance in the scales.
Biological Control
The development of effective biological controls has greatly reduced the importance of C. aonidum in many citrus-producing countries, such as Israel, Mexico and Australia. In South Africa, expenditure on insecticide sprays to control the scale on citrus was high until the introduction of effective biological control of C. aonidum reduced it by over 50% (Bedford, 1989).
Two parasite species, Aphytis holoxanthus and Pteroptrix smithi, are the main biological control agents used against C. aonidum. These were introduced to Israel from Hong Kong in 1956-57 and both established. A. holoxanthus is a very efficient parasite (Steinberg et al., 1987) and quickly brought populations of C. aonidum under control, but subsequently P. smithi has become dominant in the coastal strip, possibly because A. holoxanthus populations fall in autumn and winter in these areas whereas P. smithi populations remain high throughout the year (Steinberg et al., 1986).
A. holoxanthus was introduced from Israel to Cyprus and Texas in 1959, to California in 1960 and to Queensland, Australia in 1974, where it displaced the native parasitoid, Aphytis columbi (Smith, 1978). From California, it was introduced to Florida in 1960, to Brazil in 1962, to Egypt in 1963 and to Lebanon in 1965, but the success of the last three introductions is not known (Rosen and DeBach, 1978).
Comperiella bifasciata was introduced to South Africa accidentally and was mass-reared and released, but could not compete with A. holoxanthus and apparently died out (Bedford, 1989). The coccinellid, Rhyzobius lophanthae, is recorded as a useful predator of C. aonidum in South Africa (Bedford, 1989).
In Texas, C. aonidum used to be controlled by the parasites Pseudhomalopoda elongata and P. prima, but these died out in the winter of 1951 and had to be re-introduced from Florida in 1955 when scale populations escalated. However, control by these parasites was inadequate and led to the introduction of A. holoxanthus from Israel in 1959 (Rosen and DeBach, 1978).
Damaging outbreaks of diaspidid scales, including C. aonidum in the Seychelles in the 1930s, were controlled by introduction of the coccinellid Chilocorus distigma from East Africa in 1936 and C. nigritus from Coimbatore, India in 1938 (Veysey-Fitzgerald, 1941, 1953).
In the laboratory, C. aonidum can be mass-reared for biological control purposes on Citrullus spp. (melons or watermelons) (Bedford 1989) or on potato tubers or pumpkins (Li and Liao, 1990). In tests, Ceballos and Hernandez (1986) found mass-rearing most successful on Cucurbita pepo.
IPM Programmes
Since the successful introduction of biological control against C. aonidum in Israel, effective integrated pest management programmes have been developed in Israel (Steinberg et al., 1986, 1987) and South Africa (Bedford, 1989).
In South Africa, Aphytis holoxanthus has proved highly effective against C. aonidum on citrus, sometimes giving parasitism levels as high as 90-100% (Bedford, 1989). It is used as part of an integrated pest management programme and has reduced the cost of pesticides to the industry by at least 50%; C. aonidum is now rare in South African orchards.
Information provided by CABI, 2003. Crop Protection Compendium. Wallingford, UK: CAB International.Page last updated: 7 August 2008