Pest of the Month
Coconut Scale Insect Aspidiotus destructor Signoret(Hemiptera:Dispididae)

The coconut scale insect (CSI) is recorded in tropical and subtropical regions worldwide and is present in nearly all countries where coconuts are grown.

CSI infests stems, branches, leaf-petioles, leaves and fruits of their host plants. On leaves they are found mostly on the undersides. During severe outbreaks, scale insects completely cover the underside of leaves, defoliate and even kill their hosts.

After accidental introduction to various Pacific islands, CSI became a very serious pest of coconuts until the initiation of biological control, following which, almost invariably, its pest status in many countries has been greatly reduced. Recently it has been found in new areas and in the absence of the natural enemies it has become a serious pest.

Distribution in the pacific region
The following countries in the region record the presence of the CSI. American Samoa, Fiji, French Polynesia, Guam, Marshall Islands, Federated States of Micronesia, New Caledonia, Northern Mariana Islands, Papua New Guinea, Samoa, Solomon Islands, Tuvalu, Vanuatu, Wallis and Futuna Islands.

Host range and pest status
CSI hosts (as in list of hosts) are typically perennial species and include many species of fruit trees, such as avocado, breadfruit, mango, guava and papaya. Other major hosts are cocoa, cassava, cotton, kava, oil palm, papaya, rubber, sugarcane and tea. It also attacks a range of ornamental plants including roses.

Coconut is its preferred host. On coconut palms, CSI is found mainly found on the undersides of the leaves, but frond stalks, flower clusters and young fruit can also be attacked. During severe attacks, the leaves dry up causing leaf fall-offs,  the crown dies which lead to collapse of the infested plant. Attacks of fruits cause shrivelling of nuts leading to premature nut falls. Mostly young coconut trees of upto 10-15 years are vulnerable to CSI damage.

On bananas, the damage can occur at all stages of the plants. Attacks on suedo-stems, leaf petioles and leaf cause stunting of the plants. Infestation of fruit bunches causes deformed fruits which are unmarketable. Severe attacks can kill the plants.

Damage on fruit trees cause leaf fall, stunting and dieback of the plants. Scale presence on fruit cause blemishes, fruits are deformed and unmarketable.

Shrubs such as roses can be severely affected by CSI attacks causing stunting and be subjected to secondary infections and finally collapse of the plants.

Secondary effect of the CSI is development of shooty mould on plant surfaces, especially on leaves, which curtails photosynthesis. Sweet excretion attracts ants, which drive away the natural enemies of the scale and assist in spread of the crawlers.

Damage symptoms
On leaves, A. destructor causes scales and yellow spots to develop where the crawlers have settled and form into adults. Entire leaves may turn yellow to brown and fall. Sooty mould may develop. The bright yellow colour of affected coconut palms is clearly visible from a great distance. In extreme cases, the leaves dry out, entire fronds drop off and the crown dies. Heavy attacks result in stunning of new leaves and reduction of crop yield, even to a point of complete failure. Infested coconut fronds exhibit yellow areas on the upper surface, formed by numerous yellow spots each marking the position of the coconut scale on the under surface. The yellowing will become so extreme that it becomes conspicuous from a distance.

Description, biology and ecology

The life cycle of A. destructor typically lasts for 32-35 days. The larvae (crawlers) and the adult males are the only mobile stages of the scale. The first-instar crawlers leave the maternal scale and begin feeding by inserting ‘feeding tubes’ on the affected part of the plant surfaces. The crawlers of both sexes are mobile. They, however, can move only to a limited distance on the affected plants. They are mostly dependent on other agents, such as wind, birds, other animals, ants, etc. for dispersal. Heavy rains wash off the crawlers from their hosts.

The eggs are yellow and very small. They are laid underneath the scale around the body of the female. The female deposits 20-50 eggs over a period of few days. The incubation takes from 7-8 days. After hatching, the nymphs crawl out the females scales into the open and colonize the plant surfaces.

The first-instar crawlers are about 1mm long, yellowish-brown, oval and translucent. Second-instar females become immobile and secrete a translucent wax scale cover. The second-instar males are smaller than the females. They group together, secrete a filamentous waxy material and become immobile. The male pre-pupal and pupal stages are spent under the scale produced by the second-instar stage.

The females have two nymphal stages. The males have four, two feeding stages, a pre-pupal and a pupal stage.

The scale cover of the adult female is oval to circular, 1.5-2.0 mm across, fairly flat, very thin and translucent. The pale yellow exuviae are more or less central on the scale. The yellow adult female under the scale is 0.6-1.1 mm long. The adult male scale is redder than the females, but much smaller and more oval. The male has one pair of wings and is motile.

Control measures
CSI can easily be re-introduced, even if it is successfully controlled on the primary host crop, because of its wide host range.

Phytosanitary Measures

CSI is spread by people moving infested plants. Quarantine border surveillance should be restricted on the movement of planting materials of the host plant to avoid spread of the scale.

Cultural Control

CSI may be eradicated from new areas by destroying infested plants and plant parts. The scale, however, is known to establish very quickly because of its wide host range and therefore is difficult to eradicate. Healthy plants in well-drained soils are seldom seriously infested. Weak and neglected plantations are particularly susceptible to damage by A. destructor.

Biological Control

Biological control of CSI is the long term solution. In countries where the scale is present, the pest population is controlled by introducing natural enemies. In recent years introduction of natural enemies has controlled the pest in Tuvalu and on the island of Rotuma in Fiji.

Natural enemies of other scale insects may adapt to feeding on A. destructor as it colonizes new areas. The scale is attacked by a few specific and a large number of non-specific parasitoids and predators.

Predators rather than the parasitoids play a significant role in limiting CSI populations (Waterhouse and Norris, 1987). The most common are the coccinellid beetles, Chilocorus spp., Cryptognatha nodiceps, Pseudoscymnus anomalus , Rhyzobius spp. and Telsimia nitida. Parasitoids are not very well studied and known, but Aphytis spp. and Encarsia spp. do contribute significantly in the maintaining of CSI populations in the field.

To cater for occasional peak demands from Pacific Islands for CSI natural enemies, the predators are reared under laboratory conditions at SPC Plant Protection Service in Fiji and field released where necessary.

Reference:

For further information:
Please contact PPS Entomologist - Sada Nand Lal.