Coconut scale & false scale

Common name

Coconut scale
Coconut (false) scale

Scientific name

Aspidiotus destructor
Aspidiotus rigidus

Eggs

Eggs are laid by adult females.

Eggs can be found all around the adult female's body (A. destructor), or in a crescent shape (A. rigidus), under protective cover on the underside of leaves.

The eggs are smooth and slender (0.22 mm long by 0.09 mm wide). The eggs are initially a whitish colour and slowly turn a pale yellow. 

Over a week Aspidiotus rigidus lays around 12 eggs while A. destructor lays 28-65. 

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Nymphs

Female nymphs have two growth stages (instars), while males have four.  

Nymphs newly hatched from the egg are called crawlers. This is the most mobile life stage.

Crawlers have oblong shaped bodies.

For 2 - 48 hours the crawlers move over and between leaf surfaces to find feeding sites. They will stay there for the remainder of their lives. 

The crawlers' waxy protective cover is a clear, light green to yellowish brown colour. They are tiny - around 0.23 mm long by 0.11 mm wide (A. rigidus is slightly larger and tougher).

Nymphs have legs and antennae with a pair of short appendages at the end of their body.

Females

The second female growth stage (instar) is also oval and a clear pale yellow. However, the nymphs are now larger (0.6-1.1 mm long). The second stage lasts for 8-10 days.

Males

Male nymphs look like the second female instar but half way through development start to show male characteristics.

The protective cover becomes more egg-shaped and red-brown.

Under the second instars protective cover, the male will undergo  non-feeding pre-pupal and pupal stages before becoming an adult.

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Adults

Females

Adult females have a flat and broad, clear oval or circular cover ~2.0 mm in diameter).

In the centre of this cover there is a pale exuviae (skins shed from previous growth stages). 

The adult female is pear-shaped and 0.7-1.2 mm in length with  hardened (sclerotised) plates on its back.

The long plates on the flat and fringed body are visible under a micrtoscope with three pairs of lobes. 

Males

Adult males look very different to females. The males look more like a small fly than a scale.

The males have two-wings, antennae and three pairs of legs. They are a reddish colour (A. destructor) or yellow (A. rigidus)  and have a pair of long appendages at the end of their body.

The males have no mouthparts, and can not feed so have a short lifespan.

Once mature the males fly off to mate with females, attracted by the pheromones the females produce.

However, A. destructor females can also reproduce without male fertilisation. The same is speculated for A. rigidus.

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Aspidiotus rigidus adult female, showing the eggs in a crescent shape under her cover (© Merle Shepard and Gerald Carner, Clemson University)

Aspidiotus adult female with a flat, clear oval cover with pale exuviae (skins shed from previous growth stages) near the centre surrounded by eggs. Note the pear shaped body (© Salahud din, University of Florida)

Aspidiotus destructor red-brown male pupa, removed from its cover (© Salahud din, University of Florida)

Yellowish brown coloured Aspidiotus nymphs with little to no cover, among adult females (A. destructor) (© Lyle Buss, University of Florida)

Aspidiotus adult male that looks like a small fly (© Salahud din, University of Florida)

Yellow spots concentrated on an area of a leaflet surrounded by green healthy tissues. Note red-brown spots starting to appear (© TNAU)

Leaf tissues looking yellowed or brown and wilted (© Merle Shepard and Gerald Carner, Clemson University)

The yellowing of the palm can start by being concentrated in one section of the leaflet surrounded by unaffected green areas.

The leaf tissues may then look twisted, wilted and dry.

Alternatively  the whole leaflet will turn yellow then brown and fall off.

Entire fronds may be damaged and fall off, which can be  followed by the death of the crown. 

Whole plantations may be symptomatic with many yellowed crowns (see image on right).

The yellowing of palms can be seen from a distance.

When infestations are severe, a crust of scale insects can form over the whole leaflet (usually on the underside - see image below right).

The crust may also form over frond stems, flower spikes and young nuts that are infested.

Seedlings and young palms are likely to die when infestations are severe.

The nuts may also turn yellow-brown, have stunted growth and fall before maturing.

Coconut milk may have a noticeable sour taste.

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Impacts

The coconut scale was considered a serious pest of coconut and other important crops until biological controls were put in place to reduce populations. 

However, before this copra production was reduced from 1400 to 500 tons annually in Principe (off the west coast of Africa).

A reduction in yields of at least 25% over 2-3 years occurred after an outbreak in Côte d'Ivoire.

If the coconut scale was introduced in to new regions, outbreaks could severely reduce coconut production.

The feeding of the coconut scale causes cosmetic damage to coconuts said to reduce the products quality and making it less marketable. 

The stunted growth and early fall of nuts, producing less copra and a sour milk also reduces the economic outputs.

This youtube video from the Philippines has information on symptoms and impacts of coconut scale. 

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A coconut plantation showing many yellowed palms (© Merle Shepard and Gerald Carner, Clemson University)

A leaf with a crusted surface due to a heavy coconut scale infestation (© TNAU)

Young coconuts infested by coconut scale colonies can have cosmetic damage (© Bernhard Loehr, ICIPE, Bugwood.org)

Natural

Wasp parasitoids have been introduced from Java to successfully control A. destructor.

These parasitoid predators include Aphytis melinus and Aphytis lingnanensis in Fiji and the Philippines.

Argentina, California and Hawaii have also introduced Aphytis melinus and Comperiella bifasciata.

Ladybird beetles (Coccinellidae) predators from Trinidad have also been introduced. These include Chilocorus species, Telsimia nitida, Pseudoscymnus anomalus, Cryptognatha species (Cryptognatha nodiceps and later Cryptognatha gemellata). Rhyzobius satelles is useful when ants are not tending scale insects for their honeydew. 

The parasitic wasp, A. melanus laying eggs on a scale (© Jack Kelly Clark, formerly ANR Communication Services, University of California Statewide IPM Program)