Coconut

Adults are large weevils (40-55 mm in long), with a long snout and reddish brown in colour, and generally with 2 reddish bands on the thorax. Females lay eggs on wounds of various origins, on the mature stem as well as in the crown. Upon hatching the larvae (grubs) penetrate into the living tissues of the palm, feeding on the shoot and young leaves, where the insect completes its development in about 3 months. The damaged tissues turn necrotic and decay. Sometimes the grubs feed on the growing point killing the palm. Grubs are whitish-yellow, legless, and oval in shape; their head is reddish brown, and is armed with strong mandibles. Fully-grown grubs are 50 to 60 mm long. The pupal stage is passed within a cocoon of vegetal debris made by the grub at the end of its development. The African palm weevil usually damages young palms, yet may also, in exceptional cases, cause damage to mature crops.

The primary means of control for African palm weevil is preventative, using cultural and sanitary methods.

It is a stout beetle, about 3.5 to 5 cm long, shiny dark brown to black in colour with a curved horn on the head, hence its common name. The adult flies at night to palms and bore into the hearth of the palms spear, chewing and cutting the youngest unopened leaves and the vegetative bud.

Attacked leaves continue to develop and unfold showing a characteristic V-shape damage. If the whole growing point is eaten, the palm usually dies, particularly young palms less than four years old. The boreholes are often marked with a bundle of fibres pushed out of the hole by the beetle. This beetle is a serious pest in plantations where field sanitation is neglected. Eggs are laid in rotting plant material, especially dead palm trunks, compost heaps and rubbish dumps. The larvae (grubs) and pupae develop in rotten coconut logs and other decaying material.

Bud rot (also called heart rot) caused by the fungus Phytophthora palmivora has an extensive host range of more than 200 plant species and it is widespread in the tropics including tropical Africa. The fungus enters into the plant by infecting tender host tissues (leaves, buds or young nuts). Affected leaves turn yellow and later brown.

The heart leaf becomes chlorotic, wilts and collapses. The disease may spread to older, adjacent leaves and spathes, producing a dead centre with a fringe of living leaves. Light brown to yellow, oily, sunken lesions may be found on leaf bases, stipules or pinnae. Internally, the tissues beneath the bud are discoloured pink to purple with a dark brown border. Affected leaves progressively drop. Infected nuts show brown to black necrotic areas with a yellow border developing on the surface; internally, they have a mottled appearance. Young nuts are highly susceptible and fail to mature, they then fall off the tree; older infected nuts ripen normally.

The coconut bug (Pseudotheraptus wayi) is the most important pest of coconuts in East Africa. A related species of coconut bug, Pseudotheraptus devastans, causes similar damage to coconuts in West Africa.

Adult bugs are brown in colour and 1.2 to 1.4 cm long. They lay eggs singly on the flowers or young nuts. Nymphs are red brown to green brown in colour and have long antenna. Adults and nymphs suck on flowers and developing fruits causing flower abortion and early nutfall. The toxic saliva of the bugs causes necrotic sunken lesions (scars) and cracks on the nuts.

Attacked young nuts excrete gum. Many of the attacked young nuts fall off. Nuts older than 3 months at the time of attack may not be aborted but remain small and have scars. Yield losses are difficult to assess since many of the nuts (over 70%) fall naturally. Nuts which abort naturally, do not show scars or gummosis. Two bugs per palm can cause considerable damage. Damage is usually less serious in intercropped coconuts. The bugs also feed on cashew, mango, cocoa and guava.

The predatory red weaver ant is an efficient natural enemy of the coconut bug. Weaver ants build nests on palms and other trees by joining leaves with silk produced by their larvae. They forage on the canopy chasing away or killing coconut bugs. Palms with weaver ants are usually free of damage by the coconut bug. Good control is achieved when more than 60% palms are occupied by thriving colonies of weaver ants. Unfortunately other ants present in coconut plantations fight weaver ants, and themselves do not protect (or not as effective as weaver ants) the palms against coconut bugs. These ants such as big headed ant (Pheidole megacephala), the crazy ant (Anoplolepis custodiens), and the long legged ant (Anoplolepis longipes) among others, kill or displace weaver ants, as a result palms are severely damaged. These competing or antagonistic ants need to be managed to allow weaver ants to do their beneficial work.

This mite is tiny and difficult to see with the naked eye. When very many mites are together they appear as fine whitish dust. The coconut mite attacks and damages the upper part of the nutlets under the sepals of up to 6 months old nutlets. Attack is severe during the dry season. Attacked nuts may fall or have a scarred husk, which often splits. The nuts of coconuts showing light scarring on the husk are not seriously affected but in heavily scarred nuts there is significant damage to the nuts.

In East Africa, Marasmiellus cocophilus causes death of palms up to 8 years old, seedlings being highly susceptible on transplanting to the field. Root infections occur, leading to decay of basal tissues and finally a rot of the spear leaf. On older palms, the first symptoms are a general wilt of the fronds, which remain as a 'skirt' around the trunk. The spear leaf dies and a foul-smelling soft rot develops at the base of the leaves. A dry, reddish-brown rot with a yellow margin is typically present at the base of the bole. Cavities within these areas of rot are lined with mycelium (fungal growth) in young palms, 2-4 years old, but rare in 4-6-year-old palms, and absent in mature palms.

Fungal bodies (like small mushrooms) commonly occur on exposed roots, leaf bases of seedlings, exposed tops of seed nuts and on the soil surface around holes (growing from coconut debris) where diseased palms had been removed 2 years previously. On average, there is only about 8 weeks from the time of onset of symptoms till the death of the palm; this interval depends on the extent of fungal decay in the bole.

Spread occurs through soil, root contact between palms, infected coconut debris and probably by airborne basidiospores (fungal spores). Infection also occurs via wounds.

Symptoms include necrosis of inflorescences, yellowing of leaves and premature shedding of all fruit (nut fall) regardless of their developmental stage. Aborted nuts often develop a brown-black calyx-end rot reducing seed viability. Premature nut fall is accompanied or followed by inflorescence necrosis. This symptom is most readily observed as newly mature inflorescences emerge from the spathe. Normally light yellow to creamy white in colour; affected inflorescences are instead partially blackened (necrotic) usually at the tips of flower spikelets.

As disease progresses, additional emergent or unemerged inflorescences show more extensive necrosis and may be totally discoloured. Such symptom intensification results in the death of most male flowers and an associated lack of fruit set. Yellowing of the leaves usually starts once necrosis has developed on two or more inflorescences and discolouration is more rapid than that associated with normal leaf senescence. Yellowing begins with the older (lowermost) leaves and progresses upward to involve the entire crown. Yellowed leaves turn brown, desiccate and die. In some cases, the advent of this symptom is seen as a single yellow leaf (flag leaf) in the mid-crown. Affected leaves often hang down forming a skirt around the trunk for several days before falling.

A putrid basal soft rot of the newly emerged spear (youngest leaf) occurs once foliar yellowing is advanced. Spear leaf collapse and rot of the apical meristem invariably precedes death of the palm at which point the crown topples away leaving a bare trunk. Infected palms usually die within 3 to 6 months after the appearance of the first symptoms.

Lethal yellowing symptomatology may be complicated by other factors. For example, non-bearing palms lack fruit and flower symptoms. Foliar discolouration also varies markedly among coconut ecotypes and hybrids. For most tall-type coconut palms, leaves turn a golden yellow before dying whereas on dwarf ecotypes leaves generally turn reddish to greyish-brown.

Phytoplasmas are transmitted in a persistent (circulative-propagative) manner primarily by insect vectors belonging to the families Cicadelloidae (leafhoppers) and Fulgoroidae (planthoppers).

Mealybug infestations of above-the ground plant parts start with the appearance of crawlers (the first- instar nymphs) on the underside of the leaves on terminal shoots, stems and other plant parts. Heavy mealybug attack appears as white, waxy masses of mealybugs on stems, nuts and along the veins on the underside of leaves. Heavy infestations usually result in coating of adjacent stems, leaves and nuts with honeydew and sooty mould. Severely infested plants may wilt due to sap depletion; leaves turn yellow, gradually dry and ultimately fall off. Feeding on nuts results in discoloured, bumpy, and scarred nuts, with low market value, or unacceptable for the fresh fruit market.

In coconut palm monocultures, rodents, and especially rats, can develop into a serious epidemic, which is then difficult to bring under control again.

A number of scales feed on leaves and fruits of coconut palms. The most damaging is the coconut scale (Aspidiotus destructor). This scale is bright yellow and round (females) or reddish and oval (males) covered with a semitransparent greyish white flat scale. The scale diameter is 1.5 to 2.0 mm. Females are always wingless and remain under their scale their entire life. Adult males have one pair of membranous wings, move about actively in search of females and do not feed during the adult stage. Eggs are protected underneath the scale or shell of the mother insect until they hatch. Upon hatching the young scales leave the maternal scale, take up a position and start feeding. They do not move afterwards. They are found mainly on the undersides of the leaves, but frond stalks, flower clusters and young nuts can also be attacked.

A severe infestation of this armoured scale forms a continuous crust over flower spikes, young nuts and the lower surface of leaves. The leaves become yellow and eventually die. The crown dies leading to collapse of the infested plant. Attacks of young nuts cause shrivelling of nuts leading to premature nut falls. Mostly young coconut trees of up to 10-15 years are vulnerable to damage.

Scales may infest palms throughout the year, but damage is usually more severe during the dry season. Neglected plantations are particularly susceptible.

The coconut scale also attacks many species of fruit trees, such as avocado, breadfruit, mango, guava and papaya. Other major hosts are cocoa, cassava, cotton, oil palm, papaya, rubber, sugarcane and tea. It also attacks a range of ornamental plants including roses.

Termites can be a serious problem for coconut palms, particularly for young trees in tree nurseries or trees that have just been transplanted into the field. Termites live in the soil in hills, construct tunnel from the hills to the palms and feed on all parts of young coconut palms. Damage occurs mainly during the dry season.

Many plants have a repellent or even insecticidal effect on termites and can be applied as spray directly against termites or as barrier around the trunk of coconut palms:

• Neem: seeds and leaves (extracts or cake)
• Quassia indica or Q. amara: bark and leaves
• Sandalwood: wood
• Chinaberry, persian lilac (Melia azedarach): bark, branches, leaves, extracts

Euphorbia sp.: "In Tanzania, especially in Dar es Salaam and coast regions, farmers plant with each seedling 2 or 3 sticks of Euphorbia sp. to protect the young palms from termite attack. On nurseries the Euphorbia plants can be planted around the seedbed to prevent access of termites to the palm seedlings and young palms. When the palms have grown big the euphorbia is simply cut down. The practice is widespread and farmers are convinced that it really does keep termites off the susceptible seedlings (Z. Seguni, MARI personal communication)".