Learn about the pests that damage citrus fruits and its control.
1. Citrus Psylla, Diaphorina Citri Kuwayama (Hemiptera: Aphaiaridae):
The citrus psylla is distributed throughout the Orient and has been reported from India, China, Formosa, Japan, Myanmar, Sri Lanka, the East Indies and New Guinea. In India, it has been recorded on all species of citrus and a number of other plants of the family Rutaceae. It is the most destructive of all the citrus pests.
Damage is caused by nymphs and adults. The adult is small, 3 mm in length, active in habits, and rests on the leaf surface with closed wings, the tail end of the body being turned upwards. The insect is brown with its head lighter brown and pointed.
The wings are membranous, semi-transparent, with a brown band in the apical half of the fore wings. The hind wings are shorter and thinner than the fore wings. The nymphs are flat, louse-like and orange yellow creatures, and are seen congregated in large numbers on young leaves and buds.
Life-Cycle:
The pest is active throughout the year but its life-cycle is greatly prolonged in the winter. Only adults are to be found when it is very cold. They resume breeding in February-March and lay, on an average, 500 almond-shaped, orange and stalked eggs on tender leaves and shoots of citrus trees. The lower end of the egg stalk is embedded in plant tissue.
The eggs are laid either singly or in groups of two or three and are arranged in straight line, there being as many as 50 eggs in one place. The eggs hatch in 10-20 days in winter and 4-6 days in summer. On emerging, the light- yellow nymphs have a tendency to stick close to the egg-shell.
They are found congregated on young half-open leaves, but in the later stages of development they may also migrate to the older leaves. There are five nymphal stages and the development is completed in 10-11 days from April to September, 15-20 days in the spring and autumn, and 34-36 days in December-January.
Natural mortality among nymphs varies in different seasons and during the greater part of the year, 20-30 per cent of nymphs die a natural death. However, in December and January, the mortality may be as high as 58 per cent. The most favourable conditions for development are found in March, when there is only about 4 per cent mortality. When the nymphs are full-grown they migrate to the lower surface of leaves, where they change into adults.
The adults copulate 4-8 days after emergence and the females start laying eggs immediately afterwards. There is a great variation in the longevity of adults in various seasons. The females live longer than males, and the duration may be as long as 190 days in winter and only 12-26 days in the summer.
During the hot period of May-June, the adults are predominant although other stages may also be found. In July, they start reproducing once again, there being a second lower peak of population in August. Towards autumn, there is again a decline in the population. In summer, the life-cycle is completed in 14-17 days. There are 8-9 overlapping generations in a year.
Tetrastichus radiatus Waterston (Eulophidae) is an important parasitoid of nymphs and is distributed in all the citrus grooves. A number of lady bird beetles, viz., Coccinella septempunctata Linnaeus, C. transversalis Fabricius, Menochilus sexmaculatus (Fabricius), Chilocorus nigrita (Fabricius) and Brumoides suturalis (Fabricius) (Coccinellidae), and Chrysoperla carnea (Stephens) (Chrysopidae) larvae also feed on nymphs.
Damage:
This is the most destructive and, consequently, the most important of all the insect pests of citrus. Although there is a visible difference in the rise and fall of its population in various seasons, yet the ill-effects of its damage are so long-lasting that the trees may look sickly even when the population is not high. Thus, sooty and sickly plants seen in the winter are the victims of insects which caused damage during the previous summer.
Only the nymphs are harmful to the plants. With the help of their sharp, piercing mouthparts, they suck the cell-sap in millions. The vitality of the plants deteriorates, and the young leaves and twigs stop growing further. The leaf-buds, flower buds and leaves may wilt and die. Whatever little fruit is formed in the spring falls off prematurely. Moreover, the nymphs secrete drops of a sweet thick fluid on which a black fungus develops, adversely affecting photosynthesis.
It is also thought that the insect produces a toxic substance in the plants as a result of which the fruits remain undersized and poor in juice and insipid in taste. This insect is also responsible for spreading the citrus greening disease. If the pest is not checked in time, the entire orchard may be lost, and after a year or two of continued damage, the plants may be killed.
Control:
(i) Pest population may be reduced to a great extent by conserving the natural enemies which are quite abundant in citrus ecosystem,
(ii) Spray 400 g of thiamethoxam 25WG or 500 ml of imidacloprid 17.8SL or 3.125 litres of dimethoate 30EC or 1.5 litres of monocrotophos 36SL or 2.5 litres of oxydemeton methyl 25EC or 250 ml of imidacloprid 200SL in 1250 litres of water per ha during March with the appearance of the pest and again in the first week of September.
2. Citrus Whitefly, Dialeurodes Citri (Ashmead) (Hemiptera: Aleyrodidae):
This insect is often found in association with other allied species of whiteflies, namely Dialeurodes citrifolii (Morgan), Aleurocanthus husani (Corbitt), A. spiniferus (Quaintance), etc. The pest is very widely distributed in America, India, Pakistan, Vietnam and China.
Although it is a pest of citrus, the insect prefers to feed on certain deciduous plants such as persimmon and dharek (Melia azedarach). It is often seen on these trees in very large numbers but perishes in the autumn when the leaves are shed. It is said that probably the adults migrate to deciduous plants from evergreens, and that perhaps there is no reverse migration.
Damage is caused by adults as well as by nymphs. The adult is a minute insect, measuring 1.02- 1.52 mm, the males being smaller than the females. The antennae of this insect are six-jointed, the basal joint being thick and stout, and the last segment ending in a sharp point. The eyes are transparent, red and kidney-shaped, with the lower half covered over with bristles. The head is somewhat pointed.
The wings are more than twice the length of the body and extend beyond the tip of the abdomen. Both the wings and body are completely covered with a white waxy powder. The nymph is pale yellow, with purple eyes and its body is marginally fringed with bristles. It is distinguished from the pupa which is broadly oval, pale yellow, with an orange or yellow band in the middle of the body.
Life-Cycle:
The pest is active practically all the year round, but the pupae are generally found from October to February and in June-July. In February, the adults emerge from the pupae. They lay eggs singly on the underside of soft young leaves. The eggs are oval, pale yellow and rest on small stalks.
A single female may lay 200 or more eggs during its life which may last from 7 to 10 days. When there is severe infestation, there may be as many as 2,000 eggs on a single leaf. The greatest number of eggs is found in March-April and again in August-September. It may take 10-20 days for the eggs to hatch.
The young larva, on emergence, crawls about for a few hours and then inserts its proboscis into the succulent portion of a twig. The larvae remain there in a stationary state and transform themselves into pupae at the same spot. A nymph is full -fed in 25-71 days and then changes into a pupa. The pupal stage is of the longest duration during the hottest part of summer and the coldest part of the winter, when it lasts 114-159 days.
The adults, on emergence, avoid bright sunlight and settle on the underside of leaves, preferring the north side of a tree. They are quite common in spring and early autumn. From March to August, all stages of the pest are found. In Punjab, there are probably two generations in a year. However, in Japan, where the climate in much milder, three generations a year have been reported.
Encarsia lahorensis (Howard) and E. citufila Silv. (Aphelinidae) are the major parasitoids on this insect. A number of predators are recorded feeding on the eggs as well as nymphs of this insect, viz., Brumoides suturalis (Fabricius), Cryptognatha flavescens Motsch, Verania cardoni Weire; a predatory thrips, Aleurothrips fasciatipennis and a lacewing, Chiysopa sp.
Damage:
The pest causes damage in the larval and adult stages. It sucks the cell-sap from leaves which curl over and fall off. The honeydew excreted by the nymphs is a very good medium for the growth of a sooty mould, which interferes with photosynthesis. Thus, the trees infested with this pest deteriorate further. It has been noticed in California that a heavy infestation of whitefly is apt to be followed by an increase in the red scale of citrus, because the young scales collect under the powdery wax of whitefly for protection against bright light.
Control:
(i) For effective management of flies, close planting, water logging or any other stress condition should be avoided,
(ii) In case of localised infestation, the affected shoots should be clipped off and destroyed,
(iii) Excessive irrigation and application of nitrogen and pesticidal sprays should be avoided,
(iv) Spray 2.5 litres of ethion 50EC or 3.125 litres of triazophos 40EC in 1250 litres of water/ha during April-May and again during September-October.
3. Citrus Blackfly, Aleuracanthus Woglumi Ashby (Hemiptera: Aleyrodidae):
The citrus blackfly has been reported from India, Sikkim, Sri Lanka, the Philippines, Jamaica, Kingston, Cuba and Bahamas. In the nymphal stage particularly, it is a serious pest of citrus fruits, especially the sweet orange, Morinda tinctoria and malta, Murraya koeniglii. In addition, avocado, grapevine, mango, guava, pear, plum, etc. are also attacked.
The adult fly is dark orange with smoky wings and fore wings having four whitish areas of irregular shape. The female flies are about 1.2 mm long and the males are 0.8 mm in length. The nymphs are scale-like, shiny black and spiny, and are bearded by a white-fringe of wax.
Life-Cycle:
The adults emerge in March-April and the females lay yellowish brown oval shaped eggs which are arranged in a spiral on broad leaves. There may be 15-22 eggs in a cluster. The eggs hatch in 7-14 days and the nymphs on emergence start feeding on cell-sap. They pass through four nymphal instars and the nymphal stage is completed in 38-60 days.
They pupate on the leaf surface and this stage lasts 100-131 days. The pupa is oval, black in colour and its dorsum is arched with long black spines, and the margins have rounded black teeth. There are two distinct broods in a year. The first brood adults emerge in March-April and those of the second brood emerge in July-October.
The hymenopterous parasites recorded on this pest are Encarsia divergens Silv, E. merceti Silv. and Eretmocerus serius Silo. (Aphelinidae).
Damage:
Both adults and the nymphs suck plant sap, reducing the vitality of trees. It results in the curling of leaves and also the premature fall of flower buds and the developing fruits.
4. Citrus Red Scale, Aonidiella Aurantii (Maskell) (Hemiptera: Diaspididae):
Citrus plants and fruits in various countries of the world are known to be attacked by three species of armoured scales, viz. Aonidiella aurantii, A. citrina (Coquillett) and Chrysomphalus dictyospermi (Morgan). Of these, the red scale (A. aurantii) is the most destructive and it also feeds on Acacia, Eucalyptus, fig, grape, rose, willow, shisham (Dalbergia sissoo) and many other plants.
The scales infest all the above ground parts of the tree. The leaves, branches and fruits may be covered with them. These scales are 2 mm in diameter and have a distinct central exuviae.
Life-Cycle:
The pest is active throughout the year, but its attack is maximum during autumn. The female scales, instead of laying eggs, give birth to young nymphs. The first stage nymphs or crawlers have well developed legs and antennae, and move about for an hour or so before settling down and cover themselves with a white waxy secretion.
The female scales moult two times at 10- 20 days intervals, lose their legs and antennae, and incorporate their cast skins into waxy coverings, which become circular, depressed scales. They reach sexual maturity in 10-15 weeks, do not acquire wings and may live for several months. The male scales, on the other hand, are elongated and develop into winged adults in 1-2 months. They fly around and fertilize the sedentary females. This pest passes through a number of broods in a year.
Aphelinus sp. (Aphelinidae) parasitizes a high percentage of scales. The coccinellids, Chilocorus nigrita (Fabricius) and Scymnus quadrillum are recorded as predators on this scale throughout India.
Damage:
Injury to the infested plants is two-fold; first the scale insects feed on plant juice and devitalize the plants; second, they inject a toxic substance into the plant sap. Yellow spots appear at the point of feeding which may be on the leaves, twigs or fruits. When there is a severe infestation, all the leaves may turn pale. It is not practicable to detach all the scales from infested fruits before marketing and, therefore, their value is lowered.
5. Cottony Cushion Scale, Icerya Purchasi Maskell (Hemiptera: Margarodidae):
The cottony cushion scale is a serious pest of citrus plants in certain tropical and sub-tropical regions of the world. It is a native of Australia, where it lives mostly on Acacia spp. In India, it is present in southern States, mostly on wild plants such as wattle, casurina and gorse. It has also been recorded damaging apple, almond, walnut, peach, apricot, fig, grapevine, guava, pomegranate, etc.
The adult female is a flat oval (4.5 × 3.5 mm), brown to reddish-brown, soft bodied scale. The most conspicuous part of the insect is the large, white, fluted egg-sac which is secreted by the female. The full-grown larva is broadly oval, 3.0 × 1.5 mm, reddish-brown to brick-red in colour.
Life-Cycle:
This insect pest is active throughout the year but during the dry-hot months, its multiplication is the maximum. The males are rare and the scale generally reproduces parthenogenetically. The female lays up to 700 eggs in the ovisac held behind the body.
The reddish coloured young nymphs emerge from the eggs within 24 hours during the hot season, but may take several weeks when the weather is cold. The newly hatched nymphs move some distance before fixing themselves on leaves and twigs for feeding. They become adult females after moulting three times. The life-cycle is completed in 46-240 days, depending upon the different environmental conditions.
In south India, this pest was brought under check by the introduction and periodic releases of a large number of predaceous beetles, Rodolia cardinalis (Mulsant) (Coccinellidae) and by banning the movement of wattle from Nilgiris and Kodai Kanal.
Damage:
As a result of severe feeding by scales, the leaves and twigs firstly turn pale and then fall prematurely. Sometimes, the heavily infested young shoots and small nursery plants are also killed.
6. Citrus Mealybug, Pseudococcus Filamencosus Cockerell (Hemiptera: Pseudococcidae):
In appearance, habits and habitat the fluffy mealy bug, P. filamentosus resembles other allied species, namely P. citriculus Green found in China, and USA; P. calceolariae (Maskell) in California and Planococcus citri (Risso) found in Malaysia and Indonesia. P. filamentosus is more common in southern India than in the north. The mealy-bugs are known to feed on a number of plants, often not closely related to citrus. In the gardens, they are seen on Cactus spp., ferns, begonia, gardenia, poinsettia and other flowers.
Damage is caused by nymphs and females. The adult female is wingless, with a flattened body and short, waxy filaments along the margins. It has piercing and sucking mouthparts. The male is winged, midge-like, with long antennae and no mouthparts; consequently, it does not feed. The nymphs are amber with a whitish waxy coating and filaments.
Life-Cycle:
The females lay eggs in clusters on citrus plants, which are found in protective cotton-like masses. There may be 300 eggs or more in one mass. The eggs hatch in 10-20 days and the nymphs crawl out and start feeding by inserting their mouthparts in the lower surface of the leaves. A waxy white covering is soon formed on their bodies.
A female nymph is full-grown in 6- 8 weeks. The male nymphs spin cotton-like cocoons, two or three weeks after hatching, and pupate before transforming themselves into winged adults. Mealy bugs are usually found on the underside of the leaves in clusters. Since they multiply rapidly, all stages of development may be present at the same time.
Damage:
The insects feed on cell-sap and the plants become pale, wilted and the affected parts eventually die. The insects also excrete honeydew on which a black mould grows, which interferes with photosynthesis. Black ants are attracted to the honeydew and they become a nuisance. In severe cases of infestation, the citrus flowers do not set fruit.
Control:
(i) For effective management of these coccids, orchard sanitation is extremely important. Weeds act as additional hosts and these must be removed.
(ii) The infested shoots should be pruned and destroyed.
(iii) The ant colonies should be destroyed by ploughing the soil around trees and by application of quinalphos or carbaryl dust.
(iv) The coccinellid beetle, Cryptolaemus montrouzieri Mulsant, should be released @ 10 beetles per plant, to control mealy-bugs. Inoculative releases of an exotic parasite, Leptomastix dactylopii Howard, are also recommended.
(v) Spray 4.5 litres of chlorpyriphos 20EC in 1250 litres of water/ha. Repeat the spray after 15 days.
7. Citrus Caterpillar, Papilio Demoleus Linnaeus (Lepidoptera: Papilionidae):
The citrus caterpillar or the lemon butterfly is found in Africa, greater part of Asia as far as Formosa and Japan. It has been reported all over India and causes very severe damage. It can feed and breed on all varieties of cultivated or wild citrus and various other species of the family Rutaceae.
Only the caterpillars cause damage by eating the leaves. The full-gown caterpillar is yellowish green, has a horn-like structure on the dorsal side of the last body segment and, is 40 mm long and 6.5 mm wide. The adult is a large beautiful butterfly, 28 mm in length and 94 mm in wing expanse.
Its head and thorax are black, there being creamy-yellow coloration on the underside of the abdomen. Its wings are dull-black, ornamented with yellow markings. The general coloration on the underside of the wings is slightly paler and the markings are also larger. The antennae are black and have club like structures at their ends.
Life-Cycle:
In the plains, this pest is found throughout the year, whereas in the mountains where winter is very severe, it hibernates in the pupal stage. The butterflies appear in March and lay eggs on tender shoots and fresh leaves, mostly on the undersurface. The eggs are placed singly or in groups of 2-5. Sometimes, the eggs are also noticed on thorns of citrus or on other plants.
The eggs are glued firmly on to the surface of the leaf and are pale or greenish yellow, when freshly laid, but later turn brown, becoming dark grey just before hatching. They hatch in 3-4 days during summer and in 5-8 days during winter. The young larva emerges by cutting a round hole through the eggshell, which forms the first food of the larva. This scavenging habit persists throughout life and the larvae eat their own exuviae after each moulting.
The larval life lasts 8-16 days in the summer and about 4 weeks during November-December. The larvae show preference for young and shiny leaves of citrus. After making a full meal, they remain motionless while exposed, usually near the mid-rib. The black or brown and white markings make the larvae look like bird droppings.
In addition to its protective coloration, the caterpillar is also equipped with two reddish sacs posterior to the head and they are extended as a sign of danger, discharging a repelling fluid. When full-grown, the larvae have a tendency to crawl away from the host plant, and it is rare that they pupate on the plant, on which they had been feeding. The mature larva spins a supporting girdle around its body and pupates on a twig, a dry stick or any other raised structure.
The pupal stage lasts about 8 days in the summer, and 9-11 days in the spring and the autumn. The butterflies usually appear in the morning and begin to fly within one to three hours of emergence. A female usually mates once and lays, on an average, 75-120 eggs within 2-5 days. A smaller number of eggs are laid in the summer. A male lives for three or four days whereas a female lives for about a week.
The pest passes through three or four generations in a year. The butterflies that appear in March give rise to larvae which are seen in abundance towards the end of April. During the intense heat of June, the population gets a setback, but there is fresh egg-laying in July and the numbers increase in August.
The pest is most active in September and after the butterflies have laid eggs, there is again a preponderance of caterpillars in October. These larvae pupate in November and may enter hibernation in cold regions. Since there are over-lapping generations, all stages of the pest are found throughout the year, except in severe winter when only pupae may be found. In the laboratory, ten generations have been reared in a year.
The egg parasitoids associated with this pest are Trichogramma evanescens Westwood (Trichogrammatidae), Pteromalus luzonensis (Pteromalidae) and Telenomus sp. (Scelionidae), while larvae are parasitized by Erycia nymphalidaephaga Bar., Charops sp. (Ichneumonidae) and Brachymeria sp. (Chalcididae).
Damage:
The young larvae feed only on fresh leaves and terminal shoots. Habitually, they feed from the margin inwards to the midrib. In later stages, they feed even on mature leaves and sometimes the entire plant may be defoliated. The pest is particularly devastating in nurseries and its damage to foliage seems to synchronize with fresh growth of citrus plants in April and August- September. Heavily attacked plants bear no fruits.
Control:
(i) Hand picking of various stages of the pest and their destruction especially in nurseries and new orchards helps to suppress the population of the pest,
(ii) Spraying of entomogenous fungus, Bacillus thuringiensis Berliner, nematode DD-136 strain or neem seed extract (3%) also gives quite high motality of caterpillars.
(iii) In severe infesation, spray 1.5 litres of quinalphos 25EC or 2.0 kg of carbaryl 50WP in 1250 litres of water per ha during April (after fruit set) and October (after rainy season).
8. Citrus Leafminer, Phyllocnistis Citrella Stainton (Lepidoptera: Phyllocnistidae):
This insect is widely distributed in the Orient, northern Australia and India, and is known to be a serious pest of citrus nurseries in Tamil Nadu, Madhya Pradesh, Assam, Uttar Pradesh, Punjab and Pakistan. Apart from citrus, the insect also feeds on a variety of other plants such as pomelo, willow, cinnamon and Loranthus spp.
Only the larvae cause damage by making zig-zag silvery mines in young leaves. The full- grown larva measures 5.1 mm in length and is pale yellow or pale green with light-brown well developed mandibles. The adult is a tiny moth, measuring 4.2 mm across the wings. On the front wings there are brown stripes and prominent black spots along the tips. The hind wings are pure white and both pairs are fringed with hairs.
Life-Cycle:
The pest is active throughout the year and breeds on young growth. The duration of various stages depends upon the prevalent temperature. Late in spring or summer, development may be five or six times as fast as in autumn or early spring. The moths lay minute, flattened, transparent eggs on young leaves or tender shoots, usually on the lower surface, particularly near the midrib.
The eggs which are laid singly, generally two or three per leaf, hatch in 2-10 days, giving rise to legless larvae. Soon after emerging, the larvae mine into the leaf tissue and form galleries within which they remain confined for the rest of their immature life. Larvae are full-fed in 5-30 days, and when mature, they settle down in enlargements of the galleries near the leaf margin. By the time they spin cocoons for pupation; the leaves get twisted or folded over.
In body form, the pupa is not much different from the larva, but turns slightly brownish. It is partly exposed through the gallery wall and has a spine on its head with the help of which it pierces through the wall as it emerges as a moth. The pupal stage lasts 5-25 days and the moths are commonly seen resting on the trunks of the trees near the ground. The life-cycle is completed in 12-55 days and several overlapping generations are produced in a year.
The larvae are parasitized by Cirrospiloideus phyllocnistoides (Narayan), Scotolinx quadristriata Rao & Rama and Eurytoma sp. (Eurytomidae). The parasitization exceeds 30 per cent during September-October.
Damage:
Damage by this mining pest is serious on young leaves. The injured epidermis takes the shape of twisted silvery galleries. On older leaves, brownish patches are formed which serve as focii of infection for citrus canker. The attacked leaves remain on the plants for a considerably long time and the damage gradually spreads to fresh leaves.
Heavily attacked plants can be spotted from a distance and young nurseries are most severely affected; the young plants of orange and grapefruit may not even survive. In larger trees, the photosynthesis is adversely affected, vitality is reduced and there is an appreciable reduction in yield.
Control:
(i) Spray of 2 per cent neem seed extract has been found quite effective and safe.
(ii) Spray 400 g of thiamethoxam 25WG or 500 ml of imidacloprid 17.8SL or 1.25 litres of fenvalerate 20EC or 2.5 litres of cypermethrin 10EC or 3.125 litres of triazophos 40EC or 4.5 litres of chlorpyriphos 20EC or 500 ml of imidacloprid 200SL in 1250 litres of water per ha during April-May and August- September. Synthetic pyrethroids should be avoided on full-grown trees.