Biological control of citrus psylla
Dr. Faisal Hafeez, Afifa Naeem, Muhammad Arslan Ibrahim, Hafiz Muhammad Zahid Anwar
Pakistan is the 6th highest producer of Kinnow (mandarin) and oranges in the world, with 2.1 million tons’ production. Different cultivars of citrus are grown in tropical and sub-tropical atmosphere of the country. Although It is grown in all four provinces of Pakistan but Punjab, delivers more than 95% of the yield considering its more productivity, best growing conditions and satisfactory water. The area under citrus cultivation to Punjab is 4,80,000 acres. The areas which are considered best for citrus are Sargodha district, Layallpur and Peshawar valleys. Citrus basically includes Sweet oranges, Mandarin, Grape fruit, Lemon and Lime which are being grown monetarily. Commercially it is exported to Gulf countries(Saudi Arabia, UAE, Behrain) and outside the asia, thus major source of revenue. These fruits are the source of nutrients too, phosphorous, iron, vitamin A,C and calcium are the main companents.
When we talk about the limitations in citriculture, diseases and pest attack, alternate bearing, underdeveloped citrus industry, lack of information to the growers, involvement of middle man, poor management during harvesting, storing and transportation, are prominent issues. These pre-and post- harvesting problems effect directly both quality and quantity of the produce.
The aspect of diseases and insect pest attack is as important to be discussed as other problems mentioned above. Since, a considerable amount of produce get wasted annually due to insect pest attack. The important insect pest of citrus includes black flies, citrus psylla, leaf miner, citrus aphid, lemon butterfly and citrus mealy bug. Citrus psylla (Diaphorina citri) (Homoptera: Psyllidae) is the most infamous pest of citrus. It harms the plant in two ways. On the first hand, adult and immatures both sucks the cell sap and deplete the useful nutrients in plant leaving the plant helpless against different insects and infections. Additionally, they secrete honeydew on which sooty-mould develops. Sooty-mould diminishes the photosynthetic area of leaves which directly influences the food production process. On the second hand, it acts as a vector, it carries bacteria which is responsible for citrus greening disease or Huanglongbing(HLB) disease. The HLB infected trees exhibit symptoms that include chlorosis of the leaves, excessive fruit drop, misshapen, quality deterioration and some times tree’s death. Although, this insect can be control by employing following insecticides as: chlorpyrifos, cypermethrin, imidacloprid, bifenthrin. Since, continuous application of these insecticides has been brought resistance in the insect pests and these are source of several kinds of pollution too. Hence, integrated pest management is the best option to control this pest. Along with other practices, biological manipulation of the pest is basic principle of IPM. In natural control lady bird beetle (predator), Tamarixia radiata (parasitoid) and Diaphorencyrtus aligarhensis (parasitoid) are the natural enemies of psyllids. Current literature representing the importance of parasitoids and suppression of the pest naturally.
Tamarixia radiata (Hymenoptera: Eulophidae):
In Asia, T. radiata is well known parasitoid of Asian citrus psylla, thus more willing for biological control. The mass rearing of this natural enemy is required since it brings about the effective control of psylla. A single female of T. radiata having the potential to kill over 500 psyllids through feeding and parasitism mechanism. Female also feeds on the honeydew secretions.
Identification:
Adults:
Tamarixia radiata adults are very small black wasps ranging from 0.92 to 1.04 mm in size with widely spaced eyes. The adult’s head width is greater than length. The wings are hyaline in nature and having pale yellow veins. Sexual dimorphism also exists between the male and female adults. Male have 1.5 times longer antennae than the female’s. With the reference of length and wing expanse, males are slightly smaller than females.
Eggs:
The adult female lays one or rarely two eggs beneath a Diaphorina citri nymph. Sometimes more than one egg may be laid in that case only one parasitoid larva will reach to the adult stage and so that’s why it is regarded as a solitary parasitoid. An adult female can deposit up to 300 eggs during its life cycle.
Larvae:
The newly emerged parasitoid larvae feed on haemolymph usually from the underside of thorax, ultimately cause killing of the hosts i.e., nymphs of Diaphorina citri. The size of first instar parasitoid larvae is about 0.28 mm long and it is 0.11 mm wide, the fourth instar larvae are about 1.14 mm long and 0.59 mm wide.
Pupa:
Pupation usually occurs in the mummified Diaphorina citri nymphs and new adult emerges out from the hole on the thorax or head of the parasitized mummy.
Hosts:
Tamarixia radiata is not reported to attack any other psyllid species than Diaphorina citri.
Life Cycle:
The female usually give a single egg on the ventral side of D. citri nymph, hardly 2 eggs, mostly within groove between thorax and abdomen, very close to the point of attachment of hind coxa. The emerged larva sucks fluid from the site of emergence and later move towards the neighbouring sites consumes more body contents and gets mature there. The parasitized nymph remains unidentifiable for several days until almost all body contents of the parasitized nymph are consumed and just wing buds and dorsal coverings left behind. When larva got matured it attaches itself to the leaf or shoot surface with the help of fine anchoring silken threads which are later visible as mummified nymph. Pupation takes place below the mummified nymph and the adult emerges out after making a chewed circular exit hole in the integument of the thoracic region.
Diaphorencyrtus aligarhensis (Hymenoptera: Encyrtidae)
This parasitoid is another important natural enemy which having the potential to kill 280 D. citri nymphs through feeding and parasitism.
Identification:
Adults:
Adult wasps have black head and thorax and are small (~ 1-1.5 mm) in size with yellow legs and antennae. Sexual dimorphism also exists between the male and female based on antennae and abdomen. Male antennae are slightly longer with short hairs and lack clubs while female’s antennae are smooth and clubbed. Female has a large, rounded yellow abdomen with a black posterior while male abdomens are smaller, solid black, and more cylindrical.
Eggs:
Eggs are oval, with an average length of 130 ± 10 µm. Newly eggs are milky white but begin to be transparent with the passage of time even embryo development can be seen.
Larvae:
Larva is eruciform with a weakly developed head capsule. Larva do not have hairs or spiracles and has short, small, reddish-orange mandibles that are approximately 20% of the total body width. First instar larva is approximately 410 ± 7 µm long increase in size to 1190 ± 10 by the fourth larval instar. Larva prior to pupation shortens its length by constriction.
Pupa:
Pupating larva begins development process by making head and abdomen first, followed by legs and mouthparts. Very next, the thorax, eyes, and ocelli begin melanisation, followed by the abdomen, legs, and lastly the antennae.
Life Cycle:
- aligarhensis usually reproduces itself parthenogenetically, which results in all female offspring. Shafee et al. (1975) reported very small population of males from the representing samples of Asia. Female D. aligarhensis oviposit only a single egg into the host, usually through her abdomen. In case of super-parasitism like if two wasps lay an egg in the same host nymph, only a single will complete its development to become an adult. D. aligarhensis is considered as koinobiont parasitoid because host D. citri nymphs continue to feed adulthood of natural enemy. Adult female prefers to parasitize the second instar D. citri nymph.
Hosts:
It is not reported yet on any other host than D. citri.
Conservation:
Bio control through natural rivals brings very effective control. Proper infrastructure, awareness program and technology management is needed to make it successful formal control measure. The problems of insecticide resistance and pollution in natural biomes can be minimized by employing such control measures. The conservation of natural enemies is as important as its introduction at a place. Habitat manipulation, overwintering sites, use of safer insecticides and ground covers are important parameters which one should manage to conserve the natural enemies.
Ayub Agriculture Research Institute, Faisalabad, Pakistan
