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Sci Tech
Thrips in cashew
Heavily infested trees exhibit sickly and faded appearance which can be recognised even from a distance.
CASHEW (Anacardium occidentale), a high income nut crop is vulnerable to infestation and damaged by more than sixty species of insect-pests throughout its growth stages.
Among the suckling pests, thrips pose a major problem. About six species of thrips attack cashew in India.
Among the species of thrips, the flower thrips Scirtothrips dorsalis is a serious pest in cashew. The adults are minute, about 1 mm long, yellowish brown insects with fringed wings. They reproduce parthenogenitically.
Each female has a reproductive capacity of laying 30-50 eggs over a period of 4-5 weeks. The eggs are laid on the lower surface of leaves by inserting them singly in the tissues along the mid rib of tender leaves. Eggs are reniform (kidney shaped) and hyaline when laid but turn pale yellow just before hatching. The eggs hatch in about 4-6 days.
The nymphs are pale yellow and wingless. They moult 2-3 times passing through 3-4 instars in 12-18 days according to the prevailing temperature. Full grown nymphs seek sheltered places and then pass through two resting stages called pre-pupa and pupa. Pupal period lasts for 6-10 days. Since, they have shorter life span, the overlapping generation of the thrips cause serious damage to cashew.
The insects congregate abundantly on lower surface of leaves, emerging shoots and inflorescence. Rarely, are they found on the upper leaf surface also. Nymphs feed in company with the adults by lacerating the tender tissues and suck the oozing sap.
Due to sustained feeding by large number of thrips, the terminal leaves begin to curl downwards from the margin towards mid-rib. In due course, the young leaves fall from the plant.
Heavily infested trees exhibit sickly and faded appearance which can be recognised even from a distance. Young trees suffer comparatively more than the matured trees during summer months.
Affected nuts are unfit for consumption.
Severe drought coupled with reduced moisture content in leaves favour thrips development at a faster rate. Its population flourishes with increase in the temperature during hot weather season.
With the commencement of rains, the population of thrips decline.
In affected trees, spraying of water twice or thrice during thrips out break helps to reduce its population load appreciably.
In cases of severity, spray the infested trees with monocrotophos 36 WSC (0.05 per cent) (1.5 ml/litre of water) or endosulfan 35 EC 0.05 per cent (1.5 ml/litre) or quinalphos 25 EC 0.05 per cent (2.0 ml/litre) alternatively on need basis.
V. Ambethgar, V. Lakshmanan and S. E. Naina Mohammed
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