Thrips in Cotton Crop

Thrips in Cotton

Common Name: Thrips

Local Name: Phulkeede

Scientific Name: Thrips tabaci Linde.

Family: Thripidae

Order: Thysanoptera

Pest Category: Sap feeder

Description of Insect Stages:

Life Cycle of Thrips in Cotton

Egg:

Eggs are minute, kidney-shaped laid in slits in leaf tissues.

Nymph:

Nymphs are creamy to pale yellow in color, resemble adults but wingless.

Adult:

Adult Adults are straw-colored, yellowish band elongated measuring 1mm in length adults are slender and lice like Antennae have seven segments with the first segment paler the second which is usually dark. A brown band marks the anterior edge of the abdominal tergites. There is a single pair of pores on tergite nine.

Bionomics:

Adults small, slender, yellowish to brown with fringed wings and drift away on disturbance. Nymph very minute, slender, yellowish and microscopic.

ETL : 1 No. /leaf

Growth Stages of Thrips on Cotton

Nature of Damage:

Nymphs and adults lacerate the tissue and suck the sap from the upper and lower surfaces of leaves. They inject saliva and suck the lysed contents of plant cells resulting in silvery or brown necrotic spots.

Both nymphs and adults lacerate the underside of leaf tissues and imbibe the oozing sap.
These lacerate the epidermal cells and suck the sap.

Symptoms:

Seedlings infected with thrips grow slow and the leaves become wrinkled, curl upwards and distorted with white shiny patches. Rusty appearance in patches develops on the undersurface of leaves. Thrips infested crop in a field present rusty appearance from a distance. Higher infestation during vegetative crop growth results in late bud formation. During the fruiting phase, there is premature dropping of squares, and the crop maturity is delayed combined with yield reduction. The feeding by thrips on the developing bolls late in the season causes spots or wounds on the pericarp but that does not affect the ripening of the boll or the quality of the seed.

Leaves with white specks initially which later coalesce and result in silvery shining on the surface along the veins of leaf and leaf curl up.
The silvery undersurface of leaves
Leaves lose the natural luster
Leaf loses its succulence and at the lower surface, bronzing is seen.
Leaves appear sickly and may show uneven surfaces.

Life History:

Thrips thrive on the weeds during the offseason and migrate to cotton as soon as the seedlings emerge above ground. Males are rare and the reproduction is parthenogenetic. Eggs hatch in 5 days time, nymphal and pupal period lasts for 5 and 4-6 days, respectively. The preimaginal stage is spent on soil without feeding. The adults survive for 2-4 weeks. The life cycle of T.tabaci from egg to adult lasts or 13-19 days and they have about 15 overlapping generations per year including their development on wild plants. Thrips inhabit on leaves of cotton up to the midseason and colonize on bolls during the late season.

Effect of Thrips Symptoms in Cotton

Seasonal Dynamics:

T.tabaci occurs alongside jassids on cotton with no competition and their association is mutually exclusive. However, thrips have a negative association with the aphids that are aggregate colonizers. Population density varies highly in relation to crop growth and weather. In the last five cotton seasons, the mean seasonal incidence has been on the rise but for 2005. Direct relation exists between the weediness of the field and the population of T.tabaci. In cotton fields of Central, India T.tabaci has population peaks during July-August months modulated by the rainfall distribution and the dry spells with higher temperatures. Depending upon the seasonal conditions, the number of generations ranges between four and seven. Temperatures of 35°C are much more favorable for the development of T.tabaci. Relative humidity of 54 % or less is optimal for population development. Frequent rains and prolonged cool weather reduces the population of thrips drastically. Determination of the level of thrips infestation on the crop should be qualitative, based on leaf damage as too low, medium and high instead of counting. Presence of thrips with no symptoms, shiny white oily patches on leaves above mid-canopy vis-a-vis activity of thrips on the terminal leaves and upward cupping, light to severe browning and stiffness of leaves above mid-canopy constitute low, medium and high injury categories, respectively. Sampling to determine the extent of thrips infestation should be carried out in the early morning.

Pest Management Options:

Management of thrips should be attempted with a view to maintaining optimum plant stand during the very early crop growth stage. Insecticides of neonicotinoid group such as Imidacloprid and Thiamethoxam applied as a seed treatment is efficient in suppressing the population of other sucking pests on cotton seedlings. However pre-sowing seed treatment with systemic insecticides leads to a higher attack by thrips during the pre-flowering stage when the weather conditions are conducive for the pest. Therefore, keeping a close watch of crop growth and weather conditions, post-emergence of sucking pest management Should be done on a need basis. Even if the population of the thrips is high, insecticidal sprays are of no use when there is a likelihood of rain in a day or two that causes natural reduction of thrips. Maintaining weed-free conditions in cotton fields from the beginning of crop growth reduces the spread of the development of thrips. Insecticidal options should be made when thrips infestation results in high-grade injury during clear sky periods with no anticipated rains. A spray of 5% neem seed kernel extract prepared on-farm or crude neem oil spray @ 1% suppresses thrips population during pre squaring crop stage. In both cases, detergent/soap powder @ 1 gm/liter of the spray fluid is to be added for getting uniform spray suspension. Chemical insecticides such as Imidacloprid 200 SL @ 0.5 ml/lit and Thiamethoxam 25 WG@ 1 1.5 gm/lit of water can be used only when there is a high degree of symptoms, indicative of high thrips population. The conventional systemic insecticides should be alternated if more than one spray of systemic insecticides is to be sprayed. Sprays should be undertaken when the population of adults is more. Systemic insecticides are effective for 2-3 weeks Thrips infestation during reproductive crop growth can be managed using the non-systemic conventional insecticides like Endosulfan @ two ml/ lit of water when applied coinciding with active stages of thrips. The table of the spray volumes for field use at different crop growth stages is given below.

Monitor the nymphs and adults from the 14th day after sowing.
Spray any of the following insecticides with 500 L water/ha

Buprofezin 25 SC 1000 ml	Profenofos 50 EC 1000 ml
Diafenthiuron 50 WP 600 g	Thiacloprid 21.7 SC 100-125 ml
Fipronil 5 SC 1.5-2.0 L	Thiamethoxam 70 WS 430 g
Imidacloprid 70 WG 30-35 g or 48 FS 500-900 ml (or) 70 WS 500-1000 g or 30.5 SC 60-75 ml or 17.8 SL 100 -125 ml	Lambda cyhalothrin 2.5 EC or 600-1000 or 5 EC 300-500 ml
Methyl demeton 25 EC 500 ml	Dimethoate 30 EC 500 ml.