Cotton is one of the most important commercial crops in the world including in India. Among the cotton-growing nations, India has the largest area of 9.0 million hectares grown under varied agro-ecological areas. It also provides basic raw materials i.e. cotton fiber to the cotton textile industry. In addition, cotton provides direct employment to around 6 million farmers & about 40 to 50 million people are working in the cotton trade and its processing.
In India, there are 10 important cotton-growing states that are divided into 3 zones - north zone, central zone & south zone. The North zone includes Punjab, Haryana & Rajasthan. The central zone comprises Madhya Pradesh, Maharashtra & Gujarat. The South Zone consists of Andhra Pradesh, Telangana, Karnataka & Tamil Nadu. Besides these 10 States, cotton cultivation has gained momentum in Orissa too. Cotton is also cultivated in small regions of non-traditional States like West Bengal, Uttar Pradesh & Tripura.
In India Nangpal (1948) recorded almost 110 species of insects & mites that infest cotton. Luckily only a relatively small number of insects are of major economic importance, but individually or by their combined effect they can lead to significant yield loss. Amongst the phytophagous cotton pests, 24 have achieved the pest status and the main can be classified as sap-feeding insects, bollworms, leaf-feeding insects & mites.
Types of Cotton Sucking Pests - Its Management & Control
Cotton Aphids
Cotton aphids scientifically known as Aphis gossypi are a cosmopolitan species broadly distributed in all the cotton-growing states of India. In the past, it was considered an early-season minor pest but now causes severe damage mainly in cotton fields after the use of synthetic pyrethroids that include the resurgence of this pest. The scale of damage depends on the period of attack, insecticides formerly used, and the weather conditions. Arid weather with extended drought favors the fast build-up of this pest. Aphids stay in colonies on the undersurface of leaves & terminal shoots and suck the plant sap affecting the general vigor of the plant.
The shedding of fruiting bodies & quality of the fibre is also get affected. Besides the direct damage, the ‘Honeydew’ emitted by aphids is deposited in the upper surface of lower leaves, in which sooty mould develops that further interferes with the photosynthesis of the plant. Honeydew could also drip onto the open bolls causing ‘stickiness’ of lint (fibre).
Aphids Management
Aphids are preyed upon by various species of chrysopids, coccinellids and syrphids. The parasitoid Aphelinus gossypii also plays a vital role in reducing the population during the cooler months of the year. A high degree of control is generally exerted by these natural enemies. Growing cowpea under intercrops or on the irrigation bunds increases the natural enemy build-up.
Cotton Thrips
Thrips are usually one of the main early-season cotton pests. They at first damage the cotyledons and then various other parts that include - the bolls. The types of damage differ according to the parts of the plant attacked. Most damage occurs in the early vegetative stage of the crop when the nutritional quality of tissues is perfect for these insects. The adult as well as nymphs usually stay on the below surface of leaves, cut the tissues, and suck the cell sap. The affected leaves turn thick, sore, and brown due to constant feeding. Feeding on developing bolls, makes them turn tanned because of the development of necrotic patches. Thickening of boll rind can also be observed when bolls are assaulted, boll opening is affected.
Thrips Management
Populations of thrips are checked by certain predatory thrips. The predators on the thrips include Lygaidae, Anthocoridae & mites. But their role is very limited. The population & damage are more in the dry period. Spraying systemic insecticides such as acetamiprid, methyl-o-demeton, imidacloprid and acephate can control the thrips.
Cotton leafhoppers
Amrasca devastans is the Indian cotton jassid or leafhoppers, previously referred to as A. biguttula or Empoasca devastans. Though 8 species of jassids have been reported to feed on cotton, A. devastans is the most dominant & distributed in all the cotton-growing areas of the country.
A leafhopper not only feeds on cotton but on a wide range of other host plants. They actually introduce a toxin that damage photosynthesis in proportion to the amount of feeding & this causes the ends of leaves to curl downwards - it becomes yellowish and then goes red. Harsh ‘hopper burn’ leads to detaching of reproductive parts and can severely stunt young plants & reduce yields.
Leafhoppers Management
Cotton varieties with glabrous leaves are very much susceptible to jassids if compared to the hairy (hirsute) varieties. The vulnerability is correlated with both density & length of hairs on the below surface of the leaf mainly on veins where the jassid feeds & lays eggs. High hair thickness without length is ineffective. Some of the popular resistant varieties grown in India are - SRT 1, MCU 5 and LRK 516.
Natural enemies are not considered to have a considerable effect on jassids’ population, though a number of egg parasitoids have been recorded. The parasitoid Anagrus sp has been traced, but it does not play an important role in reducing the population. Insecticides are usually required when the jassid population reaches 2 to 3 nymphs/leaf while for glabrous cultivars the threshold may be lowered to 1/leaf. Use of any of the systemic insecticides such as imidacloprid, methyl-o-demeton, thiomethoxam, or acetamiprid gives good control of leafhoppers. The insecticides like quinalphos, chlorpyriphos, and endosulfan are not of use against this pest. Very often the cotton fields treated with these insecticides recorded high jassid population than the untreated ones. If you treat the seeds with imidacloprid / thiomethoxam - 7 to 10 g/kg of seeds before sowing cotton it will provide good control of this pest up to 45 days after planting.
Whitefly and Its Management
Whitefly spoil cotton plants in 2 ways - firstly by sucking the juice and secondly by excreting honeydew in which sooty mould develops. Damage caused by direct feeding decreases the photosynthetic activities of the plant & hence the yield. Indirect damage results from lint contamination with honeydew & related fungi and via transmission of leaf curl virus disease. Late season harshness affects the seed development & the lint quality.
Whitefly management must be done when their populations are at low levels via cultural practices. Maintenance of good field sanitation by wiping out and removing the crop residues and weeds is an effective practice against the whiteflies. Growing vegetables in small periods & allowing maximum time between host crops of whitefly cuts its pest status on cotton. A blend of cultural practices and need-based insecticidal applications can prevent whitefly populations.
Frequent applications of insecticides in early and mid seasons lead to a resurgence of whiteflies and hence a highly judicious chemical application is very important. Neem oil (1%), fish oil resin soap (2.5%), and neem seed kernel extract (NSKE) 5% give effective control of whiteflies.
The severity of whiteflies can be seen after the crop growth crosses 10 nodes on the main stem. Hence, the amount of spray fluid while spraying the insecticides must be greater than 250 l/ha using power sprayers. Proper coverage of the underside of leaves during the insecticidal sprays effectively decreases the whitefly population.
