Muhammad Daud
WHITEFLY: The whitefly was first reported in Greece 125 years ago. The scientific name of the whitefly is Bemisia tabaci. Whitefly is a small white insect of 1.0 mm length. It feeds on more than 500 plant species and transmits a range of viral diseases in plants. Whiteflies suck sap from under surface of leaves causes yellowing and upward curling of the leaves. Though, the ideal conditions for growth are 27oC and 71% relative humidity, hot and humid conditions favour the insect. The insect excretes sticky honeydew which promotes a fungal sooty mould formation on leaves and cotton bolls. The black mould interferes with photosynthesis in leaves and reduces quality of the cotton fiber. Sticky cotton is not accepted by ginners and the textile industry. Cotton losses were estimated to be in the range of 15-20% and sometimes up to 30%. The presence of a wide range of hosts such as vegetables, pulses, citrus all through the year helps the whiteflies to survive. Whiteflies are present continuously in Pakistan due the availability of wide range of crops all through the year.
COTTON LEAF CURL VIRUS (CLCuD): The whitefly transmits the dreaded cotton leaf curl virus disease in Pakistan and north India. There are no control measures for the leaf curl virus. Disease affected plants are stunted with fewer number of bolls and reduced yields. Infected plants serve as source of inoculum and infestation for the remaining healthy fields.
WHAT CAUSES OUBREAKS: Whitefly is an invited guest and an induced pest. Human interventions are responsible for the insect to survive, reproduce, spread and proliferate. A combination of factors such as a) susceptible hybrids, b) hairy or bushy genotypes, c) late sowing, d) high nitrogenous fertilizers, e) inadequate phosphorus and Potassium in the soil, f) indiscriminate use of pyrethroids, acephate, fipronil and mixtures, g) whitefly resistance to insecticides, i) improper spray application methods and j) favourable weather.
HOW CAN INSECTICIDES INDUCE PEST OUTBREAKS?: Of course they do. At CRI, Multan (Pakistan) we found that synthetic pyrethroids induce bollworms and whiteflies and; spinosad induces mealybugs. Insect pests, including whiteflies are naturally controlled by predators and parasites called ‘natural enemies’ in the field. The natural enemies are generally more susceptible to insecticides than the insect pests. For example, the whiteflies have a waxy coating over the body which protects against insecticides; but its natural enemies are not protected. The whiteflies feed from under surface of the leaf where insecticides do not easily reach. The natural enemy predators and parasites are generally present all over the plant and get exposed to insecticides. Insect pests become resistant quickly to insecticides whereas natural enemies take a longer time for resistance development.
INSECTICIDE RESISTANCE: Studies showed that whiteflies in lower punjab, Pakistan have developed resistance to all the commonly used insecticides. There are very few insecticides that are effective. This has resulted in excessive insecticide sprays that disrupted ecosystems, which led to the severe whitefly outbreaks and further development of resistance.
NEED FOR A ROBUST POLICY: If cotton has to survive in Pakistan, it is important that some policies are formulated. 1. Never allow CLCuD susceptible varieties/hybrids to be permitted for cultivation. 2. Insect pest management must be based on sticky traps, reflective sheets, suction traps, soap emulsions of neem oil, castor oil, fish oil rosin soap and insect and insect growth regulators. 3. Appropriate spray methods must be used to ensure that the spray fluid covers the under-surface of leaves. 4. Fields and vicinity must be kept weed free.
