Study on Thrips tabaci Lind. as a pest of cotton.
Mass increases in the numbers of thrips infesting cotton have been observed over the past ten years in Bulgaria. Thrips tabaci Lind. is the most injurious species, and the form T. tabaci pullus Uzel has also been observed. T. flavus Schr. forms 12-15% of the total population. In early spring T. tabaci invades Sinapis arvensis, Atriplex nitens, Anthemis arvensis, Amarantus retroflexus, Cirsium arvense, Euphorbia sp., Veronica campilopoda and Cardaria (Lepidium) draba, and from these weeds and from onion and sunflower [Helianthus annuus] the insects migrate to cotton after its germination. A direct connection was found between the densities of T. tabaci on weeds and on cotton, and it is possible to forecast the intensity of attack on cotton by examining thrips density on weeds in early spring. Cotton infested by Thrips spp. is slow in growth and development, with late bud formation, blooming and boll ripening and yield reduced by 30-62%. In the climatic conditions of Bulgaria, T. tabaci has 7-9 generations a year; it is favoured by warm dry weather and mass emergences are prevented by frequent rain or prolonged cool weather. During preimaginal development, T. tabaci spends over half the period in the soil without feeding, and insecticide sprays on the plant can be applied only against the adults. Good mortality is obtainable up to seven days after treatment, but as this is followed by considerable increases in adult numbers, a second application is required 8-10 days after the first.<new para>ADDITIONAL ABSTRACT:<new para>The main species of Thrips damaging cotton plants in Bulgaria is T. tabaci. The damaged plants grow slowly, flowering and boll ripening are retarded and yields of seed cotton can fall by 30-62%. Soil spraying with insecticides controls the insects only temporarily and repeat spray 8-10 days after the first, is necessary.