Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) has been present on many crops in Pakistan but was not a major pest prior to the mid 1980s. Since 1992, cotton leaf curl virus (CLCuV) disease transmitted by B. tabaci has decreased yields significantly. Three types of damage may be caused by this pest: 1) direct damage by sucking sap from plants, 2) indirect damage by excretion of honeydew on fiber and 3) virus transmission. Chemical pesticides are considered useful and powerful tools for the control of B. tabaci. Unfortunately, indiscriminate use of insecticide has resulted in the development of pesticide resistance, resurgence of secondary pest in the absence of natural enemies, and other environmental and human health issues. To overcome the problems being caused by chemical control, researchers and growers are interested in optimizing non-insecticidal methods such as conserving and enhancing beneficial organisms on alternate host plants, host plant resistance and selection of soft insecticides.
Bemisia tabaci (Gennadius) is a serious pest of numerous crops, vegetables, and ornamentals. It does not have a resting or dormant period and its year-round survival depends greatly on the availability of suitable host plants for feeding and shelter. These hosts provide shelter to whitefly as well as their natural enemies during unfavorable environmental conditions. During the main cotton growing period (June-October) B. tabaci and its parasitoids were recorded on Gossypium hirsutum, Solanum melangena, Bauhinia pupurea, Lantana camara, Achyranthus aspera, Convolvulus arvensis, Glycine max, Albizzia lebbek, Helianthus annus and Cucumis melo. Its population remained high on G. hirsutum and L. camara and parasitism was higher on G. hirsutum and C. arvensis. After the cotton season (November to February), B. tabaci overwintered in low numbers on S. melangena, B. pupurea, L. camara, A. aspera, Morus alba, Cucurbita pepo melopopo, and C. arvensis. The highest number of B. tabaci were recorded on S. melangena, and the highest level of parasitism was on A. aspera. Before the cotton season (March to May), B. tabaci was recorded on S.
melangena, B. pupurea, L. camara, A. aspera, M alba, C. pepo melopopo, C. arvensis, G. max, A. lebbek, H annus and C. melo. These hosts play an important role in the movement of B. tabaci along with its parasitoids to the cotton crops. Higher levels of whitefly were recorded on G. max, and C. melo. Parasitism was high on C.
melo, C. arvensis and B. pupurea. It has been concluded that not all of the investigated hosts are equally suitable for the development of whitefly and its parasitoids. Hosts like S. melongena, C. pepo melopopo, L. camara, C. arvensis, A. aspera and M alba are extremely important in providing shelter to parasitoids in extreme cold weather conditions. After that, hosts like S. melongena, G. max, H. annus, C. melo, L. camara, C. arvensis, A. aspera and B. pupurea play an important role in shifting parasitoids from their alternate host to the cotton crop. In an attempt to enhance the availability of parasitoids earlier in the cotton, host plants like L. camara, B. pupurea, A. lebbek, M alba can be grown in cotton growing areas.
Population dynamics of the whitefly and its natural enemies were studied throughout the season in treated and untreated cotton fields in three major cottongrowing areas of the Punjab. Timing of the first spray had a greater influence on population buildup of whitefly and reduction of its natural enemies. The population of whitefly was low before the start of spraying and its population started increasing after the initial spray and reached its maximum in the month of September. However, the whitefly population remained below ETL in untreated fields throughout the season. Similarly, the number of predators was high before the start of spray, significantly decreased during the spray period, and recovered after the cessation of spray applications. The parasitoid population was also affected by pesticide application but a high population persisted in untreated plots throughout the year.
Host plant resistance is an important component of integrated pest management for the suppression of whitefly populations. For the preference of whitefly (B. tabaci), jassid (Amrasca devastans), thrips (Thrips tabaci), predators (Orius spp., Geocoris spp., Chrysopa carnea, coccinellid spp., and spiders) and parasitoids (Encarsia and Eretmocerus spp.) eleven cotton varieties with different hair density and length were evaluated under unsprayed condition. Populations of B. tabaci were higher on varieties having a hair density ranging from 758 to 900 per cm2 and lower on varieties having a hair density of less than 700 per cm2. Amrasca devastans numbers were significantly higher on varieties having a hair density ranging from 562 to 737 early in the season and shifted to nearby more green hairy varieties later in the season. Thrips tabaci attack was greater on relatively smooth leaf varieties. The number of predators was similar on all the varieties, whereas the level of parasitism was higher on varieties having the maximum number of whiteflies.
The effect of seed treatment insecticides, imidacloprid and thiamethoxam on parasitoids and predators of B. tabaci was studied. Percent parasitism in imidacloprid (16.7%) and thiamethoxam (13.2%) was slightly less compared with the untreated check. Some population reductions in Chlysoperla carnea (4.9%), coccinellid (0%), Geocoris spp. (13.9%), Orius spp. (27.7%) and spider (15.5%) were observed in the plot treated with imidacloprid. Comparatively thiamethoxam proved less toxic to parasitoids.
The impact of both broad-spectrum and selective insecticides on B. tabaci and its predators and parasitoids was studied under field conditions. In plots treated with pyrethroids, organophosphates and others, the number of whitefly was significantly less in thiacloprid and Agri-50 and highest in black warrant and cypermethrin treated plots. Number of predators was higher in Agri-50 and spinosad and lower in cypermethrin and lambdacyhalothrin treated plots. The level of parasitism was higher in thiacloprid and lower in methamidophos and Agri-50 treated plots. In plots treated with organochlorines, neonicotinoids, thiourea and insect growth regulators, number of whitefly was significantly less in buprofezin and pyriproxyfen treated plots. Number of predators was higher in pyriproxyfen and endosulfan and level of parasitism was higher in pyriproxyfen and imidacloprid treated plots. In plots treated with tank mixtures of different insecticides, number of whitefly was lower in a mixture of buprofezin + fenpropathrin and higher in plots treated with a mixture of thiamethoxam + chlorpyrifos. Number of predators was higher in buprofezin + fenpropathrin and lower in deltamethrin + triazophos treated plots. Percent parasitism was maximum in mixture of pyriproxyfen + chlorpyrifos and lowest in plots treated with a mixture of imidacloprid + chlorpyrifos.
Eight different insecticides were studied in different spray regimes to determine their efficacy on whitefly, predator and parasitoid populations. One week after spray, minimum number of whitefly was observed in regime esfenvalerate + bifenthrin + black warrant and higher level of parasitism was found in regime deltamethrin + fenpropathrin + cypermethrin. In the second week after spray, comparatively less number of whitefly and higher level of parasitism were recorded in regime deltamethrin + fenpropathrin + cypermethrin.