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Abstract
SUMMARY
Cotton is a plant that seems to be designated specially to attract a wide rang of insect pests. Cotton, the worldۥs most important fiber is grown on more than 33.9 million hectares in about 100 countries. The pink bollworm (PBW) Pectinophora gossypiella (Saunders) is a worldwide pest of cotton and in some regions of the world is the key cotton pest. The PBW is a well-adapted herbivore of cotton, feeding throughout the growing season on the cotton fruit system (square, flowers and bolls) and burrowing habits. Relative suitability of different rearing environments and the actual increasing rate of pink bollworm under different conditions are given by the life table parameters (total number of the laid eggs, hatching %, survival ratio of the immature stages, rate of development and the sex ratio), so the life tables are considered as the basic parameter which may be established for an insect population under specific physical conditions.
Three different groups of insecticides; organophosphate, bio-insecticides and notionally origin insecticides; were compared against the newly hatched larvae of Pectinophora gossypiella using film residue method, while diet incorporating was utilized for Bt (Subsp. Kurstaki).
The present investigation aimed to study:
1- The toxicity of two modern insecticides (spinosad and emamectin benzoate), Bt (Subsp. Kurstaki) and neem - azadirachtin comparing with the intensively used conventional insecticides in controlling bollworms in Egypt ( chlorpyrifos and profenofos).
2- The effects of these insecticides on the survival and development of pink bollworm comparing with that conventionally used.
3- The histopathological effects of the tested insecticides.
The obtained results of this study could be summarized as follows:-
5.1. Bioassay of certain insecticides against Pectinophora gossypiella
5.1.1. Comparative toxicity study of certain pesticides against the newly hatched larvae of, Pectinophora gossypiella (LC25):
The LC25 values of chlorpyrifos, profenofos, spinosad, emamectin benzoate, Bt (Subsp. Kurstaki) and neem (azadirachtin) were 0.124, 0.328, 0.0006, 0.0001, 0.506 and 0.0022 ppm, respectively. The Ld-P line indicated that, the toxicities of the previous mentioned insecticides were in the following descending order: emamectin benzoate, spinosad, neem (azadirachtin), chlorpyrifos, profenofos and then Bt (Subsp. Kurstaki).
5.1.2. Comparative toxicity study of certain pesticides against the newly hatched larvae of Pectinophora gossypiella (LC50).
The LC50 values of the two organophosphate insecticides, chlorpyrifos was 0.369 ppm and profenofos was 0.645 ppm with, while the LC50 values of the three bio-origin insecticides were 0.0066 ppm for spinosad, 0.002 ppm for emamectin benzoate and 1.456 ppm for Bt (Subsp. kurstaki), also; the plant origin insecticide, neem (azadirachtin) LC50 value was 0.036 ppm.
The Ld-P lines indicated the toxicity of these insecticides against the pink bollworm could be arranged in the following descending order: emamectin benzoate, spinosad, neem (azadirachtin), chlorpyrifos, profenofos and then Bt (Subsp. Kurstaki).
5.2. The latent effect of the certain pesticide treatments on the newly hatch larvae of Pectinophora gossypiella.
The calculated LC50 and LC25 of these insecticides were selected and tested against the 1st instar larvae. The percentages of the survival pink bollworm insects which were treated with LC25 of the tested insecticides and also the untreated check throughout 26 days. The highest effect after the 8th day was observed by the treatment of profenofos followed by emamectin benzoate and then with the same values for both of chlorpyrifos, spinosad and BT Subsp. Kurstaki. While the lowest effect was recorded by neem- azadirachtin. The survival percentages after the 8th day post treatment were 48, 54, 60, 60, 60, and 68% respectively.
The final efficacy order after the 26th day post treatment for the tested insecticides was: profenofos > BT Subsp. Kurstaki > neem–azadirachtin and emamectin benzoate > spinosad > chlorpyrifos with survival insect percentages of 26%, 30%, 34%, 42% and 44%, respectively. The greatest loss in the survival insect percentage, 34%, was for neem–azadirachtin (from 68% after the 8th day post treatment to 34% after the 26th day post treatment) followed by Bt Subsp. Kurstaki, 30% loss in survival insect (from 60% after the 8th day post treatment to 30% after the 26th day post treatment) and then profenofos with 22% loss (from 48% after the 8th day post treatment to 26% after the 26th day post treatment) while emamectin benzoate has a lose value in survival insect of 20 % (from 54% after the 8th day post treatment to 34% after the 26th day post treatment); also, the spinosad insecticide has a lose value in survival insect of 18 % (from 60% after the 8th day post treatment to 42% after the 26th day post treatment).
Regarding the LC50, the results showed that, the highest effect after 8th day was observed by emamectin benzoate followed by profenofos, Bt Subsp. Kurstaki, and then with the same values both of chlorpyrifos, spinosad and neem- azadirachtin. The survival percentages were 34%, 38%, 48%, and 50%, respectively.
By the final inspection day, the 26th day, the efficacy order was: emamectin benzoate, profenofos and Bt Subsp. kurstaki > spinosad > neem – azadirachtin > chlorpyrifos with survival insect percentages of 20%, 20%, 20%, 24%, 30%, and 32%, respectively.
In conclusion, the effective insecticides on the biological parameters of the pink bollworm were emamectin benzoate and profenofos followed by Bt Subsp. kurstaki, neem – azadirachtin and spinosad. The least effective insecticide was chlorpyrifos in that manner. Among the LC25, it could be showed that, profenofos and emamectin benzoate induced the superior effect resulting in 48 and 54% after 8 days post treatment, respectively. Both of profenofos and Bt Subsp. kurstaki, continued as the deleterious insecticides at the 20th day (30 and 38%) and the 26th day (26 and 30%).