Background and objective
Cotton bollworm, Helicoverpa armigera (H?bner) (Lepidoptera: Noctuidae) is a cosmopolitan and polyphagous pest that attacks many plant species including cotton. One of the most important principles of the Integrated Pest Management programs (IPM) is use of non-chemical agents such as natural enemies (parasitoids and predators), and resistant host plant to control pests. Transgenic Bt cotton is one of the substitution ways of chemical pesticide used for control of this insect pest. Production of transgenic crops was a novelty in the method of applying host-plant resistance. Crops that are genetically engineered to express Bacillus thuringiensis toxin are planted in many areas.
In the present study, effect of transgenic Bt cotton, Coker cultivar, including cry1Ab was studied on biology and digestive protease activity of the bollworm. For bioassays, gut dissection and demographic studies, H. armigera were obtained from the Division of Entomology, Department of Plant Protection, College of Agriculture and Natural Resources, University of Tehran, Karaj, Alborz, Iran. The insects were held in the growth room on the condition of 25 ± 1ºC, 60–70 % relative humidity with a 16-h day length. Proteolysis using general substrate (Azocasein 2%) was assayed after some modification. In this manner, 10 µl enzyme extract and 50 µl Azocasein 2% (substrate solution) were mixed into 40 µl of the 20 mM Tris-HCl buffer pH 8.0 and incubated during 60 min. Protease activity was measured using specific substrate with some modifications. Trypsin- and chymotrypsin-like activities were assayed using 1mM BApNA (Benzoyl-l-arginine-p-nitroanilide) and 1 mM SAAPFpNA (Succinyl-l-Ala-l-Ala-l-Pro-Phe-p-nitroanilide), respectively.
The results illustrated that the second larval instar mortality in the transgenic cotton was 68 percent and in control cotton was 6 percent. Furthermore, larvae in different instars, in transgenic Bt, had significantly lower weight and longer life time in comparison with control. Several biological characters of both treatments including survivorship, development and weight were significantly changed. On the other hand, total protease, trypsin and chymotrypsin activity in digestive lumen, after the second, fifth and sixth larval instar in transgenic cotton showed significant reduction, respectively.
Coker cotton crop with cry1Ab, will have significantly negative effects in insect biology and digestive physiology during the pest life time and can be noticeable in management of important pests such as boll worm. Bt cotton can therefore be useful in an integrated pest management program, because it resulted in a reduction in the number of insecticide applications per season