Laboratory Studies on Host Preference and Insecticide Resistance of the Cotton Bollworm,Heliothis obsoleta F.1

Pupae of the cotton bollworm, Heliothis obsoleta F., from the northern part of Iran (Gorgan and Gonbad areas) were brought to the laboratory and used as the breeding source for bollworms. The larvae were reared on soybean plants for 8 generations under laboratory conditions of 60±5 % RH and 27±2 °C. To test host preference, the adults of both sexes were released into wooden cages where they were able to freely select their host. The females laid more eggs on soybean (before and at blooming) compared to cotton (before and after blooming) and maize (before and after stem extension). Seven and 14-day-old larvae were treated with different doses of Supracide and Thiodan by topical application. Altogether, 3,000 larvae were used for this experiment and from the data collected, LD 50 values were calculated. It was found that, as the age doubled from 7 to 14 days, the LD 50 value for Supracide increased from 0.6 mg/g to 0.92 mg/g. For Thiodan, the corresponding increase during this period was from 0.52 mg/g to 1.50 mg/g. Fat body content was measured using Soxhlet apparatus and its correlation with LD 50 values was determined. Fat bodies represented 2.1 and 2.5% of the body weight in 7 and 14-day-old larvae, respectively.