m. h. safaralizadeh

The effect of Bacillus thuringiensis var. kurstaki on different larval instars of Colorado potato beetle, Leptinotarsa decemlineata (Say) and the role of two plant materials, namely, caffeine and aqueous neem extract, in the enhancement of it’s efficiency was investigated under laboratory conditions. The experiments were conducted at 25±4 oC, 65±5 %(RH) and photoperiod of 16:8 (L:D).The larvae were provided from the colony established and maintained on host plant. The different larval instars were distinguished by measuring head capsule width.To evaluate susceptibility of different instar groups of larvae to B.thuringiensis, LC50 values on (1-4) instar groups were determined. LC50 values for 1st, 2nd, 3rd and 4th instar groups were, 183.86, 377.03 ,1297 and 3096 ppm, respectively. The synergistic effect of caffeine and aqueous neem extract with B.thuringiensis was investigated separately on 3rd larvae instar. A completely randomized design with 6 treatments and 4 replications was used. The results revealed that there was a striking synergistic effect of caffeine and neem on B.thuringiensis.The larval mortality percentage for mixture 618ppm of B.thuringiensis and 4000ppm of caffeine was 80% however, this was 10% and 20% for caffeine B. thuringiensis respectively when these compounds were used alone.The initiation of larval mortalitry in all mixtures was earlier than other treatments.Also the larval mortality for mixture B. thuringiensis with 35000 ppm of aqueous neem extract was 77.5% after 144 hours however, it was 22.5 % and 25 % for B.t. and neem alone, respectively. The mean weight of larvae treated with all synergists was significantly less than the control cohort, (p<0.05).

In this study, susceptibility of the first three larval stages of Spodoptera exigua to Bacillus thuringiensis was investigated. To determine the LC50 values for each larval instar, 6 different logarithmic concentrations of Bacillus thuringiensis were used. A control was also included in each of the bioassays. The value of LC50 for 1st instar larvae, that had been treated one to four hours after hatching, was 311.617 ppm. For the 2nd and third instars, the larvae values of LC50 were 1356.95 and 2708.27 ppm, respectively. It was clear that larval susceptibility to Bacillus thuringiensis decreased with age. In order to enhance the Bacillus thuringiensis efficiency to control Spodoptera exigua under field conditions, Bacillus thuringiensis should be applied at egg hatch peaks.