Potential Of Mixed Biocide BtA Against Helicoverpa Armigera And The Effects On Its Parasitoid Microplitis Mediator

The order Lepidoptera, and more specifically the family Noctuidae, comprise a large number of destructive crop pests. One of these pests is the bollworm Helicoverpa armigera (Hiibner) (Lepidoptera:Noctuidae) which causes damage on cotton and various vegetables, a serious pest of economically important crops in many countries. H. armigera has become a major threat to cotton production in China due to its capacity to develop resistance over time to each of the major synthetic insecticides used in its management. In China, several parasitoid species attack H. armigera in the field; one of these is the braconid wasp Microplitis mediator (Haliday) (Hymenoptera:Braconidae). A revised management strategy is needed to control this pest that incorporates more tactics than insecticide alone provides. Current research is directed toward using biological pesticides, especially Bacillus thuringiensis Berliner or Bt, which are compatible with natural enemies. These findings have highlighted the importance of integrated pest management (IPM) strategies for H. armigera, in which natural enemies could play an important role. A broad complex of natural enemies acts against H. armigera in various agro-ecosystems worldwide. Recent work indicates that more selective insecticides can be developed to combine with natural enemies that are not disrupted by such chemicals. A new multiple-toxin, mixed biocide BtA, Bt toxin mixed with abamectin, produced by fermentation of Streptomyces avermitilis, was formulated, in order to enhance controlling efficiency against more target pests. BtA has overcome the disadvantages of Bt by possessing toxicity against a broader range of pests and a relatively rapid killing speed. However, there are no reports regarding the effects of BtA on H. armigera and its parasitoids M. mediator. The overall objective of this thesis was to evaluate the potential of BtA against H. armigera and the effects on its parasitoid M. mediator. In first experiment, laboratory and field evaluations were conducted to evaluate the efficacy of BtA in comparison to the seven commercial insecticides (Abamectin, Bt, (3-Cypermethrin, Phoxim, Chlorfluazuron, Fenvalerate, and Carbosulfan), against Helicoverpa armigera. Toxicity of these compounds to H. armigera by leaf dip bioassays within 24 h in the laboratory showed that Abamectin was the most toxic (LC50= 0.0062 mg/ml) to the third instar larvae of H. armigera compare to Phoxim, BtA, Fenvalerate, Carbosulfan,β-Cypermethrin, Chlorfluazuron, and Bt, respectively. The mortality of the third instar larvae exposed to Chinese cabbage Brassica rapa var. pekinensis treated with various commercial insecticides were significantly different at 24,48, and 72 h. Abamectin and Phoxim residual efficacy against the third instar larvae at 24 to 72 h after treatment, resulted in larval mortalities from 40 to 70%. BtA, Fenvalerate, Carbosulfan, andβ-Cypermethrin showed increased larval mortalities of H. armigera as time after application increased. Chlorfluazuron and Bt primarily affected H. armigera larvae at 48 and 72 h after treatment, respectively. BtA was not significantly different from the other commercial insecticides against H. armigera larvae. Properly timed treatments of BtA may be used to prevent damage by H. armigera.Second experiment was conducted for determination on the effects of BtA on the development of parasitoid Microplitis mediator (Haliday) (Hymenoptera:Braconidae) and its the bollworm host, Helicoverpa armigera (Hubner) (Lepidoptera:Noctuidae), were evaluated in the laboratory. Weight gain in larvae of H. armigera was initially delayed, but larval developmental period increased and pupal weight increased when they were fed on a diet containing BtA. Due to increased longevity of the host larvae, the susceptible period to parasitization of H. armigera by M. mediator increased when the host larvae were reared on the diets containing BtA at concentrations of 0.5,1,2, and 4μg/g. Longevity of female and male parasitoids significantly decreased when newly emerged wasps were fed on a honey solution containing 200μg/ml BtA in comparison with those fed only on a honey solution. Mean longevity was significantly prolonged when parasitoids were fed on honey solution and BtA-honey solution in comparison with those fed on BtA-distilled water solutions, or distilled water, or nothing. There were no significant differences compared with the control in any biological characteristics for the offspring of female parasitoids fed on the honey solutions containing BtA at concentrations of 50,100, and 200μg/ml; characteristics measured include the egg-larval period, pupal weight, male and female pupal periods, adult fresh weight, and adult longevity. When the female parasitoids parasitized host larvae that had been fed on the diets containing BtA, their male and female pupal periods were significantly prolonged compared to the control (without BtA).Final experiment was conducted for the effects of BtA to the seven commercial insecticides (Abamectin, Bt,β-Cypermethrin, Phoxim, Chlorfluazuron, Fenvalerate, and Carbosulfan) on the pupae and adult stages of M. mediator were conducted to contact toxicity of residual effects in the laboratory by direct-dip and residual bioassays, respectively. Results of direct-dip bioassay showed Fenvalerate was the highest toxicity (pupae mortality values of 73.17%) to M. mediator pupae and significantly higher than pupae mortalities of the other commercial insecticides. BtA, Abamectin, and Phoxim were low contact toxicity to M. mediator pupae and not significantly different. For residual bioassay, Phoxim and Carbosulfan were extremely the highest toxicity (adult mortalities values of 100%) to female of M. mediator adult and significantly higher than adult mortality of the other commercial insecticides. BtA showed the lowest toxicity with mortality value less than 10%. Bt and Chlorfluazuron were no residual effects on M. mediator pupae, with mortalities were not significantly different compare with the control.Therefore, these results suggest that (1) BtA had moderate toxicity of lethal effect (LC50 = 0.7364 mg/ml) on the third instar of H. armigera larvae compare with the other commercial insecticides (Abamectin, Bt,β-Cypermethrin, Phoxim, Fenvalerate, Carbosulfan, and Chlorfluazuron) at 24 h by leaf residue bioassay in the laboratory. (2) BtA was not different efficacy in the field from the other commercial insecticides (Abamectin, Bt,β-Cypermethrin, Phoxim, Fenvalerate, Carbosulfan, and Chlorfluazuron) against H. armigera larvae at 72 h after treatment. BtA showed increased larval mortality of H. armigera as time after application increased. Trend was for increased larvae mortality of H. armigera in residual activity of BtA compared to Bt alone and the control. (3) BtA had affect on the host larvae H. armigera of parasitoid M. mediator when the host larvae fed on the diets containing BtA. Larval weight of H. armigera after feeding on the diet containing 0.5-4.0μg/g BtA for six days was significantly lower compared to the control. Larval period was significantly prolonged at all concentrations of 0.25-4.0μg/g BtA diets. Pupal weight was significantly increased from the control when H. armigera larvae fed on the diets containing 1.0,2.0, and 4.0μg/g BtA. (4) BtA had extended the susceptible larval period of the host larvae H. armigera for parasitization by parasitoid M. mediator and period more opportunity for M. mediator parasitization to occur. The susceptible period increased when the host larvae were reared on the diets containing BtA at concentrations of 0.5-4.0μg/g. (5) BtA had affected on the adult longevity of M. mediator. Female and male longevities were significantly decreased when they fed on a honey solution containing 200μg/ml BtA comparison with a honey solution. (6) BtA was no effect in any biological characteristics (egg-larval period, pupal weight, male pupal period, female pupal period, adult fresh weight, male and female longevity) for the offspring of female parasitoid M. mediator when their mother fed on the honey solutions containing 50-200μg/ml BtA. (7) BtA had affected on the development of the parasitoid M. mediator in the host larvae H. armigera fed on the diets containing BtA. The egg-larval period and pupal weight were significantly decreased when the host larvae fed on the diets containing 0.5-4.0μg/g and 4.0μg/g, respectively. Male and female pupal periods were prolonged when the host larvae fed on a diets containing 4.0μ.g/g. Adult fresh weight was significantly decreased when the host fed on the diets containing 2.0-4.0μg/g. (8) BtA was low effect of contact toxicity to the pupae and adult stages of M. mediator comparison with the other commercial insecticides (Abamecin,β-Cypermethrin, Phoxim, and Carbosulfan) in the laboratory. BtA was significantly lower contact toxicity (mortality value of 13.82%) to M. mediator pupae at the time till emerging thanβ-Cypermethrin, Phoxim, and Carbosulfan within 24 h by direct-dip bioassay. And then, BtA showed the lowest contact toxicity to adult of female M. mediator compare with Abamectin,β-Cypermethrin, Phoxim, and Carbosulfan with mortality value less than 10% within 24 h by residual bioassay. (9) BtA is more conservation of parasitoid M. mediator than Abamectin and the other commercial insecticides used at the same rate in the laboratory. (10) BtA should be included in the developing biological control strategies in crop fields. Because BtA to maximize the positive effects on the target pests and to minimize the negative impact on natural enemies and other non-target organisms.