Effects Of Garlic Intercropping On Bio-communities In Tobacco Fields

Field trials were carried out in 2008 and 2009 at Longyan, Fujian province, China, to evaluate the role of garlic, Allium sativum L., as a deterrent crop to help control the green peach aphid, Myzus persicae (Sulzer), in fields of flue-cured tobacco. The effects of garlic intercropping on arthropod communities in tobacco fields were primarily analyzed from the perspective of community ecology. Moreover, the effects of garlic intercropping on the chief leaf diseases of the tobacco plant and on the economic benefits associated with tobacco leaves were investigated in the same fields. Indoor and outdoor experiments were conducted to investigate the bioactivities of garlic extracts on the main pests of flue-cured tobacco. The allelopathic potential of garlic on different flue-cured tobacco varieties was also studied. The purpose of these experiments was to obtain information that would contribute to the control of the main pests found in fields of flue-cured tobacco (e.g., the green peach aphid) and that would help to develop a method to protect tobacco through the use of whole garlic plants. The results of the research were as follows.1 The allelopathic potential of garlic on different flue-cured tobacco varietiesThe aim of this component of the study was to investigate the allelopathic effects of A. sativum and to identify the best use of the plant (e.g., intercropping, the relay and rotatation system) to serve tobacco production in the field environment. To accomplish this goal, volatile compounds, aqueous or ethanol-soluble extracts and soil (0-10 cm) in which garlic had been grown were prepared and used as treatments to assess their effects on seed germination and seedling growth in flue-cured tobacco plants (K326, Yunyan 85, Zhongyan 100, Nc 89, CB-1 and Minyan 35). The results of this investigation indicated that garlic had an inhibitory effect on seed germination and seedling growth of the tobacco varieties tested. Furthermore, the allelopathic responses were species-specific. Volatile compounds prepared from fresh A. sativum plants inhibited the accumulation of fresh organic matter in different flue-cured tobacco seedlings by 33.91%-57.89%. Generally, ethanol-soluble extracts had stronger or more consistent effects than aqueous extracts on seed germination and seedling growth. The soil (0-10 cm depth) sampled from the immediate vicinity of A. sativum inhibited the growth of the test tobacco plants by 8.35%-60.96%.Extracts of garlic and of garlic soil (0-10 cm) from tobacco fields were also prepared. Experiments were conducted to assess the effects of these extracts on the growth of adult seedlings for the purpose of practical production. The results of these experiments showed that the adult seedlings of different tobacco varieties were not significantly inhibited by the extracts. 2 The effects of garlic intercropping on arthropod communities in fields of flue-cured tobaccoField experiments were conducted at Longyan, Fuanjian in 2008 and 2009 to determine the effects of garlic intercropped with flue-cured tobacco on the arthropod communities in the tobacco fields. Four treatments were used in the study. In treatment A, two rows of garlic were planted on the same edge of a ridge. In treatment B, two rows of garlic were planted on opposite edges of the same ridge. In treatment C, garlic was planted between two individual K326 tobacco plants on the ridge. In treatment D (the control), all ridges were planted with K326.A total of 46,002 arthropods were collected. These organisms belonged to 2 classes, 11 orders, 44 families and 65 species. Numerically, herbivores generally represented the most abundant guild in the arthropod community. The relative abundance of herbivores in the samples was 68.90% in 2008 and 67.12% in 2009. Parasitoids and predators represented the least abundant guilds. The relative abundance of these two guilds in the samples were 1.01% and 2.77%, respectively, in 2008 and 1.39% and 3.29%, respectively, in 2009. Trophically intermediate guilds were present at an intermediate level of relative abundance. The relative abundance of these intermediate or neutral guilds was 27.32% in 2008 and 28.20% in 2009. The results of the experiment showed that the numbers of individuals in arthropod communities could be decreased by intercropping garlic in tobacco fields. The treatments did not have significant effects at the ordinal level, but the effects at the family or species level showed that numbers of families or numbers of species could be increased by intercropping garlic in tobacco fields. The results also indicated that the intercropping of garlic in tobacco fields decreased the numbers of individuals and species belonging to herbivorous guilds, whereas intercropping increased the numbers of individuals and species belonging to parasitoid or predatory guilds. The occurrence or the numbers of individuals of tobacco aphids were significantly delayed in time or decreased by garlic intercropping. The diversity, evenness or stability indexes of the arthropod communities were significantly higher in the fields with garlic intercropping than in the fields with monocultures. Moreover, the dominance indexes of the arthropod communities were significantly lower in the fields with garlic intercropping than in the fields with monocultures.3 The bioactivities of garlic extracts on the main pests of flue-cured tobaccoThe effects of ethanol extracts of garlic on primary tobacco pests differed significantly across the experimental conditions. Indoor experiments were conducted to determine the relative bioactivities of different extracts. The bioactivities of 9 g DW?100 mL-1 and 7 g DW·100 mL^-1 extracts against the aphid M. persicae were stronger than the bioactivities of the other extracts used against the same aphid. The antifeeding rates that resulted from the use of the two extracts having the highest bioactivities were 77.81% to 85.55% for the 9 g DW?100 mL-1 extract and 58.37% to 70.70% for the 7 g DW·100 mL^-1 extract. The deterrent rates found for these extracts were between 68.96% and 100.00%. The corresponding mortalities were over 96.34%. The effects of ethanol extracts of garlic on Bemisia tabaci (Gennadius) adults were significant over a 72 h period. The deterrent rates of the extracts on this species were between 69.44% and 100.00%. The effects of the extracts on M. persicae were stronger in nonselective antifeeding trials than in selective trials. In the field experiments, ethanol extracts of garlic produced apparent interference effects on M. persicae or on B. tabaci populations. The antifeeding rate produced by the 9 g DW·100 mL^-1 extract in M. persicae was 65.47%, and the corresponding rate for the 7 g DW·100 mL^-1 extract was 50.16%. The antifeeding rates produced by these two extracts in B. tabaci were 68.63% and 54.83%, respectively. The population control interference index values (IIPCs) of these two extracts in M. persicae were 0.345 and 0.498, respectively. The corresponding rates in B. tabaci were 0.314 and 0.452, respectively.4 Effects of garlic intercropping on the major diseases occurring in fields of flue-cured tobaccoThe study found significant effects of garlic intercropping on tobacco mosaic viruses (TMV, CMV and PVY) and on tobacco bacterial wilt. The occurrence of the two diseases was apparently delayed by the intercropping of garlic in tobacco fields. Moreover, the incidences of both diseases were lower for field treatment B (two rows of garlic planted on the opposite edges of a ridge) than for field treatment D (all ridges planted with K326). The values of disease incidence found in the other field treatments with garlic intercropping were between the values observed for treatment B and treatment D. The values of the economic benefit measures determined for the tobacco leaves were higher in fields with intercropping than in the fields with monocultures. The economic benefit values for field treatment B (two rows of garlic planted on the opposite edges of a ridge) were higher than the values determined for the other treatments.The study found that the populations of tobacco aphids in fields of flue-cured tobacco could be affected by the intercropping of garlic in the fields over the 2-year study period. The major diseases affecting the crop could also be controlled by the practice of garlic intercropping. The diversity or stability of the arthropod communities was markedly increased by the intercropping of garlic in the fields. An especially noteworthy finding of the study was that the economic benefits associated with tobacco leaves in the fields could be increased by the practice of garlic intercropping. Moreover, the adult seedlings of different tobacco varieties were not significantly inhibited by garlic extracts or by soil taken from a depth of 0-10 cm under growing garlic plants. Consequently, this study suggests that garlic might be used as a deterrent intercrop to control green peach aphids and stabilize the arthropod communities in tobacco fields. In addition, garlic may represent a potentially valuable material. An effective agent for protecting tobacco may be developed from whole garlic plants.