Effects Of Intercropping On Arthropod Community And Major Pest Insects In Chinese Cabbage Fields

Intercropping is a widely employed conventional practice throughout the world. It can enhance the nutrient cycle, soil fertility utilization, land and sunlight use efficiency, crop productivity and pest damage reduction. This dissertation presents the results on the study of the effects of intercropping garlic (Allium sativum), lettuce (Lactuca sativa) or green cabbage (Brassica oleracea) with Chinese cabbage (B. chinensis) on the arthropod community, metabolism of the Chinese cabbage, development and growth of diamondback moth (Plutella xylostella) (DBM), as well as the enzyme activities in the. DBM's larvae .1．Effects of intercropping on the structure of arthropod communities in Chinese cabbage fieldsField trials were carried out at Langqi, Fujian, P. R. China in 2004, to determine the effects of intercropping Chinese cabbage with another crucifer species, green cabbage, and two non-cruciferous crops, i.e., garlic and lettuce, on the arthropod communities. Two intercropping plot designs were used in this study. Design 1 planted 2 ridges of the Chinese cabbages with 1 ridge of accompanying plants. The accompanying crops were garlic (CG1) , lettuce (CL1) or green cabbage (CB1) . Design 2 under-sowed the Chinese cabbages in the center (100 cm wide) with garlic (CG2) , lettuce (CL2) or green cabbage (CB2) on both edges (25 cm wide) of a same ridge. The monoculture of the Chinese cabbages (CK) was the control for comparisons in both design 1 and 2．The results obtained from the experiments showed that:(1) A total of 53,392 arthropod individuals belonging to 3 classes, 98 families, with 111 genera and 175 species were collected. In general, no significant differences were found in the number of orders (11 or 12 orders), families or genera between CK and the various intercropped treatments on the experimental lot; but significantly lower number of species was observed in CK than the intercropping treatments;(2) The greatest number of the species collected belonged to the predator guild (i.e., 59 species and 33．71% of the total). The neutral insect guild had the least number of species (i.e., 27 species at 15．43%). The abundance of each guild was as followed: neutral insects (i.e., 62．13% of the total number of individual insects)>herbivores (i.e., 27．24%)> predators (i.e., 6．79%)>parasitoid (i.e., 3．84%);(3) Species richness of herbivorous guild was higher in the intercropping treatments than CK. There were significant differences in diversity index between CK and the intercropping treatments, except CG2．No significant differences on dominance indices were found in CL1, CB1 and CK.(4) Species richness of parasitoid guild decreased in the order of CL1 (33 species)>CG1 (32 species)>CG2 (27species)> CL1 (26 species)> CK (23 species)>CB2 (21 species)>CB1(20 species). Significantly higher abundance of parasitoids was found in CG1, CL1 and CG2 than CK. Although significantly higher diversity index of parasitoid species was found in intercropping treatments than CK, no significant difference of evenness and dominant indices were found between them. (5) In regard to the predatory guild. the highest species richness was found in CG1 and the lowest in CK. The highest abundance was found in CL1 (i.e.. 141．67 predators plot). whereas the lowest in CB1 (i.e.. 97．67 predators plot). Except CL1．significantly higher diversity indices were found in intercropping treatments than CK. The majority of spiders found in the fields belonged tothe Theridiidae family (i.e.. 34．04%) and Lycosida family (i.e.. 30．57%).(6) Higher abundance and dominance indices in neutral insect guild and lower species richness, diversity and evenness indices were found in CK than the intercropping treatments. The most abundant species in the neutral insect guild were springtails (i.e., 87．35%), followed by dipters of the Ephyridae family. Overall, fewer dipters were caught in CK and CB2．Therefore, it was suggested that intercropping the Chinese cabbages with non-cruciferous crops might increase the abundance and diversity of the predators and parasitoids. and that the effects brought about by intercropping might be more pronounced on the natural enemies than on the herbivores.2．Effects of intercropping with non-crucifers on the metabolism of Chinese cabbageThe physiological and biochemical indices of the Chinese cabbage consisted of protein, sugar, reducing sugar. nitrate-N and chlorophyll contents, as well as four plant metabolic indicators, including nitrate reductase (NR). superoxide dismutases (SOD). peroxidases (POD) and catalase (CAT) activities, were compared among the Chinese cabbages grown under different intercropping treatments. In general, higher average protein and nitrate-N contents were found in the intercropped Chinese cabbages than in CK. Significantly higher chlorophyll-a content was found in the cabbages from CL than CK. during the later half of the growing season. No significant differences in sugar concentration were found in CG and CL. as compared to CK. Reducing sugar content varied over the growing period of the Chinese cabbages in CG and CL. In terms of enzyme activity, NR, SOD and CAT activities increased in CG, and SOD, POD and CAT activities increased in CL, as compared to CK. The results suggested that the Chinese cabbages intercropped with non-crucifer plants not only increased plant nutritional quality, but also affected the plant's metabolism.3．Effects of intercropping garlic or lettuce with Chinese cabbage on the growth and development of Plutella xylostella (Lepidoptera: Plutellidae)During 2005, the effects of intercropping on the growth and development of the diamondback moth over 5 generations were examined in Fuzhou. China. The duration of the larval development was generally significantly longer in both intercropping treatments than CK. Intercropping did not affect the larvae pupating rate. However. it did affect the weight of pupa. Pupae in the intercropping lots tended to be 10%-15% larger for three of the five generations. Despite the size increase, pupa's mortality rate was 2-5 times greater in the intercropping treatments than CK. While female DBM laid similar numbers of eggs in all treatments, the egg stage in 3 of the 5 generations was significantly longer in the lettuce intercropping. This suggested that intercropping with different accompanying crop might cause a reduction in the annual number of DBM generations due to prolonged developmental stage and decreased adult emergence rate.4．DBM enzyme activity in relation to intercroppingActivities of three insecticide related enzymes, acetylcholinesterase (AChE), carboxylesterase (CarE) and glutathione S-transferase (GST), of DBM were compared among third instar larvae for 5 successive generations that had been fed on the Chinese cabbages from one of the 3 treatments: CL, CG or CK. Enzyme activities in 30 larvae from each treatment were measured by spectrophotometry. The AChE, CarE and GST activities in the larvae, which were fed with treatment cabbage foliage, decreased significantly, as compared to CK. Therefore, potentially, intercropping could increase delay DBM's insecticide sensitivity on organophosphate. carbamate and pyrethroid insecticides.In conclusion, our study suggested that intercropping the Chinese cabbage with garlic or lettuce could increase the efficiency of nature enemies and decrease that of pest insects. Proper intercropping could not only reduce DBM population but also increase the insect's susceptibility to insecticides by disrupting its metabolism.