Effects Of Wheat-Alfalfa Intercropping On Dynamics Of Macrosiphum Avenae (Homoptera: Aphididae) And Its Natural Enemies

Intercropping of two or more plant species is a common cultural practice to reduce crop losses in many counties, particually; some of its potential advantages are its effects on the population dynamics of pests and natural enemies which may minimize crop damage. In the study, we intercropped alfalfa into wheat to examine ecological effects of this intercropping system on the structure of arthropod community and on the dynamics of pests and natural enemies, as a result, in agrecosystem, polyculture has no significant effects on biodiversity in community level, but distinct ecological efficacy on dynamics and abundance of predominant arthropod species in population level. Effect of intercropping in this system on structure and diversity of invertebrate community was faint so that there were no differences in component and index of diversity of the community between wheat-alfalfa intercropping and wheat monoculture fields. M. avenae was the dominant herbivore in both treatment and contract, and the dominating natural enemies included Coccinella septempunctata, Propylaea japonica (Thungbergs), Aphidius gifuensis Ashmead, Harmonia axyridis Pallas and Allothrombium ovatum Zhang & Xin.Influence of wheat-alfalfa intercropping on the population of M. avenae was significant in 2003. There was a much higher abundance of English grain aphids in wheat monoculture than wheat-alfalfa intercropping fields which led to lower losses in yields and biomass of wheat in wheat-alfalfa intercropping fields. Therein, the numbers of macropterous aphids were not different significantly between intercropping and monoculture, but the difference lied in the abundance of apterous aphids. So we suggested that intercropping of wheat and alfalfa could suppress the vast reproduction of adult aphids on wheat plants which lead to consequently the effective reduction on enlargement of aphid population. Effect of intercropping on dynamics of parasitoids A. gifuensis was also studied by comparison the number of them between wheat-alfalfa intercropping and wheat monoculture fields. The abundance of A. gifuensis was higher in monoculture than in intercropping in both 2002 and 2003, but significant difference existed in 2003. Comparison with the ratio of the number of the parasitoid to that of English Grain aphids between intercropping and monoculture, we learned that the ratio was a little higher in intercropping which indicated that alfalfa intercropped into wheat could be more propitious to parasitoids, under the condition of the same density of hosts. Moreover, higher parasitism of A. gifuensis to M. avenae was consistence with the higher ratio of parasitoids to aphids in intercropping.Intercropping system also significantly influence the dynamics and abundance of coccinellids C. septempunctata, P. japonica and H. axyridis. The numbers of coccinellds were much more enormous in wheat monoculture than in wheat-alfalfa intercropping fields in sampling dates during June and July in both 2002 and 2003, significant differences only in 2003. Aggregation of coccinellids in wheat monoculture with more preys resulted in higher ratio of coccinellids to aphids and decline of that in intercropping. The trombidiid mite Allothrombium ovatum, is well known ectoparasites of aphids, as well as other small arthropods in larval stage. A. ovatum plays an important role in suppressing aphid population in earlier period of occurrence. Wheat-alfalfa intercropping system created a more appropriate microhabitat for oviposition of adult mites. There were much more abundant egg pods in intercropping and which was more 78 fold than in monoculture. Consequently, the parasitisms of aphids by A. ovatum were much higher in treatment than in contrast which resulted in lower growth rates and lower density of aphids in wheat-alfalfa intercropping fields. What's more, A. ovatum was the sole natural enemy which had synchronization in occurrence with its host. In other words, it had not time-log and following effects comparing with coccinellids and parasitoids. As a result,...