| Yellowtop (Flaveria bidentis (L.) Kuntz), originated from South American, is an invasive plant in China. It is widely spreading from south of Hebei province, a semi-humid cotton area, to Tianjin, Henan and Shandong provinces, with 23000 hm2 infested area. Bescouse of the same emergence time, similar habitats and the same distribution area with cotton, F. bidentis is considered the potential killer to cotton. In this paper, the competition and allelopathy interference between F. bidentis and cotton, the biological activity and the chemical control method of F. bidentis were studied. The main results were as follows:1. With the competition of F. bidentis, cotton height increased by 5.7% in budding stage. However, the number of fruit-bearing branch and bud, the small boll, the fruit-bearing branch and the big boll decreased by 6.25%-36.7%, 62.8%, 11.6% and 5.59%, respectively, in the budding to bolling stage. In the stage of maturation, the stem diameter decreased by 18.8%. The available boll per cotton was only 4.4, decreasing by 36.6%. The density of 20 plans/8 m row or 4 weeks interference duration of F. bidentis decreased the stem diameter of cotton by 2.8%-16.0%. The cotton yeild decreased evidently after 8 weeks interference. The available boll per cotton, boll per mu, seed-cotton yield and lint yield decreased by 63.7%, 61.2%, 62.8% and 61.3%, respectively, after 10 weeks interference. In conclusion, F. bidentis was strong invasive and competitive in cotton, and it had to be controlled before a density of 4 plants/8 m row and 8 weeks after emergence. The economic threshlod was 2 plants/8 m of row.2. The aqueous extracts of F. bidentis collected in different months had different germinate inhibition effect to the tested weed species in the cotton field. The allelopathy ability of the aqueous extracts collected from the plant sin May to April was stronger than that extracted from the plants in August to October. The inhibitory effect of F. bidentis on weed seed germination increased with the increasing of the concentration of the aqueous extracts. The results showed that the low concentration of aqueous extracts promoted the germination of barnyardgrass (Echinochloa crusgalli) and the high concentration inhibited. The aqueous extracts of root promoted the growth of radicle and plumule of E. crusgalli. However, the aqueous extracts of flower could promote the growth of radicle and plumule of E. crusgalli in low concentration, and inhibited the growth in high concentration. Seed germination rate of E. crusgalli reduced with the increasing of the aqueous extracts concentration of F. bidentis. Inhibition of seed germination of velvetleaf (Abutilon theophrasti) by the extracts from different organs of F. bidentis was as follow:leaf>flower>root and stem. The inhibition effect on radicle and plumule of A. theophrasti become stronger with increasing of the aqueous extracts concentration from root and stem of F. bidentis, but the low concentration of aqueous extracts could promote the growth of radicle and plumule. The inhibition of root extracts to seed germination of Eclipta prostrata was weaker than that from leaf, stem and flower. The germination of E.prostrata was inhibited by all treatments except the low concentration of the aqueous extracts from F. bidentis root, and the inhibition became stronger with increasing of the concentration. Among the aqueous extracts from different organs of F. bidentis, the inhibition of seed germination of cotton was as follow: leaf and flower>stem>root. The aqueous extracts of F. bidentis root, stem and leaf could promote the growth of cotton radicle and plumule. The aqueous extracts of flower promoted the growth of radicle and plumule of cotton cultivar CCRI 36 and the growth of radicle of cotton cultivar CCRI 75. But the same concentration inhibited the growth of radicle and plumule of cotton cultivar CCRI 76 and CCRI 75's radicle. Obviously, F. bidentis has competitive advantage and high-risk invasion in cotton field.3. 72 hours after directly spraying of F. bidentis aqueous extracts at the concentration of 1g·ml-1, the survival rate of Sprodenia litura Fabricius was 54.17%, and the survival rates were 70.00%-80.00% and 62.00%-74.00% 24 and 48 hours after leave soaking test. The death rate of cotton bollworm (Helicoverpa armigera Hübner) and S. litura was 5.0% and 3.33%, respectively, by feeding the mixture of F. bidentis flower powder with artificial diet at rate of 1:6. Moreover, the ratio of 4th-instar larvae of H. armigera reduced by 5.84% and 25.00% after eating the artificial diet with leaf or flower powder. Feeding with leaf powder and the flower powder,, the ratio of 4th-instar larvae of S. litura was 75.83% and 13.33%,respectively, comparing with the check of 63.33%. The inhibition ratio of larval weight was 57.26% and 10.07% by feeding with artificial diet with flower and leaf powder. The inhibition ratio of pupa weight was 3.60%-12.14%, and the eclosion rate was 64.00%-72.00%. Meanwhile, the inhibition of flower was stronger than that of leaf and stem. The peak of eclosion of S. litura was delayed by 3 days after feeding the mixture of the leaf powder with artificial diet and 10 days delay by feeding the flower powder. The peaks of eclosion of male and female were delayed by 5 days and 3 days, respectively, by feeding the mixture of the stem powder of F. bidentis with artificial diet. The peak of eclosion was delayed about 2-3 days by feeding the mixture of leaf and flower powder.4. S-metolachlor 720.0-1440.0g a.i.·hm-2 had the best control effect on F. bidentis among the soil applied herbicides. The inhibition ratios of seed germination, radicle length and plumule length were 97.11%-99.32%, 73.41%-89.40% and 63.09%-93.85%, respectively. Trifluralin 1081.1-1621.6g a.i.·hm-2 had the highest inhibition ratio on F. bidentis radicle with the control efficacy 78.97%-81.87%. Cetochlor 1687.5-3375.0g a.i.·hm-2 had the highest inhibition ratio of plumule length with the control efficacy 60.84%-73.41%. Prometryn 1404.5-2343.8g a.i.·hm-2 also had excellent control effect to F. bidentis. The foliar-applied herbicide, Fomesafen, provided control efficacy 89.06%-100% in density and 95.90%-100% in biomass at the dosage of 281.5-563.1 g a.i.·hm-2 21days after application, Paraquat, Trifloxysulfuron sodium, Pyrithiobac-sodium and Glyphosate also performed excellent control on F. bidentis at the dosages of 375.0-1127.8, 14.1-28.1, 562.5-1125.0 and 768.8-2312.0g a.i.·hm-2, respectively. Fluoroglyco and Napropamide provided relatively poor control on F. bidentis.
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