| Ragweed, Ambrosia artemisiifolia is one of the most important worldwide harmful weed. It is seriously harmful to agriculture and human health. Its control is brought to a great difficultity because of a large amount of its seeds with secondary dormancy characteristics. Epiblema strenuana and Ophraella communa are two host-specific natural enemies against A. artemisiifolia, and they suppressed significantly A. artemisiifolia population. In order to understand the applied prospects of the two natural enemies in China, the effects of different temperature and relative humidity on the development, survival and fecundity of O. communa were studied in the laboratory, which would be used to make prediction of its adaptable climatic condition; co-control efficacy of E. strenuana and O. communa on A. artemisiifolia in different co-density treatments were studied, the findings may estimate co-control of A. artemisiifolia with the two species in the field. The main results are as follows:1 The survival rates of different stages, the developmental periods of different stages, the longevity and fecundity of O. communa differed significantly among the different temperatures (20, 22, 25, 28, 30, 32℃). The hatchability of eggs was the lowest at 30℃, which was only 61.9%, the highest was up to 85.9% at 25℃. Both the survival rates of larval and entire immature stages were the lowest at 20℃, with the values of 14.5% and 9.6%, respectively, and the highest were 79.3% and 61.4%, respectively, at 28℃. The survival rate of pupae reached the lowest (90.3%) at 20℃, and the highest (99.0%) at 25℃. Except for larval stage, the developmental periods of different stages and longevity of O. communa were shortened along with the increasing temperature. The parabolic relationship between female's fecundity and temperature was observed in this experiment. The fecundity of female was the least at 20℃, which was 521.8 eggs per female and reached the maximum at 28℃, of 2712.3 eggs per female. The optimum temperature range for the development of O. communa was 25℃to 28℃.2 The difference in the survival rates of different stages, the developmental periods of different stages of O. communa among different humidities (60%, 75%, 90% RH) was signifcant, and the exception of pupal stage was noticed. But the longevity and fecundity of O. communa showed no significant difference in the different temperature treatments. The hatchability of eggs was the lowest at 60% RH, which was only 55.57%, and the highest was up to 87.72% at 90% RH. The survival rate of larvae reached the lowest (72.81%) at 60% RH and the highest (82.63%) at 75% RH. The survival rates of pupae had no significant difference among different humidity, which were from 96.18% to 98.72%. The developmental periods of egg, larval and entire immature stages were the longest at 60% RH, with the values of 6.85 d, 16.84 d and 30.49 d, respectively, and the shortest at 90% RH, of 6.03 d, 11.27 d and 24.88 d, respectively. The developmental period of pupal stage reached a maximum (7.66 d) at 90% RH, which were shorter at 60% RH and 75% RH (7.04 d and 7.13 d, respectively). The number of eggs laid by one female was 1329.7 to 1490.5 eggs at different relatively humidities, and longevity was between 74.40 and 79.65 d. The optimum humidity range for the development of O. communa was 70% to 90% RH.3 In the adult period, when the density of A. artemisiifolia was 3.3 plants/m~2, the plant height of A. artemisiifolia was weekly inhibited by O. communa and E. strenuana in the different co-density treatments, and showed on significant difference among the different treatments in the fields. The two natural enemies might to some extent suppress the branches of A. artemisiifoli, and the relative inhibition rate was up to 58.37% at the high co-density (16 E. strenuana+12 O. communa). In the co-control treatment, the leaf control indices of O. communa on A. artemisiifoli increased with time, and 45 days after releasing natural enemies, the leaf control indices in the co-density treatment of 10 moths+6 beetles and co-density treatment of 16 moths +12 beetles were 0.936 and 1.0, respectively, which was significantly higher than that of Roundup. The two natural enemies could kill all A. artemisiifoli plants. The seed yield of A. artemisiifoli was 0 in the highest co-density treatment, and the decreased rate of seed yield was 99.50% in the second highest co-density treatment. The control effects of the two high co-density treatments of the two natural enemies on A. artemisiifoli were better compared with Roundup and other treatments. The decreased rates of the weights of aboveground and underground in the highest co-density treatment were 59.31% and 60.28%, respectively, which were significantly higher than those in other treatments. As long as the co-density of O. communa and E. strenuana is appropriately enhanced, A. artemisiifoli will be well suppressed even if A. artemisiifoli is in the mid and late period.
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