Lettuce was a leaf vegetable with short growing term and cold-tolerant plant. We grew it the whole year. The price of lettuce is the most expensive from July to September. But damages by lots of insect pests appear to greenhouse grown lettuce in summer season. In this study, we observed biological controls of western flower thrips(Franklinella occidentalisi) by the predator mites, Amblyseius swirskii and Neoseiulus cucumeris in lettuce greenhouse from July 7 to September 10. Hypoaspis aculerfer larvae and adults were released 30 individuals per m2 at the first one time as lettuce was planted on the greenhouse. We checked the density of F. occidentalis every 10 days on the 30 lettuce leaves. When the population of F. occidentalis reached 1~5 individuals per a leaf, A. swirskii larvae and adults were released 35 individuals per m2 at 1~3 times and Neoseiulus cucumeris larvae and adults also were released 150 individuals per m2 at 2~3 times in each lettuce greenhouse. During the high temperature season, Amblyseius swirskii was more effective biological controls of F. occidentalis than Neoseiulus cucumeris.

This study was conducted on the biological control effects of the natural enemy, Amblyseius cucumeris , against Polyphagotarsonemus latus. The broad mite moved to a new habitat after the carrying capacity of the preferred habitat on the upper leaf has been reached. Population dynamics between predator, Amblyseius cucumeris, and prey, P. latus, were quite well synchronized since the predator followed the movement of the prey. In predation test by Amblyseius cucumeris on P. latus, A. cucumeris fed on 8.8, 15.2, 27.0 and 30.3 individuals/day under the temperatures of 15, 20, 25, and 30℃, respectively. The A. cucumeris larva, protonymph, deutonymph, adult male, and adult female fed on 0.5, 1.6, 5.2, 3.8, and 27 individuals of adult P. latus per day, respectively, under 25℃ condition. The functional response curve of adult P. latus to the densities of A. cucumeris showed Holling’s Type Ⅱ curve. The consumption of prey by A. cucumeris increased with the prey density increase, but the consumption rate decreased. The critical ratio of predator vs. prey effectively to suppress (98% over) the population of P. latus by releasing A. cucumeris was 1:40 on red pepper.

Effectiveness of Orius strigicollis (Poppius) and Amblyseius cucumeris (Oudemans) as natural enemies against thrips were evaluated on greenhouse green pepper, sweet pepper and cucumber respectively. Control efficacy was calculated by the formula, (D<SUB>control</SUB> - D<SUB>treatment</SUB>)/D<SUB>control</SUB> × 100, where D<SUB>control</SUB> is the average density of thrips on the plots in which any natural enemy was not released and D<SUB>treatment</SUB> is the average density of thrips on the plots in which natural enemies were released respectively. As a result, control efficacies of O. strigicollis against Frankliniella occidentalis (Pergande) on green pepper and sweet pepper were 14.3~99.5% and 21.6~98.3%, respectively. In addition, control efficacy of it against Thrips palmi Kany on cucumber was 61.2-74.4%. Control efficacies of A. cucumeris against F. occidentalis on green pepper and sweet pepper were 12.9~38.3% and 17.1~87.0%, respectively. Control efficacy of it against T. palmi on cucumber was 90.4~97.4%. Field evaluation showed that the prompt applications of natural enemies were effective to reduce the density of thrips. In detail, to control F. occidentalis effectively on green pepper and sweet pepper in spring season, five to six individuals of O. strigicollis per crops should be released three to six times continuously. To control T. palmi effectively on cucumber in autumn, more than 100 individuals of A. cucumeris per crop should be released four times repeatedly.