EFFECTS OF NEEM PREPARATIONS ON PLUTELLA-XYLOSTELLA L AND ITS NATURAL ENEMIES OF THE GENUS TRICHOGRAMMA

P. xylostella (Lepidoptera: Yponomeutidac) is one of the most important cabbage pests in tropical agriculture. To combat this pest, effects of neem seed kernel water extracts 25 g or 30 g seed kernels/liter of water, on the pest and its natural enemies T. pretiosum and T principium (Hymenoptera: Trichogrammatidae) were studied by laboratory and a field experiment in Taiwan. Surprisingly, treated cabbage plants were more attractive for egglayng by P. xylostella (1073.25 eggs/plant) than untreated plants (351.75 eggs/plant) in the laboratory as well as in the field (treated = 33.40 eggs/plant; untreated = 9.47 eggs/plant). In the field study, the number of larvae were significantly reduced by application of neem to 4.98 and 5.92 larvae/plant at the end of the experiment compared to the other treatments: 18.57 larvae/plant (control) and 25.17 larvae/plant (release of T pretiosum). Development of P. xylostella pupae was generally low and about zero on neem-treated plots. Parasitization of the eggs of P. xylostella by T principium was not affected by neem treatment (99.75 parasitized eggs/plant) compared to control (78 parasitized eggs/plant) in the laboratory, as well as in the field (T pretiosum). Natural parasitism of T chilonis was evident but low in the field. The effectivity of release of T. pretiosum on parasitism was high in all variants and replications and reached an average of 47.03 % of P. xylostella eggs after two releases on neem-treated and untreated plots, respectively. Parasitism (%) was not sufficient because of a relatively low number of released parasitoids (50/m2). There was a positive correlation (r = + 0.62) of parasitization to high egg densities. On the other hand, a negative correlation of high egg densities (r = -0. 17) to percentage of parasitism was obvious. The more eggs were laid on plants the more eggs were parasitized but parasitism (%) decreased. In che laboratory, however, neem-treated host eggs were less parasitized (17.17 eggs/female of parasitoid) than untreated host eggs (33.63 eggs/female of parasitoid). After two treatments, adults of neem-treated Trichogramma-pupae died not emerge. Synthetic sexual pheromone use had a positive effect on parasitization (+ 12.92 parasitized eggs/female) as well as a methanolic neem seed kernel extract (+ 7.42 parasitized eggs/female). There was a negative effect on parasitization after a treatment with neem oil (- 13.33 parasitized eggs/female) and SAN 415 (Bacillus thuringiensis) (- 5.83 parasitized eggs/female). In addition, neem oil reduced the emergence rate of T principium by 45.13 %.