Biological control of Thrips tabaci in the field – possibilities and practical limits

Field trials against Thrips tabaci, an economically important pest insect, were conducted in 2003 at four different sites in Germany. The effect of the combined use of entomopathogenic nematodes and fungi on thrips should be investigated, in comparison to the effect of either pathogen alone. Commercially available products, Mycotal® (Koppert NL), PreFeRal® (Biobest BE), Naturalis L® (Intrachem IT), Nemaplus® and Nemagreen® (both E-Nema DE), were used. The trials were performed in onion, leek and chives, according to the EPPO guideline PP 1/85(3). Perfekthion® was used as chemical standard in onion. In leek, Spruzit® and Neudosan® were applied alternately. No chemical standard was applied in chives. The biocontrol products were applied either alone or in a mixture using common spray equipment. The nematodes were applied at a rate of 1x106 infective juveniles/m2. The dosage of the fungi was 1 kg (1.5 l in the case of Naturalis L®)/ha. The treatments started mid June and were repeated up to six times in weekly intervals. Insect pressure was low at all four sites, throughout the summer. T. tabaci was recorded only in medium numbers (mean of 30 specimen/plant max.). In two trials, no differences were detected between the treatments and the control. In the third trial (onion), a significant reduction was recorded for the treatment ‘PreFeRal+Nemaplus’, both in the number of thrips/plant (2 compared to 6 in the control) and the frequency of infestation (38 % compared to 93 % in the control), one week after the final application (3 times in total). Also in the fourth trial (leek), the number of thrips/plant was lowest for the treatments ‘fungi+nematode’ (4 compared to 7 in the control), one week after the final application (6 times in total). In this trial, yield was measured additionally, and the weight/plant registered at harvest was 20 % higher for the treatments ‘Nemaplus’ and ‘Mycotal+Nemaplus’ than in the control (425 and 412 g/plant compared to 345 g respectively). The results presented here, show that the use of biocontrol nematodes and fungi need not be restricted to protected environments, field application against insects on the upper parts of plants is possible. Moreover, their combination has the potential to give a better reduction of the pest population than each agent alone. In consideration of the extreme weather conditions in Germany in 2003, and in comparison to the effect of the chemical standards, the results are regarded as promising. However, each crop situation needs its own proof of feasibility, since factors like the microclimate, created by the plant’s individual characteristics and the canopy it forms, may influence the activity of the pathogens and by this their efficacy.