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Entomopathogenic nematodes and soil-dwelling predatory mites: Suitable antagonists for enhanced biological control of Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae)?

Entomopathogenic nematodes and soil-dwelling predatory mites: Suitable antagonists for enhanced biological control of Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae)?

Leaders:  Prof. Dr. H.-M. Poehling, Dr. R. Meyhöfer
Team:  Oliver Berndt
Year:  2002
Date:  01-05-98
Sponsors:  DFG
Lifespan:  1998-2002
Is Finished:  yes

Summary

Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) (estern lower hrips, WFT) is one of the most important pests damaging a wide range of economic important crops in protected cultures worldwide. Its cryptic life cycle, combined with a very short generation time and the ability to rapidly develop resistances against insecticides, are characteristics that make pest control extremely difficult. Moreover, commonly used natural enemies often do not lead to sufficient control levels. However it has been repeatedly reported that the life cycle of includes a soil passage, which is still neglected in biocontrol strategies. Subjects of the present study were the quantification of the soil passage of and the evaluation of the effectiveness of predatory soil mites and entomopathogenic nematodes (EPN) as natural enemies against WFT. Thus, microcosm experiments were carried out on individual potted plants, which had been covered with acryl-glass tubes. The extent of the soil passage and the control success for synchronised and mixed WFT populations was quantified by the use of photo eclectors. Additionally, the suitability and susceptibility of the soil-dwelling thrips instars towards predation or infestation by predatory mites or EPN was investigated in arena experiments. The soil passage was quantified with and three ornamentals (, and ) as host plants. The results show that only a small part of a population pupates on the plant and that the variation between host plant species is small. On only 1-3 % of the thrips population remained on the plant for pupation. With flowering ornamental host plants the proportion increased slightly on D. grandiflorum (4.6%) and (6.8%) and was highest on (7.15%). Flowers were not preferred as the pupation site, a maximum of one third of the thrips that pupated on the plant were located in the flowers.

The introduction of five predatory mites to the soil caused a thrips mortality of at least 44.9%. Doubling the predator density to 10 increased thrips mortality to 61% A maximum thrips mortalityof 80.5% was achieved by 20 . Over all, showed a higher efficiency as a natural enemy against soil-dwelling thrips instars than . Nevertheless, at least a small part of the thrips population escaped from predation in spite of the fact that in arena experiments high predation rates of spp. were observed. All thrips instars were suitable as prey and for the maintenance of reproduction for both species. The number of killed thrips larvae, prepupae or pupae was similar for both and . On average, females of the latter preyed on 3.5 thrips instars and laid 2.5 eggs per day, whereas females of preyed on 1.61 thrips and laid 0.8 eggs per day. Males of both species killed 0.6 thrips per day. Cannibalism, as a possible reason for a reduced efficacy of the predatory mites against WFT, could be excluded since the cannibalism rate of both spp. was very low and rarely exceeded one conspecific individual within three days. Only nymphs showed a cannibalism rate of on average one conspecific egg per day. In the presence of alternative prey, cannibalism never occurred. Conceivable reasons for the limited thrips control in microcosm experiments are discussed.

Possible alternative natural enemies of thrips instars in the soil are EPN. To investigate the efficiency of EPN against thrips instars in the soil, six different strains were selected which included two strains ( HK3 (H.b H) and HB Brecan (H.b B)), three strains (. Sylt (S.f S), OBSIII (S.f O) and strain CR (S.f C)) and one strain (DD136 (S.c D)). All tested thrips instars were susceptible to all EPN strains. The most virulent strains were S. (S), S. (D) and H. (H), but a high concentration of 400 infective juveniles (IJ) per cm2 was necessary to obtain high thrips mortality rates of at least 65%. Nevertheless, dose rates of 100-200 IJs/cm2 already caused 30-50% mortality in WFT. The efficacy of S. (S) against different thrips instars in the soil revealed that improved thrips control was realized if the proportion of second instar larvae in the thrips population was low. WFT prepupa and pupa were similarly susceptible to and their proportion in the population did not affect the mortality caused by EPN. The highest mortality rate (80%) was recorded for populations consisting only of prepupae and/or pupae. In microcosm experiments, the impact of S. (S), S. (D) and H. (H) at concentrations of 400 and 1000 IJ/cm² was investigated against the thrips stages in the soil. All tested EPN strains at both dose rates significantly reduced WFT populations. Up to 70% reduction in the WFT population was obtained with the higher EPN concentrations.

Over all, both species and the selected EPN strains can substantially reduce a thrips population and might be important antagonists to optimise control in protected crops. However, both soil dwelling antagonists seem not to be able to keep below an economic threshold level. It is recommended to combine the tested soil foraging thrips antagonists with predators acting on the plant parts above soil.

Keywords: , Biological Control,  spp., Entomopathogenic Nematodes