Browsing by Subject "Plants-Insects-interactions"

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Inter- and intraspecific variation of nutritional and environmental adaptation of egg-parasitoids of the genus Trichogramma (Hymenoptera, Trichogrammatidae)
(2013) Alkarrat, Hamdow; Zebitz, Claus P. W.
The biology and the use of egg parasitoids of the genus Trichogramma (Hymenoptera: Trichogrammatidae) as a biological control agent has been studied since 1904 and more than one thou-sand scientific papers on Trichogramma have been published. However, there are still open questions about the impact of biotic and abiotic environmental factors on the biology of these beneficials, such as nutritional ecology, diapause, cold storage, plant-pest-Trichogramma-interactions, and, particularly, the intraspecific variation of environmental adaptations. These parameters can seriously affect the effectiveness and the practical application of these parasitoids in many crop production systems. In this study, we have investigated the effect of: (i) different of diets, (ii) properties of pest?s host-plant surfaces and (iii) overwintering and cold storage on the performance of different species and strains of Trichogramma to select the best candidate for biocontrol of the Codling moth, taken here as a model target. Experiments with different diets (honey, pollen and honey with pollen) were carried out and their effects on life-table parameters of four strains of Trichogramma cacoeciae Marchal (HOH 1, HOH 2, KIO 2 TN and SY 98 01), and one strain of T. evanescens Westwood (195 DE 06) and T. oleae Voegelé (See 2) was evaluated under laboratory conditions. Honey diet im-proved significantly adult mean longevity, mean fecundity, mean fertility, and net reproduction rate (R0). Significant intraspecific variability was shown only in mean fecundity and mean fertil-ity, but not in longevity. The best performing strain of T. cacoeciae (HOH 1) in these experi-ments has been taken to assess the nutritional value of pollen collected from birch, maize and sunflower. Pollen as honey additive had a significant positive effect on female longevity as com-pared to honey alone, but longevity did not differ significantly between the honey-pollen diets. Pollen added to water had no ameliorative effect on longevity except sunflower pollen, which had a significant detrimental effect. Mean total fecundity and fertility of females was not im-proved when fed honey and birch or maize pollen. Sunflower pollen added to honey, however, had a pronounced detrimental effect on these parameters. When these three pollens were offered with water, all parameters were not or negatively affected, revealing sunflower pollen having the worst nutritional value. Tests on the suitability of bee-pollen to ameliorate Trichogramma mass-rearing were done with T. cacoeciae, T. evanescens and T. oleae. Except fertility, longevity and fecundity differed significantly between the species. Basic diet (water or honey) and pollen added showed interactive effects on the performance of Trichogramma species. Interactions between species and added pollen only revealed significant differences in fecundity and fertility, whereas longevity was only slightly affected. Species - basic diet - pollen interactions could only be shown for fertility. Trichogramma oleae had a shorter adult lifespan and the lowest number of parasitized eggs compared with the other two species, but it had the highest rate of emergence when fed on honey mixed with bee pollen. The results indicate that (i) Trichogramma adults are feeding upon pollen, and (ii) nutritional quality of pollen differs significantly between plant spe-cies, and (iii) honeybee-pollen may be used to improve mass-rearing of Trichogramma species. Properties of the pest?s host-plant surfaces (apple, cv. ?Golden Delicious?, ?Boskoop? and ?Topaz?) were evaluated in their effect upon searching behaviour and foraging success of three strains of T. cacoeciae (HOH 1, KIO 2 TN and SY 98 01) when eggs of the Codling moth, Cydia pomonella (Lepidoptera: Tortricidae) were offered as hosts. Foraging time differed significantly between the strains tested, showing strain HOH 1 performing best. Host handling time and time spent post-parasitization did not differ between the strains. The apple cultivar did not affect any of these parameters. Leaving time was significantly affected by chemical cues comprising lepidopteran egg and scale volatiles or contact chemicals, host pheromones, host frass or female moth accessory gland secretions abundant on the leaves, where giving up time was shortest on blank leaves and, compared to blank leaves, significantly longer on leaves with adult stimuli only but no host eggs. Leaving time was further affected by apple cultivar and T. cacoeciae strain tested. Compared with the other cv. tested, time spent on leaves of cv. ?Golden Delicious? was longest for any treatment. Trichogramma cacoeciae strain HOH 1 always spent the shortest on any cultivar and treatment. In another experiment, parasitization of eggs on apples placed at different heights (0, 25, and 50 cm above cage ground) was assessed. The parasitization rate of Cydia eggs was slightly affected by apple varieties; however the differences were not consistent. Parasitization rates decreased with height of apples above ground, with the highest rate on apples placed on ground, near the releasing point of Trichogramma females. Significant differences in parasitization rates between strains could be detected, but they were not consistent also. One German and one Tunisian strain of Trichogramma oleae and two German, one Tuni-sian, and one Syrian strain of T. cacoeciae were hibernated from October 10th 2007, up to parasi-toid emergence in 2008 under field conditions in Stuttgart-Hohenheim, Germany. 60 days after exposure to outdoor conditions and after complete hibernation life table parameters of survivors was assessed. Our results demonstrate that all strains studied were able to overwinter successful-ly on eggs of the factitious host Sitotroga cerealella Olivier (Lep.: Gelechiidae). Mean develop-ment time of trichogrammatids until adult emergence after full hibernation ranged between 183 and 189 days. Emergence rate ranged between 53.59 and 60.98 %. Net reproduction rate (R0) of T. oleae strains decreased with duration of field exposure, whereas in T. cacoeciae strains R0 was lowest after 60 days field exposure. Longevity of all adults emerged from parasitized eggs was affected by field exposure compared to the control. Total fecundity, total fertility and percent fertility after 60 days and/or after hibernation showed neither significant difference between strains nor within the strains studied. Total fertility of T. cacoeciae strains decreased significantly after midterm and/or after full hibernation compared with the control, while no significant differences were detected within T. oleae strains. Time until adult emergence, mortality, and percentage of deformed adults increased with storage duration at 4 °C. Trichogramma oleae (German strain) showed best adaptation to cold storage for 50 days. Finally and based on this study, our data suggest that the diets of honeybee-pollen could be used as food to improve Trichogramma mass-rearing, and also the German candidate T. cacoeciae HOH 1 could be a potential candidate for future mass rearing and field release programs for biocontrol of different lepidopteran pests in orchards. Furthermore, it could be proved that even allochthonous Trichogramma species / strains originated from countries with a higher annual average temperature (Syria and Tunisia) are theoretically able to establish and survive winter conditions in southwestern Germany. At least this possibility requires an environmental risk-analysis for these species/strains or any potential other Trichogramma species intended to be used as biocontrol agent in Germany.