Search

Precipitation events can induce episodic CO 2 emissions, so called the “Birch Effect”, which could be affected by soil microbial activity in dryland cropping systems. Our objective was to evaluate the effect of precipitation frequencies on soil CO 2 fluxes and the mechanism responsible for the Birch Effect during fallow after winter wheat harvest in the Loess Plateau of China. A simulated precipitation of 240 mm was applied at 5, 10, and 20 days intervals (I5, I10, and I20, respectively) for 60 days and soil CO 2 flux, K 2 SO 4 -extractable C (EOC) and microbial biomass C (MBC) were measured. The CO 2 flux increased immediately following precipitation events, with peak fluxes of 7.9, 8.2, and 7.7 μmol CO 2 m −2 s −1 for I5, I10, and I20, respectively. Cumulative CO 2 flux from days 55–60 was greater for I5 and I10 than I20. The EOC and MBC were not affected by precipitation simulation at the first event, but MBC increased from the first to the last precipitation event, especially for I5 and I10. The CO 2 flux was strongly correlated with EOC at the first event ( r = 0.73, 0.85 and 0.90 for I5, I10 and I20, P < 0.01, respectively), but not correlated with EOC at the last event for I5 and I10 and the correlation decreased from the first to the last event ( r = 0.47, P < 0.01) for I20. In contrast, the correlation coefficient of CO 2 flux with MBC was not significant at first event and increased to 0.81 ( P < 0.01), 0.44 and 0.47 ( P < 0.05) at the last event for I5, I10 and I20, respectively. Higher precipitation frequency at shorter intervals increased CO 2 emissions compared to lower frequency at longer intervals due to enhanced microbial activity as a result of increased substrate availability from accelerated soil aggregate destruction. The dominant mechanism for the “Birch Effect” shifted from “substrate supply” at the first precipitation event to “microbial stress” at the last event, especially for shorter precipitation intervals. [ABSTRACT FROM AUTHOR]

Trichogramma dendrolimi is one of the most successful biocontrol agents in China. However, an inundative condition is necessary to obtain acceptable parasitism effect. A good solution to this is the application of its thelytokous counterparts which unfortunately are scarce in field. We here report the first case of a natural T. dendrolimi population in China comprising both bisexual wasps and an extremely low proportion of thelytokous wasps. These two forms of T. dendrolimi are phylogenetically related based on the reconstructions of ITS-2 and COI genes. Also, the phylogenetic results suggested a potentially Wolbachia-drived ITS-2 variation. The expression of thelytoky was hardly affected by temperature, which might help control Asian corn borer and Dendrolimus punctatus. Wolbachia are responsible for current thelytoky according to phylogenetic analyses, antibiotic treatment and introgression experiment. We also present the third case of paternal sex ratio chromosome that restrains the expansion of Wolbachia. Moreover, the low frequency of thelytoky may be common in natural populations. Consequently if for biological control it is determined that a thelytokous strain is to be preferred, then large number of field collected females should be set up as isofemale lines, to detect the rare thelytoky. [ABSTRACT FROM AUTHOR]

Thelytokous Wolbachia-infected Trichogramma spp. are widely used egg parasitoids against lepidopteran pests in biological control programs. Wolbachia may manipulate host wasps for superparasitism and is sensitive to temperature. To explore effects of temperature and superparasitism, we compared fitness parameters and Wolbachia-mediated phenotype of thelytokous Wolbachia-infected Trichogramma dendrolimi between those emerging from superparasitised or single-parasitised hosts at 17, 21, 25, or 29 °C. Infected mothers of T. dendrolimi showed reduced superparasitism and parasitism increased with temperature. Wolbachia titre decreased with temperature when females emerged from singly-parasitised hosts, but there was no correlation in superparasitised hosts. Females showed higher Wolbachia titres at 21, 25, or 29 °C when developing from superparasitised hosts. The daily male ratio of offspring increased with temperature, and the day-age threshold for 5%, 50%, or 95% daily male ratio decreased with temperature in both parasitism forms. Females that emerged from superparasitised hosts had a shorter life span and reduced fecundity. These results indicate that Wolbachia may affect host behaviour by increasing superparasitism to enhance its spread, but this has negative effects on thelytokous Wolbachia-infected T. dendrolimi. [ABSTRACT FROM AUTHOR]