Mun Il Ryoo

Effect of host density on egg dispersion and the sex ratio of progeny of Bracon hebetor (Hymenoptera: Braconidae)

Egg dispersion and the sex ratio of progeny were studied in relation to the host density of the parasitoid, Bracon hebetor Say, infesting larvae of Plodia interpunctella (Hubner). Females appeared to allocate eggs in relation to host density to avoid laying more eggs than could complete development on a host. The dispersion pattern of the parasitoid ovipositions among hosts was influenced by host density. Multiple visitations and ovipositions by females on hosts caused a highly aggregated pattern at low-host densities. Hatch rate of eggs decreased as the number of eggs on a host increased. Females seemed to regulate progeny sex ratio (male/total) based on the number of eggs on the hosts and the clutch size of the hosts they encountered. However, the overall progeny sex ratio remained at approximately 0.5 regardless of host density, probably because the allocation of eggs was related to host density.

Biology of Paronychiurus kimi(Collembola: Onychiuridae) under the influence of temperature, humidity and nutrition

Summary We studied the influence of temperature, soil moisture content, and nutrition level on the biology of Paronychiurus kimi (Lee). The hatchability and juvenile survival rate were not related to temperature in the range tested from 17 °C to 28 °C. The juvenile developmental rate increased linearly as the temperature increased up to 28 °C, and the lower threshold temperature for development of P. kimi was estimated to be 8.6 °C. The intrinsic rate of natural increase ( r m ) increased with increasing temperature up to 25 °C, but reduced beyond that temperature. An empirical nonlinear model describing the relation between temperature and r m suggests that a P. kimi population is able to develop in the temperature range of 7.9 °C – 30.3 °C, with an optimum at 25 °C. The soil moisture content and nutrition level significantly affect the reproduction of P. kimi . The P. kimi population is unable to increase at 10% soil moisture content except when the springtail is provided with large amounts of yeast. This suggests greater tolerance of well-fed springtails to low soil moisture. Based on this study, we discuss the seasonal fluctuation of the P. kimi population in a rice paddy field.

Populations of fungi and bacteria associated with samples of stored rice in Korea.

Stored rice was collected from rice processing complexes of National Agricultural Cooperative Federation of 11 regions in Korea to evaluate the occurrence of fungi and bacteria and to identify the predominant fungi and bacteria to the genus levels. Most rice samples generally produced the higher levels of fungi and bacteria than white rice. The occurrence of fungi and bacteria varied in various locations of Korea. Among fungi observed, Aspergillus spp. and Penicillium spp. were dominant in the samples and Aspergillus spp. were observed more frequently than Penicillium spp. Predominant bacteria from rice and white rice samples tentatively belonged to the Genus Bacillus, Pectobacterium, Pantoea, and Microbacterium according to BIOLOG and FAME analyses. The results of this study showed that rice in Korea was contaminated in a relatively high level by two dominant storage fungi such as Aspergillus spp. and Penicillium spp. In addition, occurrence of mycotoxins in rice by the fungi could be possible and thus it is necessary to control the storage fungi.

A matrix model for predicting seasonal fluctuations in field populations of Paronychiurus kimi(Collembola: Onychiruidae)

A matrix model for the population dynamics of Paronychiurus kimi in the field, based on life table statistics of P. kimi in a laboratory at constant temperature under optimum environmental conditions is described. The temperature-dependent development and fertility schedule of P. kimi were estimated in terms of degree-days (for temperatures ≥ 7.83 ◦ C). The model simulates reasonably accurately the seasonal fluctuation of P. kimi in Ichon, Korea. It predicted a significantly higher density than that observed in the field between mid April and early June, when the soil moisture content was lowest and with high variance. Our results suggest that soil moisture content is the limiting factor suppressing the population below the density level predicted. The validity of the model as a standard phenology of P. kimi in the field is discussed, including the capability to account for factors other than temperature.

A model for the temperature-dependent developmental rate of Sitophilus oryzae L. (Coleoptera: Curculionidae) on rice

Abstract The development of Sitophilus oryzae (L.) in relation to temperature was studied on rice. The total developmental periods on rice were similar to those reported on wheat. A model constructed by combinging those of Logan et al. (1976) and Wollkind et al. (1978) provided a good description of this weevils development throughout the range of lower and upper threshold temperatures. The lower and upper threshold temperatures for overall development of the weevil were estimated to be 15.0° and 34.1°C.

A modeling study of soil temperature and moisture effects on population dynamics of Paronychiurus kimi (Collembola: Onychiuridae)

The effect of soil moisture on population dynamics of Paronychiurus kimi (Collembola) was examined by combining an empirical soil moisture model with a temperature-sensitive, stage-structured population model. Field observations of soil temperature were used to drive the population model, and simulations were compared to field observations of juvenile and adult densities. Simulations without soil moisture effects produced stable, interyear population dynamics and a significant correlation between simulations and observations (n=12, r2=0.40, P<0.05) but overestimated densities during much of the year. Adding responses to soil moisture improved the fit between simulations and observations (n=13, r2=0.40), suggesting that soil moisture is a major limiting factor on field populations of P. kimi. Moreover, an observed temporary decline in field populations during spring could be explained partly by the effects of an herbicide, paraquat (N,N′-dimethyl-gamma,gamma′-bipyridylium dichloride) (r2=0.45), suggesting that paraquat imposes additional limits on collembolan populations.

Morphological and Molecular Identification of Penicillium islandicum Isolate KU101 from Stored Rice

We have previously obtained a representative isolate KU101 of the predominant Penicillium species from rice under indoor storage conditions. In this study we attempted to characterize isolate KU101 using its morphological and molecular characteristics. When the micro- and macroscopic characteristics of isolate KU101 were compared with the P. islandicum reference isolate KCCM 34763, isolate KU101 was generally identical to those of isolate KCCM 34763, however, isolate KU101 grew faster and produced more orange to red pigments than isolate KCCM 34763. In a molecular-based identification, the nuclear sequence of the ITS1-5.8S-ITS2 region of isolate KU101 was most closely related to that of P. islandicum. Therefore, these results indicated that isolate KU101 from stored rice could be identified as P. islandicum, some isolates of which are known to produce mycotoxins.

Fitness and Sex Allocation of Anisopteromalus calandrae (Hymenoptera: Pteromalidae): Relative Fitness of Large Females and Males in a Multi-Patch System

Abstract The host quality model for explaining the sex ratio of progeny of hymenopterous parasitoids was tested with Anisopteromalus calandrae (Howard) parasitizing Sitophilus oryzae (L.). The life span of females and males did not show any influence of body size: 15.3 ± 6.14 and 15.9 ± 4.16 d for females of small and large size, and 5.4 ± 1.90 and 6.0 ± 2.30 for males. However the reproduction rate of large females was twice as high as for small females (80.9 ± 5.78 versus 37.4 ± 3.16). Large males mated with twice as many females as did small males (19.2 ± 3.8 versus 8.1 ± 3.8), suggesting that females do not gain greater fitness by being large than males. In a system in which the parasitoid was permitted to search among randomly dispersed host patches, large females showed a higher net reproduction rate than small females. This was not influenced by the size of male the females mated with: 132.8 ± 9.81 and 132.5 ± 7.41 for large females mated with large and small males, respectively, and 70.4 ± 13.72 and 54.07 ± 6.17 for small females mated with large and small males. The size-dependent dispersal ability among host patches of females, which is the active dispersal sex, is another factor that influenced oviposition behavior of A. calandrae, which assigns daughters on large hosts and sons on small hosts.

Microbial Population, Aflatoxin Contamination and Predominant Aspergillus Species in Korean Stored Rice

We evaluated microbial populations and aflatoxin production in unhulled and white rice from rice processing complexes of the National Agricultural Cooperative Federation in five regions in Korea and identified three predominant Aspergillus species. Fungal and bacterial populations in rice samples were significantly different between regions in 2007. Aflatoxins were also detected and varied at the levels of 2.45 - 3.43 ng per g unhulled rice grain and 1.29 - 2.09 ng per g white rice grain. Unhulled rice generally detected higher level of aflatoxins than white rice regardless of sampling regions; however, no significant differences were found in Anseong and Cheonan in 2005 and Cheonan and Gimpo in 2007. Aflatoxin production between sampling regions was not different regardless of rice type and sampling year. Although the fungal diversity was highly distinct from region to region, three Aspergillus isolates were predominant in the rice samples; thus, representative isolates AC317, AF57, and AF8 were selected and identified based on their morphological and molecular characteristics. Consequently, isolates AC317, AF57, and AF8 were identified as A. candidus, A. flavus, and A. fumigatus, respectively. These fungi can produce mycotoxins that are harmful for consumers and thus it is important to detect and reduce the population of storage fungi in rice.

Geographical variation in the population dynamics of Thecodiplosis japonensis: causes and effects on spatial synchrony

Geographical variation in population dynamics of a species offers an opportunity to understand the factors determining observed patterns of spatial dynamics. We evaluated the spatial variation in the population dynamics of the pine needle gall midge (PNGM), Thecodiplosis japonensis, which is a severe insect pest in pine forests in Korea, and studied the influences of weather factors that could affect its population dynamics. Results revealed that PNGM population dynamics were classified into five clusters based on the analysis of autocorrelation function and self-organizing map, which is an artificial neural network. We also quantified spatial synchrony in the population dynamics of PNGM using the nonparametric covariance function. Variation in spatial synchrony was strongly related to differences in maximum temperature and precipitation in Random Forest analysis, suggesting that the synchrony in PNGM population dynamics is largely the result of the Moran effect. In addition, spatial differences in population dynamics could be influenced by transient process of synchronization following invasion. Finally, the present results indicate that differences in population dynamics can be induced by interactions among several factors such as maximum temperature, precipitation, and invasion history of species.