Since I got a research position at the Japanese Ministry of Agriculture, I have been attracted by nematode ecology, although my concern was for water bugs' ecology when I was a student. The reason why I have changed my research area from entomology to nematology was that nematodes contain a lot of species and they show great variety of life styles as well as insects do. Some nematode species are hated by farmers because they sometimes cause severe crop damage, while other species, such as entomopathogenic ones, are preferred because insect pests can be controlled with them. Moreover, a nematode community as a whole can be used as an environmental indicator for chemical contamination, physical disturbance at crop fields, grasslands or forests. This broad range of nematode "activities" charmed me to this science. Indeed my interests are given to ecology of plant parasitic, insect parasitic and free-living microbivorous nematodes.
We can't understand exactly what nematodes are doing just by examining nematodes only. Nematodes are living in relationships with their food, enemy, competitor and host organisms. In addition, nematodes are affected by soil physical and chemical factors such as temperature, pH, and organic matter content. My research goal is to understand nematodes' lives as affected by these biotic and abiotic factors at individual, population and community levels, and make use of the obtained knowledge and information for agricultural, biological and environmental sciences. Actually my research subjects currently are (1) analysis of effects of different cropping systems on soil nematode communities, (2) analysis of interactions between fungi and fungal-feeding nematodes.

Research projects running

1. Characterization of nematode communities under different types of crop managements (2000-2003).
2. Assessment of organic farming impact onto soil nematode community structures (2003-).

Publications

Refereed papers .

14, Okada, H., Harada, H., Tsukiboshi, T. and Araki, A. 2005 Characteristics of Tylencholaimus parvus (Nematoda: Dorylaimida) as a fungivorus nematode. Nematology (in press).

13, Kadota, I., Oto, Y., Okada, H., Ishiguro and K.,Hagiwara, H.リンドウ「こぶ症」発症株の根部に見られる生育障害 (in Japanese)　Annual Report of Plant Protection in Northern Japan（in press）

12, Okada, H., Harada, H., Kadota, I. 2005 Fungal-feeding habits of six nematode isolates in the genus Filenchus. Soil Biology & Biochemistry 37, 1113-1120.

11, Okada, H., Harada, H., Kadota, I. 2004 Application of diversity indices and ecological indices to evaluate nematode community changes after soil fumigation. Japanese Journal of Nematology 34, 89-98.

10, Ahmad, W., Okada, H., Araki, M. (2003). Description of Nygolaimoides andrassyi sp.n. (Nematoda: Dorylaimida) from northern Japan. Jornal of Nematode Morphology and Systematics 6 (1): 37-42.

9, Okada, H., Kadota, I. (2003) Host status of 10 fungal isolates for two nematode species, Filenchus misellus and Aphelenchus avenae. Soil Biology and Biochemistry 35, 1601-1607.

8, Okada, H., Tsukiboshi, T. and Kadota , I. 2002 Mycetophagy in Filenchus misellus (Andrassy, 1958) Raski & Geraert, 1987 (Nematoda: Tylenchidae), with notes on its morphology. Nematology 4, 795-801.

7, Okada, H., Ferris, H. 2001. Effect of temperature on growth and nitrogen mineralization of fungi and fungal-feeding nematodes. Plant and Soil 234, 253-262.

6, Okada, H. 1998 Rearing of larva of yellow striped flea beetle, Phyllotreta striolata Fabricius, with flesh leaves of crucifer vegetables (in Japanese). Annual Report of Plant Protection in Northern Japan. 49, 161-163.

5, Okada, H. 1996 Effects of initial population density of Meloidogyne arenaria on yield and growth of soybean plant (in Japanese). Annual Report of Plant Protection in Northern Japan. 47, 105-106.

4, Okada, H. 1995. Propagation of two fungivorous nematodes on four species of plant-pathogenic fungi. Japanese Journal of Nematology 25, 56-58

3, Okada, H. and Nakasuji, F. 1993 Patterns of local distribution and coexistence of two giant water bugs, Diplonychus japonicus and D. major Esaki (Hemiptera: Belostomatidae) in Okayama western Japan. Japanese Journal of Entomology 61, 79-84.

2, Okada, H and Nakasuji, F. 1993 Comparative studies on the seasonal occurrence, nymphal development and food menu in two giant water bugs, Researches on Population Ecology 35, 15-22.

1, Okada, H., Fujisaki, K and Nakasuji, F (1992) Effects of interspecific competition on development and reproduction in two giant water bugs, Diplonychus japonicus Vuillefroy and Diplonychus major Esaki (Hemiptera: Belostomatidae). Researches on Population Ecology 34, 349-358.

*My lovely worms*

1,

Opposite relationships of nematodes with fungi. Oyster mushroom, Pleurotus ostreatus, is known as "nematophagus fungus", since the fungus hyphae excretes toxic chemical to paralyze nematodes, and the hyphae resolve and absorb nematodes' body. Indeed a fungal-feeding nematode, Aphelenchus avenae, is being fed upon by the fungus (left, Click to zoom out!). However, I recently found that the other nematode, Filenchus misellus, can avoid the fungal attack and feed on the hyphae (right)! For details please see my paper coming soon in Soil Biology and Biochemistry.

2,

A nematode, Aphelenchoides sp., is feeding on fungal hyphae. Fungal-feeding nematodes contribute to nitrogen mineralization under certain conditions in soil ecosystems. My photo covered a front page of Plant and Soil (Click!).

3,

A nematode of 0.4 mm length, Filenchus misellus, is feeding on fungal hyphae growing on water agar. This nematode is considered as one of the primitive species in the order Tylenchida which contains plant parasitic species such as root knot nematodes (Meloidogyne spp.) and soybean cyst nematodes (Heterodera spp.) My photo of Filenchus, on a front page of Nematology, indicates clearly for the first time the fungal-feeding habit of the nematode (Click!).