Our major research subjects are: (1) systematics of insects, (2) insect behavior including the role of bio-active chemicals, (3) biological control of insect pest, (4) population ecology of insects in agroecosystems, and (5) faunal analysis of agroecosystems. Major results in FY 1996 were as follows.

Systematic studies: Twenty five species of wasps parasitic on the legume leaf miner, Liriomyza trifolii, were listed from Japan and an illustration key to the wasp species was constructed.

Behavioral studies: The flight behavior of some pyralid and noctuid moths was analyzed in relation to the reproductive procedure. The aggregation pheromone of a stink bug,Plautia stali, was identified and a synthetic attractant of the bug for commercial use was developed, and some aspects of the ecological significance of the pheromone was analyzed.

Biological control studies: A simulation model to describe the association among thrips ( Thrips palmi ), predatory anthocorid bugs ( Orius sauteri ) and crops was developed on the basis of a general simulation model developed in the University of Wageningen. A rearing system of Diglyphus isaea, a parasitoid wasp of Liriomyza trifolii,was developed with L. trifolii larvae on the potted plants of the kidney bean as host.

Ecological studies: The influence of global warming on the occurrence of various insect pests was estimated. Estimations were made by simulations based on the temperature-insect and/or plant models and by analyzing long term insect abundance and meteorological data.

Faunal studies: Malaise traps were monthly set on a stream along paddy fields as a new monitor method to evaluate biodiversity in rural environments. Special attention was made to the trichopteran fauna for searching available bio-indicators and environmental factors affecting on faunal composition.

Topic1

Environmental Risk of an Introduced Parasitoid Species of Chestnut Gall Wasp

Torymus sinensis, a specific parasitoid species of the chestnut gall wasp, Dryocosmus kuriphilus, was introduced from China and was released in Tsukuba in 1982. T. sinensis has expanded its distribution gradually and caused reduction of this pest in eastern part of Japan. This is a very successful case of classical biological control. The environmental risk of classical biological control is becoming an important issue in many countries. T. sinensis has been demonstrated to be able to hybridize with a closely related indigenous species,Torymus beneficus. Hybrids have ovipositor sheathes of intermediate length compared to the two species (Fig.1). Interspecific competition between them was also expected when T. sinensis was introduced. Effect of established T. sinensis on T beneficus in the field was studied from this point of view.
Galls of the chestnut gall wasp were collected from nine sites in Kanto and Chubu Area and the number of emerged adults of parasitoids from galls were checked to study the species structure. In the areas where considerable numbers of both T. sinensis and T. beneficus emerged, the former species increased and the latter decreased year by year. This is considered to be a result of competition between the two species.

The extent of hybridization between these two torymid species in the sites were studied using isozyme analysis and a morphological character (ratio of length of ovipositor sheath to length of thorax). Frequencies of T. sinensis, T. beneficus and their hybrid identified by isozyme analysis are expressed in relation to the morphological character in Fig.2. Only nine hybrid individuals were found from 856 individuals analyzed.Most individuals of intermediate morphologic type were identified as T. sinensis. Therefore, it is concluded that the frequency of hybridization between the two species is not very high in these sites.

Topic2

Relationship between the Number of Males Captured by Sex Pheromone Traps and the Wild Population Density in the Diamondback Moth.

In pest insect control, sex pheromones are used in three ways: (1) estimation of wild population density, (2) mass trapping, and (3) communication disruption. To ensure a more accurate estimation of population density, we need to understand the relationship between the number of insect individuals captured by the sex pheromone trap and the actual wild population density in the field. However, this relationship has been little investigated in any insect species,because estimating wild population density is very laborious.

With the diamondback moth, Plutella xylostella, a serious insect pest of cruciferous vegetables, nine mark-release-recapture experiments (five in the summer and four in the autumn) were conducted using male moths marked with fluorescent dye (Uvitex-OB and Rhodamine-B) and sex pheromone traps (Fig.3). On the basis of the mark-recapture data, wild population density of the male diamondback moth was calculated by the Yamamura mathematical model. The number of male moths recaptured per sex pheromone trap per day (X) showed a significant positive correlation with the estimated wild population density per 100m² (Y)( r=0.771 ) (Fig.4). Here, it was estimated that about 150 male moths would be present per 100m2 in the vegetable field, if ten male moths were captured per pheromone trap per day.

In many insect pests including the diamondback moth, the number of individuals captured by sex pheromone traps often vary from day to day and field to field. To compensate for this defect,some caution must be exercised. For example,some pheromone traps should be placed evenly in the vegetable fields, and the mean number of moths captured for three to five days should be used as the number of moths captured daily per pheromone trap (X-axis in Fig.4).

A mark-recapture experiment provides various important information in field ecology.Another example of a mark-recapture experiment with a more extended pheromone trap network indicated that the adult male diamondback moths were capable of flying 300 to 600 m per day and they were active fliers in the vegetable field.

Fig.1 Females of Torymus sinensis (left), T. beneficus (middle) and their hybrid (right) showing different length of the ovipositor sheath

Fig.2 Frequency distributions of T. sinesis, T. beneficus and their hybrid expressed in relation to a morphological character

Fig.3 Components of the sex pheromone of the diamondback moth (upper) and a pheromone trap in a cabbage field (right)

Fig.4 Relationship between the number of males captured by the sex pheromone trap per day (X) and the wild population density (Y)