Katsuya Shima

Relative importance of weather and density dependence on the dispersal and on-plant activity of the predator Orius minutus

Abstract The present study evaluated the relative effects of abiotic (weather: temperature, wind speed, and rain) and biotic (intra- and intertrophic density dependence: densities of conspecifics, prey per predator individual and leaves on plants) factors on the dispersal and on-plant activity (foraging and oviposition) of the predatory bug, Orius minutus, under seminatural field conditions. Experiments were conducted at plots, each comprising 25 potted azuki bean plants, placed symmetrically in concentric circles, in 1998 and 1999. At the central pot within each plot, 5 marked females of O. minutus (with a male in 1998) were released, and their location and activity were recorded hourly up to 24 h. A total of 78 individuals were released. Stepwise multiple logistic regression was used to select among weather, density dependence, and time variables. Hourly dispersal probability of individuals was positively correlated with temperature and negatively with time since release and with prey density per individual O. minutus. Hourly probability of individuals being active was positively correlated with temperature and negatively with number of leaves of visited plants and conspecific density per leaf. Between-year difference was observed in the probability of individuals being active, which was higher in 1998 than in 1999, probably generated by hunger and higher age. By contrast, diffusion rate was estimated to be lower in 1998, suggesting a trade-off between foraging/oviposition and dispersal by flight. The results indicate that dispersal is affected by temperature and intra-/intertrophic-level density dependence within and between trophic levels, as are foraging/oviposition. The importance of incorporating both abiotic and biotic factors should be stressed when modeling predator–prey metapopulation dynamics on a greenhouse scale.

Cage evaluation of augmentative biological control of Thrips palmi with Wollastoniella rotunda in winter greenhouses

Cage trials of an anthocorid predator, Wollastoniella rotunda Yasunaga et Miyamoto, as a biological control agent of Thrips palmi Karny were conducted in Fukuoka, Japan, under winter greenhouse production conditions. Females of W. rotunda were released on caged eggplants, and placed in two greenhouses on 27 October. The development, population growth, and effectiveness of W. rotunda were observed until early March. Results from the cage trials showed that W. rotunda successfully developed, reproduced, and suppressed T. palmi populations under the conditions found in winter greenhouses. During the experiment, one full generation and a second generation of adult predators occurred. The T. palmi population which was exposed to predators remained at a low density throughout the trial period, but it increased dramatically on eggplants without W. rotunda. The maximum difference between predator treatments and controls was approximately 10‐fold by the end of January. Wollastoniella rotunda has the potential to be an effective control agent for T. palmi on eggplant, even during the winter in temperate regions.

A simple criterion for successful biological control on annual crops

Population dynamics of pest insect-natural enemy systems on annual crops is quite different from those seen in classic biological control programes. On an annual crop, for example, the persistence of populations of pest insects is forced to terminate when crops are harvested. Pest control on annual crops aims to suppress the maximum density of the pest below a certain level, and a low level equilibrium is not always the aim. It is important to determine the initial impact just after release of a natural enemy in order to determine the success of a biological control program. Therefore, effectiveness of natural enemies should be evaluated by prediction of such short-term population dynamics. This paper presents a new and simple analytical model for successful biological control on annual crops. A criterion of successful biological control is given as the ratio of the pest and natural enemy populations just when the pest begins to decrease. This ratio is derived from the intrinsic rates of natural increase of both populations and the daily total predation by natural enemies. Using this model, criteria on appropriate number and time of release of natural enemies are obtained. The practical applications of this model are discussed with respect to evaluating the success or failure of natural enemy releases in future biological control programs.