Biological control of two-spotted spider mites using phytoseiid predators

The searching behaviour of individual predators of four phytoseiid species ( <em>Phytoseiulus persimilis</em> , <em>Amblyseius p</em> o <em>tentillae</em> , <em>Amblyseius bibens</em> , <em>Metaseiulus occidentalis</em> ) is investigated in relation to the two-spotted spider mite ( <em>Tetranychus urticae</em> ), which infests greenhouse roses. Especially the role of spider- mite webbing in the predator-prey relation is studied. Webbing interferes with searching, decreasing the rate of encounter per unit prey density. Low walking speeds and activity in webbing ensure that the predator is rarely disturbed after contact with other mites. Webbing also positively influences searching, as spider mites aggregate within the webbed area: prey density, defined here as the number of prey per square centimetre of webbed leaf area, is high, as is the rate of encounter with prey. The ability to capture a prey after tarsal contact depends on the food content of the gut, the prey-stage and, in two specific cases, the webbing; the success ratio of <em>P. persimilis</em> increased on a webbed substrate, that of <em>A. potentillae</em> decreased.<br/>Models to simulate rate of predation on the basis of the dynamics of the motivational state and the state dependent rate of successful encounter are proposed. The food content of the gut is chosen as an indicator of the motivational state. A stochastic queueing model simulates predation as accurately as a Monte Carlo model or a compound simulation model. The queueing model is preferred because of its economic use of computer time and the relatively few variables used. The model was validated in predation experiments.<br/>Systems analysis showed that the effect of temperature on the rate of predation is largely determined by its relation with the relative rate of food conversion into egg biomass and not by behavioural changes related to temperature. Also, it was shown that webbing has an important influence on the predation rate. A new model for the analysis of prey-stage preference is proposed.<br/>Predators invade the webbed leaf area after contact with the silk strands, irrespective of the presence of prey. The residence time in the prey colony is determined by prey density. Simulation of experimentally defined walking behaviour shows that predators remain in profitable prey patches by turning at the edge of the webbed leaf area. However, when predator density increases, the tendency to leave the prey colony also increases, even at high prey densities. Only <em>A. potentillae</em> avoided the webbed leaf area, preferring the thickest parts of the leaf ribs or other protected places on the plant.<br/>A survey of references on life history data is presented; emphasis is given to the role of food, temperature and relative humidity. Experiments by the author show that oviposition history of predatory females is a major factor in determining the actual rate of food conversion into egg biomass; and that the egg stage of the predators is very vulnerable to relative humidities below 70%, though the evapotranspiration of the plant and the hygroscopic properties of the webbing buffer this to some extent. As the juvenile mortality of the phytoseiids increases above 30°C, and that of the two-spotted spider mites above 35°C, spider-mite control at temperatures above 30°C is not effective.<br/>The four phytoseiid species are ranked on their capacities for numerical increase and predation: <em>P. persimilis</em> , <em>A. bibens</em> , <em>M. occidentalis</em> and <em>A. potentillae</em> . On capacity to survive on alternative foods they are ranked: <em>A. potentillae</em> , <em>A. bibens</em> , M. occidentalis and <em>P. persimilis</em> . Some trials with alternative food supply did not improve survival rates established for prevailing greenhouse conditions.<br/>The rate of increase of the webbed area per individual spider mite is quantified by experiment. This knowledge will enable continuous monitoring of the prey density during simulations of the predator-prey interactions on the population level.<p/&gt