Dispersal and population simulation model of spider mites and a phytoseiid predator in the corn plant microenvironment

Within plant dispersal of Banks grass mite (BGM), Oligonychus pratensis (Banks), and twospotted spider mite (TSM), Tetranychus urticae Koch, was evaluated through field experiments. Mites were introduced onto host plants at three levels in the canopy: flag leaf, ear leaf and third lowest leaf. Mites introduced onto the flag leaf survived poorly and moved to the lower parts of the plants. Both BGM and TSM did the best on the middle and lower leaves. TSM was more likely to disperse both upwards and downwards from the infested leaf than BGM. Dispersal behavior of the predatory mite (NEO), Neoseiulus fallacis (Garman), was evaluated by simulating walking patterns for various prey and temperature combinations. The results were that NEO exhibited a random walk type of search more frequently at moderate to high prey densities (search for prey within a prey patch) and a very non-random, directional search more frequently at low prey densities (search for new prey patches). The non-random search allows the mite to closely follow leaf edges and quickly travel substantial distances in search of new prey patches. A simulation model of the mite predator/prey system consisting of BGM and NEO was developed and validated. This model included the effects of temperature, humidity and predation and was coupled to a detailed plant canopy model. Results demonstrated the importance of using leaf surface conditions instead of weather station conditions to simulate the mite system on corn in Nebraska. Also, humidity was determined to be critically important (in addition to temperature) in NEO/BGM population dynamics. The temperature and humidity at the leaf surface of moderately drought stressed corn (compared to well watered corn) resulted in higher populations of BGM. Simulation studies also showed that colonization of a corn field by less than 1 adult female BGM/plant in June can result in crop destruction by August