1. Biological control of twospotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), using Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseidae) on impatiens F. J. Alatawi, J.R. Nechols, D.C. Margolies Department of Entomology, Kansas State University, Manhattan, Kansas, 66506-4004, ABSTRACT The effectiveness of the predatory mite, Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseidae), as a suppressive agent of the twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), was evaluated on impatiens plants at predator:prey release ratios of 1:27, 1: 17 and 1:4. To determine if initial pest population size influenced the suppressive ability of the predator, releases at each predator: prey ratio were made at low and high T. urticae densities. At these two pest densities, only the release ratio of 1:4 significantly reduced both pest numbers and plant damage after short time. After only one week, P. persimilis released at a ratio of 1:4 were able to reduce T. urticae population densities to almost negligible levels. Although the release ratio of 1:17 was able to prevent further increase in pest populations, the predators did not reduce pest numbers or the level of damage especillay at the high TSM density. Therefore, for successful biological control of T. urticae on impatiens, a 1:4 release ratio P. persimilis is recommended. INTRODUCTION Greenhouse-grown impatiens is one of important ornamental plants in the U.S.A. and represents part of profitable local market[NASS 2003] The twospotted mite ((TSM) is one of major pests of floricultural (ornamental) crops of economic importance and controlled using acaricidae sprays (Skirvin and Williams 1999) However, the pressure for the less of pesticides and the fact that TSM has been showing resistance to most major acricides have led to increased use of biological control Skirvin and Williams 1999 The predatory mite phytoseiulus persimilis,in the family phytoseiidae, is an examples of biological control agents that has been reared and sold for biological control of (TSM) The key stumbling block that remains in the adoption of P. persimilis is lack of grower management expertise and the quality of the plants produced is the most important to growers of ornamental nursery stack (Skirvin and Williams 1999) One way of reducing the ration of cost to potential loss is to determine the right predator: prey release ration that gives sufficient pest control at a low pest density As a result, A predator with a predator prey ration that works well at low pest density is a good candidate for developing an IPM program in which only limited releases of the predator are performed (Castane et al 1996) Because sales are based on aesthetics, control TSM to minimize visible plant damage is critical in floricultural crops. RESELTS AND CONCLUSION Over time, there was significant difference in the means of TSM number between the three different release ratios on the same inoculation rate( df = 9;101,f = 12.5, p< 0.001). Also, there was interaction between week (times) and different release ratios (df = 6; 101, f =76.9, p<0.001.) At low TSM density and after 21 days from the time of release predators, there was no differences between the release ratio (1:4) and ( 1:7) in both number of TSM (df =101, p =0.9 ) and damage( df = 107, p = 0.06) while damage was highly significant in high TSM density( df =107, p <0.001) at the same time and ratio. Because impatiens plants are extremely low tolerance of pest damage and because that a sequence increase in the number of impatiens leaves would increase the chance of TSM escaping ( the searching area that predator should be covered is increased), it is necessarily to control the population of TSM and its damage within very short time. The release ratio 1:4 was able to reduce the TSM number and damage in the two different TSM densities within only 7 days( Graphs A,B,C, and d). Therefore, for successful control of TSM on impatiens, 1:4 release ratio P. persimilis is recommended.. PROCEDURE Four-wk-old plants of Impatiens ‘Impulse Orange’ were inoculated with 7 and 13 adult female TSM When plants were six wk old, the total number of TSM on each plant AND damage were determined Three different release ratios P. persimilis were applied 1:28, 1:17 and 1:4 For each TSM density, 4 plants were assigned as control( no predator) Weekly, number of TSM and damage were taken for each plant The experimental design was 2* 3 factorials completely randomize design run with repeated measure. REFERENCES Castane et al 1996. Management of western flower thrips on cucumber with Dicyphus tamaninii (Heteroptera: Miridae). Biol-control 7: 114-120. National Agricultural Statistics Service (NASS). 2003. United States Department of Agriculture Floriculture Crops 2001 Summary. From National Agricultural Statistics Skirvin and Williams 1999. Differential effects of plant species on a mite pest (Tetranychus urticae) and its predator (Phytoseiulus persimilis): Implications for biological control Exp.Appli. Acarology23: 497-512 High TSM Density OBJECTIVE Determine the predator prey ratio that works well for controlling both TSM population and its damage within short time after release predators on impatiens DAYS Low TSM Density Number of TSM/ plant Control(0)1:281:171:4 Number of TSM / plant (A) Number of TSM (B) Damage (D) Damage(C) Number of TSM DAYS Proportion of damaged leaves DAYS ACKNOWLEDGMENT We thank the following individuals from Kansas State University for their contributions: Kimberly Williams, Kiffnie Holt, Rebhi Bsharat, Aqeel Ahammed, and Xiaoli Wu.