1. Spatial distribution of predators and prey affect biological control of twospotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), using Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseidae) on impatiens F. J. ALATAWI, D. C. MARGOLIES, AND J. R. NECHOLS Kansas State University, Department of Entomology, 123 West Waters Hall, Manhattan, Kansas, 66506-4004 ABSTRACT The twospotted spider mite, Tetranychus urticae, initially occurs in clumps or “hot spots” in commercial greenhouses. Growers concerned about mite damage are often advised to focus on “hot spots” either with chemicals or natural enemies. However, many growers spread natural enemies evenly around the greenhouse. We investigated the impact of spider mite distribution among plants on the effectiveness of two release strategies of the predatory mite, Phytoseiulus persimilis for biological control. The experimental unit consisted of 16 impatiens plants arranged in a square. Each unit started with, relatively, the same numbers of spider mites; predators were released at a 1:4 predator:prey ratio. The experiment was designed as a 2 x 2 factorial; spider mites were established either in an even or clumped distribution and predators were released either in an even or clumped. Predators were able to totally control spider mite populations to 0 in less than 9 days in only in Clumped (pest) X clumped (predator) while spider mite populations were highly reduced in the Even X Even and Even X clumped treatments comparing to Clumped X Even. Yet, even in this (Clumped X Even) case the number of mites was totally controlled after 18 days as well as other treatments. However, significant damage was either not controlled or not significantly reduced except when both the pest and predator were evenly distributed. Releasing predators in different patterns demonstrated that they have the ability to spread and control the spider mite. For that reason, growers may only need to release predator in “hot spots,” at least early in spider mite infestations. INTRODUCTION Density and spatial distribution of prey are two main factors that affect the release strategy of predator because they effect all the other factors However, Eveleigh and Chant (1982) suggested that the searching ability of a predator must be evaluated not only in prey density per unit but also in terms of the relative distribution of the prey population in that area The important factors affecting the outcome of the predator- prey under commercial greenhouses were not extensively studied (Jarosik 1990) To ascertain the possibilities of biocontrol, further knowledge of predator- prey interaction under commercial glasshouse conditions is necessary To explores the effects of prey and predators spatial distribution on efficiency of biocontrol on impatiens plants,the design was suggested to imitate the real situation in commercial greenhouses The overall goal is to develop a realistic biological control system of controlling T. urticae in impatience plants in greenhouse based on the technical and economical feasibility of pest management DISCUSSION Results strongly show the high ability of the P. persimilis to find its prey then control or at least reduce its population within short time under different distributions( Table 1) when both the prey and predator have same distribution( i.e E X E or C XC), more prey were killed by the predator when prey were clumped followed by even distribution (Table 1) This is because that prey was more easily discovered by their predators when the former occurred in groups and the distance between them was small (Eveleigh and Chant,1982) This indicates that the actual distance between the preys is important in determining the searching success of the predators However, still this will depend on the prey population density because at high prey densities per unit area the spatial distribution may have little effect on the predator’s searching success due to the distance between being short under such condition (Eveleigh and Chant,1982) Even though the number of T. urticae was completely controlled in the end of the experiments (time 2) in E X C (Table 1), the damage was not reduced comparing to the time (0) (Table 2) This shows the importance of controlling T. urticae within short time which is considered as the key of having successful biological control program especially for those cultivars that have short life cycle like impatiens It was clear that the predators in C X E, especially those were released on uninfected plants, had longer searching time than those in E X C( because of even distribution of prey in all neighbor plants) and they were searching randomly This difference could be explained based on that fact that predators respond to herbivore-induced plant volatiles and the intensity of the volatiles is directly related the number of potential prey in an area (Pallini et al. 1996) This would give a high chance for prey to cause damage to clean plants while most of the predators are searching randomly. Therefore, this strategy should be avoided while applying biological control program. METHOD The experimental unit consisted of 16 impatiens plants arranged in a square (in a try)( Fig 1) Spider mites were established either in an even or clumped distribution and predators were released either in an even or clumped Predators were released at a 1:4 predator: prey ratio The responses were the total number of T. urticae count and the average of damage in each try Spider mites were sampled and damage was recorded three times, beginning when predators were released (time 0) and then after 9(time 1) and 18 (time 2) days. The experimental was a randomized complete block design (as a 2X2 factorial: prey and predators spatial distributions) with three blocks Multiple comparisons procedure (ANOVA one and two ways) of the means for the different four treatments and controls in each time for each response was used Procedure of creating different patterns of T. urticae and predator P. persimilis At Four-wk-old plants of Impatiens ‘Impulse Orange’, randomly and individually, some plants were each inoculated with six adult females whereas others were each inoculated with 24 adult females Twelve days after inoculating plants with T. urticae plants were gathered to form theses different units. Treatments of Even (pest) X Even (predator)(E X E), Clumped X Clumped (C X C), Even X Clumped (E X C), and Clumped X Even (C X E) were created ( for details see Fig 1) In addition, tow controls for even and clumped units (i.e. no predators were applied on them) OBJECTIVE Determine the best strategy of releasing the predators by measuring the ability of P. persimilis to control T. urticae and protect plants from visible damage under different distributions RESULTS Nine days from the time of releasing predators (time 1): while there was highly significant difference between (C X C) and other two treatments ( E X E) (t= 4.4; df= 6 ; P= 0.004) and (C X E) (t= 9.2; df= 6; P< 0.000), the different between the (E X E) and (C X C) treatments was not that high significant (t= 2.6; df= 6 ; P= 0.043) (Table 1). Pest number was completely controlled only in (C X C) (Table 1). While there was high significance in damage between the E X E and E X C (P= 0.003), there was no difference between other two treatments X C and C X E) (P= 0.23) At the end of the experiments (time 2):the data showed that there was no significant difference in the number of prey killed,which was completely controlled ( 0), under different even and clumped of prey and predator distributions (F= 0; df =1,6; P= 1) ( Table 1) At this time still there was highly significance different in damage between E X E and E XC (P= 0.0004) while there was no different between C X C and C X E (P= 0.16). Also there was highly significant different between the damage of C X E comparing to the damage on other treatments which they were not different from each other ( Table 2 ).Damage has been clearly reduced within times was only in the E X E ( Table 2). A Treatment 0 day9 days18 days Even X Even Even X Clumped Clumped X Even Clumped X Clumped Control (Even) Control (Clumped) 420 ± 17 a 415 ± 22 a 422 ± 27 a 427 ± 12 a 440 ± 26 a 449 ± 14 a 79 ± 73 a 93 ± 47 b 287 ± 46 c 0 ± 0 d 1666 ± 249 A 2805 ± 582 B 0 ± 0 a 3383 ± 224 A 4392 ± 827 A Table1. Mean (± SD) number of T. urticae (pest) within three different times * Even X Even Even X Clumped Clumped X Clumped Clumped X Even Control (Even) Control (Clumped) 2.1 ± 0.13 a 2.05 ± 0.10 a 0.72 ± 0.03 b 0.73 ± 0.03 b 2.03 ± 0.13 a 0.72 ± 0.03 b 1.37 ± 0.38 a 2.26 ± 0.05 b 0.75 ± 0 a, c 1.01 ± 0.32 c 3.45 ± 0.26 A 2.84 ± 0.28 B 0.77 ± 0.33 a 2.02 ± 0.03 b 0.53 ± 0.03 a 0.87 ± 0.42 a 4.6 ± 0.44 A 4.03 ± 0.15 A Treatment 0 day9 days18 days Table2. Mean (± SD) of damage caused by T. urticae within three different times * * ( N= 3 per each treatment ) In each column when any two or more means have same letter, they are not different from each other and capital letters indicate that the compression only between controls. Capital letter only for control compression Fig.1. A view of the plants arrangement in one unit: A central plants: When even (pest): each plant in the unit inoculated with 6 mites When clumped (pest): only the central plants each inoculated with 24 mites while the other 12 plats were clean (uninfected) When even (predator): each plant in the unit received same # of predators When clumped (predator): only the central plants received all predators. ACKNOWLEDGMENT We thank the following individuals from Kansas State University for their contributions: Rebhi Bshara, Kiffnie Holt, Punya Nachappa, Aqeel Ahammed, and Xiaoli Wu. We acknowledge Syngenta Seeds,Inc., for providing the plant material used in these experiments. This project was funded in part by USDAÐNRI (Project Award No. 00-35316Ð9248) and by USDADPMAP(Project Award No. 2002-34381D12146). CONCLUSION Information about the T. urticae distribution is required prior to selects and apply a correct strategy of releasing predators Economically, detecting the pest early, while in clumped, will reduces the number of predators that should be released, decrease the damage in the hotspots which keep plants with high value, and most impotently protect other plants from damage Therefore, growers are not recommended to spread natural enemies evenly around the greenhouse. However, since impatiens is highly susceptible to damage, controlling T. urticae on impatiens at very low density of T. urticae is highly recommended Yet, investigating the hot spots of prey at low density may be not noticeable. Because of high searching ability that predators showed, our results indicate that releasing the predators randomly may control both pest and its damage too. REFERENCES Eveleigh, E.S. and Chant, D.A. 1982. Experimental studies on acarine predator–prey interactions: the distribution of search effort and predation rates of a predator population in a patchy environment (Acarina: Phytoseiidae). Can. J. Zool. 60: 3001–3009 Jarosik 1990. Phytoseiulus Persimilis and its prey Tetranychus urticae on glasshouse cucumbers and peppers: key foacors related to biocontrol efficiency. Acta Entomol. Bohemoslov. 87:414-430 Pallini, A., Janssen, A., Sabelis, M. Odour-mediated responses of phytophagous mites to conspecific and heterospecific competitors. Oecologia (1997) 110:179-185