26 Seasonal incidence of spider mite, Tetranychus urticae (Koch) (Tetranychidae : Acari) on gerbera (Gerbera jamesonii) under polyhouse conditions DHARMISHTHABAHEN R. SHAH and ABHISHEK SHUKLA AINP on Agricultural Acarology, Department of Entomology, N. M. College of Agriculture, Navsari Agricultural University, Navsari-396 450, Gujarat, India Email: shuklafrs.nau@gmail.com ABSTRACT: Population of two spotted red spider mite, Tetranychus urticae Koch was recorded on gerbera under polyhouse condition from January 2013 to December 2013. Mite population remained active round the year under polyhouse conditions. The mite population was maximum during last week of July 2013 with 12.86 mites per leaf (2 cm 2 ). The correlation studies between weather parameter and mite population indicated that the mite had non-significant negative correlation with minimum, maximum and average temperature while the average relative humidity at the time of observed had significant positive correlation with mite population. The mite population was maximum on top strata as compared to middle and bottom strata under the polyhouse on gerbera. Keywords: Gerbera, polyhouse, seasonal incidence, spider mite INTRODUCTION Gerbera (Gerbera jamesonii) ranks fifth in the world trade among top ten cut flowers suitable for both export and domestic purposes. Mites are among the destructive pests of agri-horticultural crops in many parts of the world. Two spotted mite, Tetranychus urticae (Koch.) is one of the serious pests of gerbera under polyhouse conditions. The spider mites generally feed on the lower surface of the leaves as a result the infested leaves initially show speckling and later turn yellowish, finally leading to defoliation. The mites spread to all parts of the plants as the population increases especially during day periods and produce webbing over the entire plants. Moderate population may greatly affect crop production and heavy infestation results in death of the plants (Jeppson et. al., 1975). The spider mite remains active throughout the year in polyhouses and cause damage to the high value crops including gerbera. The present investigation was carried out to study the seasonal activity of the spider mite under polyhouse conditions so that the effective management tactics will be developed. MATERIALS AND METHODS The study on the incidence of spider mite, T. urticae was carried out during January to December 2013 in a polyhouse at the Department of Floriculture and Landscaping, ASPEE College of Horticulture and Forestry, Navsari Agricultural University, Navsari. The gerbera cultivar Stanza was used for the present study. The observations on the incidence of spider mite were recorded at weekly interval, beginning from first standard meteorological week (SMW) of January 2013 and continued up to the last SMW of December 2013. For sampling, three random leaves representing top, middle and bottom canopy were plucked from each of twenty five randomly selected plants. These samples were held in separate properly labeled polyethylene bags and brought to the Acarology laboratory for numerical mite counts from a leaf bit of 2 x 2 cm under stereo-binocular microscope. The data were recorded separately considering canopy and leaf surface. The data so obtained were summed up and converted to total population per leaf (irrespective of plant canopy and leaf surface). To understand the pattern of distribution of spider mites on the plant and their preference to leaf surface, the data recorded on mite counts were summed up separately considering plant canopy. Correlation studies between spider mite, T. urticae population and prevailing abiotic factors of polyhouse were also made. The impact of abiotic factors viz. maximum, minimum and average temperature as well as relative humidity of polyhouse at the time of observation was made by calculating the simple correlation (r). RESULTS AND DISCUSSION It is evident from the data (Table 1) that the population of mite, T. urticae remained active throughout the year under polyhouse condition. The mean mite Pest Management in Horticultural Ecosystems, Vol. 20, No. 1 pp 26-29 (2014)