Growth Stage Based Economic Injury Levels for Two Spotted Spider Mite , Tetranychus urticae Koch ( Acari , Tetranychidae ) on Tomato , Lycopersicon esculentum Mill .

Study on relationship between infestation of two spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) and yield loss in tomato, Lycopersicon esculentum Mill. revealed that one mite released to a tomato plant 6 weeks after planting and allowed to feed for 3, 6, 9 and 12 weeks, reduced the final yield by 0.233, 0.689, 1.291 and 1.624 g per plant, respectively. Under similar conditions with same treatments a mite day reduced 16.4, 1.80, 0.96 and 1.18 mg of the final yield, respectively. Middle stage of the crop was the most critical period and mite infestation initiated at this stage can contribute to more than 50% of total yield loss due to leaf defoliation and reduction of chlorophyll content of the leaves. Economic injury levels (EILs) for tomato has been calculated based on the number of mites initially released and mite days during different growth stages. Though calculations based on these two parameters yielded similar information, for predictive purpose, the number of mites is more useful in quick decision making. This study was able to provide quantifiable decision making tools, by which the EILs can be fixed to enable the farmers to time their by application of acaricides.


INTRODUCTION
Tomato, (Lycopersicon esculentum Mill) is grown in protected houses and in open fields for direct consumption and processing.In the world, tomato is cultivated over an area of 3.989 million hectares with a total production of 108.499 million tons and productivity of 27202 kg/ha.In India, it is cultivated on 0.52 million hectares with production of 7.42 million tons (productivity 14269 kg/ha) (Anonymous, 2004).In Sri Lanka, tomato is grown over an area of 5300 ha with total production of 40400 tons and productivity of 7574 kg/ha (Anonymous, 2003).
All parts of the tomato plant offer food, shelter and reproductive sites for many kinds of arthropods.On protected as well as field grown tomato, one of the predominant pest species is the two spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) (Lange and Bronson, 1981).This mite has been reported infesting over 200 species of plants (Perry et al., 1998).
The two spotted spider mite prefers hot, dry weather of the summer and fall months, however, may occur anytime during the year.The mite generally feeds underneath the leaves and causes yellowing of the leaves due to mesophyll collapse and necrotic spots occur in the advanced stages of the attack.
Although tomato is an important vegetable crop grown in South Asia and frequently attacked by two spotted spider mite, detailed studies on the economic injury level and crop losses are scanty.This study was undertaken to estimate the yield loss in tomato due to the two spotted spider mite and establish the economic injury level of two spotted spider mite on tomato under controlled conditions.

MATERIALS AND METHODS
This study was carried out in the laboratory and polycarbonate house at the Acarology section of the Department of Entomology, University of Agricultural Sciences, GKVK, Bangalore, and Karnataka, India during the period of June to October 2008.

Maintenance of Tetranychus urticae Koch culture in laboratory and polycarbonate house
Leaves of tomato plant infested by the two spotted spider mites in the field were brought to the laboratory and observed under a stereo microscope.After confirming the identity of the mite, the leaves were kept on mulberry leaf bits measuring 8 cm x 8 cm spread on a layer of moist sponge in plastic trays, to enable multiplication of mites.The mulberry leaf bits were changed every week with fresh leaf bits.These cultures were maintained in ambient temperature (20 -32 °C) and 50% RH in the lab.Mulberry leaf bits were regularly examined to eliminate predators.In addition, a two spotted spider mite culture was also maintained as a stock culture on potted French bean plants in the glass house.

Establishment of tomato plants in polycarbonate house
Tomato plants, variety Abhinav (Syngenta India, Limited), were used in this study.Three week old seedlings procured from commercial nursery were planted in 22 cm diameter earthen pots containing pot mixture prepared with red soil and farm yard manure (FYM) in 1:1 ratio.In each pot, one seedling was planted.One and four weeks after transplanting, Trichoderma harzianum (a beneficial fungus) culture was added to the pots to manage pathogenic soil inhabiting fungi.Chemical fertilizers (3 g of diammonium phosphate, 2 g of muriate of potash per pot) were added two, five and eleven weeks after planting.The moisture level of pots was kept at field capacity using a drip irrigation system and temperature inside the polycarbonate house was maintained below 35°C using blower-wet pad system, especially during day time.Metal wire and jute threads were used to train tomato plants.
Since six potted tomato plants were used for each treatment, 504 pots were arranged in groups of six, in the glass house, and the treatments were imposed randomly.Dicofol and Fenazaquin (10% EC) were sprayed to avoid the mites feeding after the specified period.
Observations were recorded from each plot (6 plants/ plot) by randomly selecting 15 leaflets from different canopy levels (five each from top, middle and bottom of the canopy) once a week after the release of mites.
The number of eggs, immature stages and adults were counted under a stereo microscope and the data were used for calculation of cumulative mite days, CMD (one day feeding duration of a mite) per plant as follow: CDM (adults) = Σ In addition, the number of leaves per plant, height of the plants (cm) and chlorophyll content of leaves were estimated.Chlorophyll extraction was made using dimethyl sulphoxide and acetone (80%) at 1:1 ratio and estimated using a spectrophotometer (Thermo Spectronic Genesys 10 μv) at 645 and 663 nm wave lengths and formula as suggested by Arnon (1949).

Estimation of the economic injury levels in controlled conditions
Economic Injury Level (EIL) at different growth stages of tomato was estimated as per the following equation explained by Pedigo (2002): where, C is the cost of management, V the market value of tomato, I the unit injury per pest (mite or mite day in this study), D the damage per unit injury, and K the control efficacy or proportional reduction in potential injury or damage by management.

Data analysis
Data on counts of immature stages and adults of spider mites were used to estimate the cumulative mite days of each treatment.Influence of mite days on growth parameters such as plant height, leaf numbers and fruit yield was investigated using Analysis of Variance (SAS Institute, 1989).Regression analysis (SAS Institute, 1989) was used to establish relationship between mite density, mite days and yield parameters.

Mite days
Mite days (one mite feeding in one day) and initial number of mites per plant were used to quantify yield loss of tomato due to mite feeding.The cumulative mite days were directly proportional to the duration of mites feeding on plants and the number of mites initially introduced on to the plant as well (Table 1).The cumulative mite days (CMD) per plant were highest in treatments with higher feeding duration such as 12 and 9 weeks with higher densities of mites (400 mites per plant) initially released to the plants (Table 1).This relationship was true when the mites were released either during the early growth stage of the plant or later growth stage of the plant.Although there was a slight increment in CMD when mites were released during latte growth stage of the plant, this was not significantly different from early release of mites.In plants on which mites were not released and maintained as the control, a few mites were recorded as a result of cross infestation, but these were in very low numbers (Table 1).

The effect of mite days on growth of tomato plants
The effect of mite days on growth parameters of tomato such as plant height and number of leaves was examined.Plant height was not significantly affected by mite densities, feeding intervals or cumulative mite days.
Since mite feeding causes severe damage to leaves, which severely affected the number of leaves recorded 12 weeks after planting.Maximum number of leaves (44.3 per plant) was recorded in plants which were not infested by mites, whereas the plants which were infested for twelve weeks starting from 6 weeks age had the least number of leaves.When feeding duration and mite number initially released (mite days) increased, the number of leaves remaining on the plants decreased, especially 12 weeks after planting.The best fitted mathematical relationship between number of leaves per plant and mite days computed for 14, 15, 16 and 18 WAP was a logarithmic model presented in Fig. 1.

The effect of mite days and feeding intervals on chlorophyll content
Leaf chlorophyll content was measured in mite infested tomato plants at 11 and 14 weeks after transplanting.Plants with early infestation and 12 weeks of mite feeding (A4) and late infestation with 9 weeks mite feeding (A5) had significantly low total chlorophyll (1.592, 1.597 mg/g) than other feeding durations at 11 th week.Eleven weeks after transplanting, chlorophyll b content was statistically on par in all the treatments, but chlorophyll a showed significant differences among treatments similar to total chlorophyll content (Table 2).Early infested leaves with 12 weeks of feeding duration (A4) showed significantly lowest total chlorophyll a and b content (1.016, 0.799, 0.217 mg/g) compared to other treatments 14 weeks after planting (Table 2).
Mite densities initially introduced and mite days significantly affected total chlorophyll, chlorophyll a and b content (Table 2).Plants on which 400 mites per plant were initially introduced had the lowest total chlorophyll, chlorophyll a and b contents in 11 week (1.363, 0.986 and 0.377 mg/g) and 14 week (0.898, 0.693 and 0.205 mg/g) old plants, followed by 200, 100 and no mites (control plot).Chlorophyll content was higher in 11 week old plants than in 14 week old plants, after transplanting, all the treatments (Table 2).
Damage to the plants by T. urticae is by several ways.First, feeding causes the destruction or disappearance of chloroplasts which then leads to basic physiological changes in the plant.Stomatal closure can be a primary host-plant response, and in such cases, uptake of CO2 decreases resulting in a marked reduction in transpiration as well as photosynthesis (Sances et al., 1979).T. urticae injures individual leaf cells, causing the reduction of total chlorophyll content and net photosynthetic rate of leaves (Sances et al., 1981;Park and Lee 2002).Such leaf cell damage and tissue injury alters carbon allocation patterns of plant organs (Wyman et al., 1979), often causing deformity of plants (Avery and Briggs, 1968).

Effect of mite feeding on yield of tomato
Lowest cumulative yield was observed in plants, which were infested by mites early and were exposed to feed for a longer period (A4).Cumulative yields increased with reduction of feeding period.Mite feeding on plants in early growth stages (A2 and A3) significantly contributed to reduction of yield than mites feeding on plants during later stages with same feeding duration (A5 and A6).Feeding on plants in very late growth stage had no significant contribution to yield reduction (A7).All the yields were significantly different from each other except A1 and A7 (Table 3).Number of mites initially released in different treatments had significant influence on tomato yield compared to mite free treatment (M0).Lowest cumulative yield was recorded in plants on which 400 mites per plant were initially released (M3), followed by 200 (M2) and 100 (M1) mites, respectively (Table 3).Early infestation of mites for 3 weeks (A1), 6 weeks (A2), 9 weeks (A3), 12 weeks (A4) and late infestation of mites for 9 weeks (A5), 6 weeks (A6), 3 weeks (A7) period.Different initial number of mites released per plant were zero (M0), 100 (M1), 200 (M2) and 400 (M3).Percent yield loss due to mite infestation was calculated based on yield of plants on which no mites were released.There were combined effects of number of mite initially released, stage of plant when mite infestation was initiated (early or late stage) and feeding duration (mite days).The highest yield loss (56.1%) was recorded in plants in treatment A4M3, followed by A3M3 (45.1%),A4M2 (44.2%) and A5M3 (39.9%) (Table 4).Thus early mite infestation contributed more yield loss than the late infestation, though the feeding duration and initial numbers of mites released were same.Plants infested early (A1, A2, A3) showed higher yield losses than late infested plants A5, A6, A7 with similar mite feeding durations and initial mite densities (Table 4).Initial mite population positively correlated with reduction in yield in both early and late infested plants (Table 4).There was a positive correlation between yield reduction (crop loss) and mite days (R 2 =0.911).Stacey et al. (1985) studied tomato plants, which had been subjected to varying levels of red spider mite damage at different stages in the growth of the plants.10% of leaf area damage by mite caused a 9% loss of yield with no apparent damage threshold.Similar results have been recorded by Sances et al. (1981) on strawberry plants under field conditions.Early season infestation affected physiological parameters at much lower population level than was necessary to cause similar injury later in the season.T. urticae infestations decreased leaf productivity by reducing the total number of leaves per plant.Approximately 14% reduction of total leaf area could result in a significant yield loss (Park and Lee, 2005).In the present study, 11% reduction in leaf numbers was observed.

Jayasinghe and Mallik
Three major principles behind yield loss due to spider mite infestation in various crops have been established; (1) biomass reduction (2) disturbance of water conduction, dry matter partitioning and CO2 gas exchange and (3) chlorophyll reduction and shedding of immature flowers (Sances et al., 1979;Sances et al., 1981;Park and Lee, 2002;Park and Lee, 2005).
Results of the present study agree with earlier studies.Significant defoliation occurred in tomato plants which were infested by mites for longer duration (12 or 9 weeks) and under higher mite population (400 mites per plants initially released) during early growth stages.
In this study, it was observed that plants infested with 100, 200 or 400 mites, when they were 6 weeks old, experienced higher loss in yield than plants on which mites were released 9, 12, 15 weeks after planting with the same number of mites.There was a negative relationship between duration of mite infestation and number of leaves.However, yield loss (%) could not be attributed to this relationship.This can be due to flower drop and movement of resources to the sink being affected by early infestation, whereas later infestations may depend more on unused resources in the leaf.

Estimation of economic injury levels for spider mite infestation at different growth stages of tomato, under controlled conditions
Yield loss per mite varied with the number of mites initially introduced, feeding durations and growth stage of plants when mites were released (early or late).Mite infestations during middle stage (9 to 15 weeks after planting) of tomato plant reduced the yield, maximum, whereas, infestation by a single mite, during early and later stage of growth affected the yield of plants to a lesser extent (Table 5).However, early infestation reduced the yield more than later infestation.Yield loss per mite day was also investigated.Short feeding duration during early growth stages of the plants caused high yield reduction (16.4,1.8 and 0.9 mg per mite day) than similar feeding duration during the later growing stages (2.49, 0.9 and 0.85 mg per mite day) (Table 5).
Economic Injury Levels (EILs) varied with feed duration of mites and growth stages.EILs were calculated as mites per plant using the formula EIL=C/(V.I.D.K.) mentioned above for different growth stages of tomato.Average cost for chemical pest management (C) was Rs. 5000 per crop per hectare during the study period and farm gate price of tomato (V) varied from Rs 2 -10 per kg.Yield loss per mite per plant (I.D) for different feeding durations and plant growth stages of tomato were obtained by the yield loss functions of different treatments (A4, A5, A6 and A7) for a mite and a mite day (Table 5).It was considered that 15000 plants/ ha.Since EIL values can differ with market prices.When farm gate price of tomato was increased, the EIL value decreased and vice versa.Efficacy of management (K) was considered as 1 (Table 6).The Economic Injury Level (EIL) is the most widely used decision making tool, specially in integrated pest management.There are no reports on economic injury levels of T. urticae on tomato.Park and Lee (2007) studied the Economic Injury Level for T. urticae on glasshouse cucumbers during four growing seasons based on one mite feeding and mite days.
EILs for tomato has been calculated based on the number of mites and mite days during different growth stages.Though calculations based on these two parameters yielded similar information, for predictive purpose, the number of mites is more useful since this can help in quick decision making.In fact, farmers are adopting the strategy of investing more on acaricides, if they anticipate good prices for the harvested fruits.However, this study was able to provide quantifiable decision making tools, using which the EIL can be fixed to enable the farmers to time their application of acaricides (Fig. 2).CONCLUSIONS Mite days (one mite feeding for a day) and number of mites per plant initially released were used to quantify the yield loss of tomato due to mite feeding.Middle stage of the tomato crop was the most critical period for mite damage and mite infestation, contributing to more than 50% of total yield loss due to severe damage to the leaves and reduction of chlorophyll content as well.Using yield loss per mite or mite day and considering other parameters such as cost of management and tomato prices, EIL values were estimated, for different growth stages of tomato with different feeding durations.The EIL values can be used as an effective decision making tool in spider mite management in each growth stage of the tomato crop.

Table 1 . Mean cumulative mite days in early and late growth stages of tomato plants with different levels of initial infestation of the mite.
ghMean values in each column superscripted by the same letters are not significantly different (P>0.05)

Table 2 . Chlorophyll content of tomato leaves exposed to different durations of mite infestations and number of mites initially released during different growth stages.
*Figures in parentheses indicate age of plant at the time mites were released and the duration of feeding (weeks).

Table 3 . Yield of tomato as affected by duration of mite infestation and number of mites initially released during different growth stages
WAP -Weeks after plantingFigures in parentheses indicate age of plant at the time mites were released and the duration of feeding (weeks).Mean values in each column superscripted by the same letter are not significantly different (P>0.05).

Table 6 . Economic injury levels (EIL) for T. urticae on tomato at different growth stages, where V is price of tomato and K is efficacy of control measure
*V is the price of tomato (Rs./Kg)