Effect of Silicon fertilization on two major insect pests of tomato under greenhouse conditions

Tomato (Solanum lycopersicum L) is an important vegetable crop in Iraq. This horticultural crop is attacked by several insect pest species. Among them, the whitey Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) and the tomato leaf miner Tuta absoluta Meyrick (Lepidoptera: Gelechiidae) are the major threat of greenhouse tomatoes in Basrah province in south Iraq. The management of these pests is heavily based on application of chemical pesticides. Vast application of pesticides caused harmful damage to the environment, human health and may increasing the risk of pest resistance on insect populations. One of the promising strategies which are compatible with organic farming is application of silicon for enhancing plant vigor and resistance to pest damage on various agricultural crops. Due to these facts, the experiments have been carried out at Basrah University to evaluate the effects of silicon (Si) fertilization on tomato plants for reducing damage of these two major pests. Treatments comprised two type of Si applications (Soil drench treatment and foliar spraying) with four Si concentrations (0, 0.5, 1 and 2%) of AB Yellow ® silicic acid formulation. The population density of B. tabaci and T. absoluta were studied weekly during the growth season. The results clearly demonstrated that Silicon applications signicantly decreased the population of immature of both whiteies and tomato leaf miner on tomato crop in the greenhouse; Si-Foliar spraying was more effective in reducing the population density of these key pests compared to Si- soil drench application. study Tomato seeds (variety: REDFLORA were cultivated on After one month, the seedlings have been transplanted to a greenhouse where the treatments were laid out in a Randomized Complete Block Design (RCBD) with three blocks 25 meters length, 40 cm width and a distance of 100 cm between each block and all experimental eld area was divided into 8 experimental plots. Each plot size was 3 m length, and unit-to-unit was 50 cm. The treatments comprised two types of Si applications (Soil drench treatment and Foliar spraying) with three Si concentrations (0.5, 1 and 2%) of AB Yellow ® silicic acid formulation. Untreated plots were considered as control check which received no silicon amendment. For with of the Two applications of the days The rst application was investigated organic other


Introduction
Tomato (Lycopersicum lycopersicum Mill) is one of the most economic important crop of commercial plantations in Basrah Province, Iraq. The total area planted with this crop reached 2206 hectare, with a production rate of 239.8 thousand tons (almost half of the total production in Iraq, 467.6 thousand tons) in the growing season of 2018/2019 [1]. Annually, the crop is infested by many pests, but the white y Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is the most destructive insect pest infesting the crop; It causes economic losses reaching 100% in the case of severe injury [2,3]. The economic loss is due to the serious feeding on the phloem of the infested plants, as well as the transferring of the pathogenic viruses to the healthy plants [4,5]. The tomato leaf miner Tuta absoluta Meyrick (Lepidoptera: Gelechiidae) is a serious invasive pest, which was rst recorded infesting tomato crop of Basrah in 2011 [6]. This pest can easily infest both fresh and processed tomatoes under greenhouse or eld cultivated conditions. The tomato leaf miner may scatter rapidly and attack all stages of tomato and without appropriate management; heavy infestation of the pest usually destroys the unprotected tomato elds.
Moreover, this pest can infest potatoes, sweet peppers, eggplants and several species of solanaceous crops [7][8][9]. The tomato leaf miner feeds on the leaves of tomato across all growth stages and on the fruits during productive growth stages [10,11].
For effective management of T. absoluta, various control strategies such as cultural and biological control are applied; however, chemical control is extensively used against the pests globally and are especially disseminative in agricultural systems [12]. Due to the negative effects of chemical insecticides, scienti c efforts have been focused on the successful alternatives enhancing the plant resistance against insects in the elds. These ways enable the plant to perform as an undesirable resistant plant to the pests or/and prevent herbivores from feeding and laying eggs [13][14][15][16]. One proposed action has been development application of silicon-based formulations as nutritional amendment as a part of proper integrated pest management scenario [17][18][19][20].
Silicon (Si) is one of the most abundant elements in the soil components that is mainly present in the inert state and is rarely found in the soluble state [21]. Si is absorbed by plant in the phase of silicic acid (Si(OH) 4 ) [22]. Although Si is not considered a major nutrient in plant growth, it functions in the development and production of some plant species; currently the element is recognized as an important factor increasing the tolerance of the plant to the biotic (insect and mite pests and plant diseases) and abiotic stresses (such as drought, metal toxicity, cold, lodging, high temperatures) [20,23]; Si enhances the plant`s resistance to arthropod pests due to the ability of Biosilica to accumulate in the plants cellular walls that prevents pest feeding, and decreases the digestibility of the leaves and the biological performance of the pests [24][25][26]. Moreover, Si stimulates the chemical plant defense by increasing the synthesis of phenolic compounds and lignin [27-29, 13, 30].
Commercially, silicon-based fertilizer has been found as potassium silicate (K 2 SiO 3 ) [31], which used against sugarcane yellow mite Oligonychus sacchari on sugarcane [32]. Si formulation can be supplied also as Calcium silicate (CaSiO3), which decreased the population and damage of some suckingmouthpart pests Frankliniella schultzei Trybon (Thysanoptera: Thripidae) infesting tomato plants [33]. Application of silicon fertilizers has been successful used to enhance the resistance against pest infesting different crops such as sugarcane [34,35,19,36,37], rice [38], cucumber [39], cabbage [40] and soybean [41]. Generally, calcium silicate formulations contain Si in the inert forms, which is di cult to be absorbed by the plant. The soluble state, Orthosilicic acid (H 4 SiO 4 ), has been stabilized as commercial formulations such as Silixol Granules, which used to decrease the infestation rate of Rice Stem borer Scirpophaga incertulas (Walker) and leaf folder Cnaphalocrocis medinalis on rice [42][43][44]. Several studies have indicated the vital role of silicon-based treatments improving resistance of tomato against different pests including herbivores insect T. absoluta by using calcium silicate treatments [45], and phloem feeder two-spotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae) by applying Soil and foliar application of rock dust including more than 60% SiO 2 [46]. One recently new silicon formulation has been developed by Rexil-Agro (Netherlands) and ranked as bio stimulants which recommended for different agricultural and horticultural crops under eld and greenhouse conditions [47]. The objective of the study was to evaluate the response of tomato crop to the soluble state of Si formulation that could enhance anti-herbivore resistance against white y B. tabaci and tomato leaf miner T. absoluta.

Silicon formulation
The Silicic Acid Agro Technology (SAAT) is the suitable form of stabilized silicic acid. AB Yellow® is a new silicic acid patent which is synthesized by Rexil-Agro Company (The Netherlands). It is stabilized silicic acid in which polymerization of silicon is halted. AB Yellow® is containing 2.5% plant-available silicic acid (0.8% Si) in combination with 0.3% boron, 1.5% zinc, 0.15% copper and 0.1% molybdenum [47].
Trial site, Experimental design, Silicon treatments and sampling methodology The study was conducted in a greenhouse at the agricultural research station, College of Agriculture, University of Basrah (Basrah, Iraq) during the growing season 2019-2020. Tomato seeds (variety: REDFLORA F; Company: Apollo Seeds, USA) were cultivated on August (2019). After one month, the seedlings have been transplanted to a greenhouse where the treatments were laid out in a Randomized Complete Block Design (RCBD) with three blocks 25 meters length, 40 cm width and a distance of 100 cm between each block and all experimental eld area was divided into 8 experimental plots. Each plot size was 3 m length, and unit-to-unit was 50 cm. The treatments comprised two types of Si applications (Soil drench treatment and Foliar spraying) with three Si concentrations (0.5, 1 and 2%) of AB Yellow ® silicic acid formulation. Untreated plots were considered as control check which received no silicon amendment.
For the foliar spraying, tomato plants were sprayed from the bottom to the top, with 50 ml of the concentrations. All foliar sprays were applied by a 16 liter volume knapsack sprayer (Hardi International, England). In the soil treatments, each plant was drenched also with 50 ml of the solution. Two applications of the treatments were carried out at 20 days intervals. The rst application was investigated at 30 days after the seedling transplantation. Irrigation, organic fertilization and other cultural practices were conducted as the common recommended protocol.
The population density of B. tabaci and T. absoluta were studied weekly during the grown season, starting from the second week of December (11/12/2019) until the second week of March (11/3/2020) (All pests infestations occurred naturally during growing season). Three fully expanded leaves (basal, middle and upper parts) were randomly collected from three plants per an experimental unit. Then, the sampled leaves were put in plastic bags and brought to the lab for counting the number of white ies and the T. absoluta larvae by using an accurate optical microscopy. Tuta absoluta larvae were calculated in each leaves, whereas, white y nymphs were counted from a one inch 2 area in each of the three leaves.

Data analysis
All data were analyzed for normality and homogeneity of variance (Bartlett's test), and appropriate transformations [arcsin, or log (x + 1)] were done where these conditions were not encountered, before analysis of variance was carried out. Data of the average numbers of immatures (nymphs or larvae) per leaf/ inch 2 of leaf were analyzed by two-way ANOVA; followed by a Least Signi cant Difference (LSD) test (P ≤ 0.05) using the statistical software R [48].

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The results of Table 1 indicated that white y nymph population was 0.125 nymph/ unit area of leaf (inch 2 ) at the 1st week, and the highest population of 3.04 nymph/ leaf (inch 2 ) was recorded at 12th week (F = 273.725, P < 0.00). There was a negative response between Si concentrations and the white y nymph population density; at the highest concentration of Si (2%), average nymph numbers were reduced 50% compared to the control, with average of 0.76 and 1.55 nymph/unit area of leaf (inch 2 ), respectively (F = 79.488, P < 0.00; Fig. 1). The results of foliar-applied versus drench-applied Si (Fig. 2) showed a signi cant difference in nymph population between Si applications; the density of white y nymphs was lower on the foliar application, with average of 1.11 and 1.33 nymph / leaf (inch 2 ), respectively (F = 28.119, P < 0.00). The populations of the nymph in the plots with foliar application of Silicon were the lowest throughout the growing season (F = 3.278, P < 0.00; Fig. 3). Table 1 The population density of white ies infesting tomato during the reproductive stage, 2019-2020. Week No. of nymphs/ leaf (inch 2 ) The results of response of tomato crop treated with different concentrations and application of Si to the infestation of T. absoluta ( Table 2) showed the population was 0.11 larvae/ leaf at 1st week, and increased to reach the highest density of 1.86 larvae/ leaf at 11th week (F = 74.738, P < 0.00). In general, the highest Si concentration of 2% signi cantly (F = 31.631, P < 0.00) recorded the minimum number of the larvae (0.53 larvae/ leaf) on the treated plants compared to control treatment which did not differ signi cantly than the lowest concentration of 0.05% Si, with average densities of 0.98 and 0.94 larvae/ leaf, respectively (Fig. 4). Moreover, the foliar Si application showed signi cantly lower population of larvae compared to drench application, with average of 0.73 and 0.83 larvae/ leaf, respectively (F = 13.320, P < 0.00, Fig. 5). Over all the growing season, there was no signi cant differences between the populations of tomato leaf miners on foliar-applied and drench-applied plots (F = 0.238, P < 0.997520; Fig. 6) Table 2 The population density of tomato leaf miner infesting tomato during the reproductive stage, 2019-2020. week No. of larvae/ leaf

Discussion
Silicon applications resulted in signi cantly decreased in population of immature of both white ies and tomato leaf miner on tomato crop in the greenhouse. The basic mechanism of Si applications on the pests is mechanical barriers (single or double-layer of silicon) which are connected directly under the cuticle. Silicon is accumulated and polymerized in the veins and leaf epidermal cells [49,50] that preventing the feeding of phytophagous insect and reducing the host acceptance and suitability [25,51,26]. Moreover, Si applications enhance chemical plant resistance against pests by increasing the levels of vital biochemical such as phenols. Our results have been clearly showed that application of silicic acid formulation in the form of foliar or soil drench, may reduce signi cantly the population density of silver white y B. tabaci on tomato in comparison with control. Even though, by increasing the concentration of silicon, the reduction in population density was higher. In a research study in Brazil, Ferreira et al. [52] evaluated the e cacy of silicon application on two soybean cultivars under greenhouse conditions. The authors applied silicon treatment on the vegetative growth stage of soybean. The results of their study indicated that silicon fertilization signi cantly affected the pest and enhanced nymphs' mortality. On cucumber, Correa et al. [53] investigated effectiveness of silicon application for management of B. tabaci. The ndings depicted that number of nymphs on treated plants was signi cantly lower than untreated cucumbers. In a similar experiment, Callis-Duehl et al. [39] assessed effects of silicon on B. tabaci on cucumber. The authors showed that application of potassium silicate solution, could reduce number of B. tabaci on cucumber leaves whereas the number of living white ies on untreated cucumber leaves was higher (44.5%). This nding revealed that B. tabaci populations has less preference to treated silicon plants. Silicon has negative effects on other sap-sucking insect pests on different plant crops. Ramirez-Godoy et al. [54] showed that the population density of Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) a major global threat for citrus industry, affected negatively by application of silicon. The authors concluded that application of silicon in the form of potassium silicate may signi cantly reduce the oviposition rate of D. citri up to 60% in Tahiti lime. In a recent study, Nikpay and Laane [37] assessed the effectiveness of silicic acid treatment on reduction damage of yellow mite on two sugarcane commercial varieties. The eld trial data illustrated that foliar spraying of silicic acid at different rates can decrease the number of living mites on treated varieties. However, the effectiveness of silicon treatment may increase by increasing in the number of silicon application.
There is very few data on silicon application against tomato leaf miner T. absoluta. Our results clearly showed that the silicon treatment either by foliar spraying or soil drench treatment can signi cantly reduce leaf miner larvae populations on leaves under greenhouse conditions. The only published research by Dos Santos et al. [45] indicated that application of silicon on the form of liquid foliar treatment on tomato leaves affected the midgut of the treated larvae of T. absoluta due to the toxic effect of some biochemicals that simulated by the Si-foliar application.
Other species of lepidopteran pests have also been shown similar responding to Si application; Sidhu et al. [55] found that Si application contributed to the management of the sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae) reducing the feeding injury by adversing the suitable host plant acceptance by the borers. Hall et al. [50] indicated that Si acted as a direct defensive mechanism against chewing mouthpart herbivores through enhancing the mechanical plant resistances. Moreover, Melo et al. [56] revealed that foliar application of 1% silicic acid solution (SiO 2 .xH 2 O) reduced the numbers of white y eggs and nymphs in chrysanthemum plants.

Conclusion
Applying of Silicon formulation signi cantly decreased the population of key pests (white ies and tomato leaf miner) on tomato crop in the greenhouse. Using silicon products has been broadly accepted in organic farming and may be considered as an appropriate, effective and ecologically-sound strategy for alleviating of the biotic stressors such as arthropod pests under eld and greenhouse conditions. This new potential concept should be chosen wisely in sustainable agriculture and production of agricultural and horticultural crops.