HORTSCIENCE 44(6):1616–1621. 2009. and Ravensberg, 1992). Citrus mealybug is polyphagous, feeding on many horticultural crops such as Asparagus officinalis (L.), Silicon-based Fertilizer Applications Dizygotheca elegantissima (Hort. Veitch), Euphorbia pulcherrima (Willd. ex Klotzsch), Have No Effect on the Reproduction Impatiens walleriana (Hook. f.), Malus syl- vestris (L.), Musa sp.; Rosa sp., Epipremnum and Development of the Citrus aureum (Linden & Andre´), Codiaeum varie- gatum (L.), Aphelandra squarrosa Nees., Solenstemon scutellarioides (L.) Codd, and Mealybug, Planococcus citri Risso Ficus lyrata Warb. (Blumberg et al., 1995; Gray, 1953; MacGillivray, 1921; Malais and (Hemiptera: Pseudococcidae), Feeding Ravensberg, 1992; McKenzie, 1967; Williams and Watson, 1988). on Fiddleleaf Fig, Ficus lyrata (Warb.) Citrus mealybug populations are typically controlled with either contact or systemic Brian K. Hogendorp insecticides (Dreistadt, 2001; Hatting and University of Illinois, Department of Natural Resources and Environmental Tate, 1997), which may quickly lower citrus Sciences, Urbana, IL 61801 mealybug populations to nondamaging levels and prevent future outbreaks from occurring Raymond A. Cloyd1 (Hudson et al., 1996; Pedigo, 2002; Peleg Kansas State University, Department of Entomology, 123 Waters Hall, and Bar-Zakay, 1995). However, managing Manhattan, KS 66506-4004 citrus mealybug populations exclusively with insecticides is difficult because the third John M. Swiader instar and adult females are covered with a University of Illinois, Department of Natural Resources and Environmental hydrophobic, nearly impervious waxy coat- ing that decreases penetration of contact Sciences, Urbana, IL 61801 insecticides through the cuticle, thus reducing Additional index words. pest management, interiorscapes, silicon, greenhouse, horticulture, efficacy (Copeland et al., 1985; McKenzie, potassium silicate 1967). As a result, routine applications of contact insecticides are needed early to Abstract. Mealybugs are major insect pests of greenhouses, interiorscapes, and conser- avoid extensive citrus mealybug population vatories feeding on a wide range of horticultural crops. However, mealybugs are difficult outbreaks. If not detected in time, a citrus to regulate with insecticides as a result of the presence of a nearly impervious protective mealybug infestation may result in the pres- waxy covering, which means that alternative management strategies are required. As ence of overlapping or simultaneous gener- such, this study was designed to determine the value of applying silicon-based fertilizers ations, thus making it difficult to manage such as potassium silicate to fiddleleaf fig, Ficus lyrata, plants as a means of alleviating citrus mealybug populations with insecti- outbreaks of the citrus mealybug, Planococcus citri. The study evaluated the effects of cides, parasitoids, predators, lures, or traps. applying a commercially available silicon-based fertilizer product, ProTekÒ 0-0-3 The The consequences of relying explicitly on Silicon Solution, as a drench to the growing medium at different rates (0, 100, 400, 800, insecticides have led to an increased interest and 1600 ppm silicon). We determined the effect of the silicon-based fertilizer rate in implementing other management strate- treatments on citrus mealybug life history parameters, including number of eggs laid gies to deal with citrus mealybug populations by the female, body size (mm), and development time (days) from first instar to oviposit- (Hussey and Scopes, 1985). A potential strat- ing adult female. In addition, we used a plant alkaline fusion technique to assess the egy includes the appropriate use of fertilizers concentration (mgÁkg–1 or ppm) of silicon in the aboveground tissues (leaves and stems) of to prevent citrus mealybug population out- fiddleleaf fig plants at variable time intervals (days). This technique involves dry-ashing breaks (Fennah, 1959; Hogendorp et al., plant tissue in a muffle furnace followed by alkaline fusion and then colorimetric analysis. 2006). Furthermore, an analogous cultural We found that the silicon-based fertilizer rate treatments did not negatively affect any of practice involves applying supplemental fer- the citrus mealybug life history parameters measured. Citrus mealybug female egg load tilizers to enhance plant resistance to insect ranged from 132.3 to 159.2 and the development time (days) ranged from 66.9 to 68.7 d. pests (Rojanaridpiched et al., 1984). As such, The silicon concentrations present in the fiddleleaf fig plants on the final harvest date it has been suggested that applications of were between 4419.2 and 7241.7 mgÁkg–1 silicon with fiddleleaf fig plants that received the silicon may increase plant vigor and leaf 1600 ppm silicon-based fertilizer rate treatment having the highest silicon concentration. epidermal toughness. In addition, silicon Moisture content was not significantly different among plants receiving the different applications may act as an elicitor in triggering silicon concentrations. Our results seem to suggest that fiddleleaf fig may actually be a defensive processes in plants (Gomes et al., silicon ‘‘rejector’’ and, as such, applications of silicon-based fertilizers are not beneficial 2005; Heldt, 2005; Wadham and Parry, 1981). to fiddleleaf fig plants because they do not accumulate sufficient quantities of silicon to Citrus mealybugs have piercing–sucking impact citrus mealybugs. mouthparts, which are used to remove fluids from the phloem sieve tubes. Silicon-based fertilizer applications have been demon- Mealybugs are major insect pests of strated to inhibit the feeding of piercing– interior plantscape environments such as sucking insects (Salim and Saxena, 1992; greenhouses, interiorscapes, and conservato- Salim et al., 1990; Sogawa, 1982) and stem Received for publication 11 June 2009. Accepted ries feeding on a wide variety of horticultural borers (Djamin and Pathak, 1967; Nakano for publication 21 July 2009. plants (Blumberg and Van Driesche, 2001; et al., 1961; Sasamoto, 1961; Ukwungwu, We thank Dr. Yoonseong Park in the Department James, 1937; Kole and Hennekam, 1990; 1990). of Entomology at Kansas State University (Man- hattan, KS) for reviewing an initial draft of the McKenzie, 1967). The primary mealybug Currently, there is minimal quantitative manuscript. We also want to thank the five anon- species encountered in greenhouses and con- information associated with the use of silicon ymous reviewers for their contributions. servatories is the citrus mealybug, Planococ- in preventing outbreaks of insect and/or mite 1To whom reprint requests should be addressed; cus citri (Risso) (Barlett, 1978; Blumberg pests of dicot plants grown in greenhouses. A e-mail [email protected]. and Van Driesche, 2001; Cox, 1981; Malais number of studies have correlated increased

1616 HORTSCIENCE VOL. 44(6) OCTOBER 2009 PEST MANAGEMENT silicon content in monocot plants such as rice Facility at the University of Illinois (Urbana, five fiddleleaf fig plants from each treatment and maize (Zea mays L.) with enhanced IL) and placed on three (1.4 · 2.1. m) raised was harvested to determine the total silicon insect resistance (Djamin and Pathak, 1967; wire-mesh benches. The temperature inside concentration using the plant alkaline fusion Moore, 1984; Rojanaridpiched et al., 1984; the greenhouse was maintained at 24 ± 2 C technique (Hogendorp, 2008). The mean Salim and Saxena, 1992; Salim et al., 1990; (day) and 22 ± 2 C (night), and all plants values from the subset of plants were used Sharma and Chatterji, 1971; Sujatha et al., were subject to natural daylight conditions to establish a baseline silicon concentration 1987; Ukwungwu, 1990). Moreover, addi- with no supplemental lighting. (mgÁkg–1) of the fiddleleaf fig plants for each tional studies have correlated applications of Plants were irrigated with municipal treatment before artificially inoculating the silicon-based fertilizers (as growing medium water (Illinois American Water, Belleville, plants with first instar citrus mealybugs amendments or drenches) with increased IL) for 14 d with no supplemental fertilizer to through a coleus leaf disk procedure (de- insect resistance (Gomes et al., 2005; Moore, minimize transplant shock. On 5 Mar. 2008, scribed subsequently). At the conclusion of 1984; Se´tamou et al., 1993; Wadham and the fiddleleaf fig plants received an initial the study (7 July 2008), the remaining fid- Parry, 1981). However, relatively few studies fertilizer treatment without any supplemental dleleaf fig plants were harvested and pre- have assessed applications of silicon-based silicon added using Peter’sÒ 20N–8.8P– pared for total silicon analysis using the plant fertilizers to dicot horticultural plants as a 16.6K (Scotts-Sierra Horticultural Products, alkaline fusion technique. Harvest proce- means of reducing feeding by piercing– Marysville, OH) at 200 ppm nitrogen. dures were accomplished by excising the sucking insects (Hanisch, 1980; Moraes Twenty days later, on 25 Mar. 2008, all the plant stem at the level of the growing medium et al., 2005). It is possible that silicon-based plants received silicon treatments as po- and removing the aboveground plant parts fertilizer applications may impact insect tassium silicate (ProTekÒ 0-0-3 The Sili- (leaves and stems) and storing the plant tissue feeding, negatively affecting life history con Solution; Dyna-Gro Nutrient Solutions, in a #20 brown paper bag (Commercial Bag parameters such as development time, fecun- Richmond, CA), which was incorporated and Supply, Des Moines, IA). Plant material dity, and reproductive rate. Therefore, the with the nutrient solutions. There were five was then weighed (in grams) within 2 h of purpose of this study was to investigate the silicon-based fertilizer rate treatments (0, processing and dried in a gravimetric con- effects of applying a silicon-based liquid 100, 400, 800, and 1600 ppm silicon) applied vection oven (Precision Scientific Group, fertilizer at differing concentrations on the as a constant liquid feed with 15 plant Chicago, IL) set at 62 ± 2 C. The plant life history parameters, including egg load, replicates. The nutrient solutions that were material was reweighed after 5 to 6 d in the size (mm), and development time (days) of mixed with the silicon-based fertilizer were gravimetric oven to determine dry matter citrus mealybug when feeding on fiddleleaf prepared and maintained in 68.1-L (61 · 40 · content. Moisture content (in grams) was fig, Ficus lyrata (Warb.), which is susceptible 42 cm) storage tote reservoirs (RubbermaidÒ then assessed by subtracting the weight of to this mealybug species. Home Products, Wooster, OH). The nutrient the dry matter from the initial fresh weight. solutions were initially prepared using Citrus mealybug parameters. Citrus Materials and Methods Peter’sÒ 20N–8.8P–16.6K at 200 ppm nitro- mealybugs were reared on butternut squash, gen. Then, soluble potassium silicate was Cucurbita maxima (L.), in a growth chamber This study was conducted to assess if added to the reservoir resulting in the follow- located in the National Soybean Research applications of different rates (0, 100, 400, ing treatment concentrations: 0, 100, 400, Laboratory, University of Illinois (Urbana, 800, and 1600 ppm silicon) of a silicon-based 800, and 1600 ppm silicon. The composition IL). The growth chamber was maintained fertilizer (as potassium silicate) would nega- of the potassium silicate fertilizer was 3.7% at 24 ± 5 C. On 9 May 2008, a coleus, tively impact the development and reproduc- potassium (as K2O) and 7.8% silicate (as Solenstemon scutellarioides (L.) Codd., leaf tion of citrus mealybugs feeding on fiddleleaf SiO2) with a 2:1 weight ratio. Potassium disk transport procedure was used to artifi- fig, Ficus lyrata, plants. We used a plant sulfate was also incorporated into the silicon cially inoculate the fiddleleaf fig plants with alkaline fusion technique followed by a col- nutrient treatment solutions to compensate first instar citrus mealybug crawlers (Sadof orimetric procedure to determine the concen- for the additional potassium. The concentra- et al., 2003). Coleus leaf disks (1.7 cm2) were tration of silicon (mgÁkg–1) in the aboveground tions of potassium sulfate added to each excised from leaves obtained from stock tissues of fiddleleaf fig plants associated with nutrient solution varied depending on the plants and placed on top of the infested each application rate (Hogendorp, 2008). In concentration of the potassium silicate treat- butternut squash for 30 min. The coleus brief, this technique involved determining the ment. The 0, 100, 400, 800, and 1600 ppm stock plants were located in the Plant Health concentration of total silicon in the fiddleleaf silicon treatments each received 64, 62, 48, Care Facility at the University of Illinois fig plants (in triplicate) by dry-ashing the 32, and 0 g, respectively, of potassium sulfate (Urbana, IL). Approximately 30 to 40 first plant tissue in a muffle furnace (Hythermco, incorporated into 56.7 L of nutrient solution. instar citrus mealybug crawlers infested each Pennsauken, NJ) at 550 C for a minimum of This ensured that the citrus mealybug leaf disk, which was positioned on the upper 4 h followed by alkaline fusion and colori- responses to the silicon-based fertilizer treat- side of the first four leaves most adjacent to metric analysis. The concentration of total ments were not confounded by potassium. the apical bud of the plant. Five to six leaf silicon was assessed using the reduced form The final pH of the fertilizer treatment disks were placed onto each fiddleleaf fig of silicomolybdate measured at 820 nm on solutions for the treated and controls was plant, resulting in between 150 and 240 first a spectrophotometer (Model 160; Shimadzu 5.6 and 5.8, respectively, using concentrated instar crawlers per plant. The leaf disks were Corp., Kyoto, Japan) (Hogendorp, 2008). sulfuric acid (Mallinckrodt Baker, Inc., Paris, allowed to remain on the fiddleleaf fig plants Plant parameters. The study was set up as KY). The pH was measured with the aid of a until they desiccated, which encouraged the a completely randomized design using 75 pHTestr2 Double Junction pH meter (OaktonÒ first instar crawlers to migrate from the leaf fiddleleaf fig, Ficus lyrata (Warb.) (cv. Little Instruments, Vernon Hills, IL). Every 7 d, the disk onto the fiddleleaf fig leaves. The first Fiddle), plants that were obtained as rooted reservoirs containing the silicon treatment instar crawlers are fragile and so it was cuttings from AG3, Inc. (Eustis, FL) on 20 solutions were emptied and new treatment important to minimize handling. Large initial Feb. 2008. The rooted cuttings were trans- solutions were prepared so as to avoid the populations were required to ensure collect- planted into 15.4-cm standard pots (Dillen formation of silicon precipitates and gels. ing 15 mature ovipositing females from each Products. Middlefield, OH) filled with Sun- The fiddleleaf fig plants were watered as fiddleleaf fig plant at the end of the study. shineÒ LC1 growing medium (Sun Gro Hor- needed, receiving the applications of the Two weeks after artificial inoculation, the ticultureÓ Canada Ltd.. Bellevue, WA), silicon-based fertilizer treatments concurrent fiddleleaf fig plants were inspected daily for which was composed of 70% to 80% Cana- with the nutrient solutions. At each watering, the presence of ovipositing females and egg dian sphagnum peatmoss, perlite, dolomitic plants were irrigated until leachate was sac deposition. Oviposition was determined limestone, gypsum, and a wetting agent. The observed. by the presence of a white cottony egg sac plants were grown in a greenhouse (3.0 · On 9 May 2008, 45 d after the plants had protruding from underneath the posterior por- 6.0 m) located inside the Plant Health Care received the first silicon treatment, a subset of tion of the abdomen. Once citrus mealybug

HORTSCIENCE VOL. 44(6) OCTOBER 2009 1617 females initiated oviposition, the body of mealybugs were not always collected from silicon-based fertilizer rate treatments (132.3 the female, along with the corresponding each fiddleleaf fig plant; therefore, a different to 159.2 eggs per female citrus mealybug) egg sac, was collected using a microspatula. statistical test was needed. Significant treat- resulting in a difference of 26.9 eggs (Table This made it possible to remove all the eggs ment means were separated using a Tukey- 1). Mean development time of the citrus contained within the egg sac and collect Kramer least squares mean separation test mealybugs occurred over a narrow range ovipositing females from difficult locations adjusted for multiple comparisons. The (66.9 to 68.7 d), and all of the silicon-based on plants such as petioles and branch junc- Tukey-Kramer least squares mean separation fertilizer rate treatments were statistically tions, resulting in nearly 100% recovery of test was used as a result of the variable similar to plants that received the 0 ppm all eggs. Fifteen ovipositing female citrus number of ovipositing female citrus mealy- silicon treatment (67.8 d). Mean body size mealybugs were collected from each plant. bugs collected from the fiddleleaf fig plants. was between 2.20 (800 ppm silicon) and 2.49 In the event that there were less than 15 A Fisher’s protected least significant differ- (100 ppm silicon) (Table 1), a difference of ovipositing females on the plants, then the ence mean separation test was used to com- only 0.29 mm. maximum number of ovipositing females pare total fiddleleaf fig silicon concentration There were significant differences in the was collected. (mgÁkg–1) and plant moisture content (g) total plant silicon concentrations (mgÁkg–1) Once the female body, along with the within each harvest interval with the silicon- associated with the silicon-based fertilizer corresponding egg sac, was collected, they based fertilizer treatments as the main effect. rate treatments for the preinoculation harvest were transferred into a 20-mL liquid scintil- Each treatment contained the same number of (F = 35.95; df = 4, 24; P # 0.0001) with the lation glass vial (Research Products Interna- replications within each harvest interval and highest silicon concentration (6277 mgÁkg–1) tional Corp., Mount Prospect, IL) containing there were equivalent numbers of fiddleleaf in plants that received 400 ppm silicon 15 mL of 70% isopropyl alcohol, which fig replicates among the silicon-based fertil- and the lowest silicon concentration (3775 preserved the female citrus mealybug until izer rate treatments. mgÁkg–1) in fiddleleaf fig plants that received the egg load and body size (mm) could be 0 ppm silicon (Table 2). In the preinoculation determined. The collection date of each citrus Results harvest, the fiddleleaf fig plants that received mealybug female was recorded to assess any of the silicon-based fertilizer treatments development time (days) from first instar The effects of the silicon-based fertilizer (100, 400, 800, and 1600 ppm) were signif- crawler to ovipositing adult female. After rate (0, 100, 400, 800, and 1600 ppm) treat- icantly different from the 0 ppm silicon collecting the citrus mealybugs from the ments on the citrus mealybug life history treatment. There were significant differences fiddleleaf fig plants, the contents of the 20- parameters are presented in Table 1. There in silicon concentration (mgÁkg–1) associated mL vial, including the body of the citrus were no statistical differences among the with the silicon-based fertilizer rate treat- mealybug female, corresponding egg sac, silicon-based fertilizer rate treatments asso- ments for the fiddleleaf fig plants in the and any eggs in suspension, were quantita- ciated with female egg load (F = 1.57; df = 4, second harvest (F = 18.39; df = 4, 49; P # tively transferred to a small PyrexÒ petri dish 212; P = 0.1845), size (mm) (F = 1.687; df = 0.0001). The lowest silicon concentration (60 · 15 mm) (Corning Incorporated Life 4, 212; P = 0.1559), and development time (4419 mgÁkg–1) was affiliated with fiddleleaf Sciences, Lowell, MA) and placed on the (days) (F = 1.57; df = 4, 212; P = 0.2401). fig plants that received 0 ppm silicon and the viewing stage of a Nikon SMZ1000 stereo- The lowest mean female egg load occurred highest silicon concentration (7241 mgÁkg–1) scope (Nikon Instruments, Inc., Melville, on plants treated with 0 ppm silicon (132.3 was associated with those plants that received NY). A metric ruler was placed beneath the eggs), whereas the highest mean female egg the 1600 ppm silicon treatment (Table 2). small petri dish and the body length from load was recorded on plants treated with 100 Although there were numerical differen- the anterior portion to the caudal tip of the ppm silicon (159.2 eggs). There was a narrow ces in fiddleleaf fig moisture content (g) abdomen was measured (mm) and recorded. range in mean female egg loads among all the associated with the silicon-based fertilizer With the aid of a microprobe, the egg sac was separated from the body of each female and the eggs were removed from the egg sac. Table 1. Mean (± SEM) number for citrus mealybug (CMB), Planococcus citri, life history parameters, The body of the female was then positioned including number of eggs laid by female, body size (mm), and female development time (days), as a ventral-side down and dissected. All of the function of the potassium silicate fertilizer rate treatments to fiddleleaf fig, Ficus lyrata, applied as a eggs were removed from within the body growing medium drench.z cavity. The 70% isopropyl alcohol aided the Fertilizer treatment separation of the eggs from the waxy fila- rate (ppm) n Mean ± SEM Mean ± SEM Mean ± SEM ments covering the egg sac and the body 0 43 132.3 ± 11.68 ay 2.3 ± 0.10 a 67.8 ± 0.62 a cavity. Once all the eggs were in suspension, 100 45 159.2 ± 11.48 a 2.4 ± 0.10 a 66.9 ± 0.60 a a grid was positioned beneath the small petri 400 42 149.1 ± 11.84 a 2.4 ± 0.10 a 68.5 ± 0.63 a dish. The grid, consisting of thin black lines, 800 42 146.9 ± 11.88 a 2.2 ± 0.10 a 68.7 ± 0.63 a 1,600 41 152.0 ± 12.00 a 2.2 ± 0.11 a 68.0 ± 0.62 a was constructed and printed on a plastic over- z head transparency (3äM, Ontario, Canada) n = number of citrus mealybugs per silicon treatment rate. CMB eggs CMB body size (mm). CMB 2 development time (days). resulting in 1-cm squares. This made it yMeans followed by a common letter within a column are not significantly different (P # 0.05) as possible to demarcate the counted areas determined by least square means separation test adjusted using Tukey-Kramer for multiple comparisons. of the petri dish when using the Nikon SMZ1000 stereoscope making it possible to Table 2. Mean (± SEM) silicon concentration (mgÁkg–1) of fiddleleaf fig, Ficus lyrata, plants associated with count all the eggs. each harvest interval for the different silicon-based fertilizer rate treatment (ppm silicon).z Data analysis. All data were analyzed x using SAS systems for Windows, Version Silicon fertilizer rate Preinoculation harvest, 7 May Second harvest, 7 July 9.1 (SAS Institute, 2002). The effects of the treatment (ppm) Mean ± SEM (n = 5) Mean ± SEM (n = 10) silicon-based fertilizer treatments on the 0 3,774.9 ± 132.39 by 4,419.2 ± 220.97 c citrus mealybug life history parameters, 100 6,108.7 ± 185.18 a 6,444.5 ± 193.04 b including female egg load, adult body size 400 6,277.1 ± 190.41 a 6,853.0 ± 143.59 b 800 6,150.0 ± 251.02 a 6,321.2 ± 124.01 b (mm), and development time (days) from first 1,600 6,117.4 ± 90.72 a 7,241.7 ± 447.45 a instar crawler to ovipositing adult female, zn = number of plant replicates per treatment. were analyzed using an analysis of variance yMeans followed by a common letter within a column are not significantly different (P # 0.05) as procedure with the silicon-based fertilizer determined by Fisher’s protected least significant difference mean separation test. treatments (as potassium silicate) as the main xPlants were harvested before inoculation with first instar citrus mealybug crawlers using the coleus leaf effect. Fifteen ovipositing female citrus disk procedure (Sadof et al., 2003).

1618 HORTSCIENCE VOL. 44(6) OCTOBER 2009 rate treatments in the preinoculation harvest, mean relative shoot silicon concentration centrations in fiddleleaf fig plants associated the differences were not statistically signifi- [1.728 (residual maximum likelihood)] for with the preinoculation harvest were more cant (F = 1.60; df = 4, 24; P = 0.2134) with fiddleleaf fig, ranking from 648 of 735 total narrow (2502 mgÁkg–1) compared with the the lowest and highest mean moisture con- plant species, which is a relatively high differences detected in the silicon concen- tents ranging from 17.9 to 23.2 g, a difference ranking among dicots and doubling the aver- trations in the second harvest (2822 mgÁkg–1). of only 5.3 g (Table 3). There were no age mean relative shoot silicon concentration Fiddleleaf fig plants that received the 1600 significant differences in the fiddleleaf fig for the Rosales plant order [0.780 (residual ppm silicon-based fertilizer rate treatment plants in regard to moisture content (g) for the maximum likelihood)] (Hodson et al., 2005). had the highest silicon concentration (7241 second harvest (F = 1.54; df = 4, 24; P = Lanning (1966) reported a silica deposition mgÁkg–1); however, this silicon concentration 0.2068); however, plants that received 1600 value of 3.0% in fiddleleaf fig leaves; how- did not appear to inhibit citrus mealybug ppm silicon had the lowest moisture content ever, the initial classification or ranking of feeding or restrict access to essential nutrients (41.2 g) for the second harvest. fiddleleaf fig as a possible ‘‘neutral’’ plant for development and reproduction. Correa may have been arbitrary. et al. (2005) reported that silicon applications The potassium silicate fertilizer rate treat- attributed to increased nymphal mortality Discussion ments resulted in significant differences in of sweetpotato whitefly B-biotype (Bemisia silicon concentrations in fiddleleaf fig tissues tabaci Gennadius) feeding on cucumber. It is The silicon-based fertilizer rate treat- for both harvest intervals in which fiddleleaf possible that the threshold silicon concen- ments did not significantly affect any of the fig plants that received the higher silicon- tration value for providing silicon-mediated citrus mealybug life history parameters mea- based fertilizer rate treatments appeared to host-plant resistance to insect pests with sured based on our results obtained for absorb and accumulate more silicon than piercingÁsucking mouthparts is between female egg load, adult size (mm), and devel- those plants that received 0 ppm silicon. It 7,300 and 19,000 mgÁkg–1 silicon. opment time (days) from first instar crawler has been suggested that deposits of silicon Most studies that evaluate the effect of to mature ovipositing female adult. This polymers in the intracellular and intercellular silicon applications on certain insect pests indicates that the silicon applications failed spaces, vascular elements, and on the surface involve monocots, and, in general, monocots to produce any type of resistance in fiddleleaf of leaf tissues are associated with increased accumulate substantially more silicon in fig plants to citrus mealybug feeding. These insect resistance (Keeping and Meyer, 2002; plant tissues than dicots (Jones and Handreck, results are similar to those associated with Ma and Takahashi, 2002; Sangster and 1967; Sangster and Hodson, 1986). It has citrus mealybug feeding on coleus and Hodson, 1986; Sangster and Perry, 1981; been suggested that the roots of monocot greenhouse whitefly feeding on poinsettia, Savant et al., 1997; Se´tamou et al., 1993; plant species such as oat, Avena sativa (L.), Euphorbia pulcherrima Willd. ex Klotzsch, Yoshida et al., 1962), although in our study, may contain several large pore (protein) path- in which none of the silicon-based fertilizer elevated silicon concentrations in the above- ways that allow water movement into the treatments had any effect on either insect pest ground tissues of fiddleleaf fig did not nega- symplast, coupled with uncharged nonelec- (Hogendorp, 2008). However, it has been tively affect any of the citrus mealybug life trolytes such as monosilicic acid, which is well documented that concentrations of sili- history parameters measured: female egg load, distributed through mass flow (Jones and con in monocots such as rice and sugarcane adult size (mm), and development time (days). Handreck, 1965; Raven, 1983). leads to less injury from piercing–sucking In the second harvest, untreated fiddleleaf In a study using a dicot plant, Ranger et al. insects (Salim and Saxena, 1992; Salim et al., fig plants had silicon concentrations (4419 (2009) found that higher concentrations 1990; Sogawa, 1982) and stem borers mgÁkg–1) in the tissues that would classify of silicon were present in Zinnia elegans (Djamin and Pathak, 1967; Meyer and them as silicon ‘‘rejectors’’ (Ma et al., 2001) Jacq. plants treated with potassium silicate Keeping, 2001; Nakano et al., 1961; Panda although fiddleleaf fig plants that received compared with untreated plants; however, et al., 1975; Sasamoto, 1961; Sharma and the supplemental silicon-based fertilizer the length of the prereproductive period Chatterji, 1971; Ukwungwu, 1990). The cur- treatments accumulated sufficient concen- and survivorship of the green peach aphid, rent study has demonstrated that fiddleleaf trations (6321 to 7241 mgÁkg–1) of silicon to Myzus persicae (Sulzer), were not negatively fig, which is a dicot, may be inherently dif- be classified as silicon ‘‘neutral,’’ that is, affected and total cumulative fecundity and ferent from agronomic monocot crops and plants containing silicon concentrations from intrinsic rate of increase was marginally does not benefit from supplemental silicon- 5,000 to 30,000 mgÁkg–1 (Ma et al., 2001). reduced when feeding on plants that received based fertilizer applications. However, fiddleleaf fig plants only accumu- soluble silicon. Similar to our study, this The reason fiddleleaf fig was selected for lated half of the silicon concentration in plant demonstrates that silicon applications may our study was because of the premise that tissues when compared with cucumber, not benefit dicot plants in terms of reducing fiddleleaf fig absorbs a relatively high con- Cucumis sativus L. plants (13,000 to 19,000 outbreaks of insect pests. centration of silicon (greater than 0.5%) in mgÁkg–1) (Miyake and Takahashi, 1983). Fiddleleaf fig plants, being a long-lived aboveground tissues compared with other Again, this suggests that fiddleleaf fig may woody perennial, may be able to continually dicot plant species (Hodson et al., 2005; actually be a silicon ‘‘rejector’’ plant. accumulate and increase silicon concentra- Lanning, 1966). A meta-analysis of 125 It is interesting to note that the differences tions in aboveground plant tissues over time. silicon-related studies reported a ‘‘high’’ between the highest and lowest silicon con- However, it may be that the duration of the our study was too short with silicon concen- trations being assessed 105 and 136 d after plants had received the silicon-based fertil- Table 3. Mean (± SEM) moisture content (g) of fiddleleaf fig, Ficus lyrata, plants associated with each harvest interval for the different silicon-based fertilizer rate treatment (ppm silicon).z izer rate treatments. As such, additional studies investigating the effects of silicon x Silicon fertilizer rate Preinoculation harvest, 7 May Second harvest, 7 July accumulating in fiddleleaf fig tissues over treatment (ppm) Mean ± SEM (n = 5) Mean ± SEM (n = 10) an extended time period involving multiple 0 23.2 ± 1.38 ay 49.3 ± 2.86 a or overlapping generations of citrus mealy- 100 19.6 ± 1.74 a 52.6 ± 5.23 a bug may be warranted. 400 17.9 ± 2.10 a 49.5 ± 3.30 a Although our study did not assess where 800 20.3 ± 0.92 a 52.2 ± 3.84 a 1,600 19.2 ± 1.37 a 41.2 ± 2.64 a silicon was deposited in fiddleleaf fig tissues, zn = number of plant replicates per treatment. observations indicated that citrus mealybugs yMeans followed by a common letter within a column are not significantly different (P # 0.05) as preferred to feed on new, young, succulent determined by Fisher’s protected least significant difference mean separation test. leaf tissue with relatively few citrus mealy- xPlants were harvested before inoculation with first instar citrus mealybug crawlers using the coleus leaf bugs harvested from fully expanded mature disk procedure (Sadof et al., 2003). leaves and petioles (B. Hogendorp, personal

HORTSCIENCE VOL. 44(6) OCTOBER 2009 1619 observation). Silicon is transported and Djamin, A. and M.D. Pathak. 1967. Role of silica in plantscapes in The Netherlands. The IPM deposited through the transpiration stream resistance to Asiatic rice borer, Chilo suppres- Practitioner 12:1–4. leading to increased silicon deposition in salis Walker in rice varieties. J. Econ. Entomol. Lanning, F.C. 1966. Silica and calcium deposition older plant tissues, thus resulting in a thicker 60:347–351. in the tissues of certain plants. Adv. Frontiers leaf epidermis (Marschner, 1995). Lanning Dreistadt, S.H. 2001. Knowing and recognizing Plant Sci. 13:55–66. mealybugs and scale insects, p. 207–211. In: Ma, J.F., Y. Miyake, and E. Takahashi. 2001. (1966) determined fiddleleaf fig leaves con- Flint, M.L. (ed.). Integrated pest management Silicon as a beneficial element for crop plants, tain 1.5% silicon in leaf tissues, although the for floriculture and nurseries. University of p. 17–40. 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