Silicon Fertilization Does Not Enhance Creeping Bentgrass Resistance to Cutworms and White Grubs

Carl T. Redmond, Research Analyst, and Daniel A. Potter, Professor, Department of Entomology, S-225 Agriculture Science Bldg. N., University of Kentucky, Lexington 40546-0091

Corresponding author: Daniel A. Potter. dapotter@uky.edu

Abstract

High plant silicon (Si) content, which makes grasses more difficult for herbivores to chew and digest, is associated with insect resistance in several Poaceous crops. We evaluated if fertilization with Si enhances resistance of ‘Penncross’ creeping bentgrass to two major insect pests: foliage-feeding black cutworms, and root-feeding masked chafer grubs. Prilled calcium silicate fertilizer (Excellerator) applied to fairway-height turf on silt loam soil elevated leaf Si by as much as 40% without reducing palatability or suitability for cutworms. Rates as high as 3,363 kg of product per ha also did not reduce density or weight of naturally-occurring chafer grubs. Sodium silicate drenches that elevated leaf Si content of greenhouse-grown bentgrass from 0.5 to 2.5% did not reduce cutworm feeding or survival, caused no inordinate erosion of mandibular teeth, and had only small effects on larval growth rates. Our results suggest that Si fertilization is unlikely to enhance creeping bentgrass resistance to key insect pests.

Introduction

There is a recent growing interest in silicon (Si) fertilization of turfgrasses to enhance plant performance and resistance to diseases and abiotic stress (3,9). Silicon is plentiful in most soils but is combined with other elements, forming insoluble silicates, so most of it is not in plant-available form (5). Silicon deficiency adversely affects plant growth and viability (5). Plant-available Si may be limiting in highly organic soils having little mineral matter, and in soils that are heavily leached or that have high quartz sand content, conditions that may occur on golf course greens, tees, and other turfgrass sites (3).

Silicon is absorbed as silicic acid by plant roots and transported via xylem to stems and leaves (5). Silica polymers deposited in cell walls strengthen stems and present a barrier to fungal penetration. Silicon may also elicit production of compounds involved in induced disease resistance (5). Silicon fertilization may benefit turfgrasses by reducing water use, enhancing heat and drought tolerance, producing more erect growth with increased photosynthetic efficiency, promoting cleaner mowing cuts, and rendering foliage less susceptible to fungal pathogens (3,9).

Plant tissues high in Si may be difficult for herbivores to chew and digest (12,15). In some cases, insects’ mandibular teeth may be so eroded by the abrasiveness of Si in plant cell walls that feeding ability is compromised and impaired growth or starvation ensues (7,14). Fertilizing with Si enhances insect resistance in several Poaceous crops including rice, wheat, and sugar cane (15). In contrast, applications of calcium silicate slag that increased Si content of five species of potted warm-season turfgrasses did not affect growth and development of tropical sod webworms, Herpetogramma phaeopteralis (10). No published studies have examined if augmenting Si enhances insect resistance in cool-season turfgrasses. We tested that hypothesis in field and greenhouse trials using ‘Penncross’ creeping bentgrass (Agrostis stolonifera) and the black cutworm [BCW], Agrotis ipsilon, a caterpillar that chews blades and stems. We also evaluated Si fertilization effects on field densities of root-feeding masked chafer (Cyclocephala spp.) grubs.

Does Field-Applied Si Fertilizer Elevate Si Content of Creeping Bentgrass?

Capacity for creeping bentgrass to accumulate Si was evaluated by applying a prilled calcium silicate fertilizer (Excellerator, Excell Minerals, Sarver, PA) containing 39.3% total available Si (5.8% CaMgSiO₄, 17.9% Ca₃Mg (SiO₄)₂, 8.0% Ca₂MgSiO₇, and 7.6% Ca₂SiO₄), plus micronutrients, to plots on a Maury silt loam (fine silty, mixed, mesic Typic Paleudalf, pH = 6.2) at the University of Kentucky Turfgrass Research Facility, Lexington. The turf was fertilized twice per year with 73 kg of N per ha from urea in October and December, irrigated about 2.54 cm/week, and mowed at 16 mm three times per week. Treatments included the label rate (2,242 kg of product per ha or 2000 lb/acre) and 1.5, 0.5, and 0.25× label rate, plus untreated controls, applied to 0.91- × 0.91-m plots in a randomized complete block with six replicates on 19 April 2005. Grass clippings (1 cm long; 10 g total wet weight) were harvested from the inner 0.5 m² of each plot 30 days after treatment, dried at 40°C, and ground to pass a 40-mesh screen. Sub-samples (100 mg) were digested in an autoclave with sodium hydroxide and hydrogen peroxide and analyzed for Si by inductively coupled plasma spectrometry (4).

Leaf Si content was elevated by the Si fertilizer (F = 9.0; df = 4, 20; P < 0.001) with a significant linear rate effect (polynomial contrasts, P < 0.001). Mean (± SE) percentage foliar Si was 0.57 ± 0.03, 0.72 ± 0.03, 0.68 ± 0.43, 0.78 ± 0.03, and 0.80 ± 0.02 at the 0, 0.25, 0.5, 1.0, and 1.5× rates of fertilizer, respectively, representing increases of 36.4 and 40.3% at 1.0× and 1.5× label rates.

Does Field-Applied Si Fertilizer Enhance Bentgrass Resistance to Black Cutworms or Masked Chafer Grubs?

Eight pairs of 1.83- × 1.83-m plots were marked adjacent to the aforementioned rate study in the same stand. One plot of each pair was treated with Excellerator at label rate on 19 April 2005. Tillers were sampled 30 DAT and presented to first-instar BCW in choice tests to determine if larvae would avoid feeding on Si-fertilized grass. For each replicate, six tillers, three each from treated and control plots, were arranged in an alternating, spoke-like pattern on moist filter paper in 9-cm diameter Petri dishes. Ten larvae were then added to each dish. Assays were run in a growth chamber at 25 ± 1°C (day) and 22 ± 1°C (night) and a photoperiod of 14:10 (L:D) h. Numbers of BCW feeding on tillers were recorded after 24 h. Data were analyzed by one-tailed paired t-tests (H₁: treated < control). First instars did not discriminate between tillers from treated or untreated plots. Mean (± SE) numbers feeding on treated versus untreated tillers were 4.8 ± 0.5 and 4.3 ± 0.6, respectively (t = -0.48; df = 7, P = 0.68).

Newly-hatched BCW were reared on clippings from the field plots used for choice tests, starting 30 DAT, to determine if Si fertilization reduced bentgrass suitability for growth and development. Initially there were 10 larvae per plot (160 total) individually held in Petri dishes at 27 ± 1°C and 18:6 (L:D) h photoperiod and provided fresh clippings on moist paper every 48 h. Parameters measured were 7- and 14-day larval weight and instar, pupal weight, days to pupation and moth emergence, and survival. Values for individuals reared on grass from a given plot were averaged; then cohorts fed grass from treated versus control plots were compared by paired t-tests using plots as replicates. There were no gender differences except for pupal weight, so males and females were pooled except for that parameter. Silicon fertilization had little or no effect on grass suitability for BCW (Table 1).

Table 1. Mean weight, growth rate, and survival of black cutworms reared on clippings from Si-fertilized or non-treated creeping bentgrass field plots.

Parameter		Treated	Control	t-value^x	P
Larval wt (mg) attained by	7 days	15.9 ± 0.8	16.2 ± 0.6	-0.50	0.31
Larval wt (mg) attained by	14 days	446 ± 17	484 ± 17	-1.65	0.07
Larval instar attained by	7 days	3.9 ± 0.03	3.8 ± 0.05	0.69	0.74
Larval instar attained by	14 days	6.6 ± 0.1	6.7 ± 0.1	-1.98	0.04
Days to reach	Pupa	21.5 ± 0.2	21.4 ± 0.2	0.66	0.27
Days to reach	Adult	34.4 ± 0.2	34.2 ± 0.2	0.57	0.29
Pupal wt (mg)	Females	519 ± 8	527 ± 6	-0.78	0.23
Pupal wt (mg)	Males	476 ± 13	479 ± 11	-0.18	0.43
% survival	(to pupa)	87.5 ± 3.1	90.0 ± 2.7	-0.61	0.28

^x One-tailed paired t-test, 7 df.

Additional larvae concurrently reared on grass from the same plots were sacrificed on the last day they were second or sixth instars. Head capsules of 10 larvae per treatment, per instar, were split with a scalpel and mandibles were removed with fine forceps; length (µm) of the third mandibular incisor was then measured with an ocular micrometer at 50× magnification. Mean (± SE) lengths for second instars fed on treated or control grass were 23.6 ± 1.3 and 20.0 ± 0.8 µm, respectively (t = 2.4, df = 18, P > 0.98; H₁: treated < control). Values for sixth instars were 82.4 ± 4.2 and 87.2 ± 3.6, respectively (t = -0.9, df = 18, P < 0.20). There was no inordinate wear associated with feeding on Si-fertilized grass.

Half of each treated plot was re-treated with Excellerator at 0.5 label rate on 20 July 2005 to determine if refreshing Si levels might augment resistance to masked chafer grubs. Natural grub densities were sampled on 8 Sept 2005. Sod strips (0.91 × 0.46 m, 6 cm deep) were cut from both halves of each plot and grubs were counted, identified, and weighed. Samples from halves of each control plot were averaged, whereas portions of treated plots that received one or two applications were considered separate treatments. Six core samples (5 cm diameter, 8 cm deep) were pulled from each plot concurrent with grub sampling and washed to remove soil. Roots were then cut off, dried, ground, and analyzed for Si content as described earlier.

Densities of masked chafers, the predominant grubs present, were unaffected by Si treatments (F = 1.48, df = 2, 12, P = 0.27). Mean (± SE) numbers in 0.42 m² samples from untreated plots and portions of treated plots receiving one or two Si applications were 8.3 ± 1.9, 11.7 ± 3.2, and 10.2 ± 1.6, respectively. Weight per grub also did not differ between treatments (F = 0.15; df = 2, 11; P = 0.86) averaging 390 ± 24, 367 ± 31, and 379 ± 24 mg, respectively. Root Si content was not elevated by the treatments (F = 1.0; df = 2,10; P = 0.40) averaging 6.5 ± 0.9, 7.0 ± 0.7 and 7.5 ± 0.7% dry wt for control, once-treated, and twice-treated plots.

Does Sodium Silicate Soil Drench Enhance Bentgrass Resistance to Cutworms?

Additional trials evaluated if a Si soil drench (7) might be more effective than prilled calcium silicate for augmenting bentgrass resistance to BCW. ‘Penncross’ bentgrass cores (20 cm in diameter, 12 cm deep) pulled from research plots in early December 2005 were potted and grown in a greenhouse at 23 to 25°C and photoperiod of 16:8 (L:D) h under sodium vapor lights to stimulate growth. They were watered every third day and cut at 2- to 2.5-cm height. Granular mefonoxem (Subdue, Syngenta, Greensboro, NC) at 370 ml/93 m² was applied once to discourage fungal diseases. Treatment regimes (Table 2) were each replicated six times. Sodium silicate solution (14% NaOH, 27% SiO₂) was applied in 20 ml water per pot. All treatments were applied to the surface followed by 1.0 cm of irrigation.

Table 2. Si sources and amounts applied to pots of creeping bentgrass in the greenhouse study.

Treatment	Si rate (g/m²)	No. of weekly applications^y	Si / pot / application	Total Si / pot
Excellator	88.0	1	1.76g	1.76g
Sodium Si solution (low)^x	10.0	4	0.05g	0.20g
Sodium Si solution (high)	100.0	4	0.50g	2.00g
Control	−	−	−	−

^x Contains 14% NaOH, 27% SiO₂ (Sigma-Aldrich, St. Louis, MO).

^y First application 5 January 2006; subsequent ones 12, 19, and 26 January 2006.

Tillers harvested from each pot at 35 DAT were presented to neonate BCW in paired choice tests, as above. Each treatment was tested versus untreated grass. Damaged tillers and larvae feeding on each tiller were counted after 24 h. A rearing trial, similar to above, was initiated at 35 DAT. Ten neonates were collectively reared on clippings from each pot. Survival, larval weight, and instar were recorded after 7 days. Because quantity of grass clippings was limited, three representative larvae from each cohort (ones whose weight deviated < 10% from the group mean) were individually reared on clippings from that pot for another 7 days, then weight and instar were recorded. Data were averaged to provide a single value per replicate pot and analyzed by paired t-tests (choice tests) or two-way ANOVA (rearing trial). Clippings harvested during the bioassays were dried and analyzed for Si content as described earlier.

Low and high rates of sodium silicate resulted in 2.4- and 5-fold increases in Si content of bentgrass leaf blades (Table 3). Neonate BCW nevertheless did not discriminate between treated or control tillers. Mean numbers feeding on the respective choices were 3.7 ± 0.6 and 4.8 ± 0.9 for high SiO₂ versus control, 4.7 ± 0.8 and 3.7 ± 3.7 for low SiO₂ versus control, 4.0 ± 1.4 and 5.3 ± 1.5 for Excellerator versus control (t ≤ 0.77, df = 5, P ≥ 0.23 in each comparison). Numbers of tillers fed upon also did not differ (range, 2.0-2.7 out of 3 for all treatments). BCW fed clippings from pots drenched with sodium silicate gained less weight than controls during the first week, but there was high survival in all treatments and differences were non-significant by 14 days (Table 3). Mandibles of larvae reared on Si-augmented grass showed no inordinate incisor wear. No phytotoxicity was apparent 3 DAT, but by 35 DAT there was substantial yellowing and browning of some grass plants from the high-rate sodium silicate drench. We used only green, apparently healthy tillers in the bioassays.

Table 3. Mean weight, growth rate, and survival of black cutworms reared on clippings from creeping bentgrass treated with sodium silicate soil drenches or prilled calcium silicate (Excellerator) in the greenhouse.

Treat- ment	Foliar Si (%)^x	Larvae after 7 days			Larvae after 14 days
Treat- ment	Foliar Si (%)^x	Weight (mg)	Instar	% survival	Weight (mg)	Instar
High SiO₂	2.5±0.3 a	11.3±0.8 a	2.7±0.1 a	97±2 a	256±25 a	6.2±0.2 a
Low SiO₂	1.2±0.1 b	13.9±1.7 ab	2.9±0.1 b	98±2 a	278±12 a	6.4±0.2 a
Excell-erator	0.7±0.1 c	17.1±1.4 bc	3.0±0.0 b	97±2 a	284±29 a	6.4±0.2 a
Control	0.5±0.0 c	20.4±1.9 c	3.0±0.0 b	98±2 a	320±37 a	6.4±0.2 a

^x ANOVA results: Foliar Si, F = 37.4, P < 0.001; 7-day weight, F = 5.9, P < 0.01; 7-day instar, F = 6.0, P < 0.01; % survival, F = 0.22, P = 0.88; 14-day weight, F = 0.83, P = 0.50; 14-day instar, F = 0.40, P = 0.75; df = 3, 15 for each analysis.

Discussion and Recommendations

Leaf and stem silicification has been viewed as both a constitutive and quantitatively inducible anti-herbivore defense of grasses (13,17). High plant Si content may adversely affect herbivores by abrading dentition (1,7,14), reducing digestibility (15), or contributing to pathological conditions in livestock (12). Herbivores as diverse as voles (6), slugs (18), and insects (8) may in some cases discriminate between high- and low-Si plants, feeding preferentially on the latter. Other studies, however, including one with a caterpillar feeding on Agrotis tenuis (2), did not support the hypothesis that natural or grazing-induced variation in Si acts as a defense.

Fertilizing with prilled calcium silicate increased foliar Si content in our creeping bentgrass field plots by 36 to 40%, and sodium silicate soil drenches gave 2.5 to 5-fold increases in the greenhouse. Our results differ from Uriate et al. (16) who, finding no effect of spray-applied potassium silicate on leaf Si in USGA sand-based putting greens, suggested that creeping bentgrass may be a Si excluder. Foliar Si in their untreated plots averaged 85 mg/kg, compared to 57 mg/kg in our control plots on silt loam soil.

Black cutworms, a key pest of bentgrass putting greens and tees, nevertheless readily devoured foliage from our calcium silicate-fertilized field plots with no reduction in their growth or survival. There also was no effect on abundance or weight of masked chafer grubs. Initially-retarded growth of BCW observed on high-Si grass in our greenhouse trial could translate to higher mortality from invertebrate predators (e.g., ants) that focus on early instars (11), but larvae had compensated by 14 days. Because the high sodium silicate rate caused some phytotoxicity, changes in leaf quality unrelated to Si may have occurred in the apparently healthy tillers provided to the larvae. Absence of measurable mandibular wear in our study contrasts with Goussain et al. (7) where incisors of fall armyworms fed leaves from Si-augmented corn plants became so eroded that feeding was impaired and mortality and cannibalism increased.

In conclusion, while Si doubtless contributes to general structural defense of turfgrasses by lowering food quality for insects and other herbivores, our data suggest that augmenting Si is unlikely to enhance resistance of creeping bentgrass to two of its major insect pests. Korndofer et al. (10) reached a similar conclusion for warm season grasses and tropical sod webworms. Research concerning how Si affects other grass species and insect feeding guilds (e.g., stem-burrowers, sucking pests) is nevertheless warranted.

Acknowledgments

We thank A. Wolf (Agricultural Analytical Services Lab; Penn. State Univ.) for grass Si analyses and Excell Minerals (Sarver, PA) for offsetting their cost, S. Tittle, J. George, and D. Hammons for technical help, and D. Williams for access to his turf research plots. We also thank L. Datnoff (Univ. of Florida) and D. Mueller (Excell Minerals) for critically reading the manuscript. Partial funding was provided by the United States Golf Association.

Paper 06-08-68 of the KY Agricultural Experiment Station.

Literature Cited

1. Baker, G., Jones, L. H. P., and Wardrop, I. D. 1959. Cause of wear in sheeps’ teeth. Nature 184:1583-1584.

2. Banuelos, M. J., and Obeso, J. R. 2000. Effect of grazing history, experimental defoliation, and genotype on patterns of silicification in Agrostis tenuis Sibth. Ecoscience 7:45-50.

3. Datnoff, L. E. 2005. Silicon in the life and performance of turfgrass. Online. Applied Turfgrass Science doi:10.1094/ATS-2005-0914-01-RV.

4. Elliott, C. L., and Snyder, G. H. 1991. Autoclave-induced digestion for the colorimetric determination of silicon in rice straw. J. Agric. Food Chem. 39:1118-1119.

5. Epstein, E. 1999. Silicon. Annu. Rev. Plant Physiol. Plant Mol. Biol. 50:641-664.

6. Gali-Muhtasib, H. U., Smith, C. C., and Higgins, J. J. 1992. The effect of silica in grasses on the feeding behavior of the prairie vole, Microtus ochrogaster. Ecology 73:1724-1729.

7. Goussain, M. M., Moraes, J. C., Carvalho, J. G., Nogueira, N. L., and Rossi, M. L. 2002. Efeito da aplicação de silício em plantas de milho no desenvolvimento biológico da lagarta-do- cartucho Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae). Neotrop. Entomol. 31:305-310.

8. Goussain, M. M., Prado, E., and Moraes, J. C. 2005. Effect of silicon applied to wheat plants on the biology and probing behaviour of the greenbug Schizaphis graminum (Rond.) (Hemiptera: Aphididae). Neotrop. Entomol. 34:807-813.

9. Hull, R. J. 2004. Scientists start to recognize silicon’s beneficial effects. Turfgrass Trends 8:69-73.

10. Korndorfer, A. P., Cherry, R., and Nagata, R. 2004. Effect of calicium silicate on feeding and development of tropical sod webworms (Lepidoptera: Pyralidae). Fla. Entomol. 87:393-395.

11. López, R., and Potter, D. A. 2000. Ant predation on eggs and larvae of the black cutworm (Lepidoptera: Noctuidae) and Japanese beetle (Coleoptera: Scarabaeidae) in turfgrass. Environ. Entomol. 29:116-125.

12. Mayland, H. E., and Shewmaker, G. E. 2001. Animal health problems caused by silicon and other mineral imbalances. J. Range Manage. 54:441-446.

13. McNaughton, S. J., and Tarrants, J. L. 1983. Grass leaf silication: Natural selection for an inducible defense against herbivores. Proc. Natl. Acad. Sci. 80:790-791.

14. Raupp, M. J. 1985. Effects of leaf toughness on mandibular wear of the leaf beetle, Plagiodera versicolora. Ecol. Entomol. 10:73-79.

15. Schoonhoven, L. M., van Loon, J. J. A., and Dicke, M. 2005. Insect-plant biology. Oxford Univ. Press, Oxford.

16. Uriarte, R. F., Shew, H. D., and Bowman, D. C. 2004. Effect of soluble silica on brown patch and dollar spot of creeping bentgrass. J. Plant Nutr. 27:325-339.

17. Vicari, M., and Bazely, D. R. 1993. Do grasses fight back – the case for antiherbivore defenses. Trends Ecol. Evol. 8:137-141.

18. Wadham, M. D., and Parry, D. W. 1981. The silicon content of Oryza sativa L. and its effect on the grazing behavior of Agriolimax reticulates Müller. Ann. Bot. 48:399-402.