Southern Chinch Bug Feeding Impact on St. Augustinegrass Growth Under Different Irrigation Regimes

J. Cara Vazquez, M.S. Student, and Eileen A. Buss, Assistant Professor, Entomology and Nematology Department, University of Florida, Gainesville, FL 32611

Corresponding author: Eileen A. Buss. eabuss@ifas.ufl.edu

Abstract

Blissus insularis Barber reportedly is more damaging and abundant in sunny, drought-stressed areas of St. Augustinegrass compared to more properly irrigated areas of lawns. However, little is known about the response of St. Augustinegrass to B. insularis feeding and its interaction with irrigation. We sought to quantify St. Augustinegrass growth response to three levels of irrigation (30, 60, or 100% sand saturation) and B. insularis densities (0, 30, or 200 fourth and fifth instars per 181.4 cm²). Feeding by B. insularis significantly reduced turfgrass color, density, yield of grass clippings, and dry root weight at all irrigation levels. Main effects of irrigation level, however, were not significant, and there was no interaction between B. insularis density and irrigation level. Our results suggest that increased irrigation may not prevent B. insularis damage. It is possible that long-term B. insularis feeding damage may look like drought stress, but not be a result thereof. Also, B. insularis could already be present and feeding in a lawn, but a secondary stress, like drought, may intensify the damage. We speculate that increased temperature in sunny areas or near sidewalks and roads may decrease the development time of localized B. insularis populations and the ensuing greater B. insularis densities cause more and faster turfgrass damage.

Introduction

The southern chinch bug (Blissus insularis Barber) is the most destructive insect pest of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] and can cause extensive damage or kill entire lawns (2,9). Both the adults and nymphs are damaging, and light to moderate infestations usually are aggregated in small areas of lawns (3). Blissus insularis seem to prefer open sunny areas of St. Augustinegrass, especially areas with abundant thatch (9), where they suck fluids from the crown and stem of grasses. Activity for this insect begins between March and April (5) and continues until November (or until frost occurs) in north-central Florida. Damage is often noticed in what appears to be drought-stressed areas along sidewalks, pavements, or in poorly irrigated areas. Blissus insularis move between lawns mainly by walking and large numbers have been observed crawling across sidewalks and driveways bordering heavily infested lawns (5). All life stages are distributed vertically through the turf thatch and into the upper organic layer of the soil. Densities of 500 to 1,000 B. insularis per 0.1 m² are common and infestations of more than 2,000/0.1 m² have been reported (9).

The effect of moisture on B. insularis populations and their feeding injury to the turf is equivocal. Beyer (1) suggested B. insularis “thrived” best when the grass was most tender and succulent and that B. insularis feeding prevented normal growth and caused a dwarfed condition of the grass. He also noted that warm and fairly dry weather was most favorable for hatching of B. insularis eggs. Wilson (14) reported that B. insularis injury was more evident during dry weather because the dryness reduced turf vigor and favored the rapid increase in B. insularis abundance. Kerr (5) suggested that moisture had a marked, if “paradoxical” effect on B. insularis populations. Heavy irrigation or rainfall may make the grass more succulent and able to tolerate some feeding damage, while at the same time making the grass more attractive to B. insularis. However, Beyer (1) reported that destructive outbreaks of B. insularis are sometimes prevented by heavy rainfall. It is important to determine whether or not there is an interaction between irrigation and B. insularis feeding on St. Augustinegrass since irrigation practices may be an important cultural control (13) and turfgrass quality affects pest sampling and monitoring. We sought to examine the possible interaction of irrigation levels (low, medium, and high sand saturation) and B. insularis density (0, 30, 200 per 181.4 cm²) on St. Augustinegrass growth and survival.

Plant Maintenance and Insect Collection

Forty-five plugs (15.2 cm in diameter) of ‘Palmetto’ St. Augustinegrass were planted in large plastic pots (15.2 cm in diameter × 43.2 cm high; area of 181.4 cm²) in August 2003 (10). Pots were placed on reinforced metal tables inside a climate-controlled greenhouse at the Turfgrass Research Envirotron at the University of Florida in Gainesville. Plants were maintained under a photoperiod of 14:10 (L:D) and daytime and nighttime temperatures were 27°C and 24°C, respectively. Plants were fertilized with 16-4-8 [N:P:K] water soluble ammonium nitrate at 0.23 kg N per 92.9 m² and allowed to establish for 1 mo. Plants were watered as needed.

Blissus insularis were collected from a residential St. Augustinegrass lawn in Ocala, Marion Co., FL, using a modified Weed Eater Barracuda blower/vacuum (Electrolux Home Products, Augusta, GA). Insects were transported in a mesh-covered bucket to the laboratory where they were maintained on fresh cuttings of Palmetto St. Augustinegrass under laboratory conditions for < 2 weeks.

Measurement of Irrigation Levels and Treatment Impacts on Plant Growth

Treatments included irrigation [low (30%), medium (60%), or high (100%) sand saturation] and 0, 30, or 200 fourth and fifth instar B. insularis. There were five replicates in a complete randomized block design. A mesh cage was placed 5.0 cm into the sand and around the plants, such that the grass blades could be maintained at 7.6 cm height. Initial turfgrass density and color were visually assessed and rated on a scale of 1 to 9 representing yellow and sparse to dark green and dense turf, respectively.

To determine the different sand saturation levels, pots were completely saturated with 1,500 ml of water and allowed to drain for 24 h. After 24 h, insects were transferred from vials using a camel-hair brush and cages were closed with nylon. Each pot was then weighed to determine its initial saturation. Amounts of water to apply at respective irrigation levels were determined by replacing some fraction of the water used in evapotranspiration (ET). This was determined gravimetrically by weighing the pots each week and subtracting the weekly weight from the initial saturation level, then calculating the treatment percentage level (either 30, 60, or 100%). These percentages were chosen to represent the typical irrigation levels found in the Florida landscape. ET rates were measured and plants were irrigated weekly.

To quantify treatment effects on plant growth, several factors were examined. Grass blades were cut weekly, collected, dried for 24 h in a convection oven at 55°C, and dry weights were measured. After 8 weeks, turfgrass density and color were visually assessed as previously described. Roots were cut just below the plant crown, washed and cleaned of soil and debris, and dried for 48 h to obtain initial dry weights. To determine the percent organic matter of the roots, dried roots were placed in 150-ml Erlenmeyer beakers and baked at 450°C in a Fisher Scientific Isotemp Muffle furnace. Dry ash weights were subtracted from initial dry weights to determine the percent organic matter of roots. Most B. insularis were removed from pots before root extraction using a hand-held, modified Black & Decker Dust-Buster vacuum (Bioquip Products, Rancho Dominguez, CA), powered by a portable, 12-volt DC battery pack, and any remaining B. insularis were removed by flotation (4). The number of surviving B. insularis was counted.

Turfgrass density, color ratings, and root weight data were analyzed by two-way ANOVA with irrigation and B. insularis as main effects (JMP; SAS Institute Inc., Cary, NC). Weekly grass clipping dry weights were analyzed for main effects of B. insularis, irrigation, and B. insularis by irrigation by univariate ANOVA for repeated measures by using SAS 9.1 (SAS Institute Inc., Cary, NC). The number of live B. insularis recovered at the end of the study was analyzed using a one-way analysis of variance (P ≤ 0.05) and treatment means were compared using the Tukey-Kramer HSD test (JMP; SAS Institute Inc., Cary NC).

Treatment Impacts on Plant Growth and Insect Recovery

Initial color and density ratings of St. Augustinegrass did not statistically differ among treatments, but chinch bugs had significantly reduced ratings for both parameters by 56 days after the test began (Table 1). Regardless of irrigation level, ratings of turfgrass color and density were much lower in pots caged with 200 B. insularis compared with those that were uninfested or caged with 30 B. insularis. Irrigation level itself did not affect turfgrass color, density, or numbers of B. insularis recovered at the end of the trial, nor was there significant interaction between the main factors (Table 1).

Table 1. Color and density ratings of St. Augustinegrass under different irrigation levels before and 56 days after being inoculated with two densities of B. insularis, and number of chinch bugs recovered from each treatment.

Level of irrigation	Number of B. insularis	Day 1^w,x		Day 56^y		B. insularis recovered^z
Level of irrigation	Number of B. insularis	color	density	color	density	B. insularis recovered^z
Low	0	8.0 ± 0	6.4 ± 0.2	5.4 ± 0.2	4.4 ± 0.2	0 ± 0
	30	8.0 ± 0	6.4 ± 0.2	4.4 ± 0.6	4.4 ± 0.6	22 ± 4
	200	7.4 ± 0.2	6.2 ± 0.2	0.8 ± 0.5	0.6 ± 0.4	55 ± 10
Medium	0	7.4 ± 0.2	6.0 ± 0.3	5.4 ± 0.2	4.8 ± 0.2	1 ± 0
	30	7.2 ± 0.3	5.8 ± 0.3	4.2 ± 0.6	4.0 ± 0.6	26 ± 8
	200	7.4 ± 0.2	6.0 ± 0.3	2.2 ± 0.4	2.2 ± 0.2	42 ± 9
High	0	7.6 ± 0.2	6.0 ± 0	5.4 ± 0.2	4.8 ± 0.2	1 ± 1
	30	7.6 ± 0.2	6.0 ± 0	5.0 ± 0.3	4.6 ± 0.2	19 ± 4
	200	7.2 ± 0.3	6.0 ± 0.3	1.0 ± 0.3	1.2 ± 0.3	57 ± 9

^w Color and density ratings based on 1-9 scale: 1 = yellow/brown or bare soil; 9 = dark green or dense turf. Data are means (± SEM).

^x ANOVA for Day 1 Color and Density: main effects of irrigation, initial B. insularis density, and their interaction all non-significant.

^y ANOVA for Day 56 Color: B. insularis, F = 70.4; df = 2, 36; P < 0.0001;
irrigation, F = 0.64; df = 2, 36; P = 0.53;
interaction, F = 1.59; df = 4, 36; P = 0.20.
ANOVA for Day 56 Density: B. insularis, F = 65.0; df = 2, 36; P < 0.0001;
irrigation, F = 1.49; df = 2, 36; P = 0.24;
interaction, F = 1.83; df = 4, 36; P = 0.14.

^z ANOVA for B. insularis recovered: B. insularis initial density, F = 12.4, df = 8, 36; P < 0.0001.

Leaf clipping dry weight significantly decreased each week of the study. For the different chinch bug densities, ANOVA for repeated measures indicated a significant overall treatment effect across all sample weeks (Wilks lambda, F = 4.19; df = 2, 36; P < 0.0001). Turfgrass infested with 200 B. insularis consistently produced less leaf tissue compared with those that were uninfested or caged with 30 B. insularis (Fig. 1). During weeks three and four, less leaf growth occurred on plants infested with 30 B. insularis compared to the uninfested control (Fig. 1). Effect of irrigation (Wilks lambda, F = 1.61; df = 2, 36; P = 0.10) and interaction (Wilks lambda, F = 0.88; df = 4, 36; P = 0.64) were not significant.

Fig. 1. Mean (± SEM) dry weight (g) of grass clippings by week with chinch bugs as main effect. Columns within a week with the same letter are not significantly different (P < 0.05) by the Tukey-Kramer HSD test.

Root weight was reduced at the highest chinch bug density (F = 7.37; df = 2, 36; P = 0.002), but effect of irrigation (F = 1.44; df = 2, 36; P = 0.25) and interaction (F = 0.53; df = 4, 36; P = 0.71) were non-significant. Mean (± SEM) root weights were 2.1 ± 0.1, 1.6 ± 0.2, and 1.3 ± 0.1 g for grasses with starting densities of 0, 30, or 200 B. insularis, respectively. Mean root weights at the low, medium, and high irrigation levels were 1.7 ± 0.2, 1.4 ± 0.1, and 1.8 ± 0.2 g, respectively.

After the study, fewer B. insularis were recovered from the pots that were originally infested with 30 and 200 fourth and fifth instars, but distinct densities were still present (Table 1). In addition, the uninfested control pots became slightly infested (Table 1). Of all of the B. insularis collected at the end of the study, 6% were first through third instars, 3.9% were fourth and fifth instars, and 90.4% were adults. The discrepancy between initial and final counts of B. insularis may have resulted from escape while cages were opened weekly to collect leaf clippings, some may have died from handling, competition, starvation because of turfgrass death, senescence, and/or some may have been missed during final extraction from the plant material.

Conclusions and Recommendations

Our results are consistent with observations made by Beyer (1) who reported that feeding from large numbers of B. insularis prevented normal growth and brought about a “dwarfed condition” in St. Augustinegrass, eventually leading to plant death. Feeding by other Blissus spp. can cause turfgrass wilting, chlorosis, stunting, and death through clogging of vascular transport systems (6,7,12). Although 20 to 25 B. insularis per o.1 m² has been considered a damage threshold for St. Augustinegrass (11), 30 B. insularis in just 20% of that area only caused temporary leaf growth reduction within a two-month period in our study. However, we cannot conclude that the current threshold is too low based on these data since leaf growth declined in all of the treatments. Leaf growth may have been reduced due to declining fertility, stress from limited space in pots, or growth may have naturally slowed due to the time of year. Feeding by the higher density of B. insularis (200/pot), however, significantly reduced grass color and density ratings, clipping yield, and root weight regardless of irrigation regime.

The fact that there was no interaction between irrigation and B. insularis in our study seemingly contradicts the common perception that damage is worst in drought-stressed or poorly irrigated areas. Long-term B. insularis feeding damage may look like drought stress, but not necessarily be a result thereof. Also, B. insularis could already be present and feeding in the turf system, but a secondary stress, like drought, may intensify the damage. Our results suggest that increased irrigation may not prevent chinch bug damage. Under certain circumstances though, irrigation may help suppress B. insularis populations by encouraging the fungus Beauveria bassiana (Balsamo) Vuillemin, which is pathogenic on all life stages. Impact of B. bassiana on B. insularis seems to be greatest under moist, humid conditions (8).

Increased temperature in sunny areas or near sidewalks and roads may decrease the development time of localized B. insularis populations, such that ensuing greater B. insularis densities cause more and faster turfgrass damage. Under field conditions, after infested turfgrass dies, expanding B. insularis populations spread out into surrounding turfgrass (3,5), which they were unable to do in this greenhouse test.

Acknowledgments

We would like to thank L. Wood, P. Ruppert, K. Barbara, and J. C. Turner for their help with plant maintenance and/or data collection. Special thanks go to L. Trenholm, B. Owens, J. Weinbrecht, and J. Haugh (Department of Environmental Horticulture) for their advice, use of equipment, and greenhouse facilities. We thank P. Koehler, R. Vazquez (Department of Entomology and Nematology), and V. Mergel (Department of Statistics) for their assistance with statistical analysis of data and/or review of this manuscript.

Literature Cited

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