Impact of Simulated Threecornered Alfalfa Hopper (Hemiptera: Membracidae) Induced Plant Loss on Yield of Maturity Group IV and V Soybeans
Mainstem girdling of soybean seedlings by threecornered alfalfa hoppers, Spissistilus festinus (Say), can result in several plant responses: (1) death of the plant, usually within 2 - 3 wks of injury,(2) mainstem breakage at the girdle soon after girdling through maturity, (3) partial breakage at the girdle with continued plant growth, (4) or the injured area (girdle) may heal over and the plant remain upright. Studies were conducted to simulate plant loss occurring during the reproductive growth stages which would result from threecornered alfalfa hopper injury during the early vegetative growth stages and examine the impact of this plant loss on yield of full-season indeterminate, late-planted/double-crop indeterminate, and full-season determinate soybeans, Glycine max (L.) Merr., at specific reproductive growth stages. Yield of full-season indeterminate soybeans was reduced by 6.4 - 15% when plant loss exceeded 20% at the R1–R2 growth stage. At R3 and R5, yields were reduced 7.4 - 30% and 7.2% to 32.1%, respectively, when plant density was reduced by ≥ 10%. For late-planted / double-crop indeterminate soybeans, yield losses of 10.6% to 19.3% occurred when plant loss exceeded 30% at the R1–R2 growth stage. Yields were reduced 6.9% to 20.6% at R3 when plant loss was ≥ 20. At the R5 growth stage, yields were reduced 22.4 - 40% when plant density was reduced by ≥ 30%.Abstract
Historically, soybean, Glycine max (L.) Merr., production in the midsouthern United States involved planting Maturity Group V, VI, and VII varieties in May and June. Over the last 10 - 15 years growers have transitioned to Maturity Group IV and V varieties planted in April (Early Soybean Production System) (Heatherly 1999). One principle of this system is drought avoidance. When planted in April the critical reproductive periods (bloom, pod set, seed fill) of early-maturing (Maturity Group IV and early Maturity Group V) varieties occur earlier in the season when soil moisture and rainfall are more prevalent. This can have a substantial impact on both nonirrigated and irrigated production through higher yields and/or lower production costs (Heatherly 1999). Another difference in the 2 production systems is the growth habits of the varieties. Many of the Maturity Group V varieties and the Maturity Group VI and VII varieties have a determinate growth habit. In contrast, the Maturity Group IV and some of the earlier maturing Maturity Group V varieties are more indeterminate. This shift in production systems along with increased management intensity has led to increases in yield potential (Heatherly and Bowers 1998).
The threecornered alfalfa hopper, Spissistilus festinus (Say), is a common insect found in soybean fields. In the southern U.S., it has been considered an early-season pest of soybean (Mueller and Dumas 1975, Tugwell and Miner 1967, Tugwell et al. 1972). Bauer et al. (2000) reported that threecornerned alfalfa hopper was more prevalent in an early soybean production system compared with a late soybean production system (Maturity Groups V – VII planted during May and June). Current treatment thresholds in the midsouthern U.S. for threecornered alfalfa hopper infesting vegetative stage soybeans (≤30.5 cm tall) focus on plant density reductions (seedling death). The threshold in Arkansas states that treatment is recommended when 50% of plants are girdled or if fewer than 13 - 20 ungirdled plants per row m remain for row spacings of 76.2 - 96.5 cm (wide row spacings) with nymphs present (Studebaker et al. 2013). In Mississippi treatment is recommended when plant density is being reduced below the recommended plant population (Catchot et al. 2013). In Tennessee treatment is recommended when 10% of young plants (up to 25.4 - 30.5 cm) are infested with adults and/or nymphs (Stewart and McClure 2013). Louisiana does not recommend treatment of threecornered alfalfa hopper prior to the reproductive growth stages (Beuzelin et al. 2013).
Threecornered alfalfa hopper has piercing sucking mouth parts that are inserted into plant tissue to remove phloem material. These insects feed around the circumference of soybean main stems or leaf petioles. Feeding in a localized area of main stems or leaf petioles forms a “girdle” which inhibits movement of phloem material. Girdles may encompass part of or the entire circumference of the main stem or leaf petiole. Rice and Drees (1985) reported that approx. 90% of mainstem girdling occurred from the soil line to the first internode on vegetative stage soybeans. Feeding (girdles) on the main stem of soybean seedlings can result in several plant responses: (1) death of the plant, usually within 2 - 3 weeks of injury, (2) the mainstem may break at the girdle site from soon after girdling through maturity, (3) the mainstem may only partially break at the girdle site and plant continues to grow, (4) or the injured area (girdle) may heal over and the plant remain upright (Mueller and Dumas 1975, Mueller and Jones 1983). Plants that break at the girdle may or may not contribute to yield. If breakage occurs close to maturity and the plants are supported by adjacent plants they can contribute to yield. Studies conducted to simulate plant loss due to threecornered alfalfa hopper injury during the seedling stages reported that plant density reductions of 45% at flowering (R1) reduced yield of a determinate Maturity Group VI variety and that plant density reductions of 30% and 45% at 2 wks after bloom (R3 possibly) also reduced yields (Caviness and Miner 1962). The studies reported here focused on the complete breakage response described above during the reproductive period resulting from threecornered alfalfa hopper injury that occurred during the early vegetative growth stages. Simulated damage methods were used to examine the impact of plant loss occurring during specific reproductive growth stages on yield of full-season indeterminate, late-planted / double-crop indeterminate, and full-season determinate soybeans.
Materials and Methods
Impact of plant loss on yield of full season indeterminate soybeans. Studies were conducted using indeterminate Maturity Group IV soybean varieties grown in high-yield environments and intensively-managed to represent full-season soybean production in the midsouthern U.S. These studies were planted to varieties recommended by the Extension Service in each state at recommended plant populations of approx. 271,810 plants per ha (Flinchum 2001, Ashlock et al. 2006, Levy et al. 2013, Catchot et al. 2013) during April to early May, with one exception (20 June 2008, Jackson, TN). Studies were managed with respect to agronomic and pest management practices according to Extension Service recommendations in the respective states. Plant loss treatments were imposed at the late R1-early R2, R3, and R5 growth stages. Threecornered alfalfa hopper induced plant loss was simulated by cutting plants between the soil surface and the first mainstem internode (approx. 3 - 5 cm above the soil surface) with hand-held pruning shears. Plant density was determined within 1 day prior to plant clipping, and the appropriate number of plants to clip per plot was calculated. Plants to be clipped within each plot were randomly selected. Clipped plants were not removed and were allowed to either fall over or stand if supported by adjacent plants to more closely simulate natural conditions. The proportion of plants clipped at the R1–R2, R3, and R5 growth stages were 0, 10, 20, 30, 40 and 50%. The late R1-early R2 and R3 growth stage studies were conducted during 2010 and 2011 at the Mississippi State University, Delta Research and Extension Center (Stoneville, MS) and the University of Arkansas, Rohwer Research Station (Rohwer, AR). Planting dates, harvest dates and soybean varieties are detailed in Table 1. Plot size at Stoneville and Rohwer was 2 rows (101.6 cm centers) by 3.05 m and two rows (96.5 cm centers) by 7.62 m, respectively. The R5 growth stage studies were conducted at the University of Arkansas, Rohwer Research Station (Rohwer, AR) during 2008 - 2011, University of Tennessee, West Tennessee Research and Education Center (Jackson, TN) during 2008 - 2009, Louisiana State University Dean Lee Research and Extension Center (Alexandria, LA) during 2008, and the University of Arkansas, Lon Mann Cotton Branch Experiment Station (Marianna, AR) during 2009. Plot size at Rohwer, Jackson, Alexandria, and Marianna was 2 rows (96.5 cm centers) by 7.62 m, 2 rows (76.2 cm centers) by 9.1 m, 2 rows (96.5 cm centers) by 7.62 m, and 2 rows (96.5 cm centers) by 7.62 m, respectively. Planting dates, harvest dates, and soybean varieties are detailed in Table 1. Treatments were arranged in a randomized complete block design with 4 - 5 replications.
Impact of plant loss on yield of late planted / double crop indeterminate soybeans. Studies were conducted during 2010 and 2012 at the University of Arkansas, Rohwer Research Station. Plots were planted later in the growing season to simulate a double crop scenario following winter wheat, Triticum aestivum L., production. Treatments were arranged in a randomized complete block design with 4 replications. Plot size was 2 rows (96.5 cm centers) by 7.62 m. Planting dates, harvest dates, and soybean varieties are detailed in Table 1. Plant loss treatments described above were applied at the R1, R3, and R5 growth stages.
Impact of plant loss on yield of full season determinate soybeans. Studies were also conducted during 2008 and 2009 at the Louisiana State University, Ben Hur Research Farm (Baton Rouge, LA) and during 2009 and 2010 at the Louisiana State University, Macon Ridge Research Station (Winnsboro, LA) using full-season determinate Maturity Group V soybeans. Treatments were arranged in a randomized complete block design with 3 - 6 replications. Plot size was 4 rows (76.2 cm centers) by 7.62 m at Baton Rouge and 2 rows (101.6 cm centers) by 6.1 m at Winnsboro. Planting dates, harvest dates, and soybean varieties are detailed in Table 1. Plant loss treatments described above were applied at the R5 growth stage.
Impact of initial plant density and plant loss on yield of full season indeterminate soybeans. Other experiments were conducted to evaluate the impact of simulated threecornered alfalfa hopper induced plant loss, and initial plant population on soybean yield were conducted at the R5.5 growth stage. The plant population treatments were 33, 67, and 100% of the initial plant population of 258,328 plants per ha, which is in the range of recommended soybean plant populations for the midsouthern U.S. Plots were thinned to the appropriate plant density at the VC – V1 growth stage. Simulated threecornered alfalfa hopper induced plant loss treatments of 0, 20, 40, and 60% lodging were applied at the R5.5 growth stage using methods described above. This study was conducted at the Mississippi State University, Delta Research and Extension Center (Stoneville, MS) and at the University of Tennessee, West Tennessee Research and Education Center (Jackson, TN) during 2010. Plot size at Stoneville and Jackson was 2 rows (101.6 cm centers) by 6.1 m and 2 rows (76.2 cm centers) by 9.1 m, respectively. Planting dates, harvest dates, and soybean varieties are detailed in Table 1. Treatments were placed in a factorial arrangement within a randomized complete block design with 4 replications.
Data analysis. Yields were standardized to 13% moisture and converted to kg/ha. Also, yields were converted to crop value ($/ha) based on a selling price of $0.367/kg ($10/bushel). Crop value loss was determined by calculating the difference in crop value between the nondamaged control plots and plots that received the plant loss treatments. Data were analyzed using mixed model analysis of variance (Proc Mixed, Littell et al. 2006). Type III statistics were used to test all possible fixed effects (level of plant loss) and interactions among the fixed effects. Random effects were years, locations, and replications nested within years by locations (Blouin et al. 2011). Years and locations were considered as environmental or random effects; this allowed inferences about treatments to be made over a range of environments (Blouin et al. 2011, Carmer et al. 1989). Least square means were calculated, and mean separation (P ≤ 0.05) was produced using PDMIX800 in SAS (SAS Institute 2010), which is a macro for converting mean separation output to letter groupings (Saxton 1998).
Results
Impact of plant loss on yield, crop value, and crop value loss of full season indeterminate soybeans. A significant impact on yield, crop value (F=9.50, df=5, 73, P < 0.01), and crop value loss (F=9.19, df=4, 58, P < 0.01) was observed when simulated threecornered alfalfa hopper induced plant loss treatments were applied during the late R1-early R2 growth stage (Table 2). Plant losses of ≥30% resulted in significantly lower soybean yields and crop value compared with that of plots with plant loss of ≤20%. Yield losses of 6.4 - 15% occurred when plant loss exceeded 20%. Losses in crop value ranged from $87.86/ha to $205.64/ha when plant loss was ≥30%.
A significant impact on yield, crop value (F=24.52, df=5, 75, P < 0.01), and crop value loss (F=19.32, df=4, 60, P < 0.01) was observed when simulated threecornered alfalfa hopper induced plant loss was applied at R3 (Table 3). All plant loss treatments significantly reduced yield and crop value compared with the nondamaged control. Plant losses of 30% or 40% resulted in significantly lower soybean yields and crop value compared with plant density reductions of 0%, 10%, or 20%. Plots where plant density was reduced 50% had significantly lower yields and crop value compared all other treatments. When plant density was reduced by ≥ 10%, yield losses of 7.4% to 30.0% were observed. Crop value losses ranged from $103.77/ha to $417.23/ha. Plant loss of 30% or greater resulted in significantly higher crop value losses compared with plant loss of 20% or less, with plant loss of 50% resulting in significantly more crop value loss than all of the other treatments.
A significant impact on yield, crop value (F=61.42, df=5, 145, P < 0.01), and crop value loss (F=52.76, df=4, 113, P < 0.01) was observed when simulated threecornered alfalfa hopper induced plant loss was imposed at R5 (Table 4). All plant density reduction treatments significantly reduced yield and crop value compared with the nondamaged control. Generally, as the level of plant loss increased, a significant reduction in soybean yield and crop value was observed with each increase in plant loss. Plots that received the 20% or 30% plant loss treatments produced similar yields and crop value. Yields and crop value of plots that received either of these treatments were significantly less than for plots that received treatments of ≤10% plant loss and were significantly greater than for plots that received treatments of ≥40% plant loss. Plant loss of ≥ 10% resulted in yield reductions of 7.1% to 33.8%. Crop value losses ranged from $174.45/ha to $486.63/ha. Plant loss of 20% or greater resulted in significantly higher crop value losses compared with plant loss of 10%. When plant loss exceeded 30%, each increment in plant loss resulted in significantly more crop value loss than the previous level.
Impact of plant loss on yield, crop value, and crop value loss of late planted / double crop indeterminate soybeans. A significant impact on yield, crop value (F=7.85, df=5, 33, P < 0.01), and crop value loss was observed when simulated threecornered alfalfa hopper induced plant loss was applied during the late R1-early R2 growth stage (Table 5). Plant loss of ≥30% resulted in significantly lower soybean yields and crop value compared with that of plots with ≤20% plant loss. Plant loss of 50% resulted in significantly lower yields and crop value compared with all other treatments. Yield losses of 10.6% to 19.3% were observed when plant loss exceeded 30% at the R1–R2 growth stage. Crop value losses ranged from $45.04/ha to $217.73/ha. Plant loss of 50% resulted in significantly more crop value loss compared with any of the other plant loss treatments.
A significant impact on yield, crop value (F=12.34, df=5, 32, P < 0.01), and crop value loss (F=11.05, df=4, 25, P < 0.01) was observed when simulated threecornered alfalfa hopper induced plant loss was applied at R3 (Table 6). Simulated threecornered alfalfa hopper induced plant loss of ≥20 resulted in significantly lower yields and crop value compared with the nondamaged control. Plant density reductions of 40% and 50% resulted in significantly less yield and crop value compared with reductions of ≤20% and ≤30%, respectively. Yield losses of 6.9% to 20.6% were observed when plant loss was ≥ 20. Crop value losses ranged from $24.45/ha to $221.57/ha. Plant loss of ≥40% resulted in significantly more crop value loss compared with plant loss of ≤20%.
A significant impact on yield, crop value (F=15.85, df=5, 31, P < 0.01), and crop value loss (F=9.24, df=4, 18.4, P < 0.01) was observed when simulated threecornered alfalfa hopper induced plant loss was applied at R5 (Table 7). Plant loss of ≥30% significantly reduced yield and crop value compared with the nondamaged control. Plant loss of 50% resulted in significantly lower yields and crop value compared with all other treatments. Yields were reduced by 22.4 - 40% when plant density was reduced by ≥30%. Crop value losses ranged from $66.49/ha to $354.89/ha. Plant loss of ≥30% resulted in significantly higher crop value losses compared with plant loss of 10% with plant loss of 50% resulting in significantly higher crop value losses than any of the other plant loss treatments.
Impact of plant loss on yield, crop value, and crop value loss of full season determinate soybeans. A significant impact on yield, crop value (F= 51.45, df=5, 95, P < 0.01), and crop value loss (F=49.03, df=4, 76.1, P < 0.01) was observed when simulated threecornered alfalfa hopper induced plant loss was applied at R5 (Table 8). Plant loss of ≥20% significantly reduced yield and crop value compared with the nondamaged control. Yields and crop value of plots that received the 20% plant loss treatment were similar to those of plots that received the 10% plant loss treatment. Plant loss of ≥30% resulted in significantly lower yields and crop value compared with all other treatments. Yield losses of 9.6% to 47.1% were observed when plant loss exceeded 20%. Crop value loss ranged from $31.25/ha to $453.61/ha. Plant loss of ≥30% resulted in significantly more crop value loss compared plant loss of 10% or 20%. Each increasing increment of plant loss above 30% resulted in significantly more crop value loss than the previous plant loss level.
Impact of initial plant density and plant loss on yield of full season indeterminate soybeans. No significant interaction between initial plant population and simulated threecornered alfalfa hopper induced plant loss at R5.5 was observed for yield, crop value (F=1.16, df=6, 63, P = 0.34), or crop value loss (F= 0.96, df=4, 42, P = 0.44). Also, there was no significant impact of initial plant population on soybean yield, crop value (F=0.37, df=2, 14, P = 0.70), or crop value loss (F=1.75, df=2, 14, P = 0.21). Simulated threecornered alfalfa hopper induced plant loss occurring at R5.5 significantly impacted soybean yield, crop value (F=40.06, df=3, 63, P < 0.01), and crop value loss (F=30.21, df=2, 42, P < 0.01). All levels of plant loss significantly reduced yield and crop value compared with the nondamaged control (Table 9). In this study simulated threecornered alfalfa hopper induced plant loss was applied to plots in increments of 20%. Each increment of plant density reduction resulted in significantly lower yield and crop value than the previous increment. Yield losses of 9.2% to 36.3% were observed when plant loss was ≥20%. Crop value losses ranged from $150.92/ha to $593.88/ha. Each increasing increment of plant loss resulted in significantly more crop value loss than the previous level.
Discussion
Soybean plants may respond in several ways to threecornered alfalfa hopper injury (girdling) to the main stem during the seedling and early vegetative stages. These include death of the plant usually within 2 - 3 wks of injury, mainstem breakage at the girdle site from soon after girdling through maturity, partial breakage at the girdle site with continued growth, or the injury site may heal over and the plant remain upright (Mueller and Dumas 1975, Mueller and Jones 1983). Mueller and Jones (1983) infested threecornered alfalfa hopper nymphs on the main stems of determinate Maturity Group VI and VII soybeans at the V3–V6 stage. Of the plants with mainstem girdles, approx. 34% died by 6 wks after infestation. Another 6.5% broke or lodged sometime prior to harvest and were not harvestable. This study did not indicate when breakage/lodging occurred. For plants that break at the injury site (girdle), when this occurs can impact yield. Caviness and Miner (1962) found that plant density reductions of 45% at flowering (R1) reduced yield of a determinate Maturity Group VI variety and that plant density reductions of 30% and 45% at 2 wks after bloom (R3 possibly) also reduced yields. One factor that can influence if/when mainstem breakage at girdle site occurs is wind. Mueller and Jones (1983) stated that only the most severe weather conditions, such as extremely strong winds, driving rains, or hail could cause this. However, strong winds are often associated with thunderstorms and tropical weather systems that occur in the MidSouth.
In our studies plant loss significantly reduced yield and crop value at all growth stages evaluated. Generally, the closer to maturity that plant loss occurred, lower percentages of plant loss were required to significantly reduce yield and value of full-season indeterminate soybeans. The response to plant loss was not consistent across growth stages indicating that indeterminate soybeans have some ability to compensate for reductions in plant density after flowering (R1) depending on the level of plant loss and when plant loss occurs. For late-planted / double-crop indeterminate soybeans, similar levels of plant loss were required to reduce yields at the R1–R2 and R5 stages. However, a lower percentage was required to reduce yields at the R3 growth stage. The reasons for this difference are unknown, but for the R5 growth stage may be related to the overall lower yield potential. In the current studies, plant loss of ≥20% at R5 significantly reduced yields of determinate Maturity Group V soybeans. Mueller and Jones (1983) infested threecornered alfalfa hopper nymphs on the main stems of determinate Maturity Group VI and VII soybeans at the V3–V6 stage. Of the plants with mainstem girdles, approx. 34% died by 6 weeks after infestation. Another 6.5% broke or lodged sometime prior to harvest and were not harvestable. This study did not indicate when breakage/lodging occurred.
Current threecornered alfalfa hopper thresholds for vegetative soybeans focus on plant density reductions when plants are 30 cm or less in height. Current recommendations for soybean plant populations in the MidSouth range from approx. 247,000 - 321,000 plants/ha for plantings during early April to early May. For a late-planted (June through July) indeterminate Maturity Group IV soybean variety (95cm row spacing), Ball et al. (2000) observed significantly higher yields with plant populations ≥210,000 and ≥300,000 plants/ha during 1997 and 1998, respectively, under irrigated conditions. Lee et al. (2008) observed that plant populations required to produce 95% of maximum yield for three Maturity Group IV soybean varieties ranged from 108,000 - 159,000 plants/ha. In the current studies, initial plant population as low as approx. 85,000 plants/ha did not significantly impact yield for an indeterminate Maturity Group IV variety planted during late April to early May. However, reduced plant density may delay canopy closer which would allow more time for weeds to emerge. Increased weed emergence could lead to increased weed management costs for growers and / or increased competition with the crop.
Soybean production practices, especially varietal maturity, in the MidSouth have changed drastically during the last 20 + years. These studies indicate that yield response of indeterminate Maturity Group IV soybeans to plant loss that could occur from early-season threecornered alfalfa hopper injury is not consistent across reproductive growth stages. Additional research is needed to determine the mechanism of yield compensation for plant loss. Also, research is needed to determine what percentage of girdled/injured plants will break/lodge following threecornered alfalfa hopper injury during the early vegetative growth stages, when breakage/lodging is most likely to occur, what level and/or duration of threecornered alfalfa hopper infestation on soybean seedlings that would result in yield reductions from plant breakage. This would help refine threecornered alfalfa hopper treatment thresholds for early vegetative stage soybeans.
Contributor Notes
2Mississippi State University, Delta Research & Extension Center, Stoneville, Mississippi 38756.
4University of Tennessee, West Tennessee Research & Education Center, Jackson, Tennessee 38301.
5(Formerly University of Arkansas) Dulaney Seed, Clarksdale, Mississippi 38614.
6(Formerly University of Arkansas) FMC Corporation, Monticello, Arkansas 71655.
7LSU AgCenter, Baton Rouge, Louisiana 70803.
8University of Arkansas, Division of Agriculture, Lonoke, Arkansas 72086.
9LSU AgCenter, Department of Entomology, Baton Rouge, Louisiana 70803.