This was a field experiment which was conducted in 2004 and 2005 using two Edamame soybeans cultivars, Butterbeans and Envy, which were both early and late planted in both years. Edamame soybean is a high protein leguminous green vegetable crop which is harvested and eaten when seeds are immature (R6). This study was aimed at measuring and comparing the infestation levels of Cerotoma trifurcata between the two cultivars and the damage caused. Bean leaf beetle adults were visually observed during the developmental stages of the plants up to the harvestable R6 growth stage. Butterbeans showed more bean leaf beetle counts per plant than Envy for the 2004 and 2005 first planting. Both Butterbeans and Envy had a single C. trifurcata population peak in both seasons first planting. The second planting in 2004 showed significantly higher bean leaf beetle densities per plant on Butterbeans than Envy. In 2005 there was no significantly difference in bean leaf beetle infestation levels between the cultivars. Both cultivars had two peak populations of bean leaf beetle during the second planting period. There was no difference in pod damage between Butterbeans and Envy in 2004 and 2005 first planting date. However, during the second planting dare Butterbeans cultivar was significantly more damaged compared to Envy in both years. Hence, this study has revealed that planting date and choice of soybean cultivar can affect the infestation levels of C. trifurcata on plants. Butterbeans which is late maturing attracted more bean leaf beetle than Envy (early maturing).
Keywords: Edamame, Butterbeans, Envy, Bean leaf beetle, Cerotoma trifurcata, Glycines max
The bean leaf beetle (BLB), Cerotoma trifurcata (Forster) (Coleoptera: Chrysomelidae), is endemic to North America and it has been historically regarded in the mid-western United States, as a relatively minor pest which can occasionally cause economic losses in soybeans, Glycine max. However, its pest status and abundance has increased in the recent years due to an increase in soybean acreage. Hence, there appears to be a need to find and evaluate integrated pest management tactics for this pest (Carrillo et al. 2005). This insect has been reported to be a sporadic pest of soybean, G. max, in Nebraska. The BLB adults overwinter in field crop residue and in forest leaf litter (Carrillo et al. 2005). In the spring the adults exit the overwintering sites and move to the available hosts such as alfalfa. The adults move to soybeans as soon as the first seedlings emerge and colonize them at their early vegetative stages. The insect has two generations per year in Nebraska (Lam et al. 2001).
An understanding of the phenology of this pest is essential in the development of a successful pest management program. Important applications of this information include improved timing of scouting procedures and proper implementation of management tactics (Smelser and Pedigo, 1991). The early-season management of overwintered adult C. trifurcata has been suggested as an important tactic to reduce population buildup and crop damage (Krell et al. 2004). Bean leaf beetle is the pest most associated with pod injury on a regular basis in the Midwest. This insect chews smooth-edged, round holes in newly expanded leaves and pods reducing yield and seed quality. It may feed on blossoms and stems. Its larvae feed on roots, nodules and the underground stem. The beetle is capable of transmitting diseases such as bean pod mottle virus (BPMV). Adult bean leaf beetles feed mostly on leaves and pods, with the most serious losses caused by pod feeding. Pod lesions caused by the beetles allow excess moisture and secondary plant disease organisms to enter the pod, causing seed shriveling and discoloration. As a result, the seed quantity and quality are reduced (Zheng et al. 2005).
Host plant phenology affects the distribution and abundance of herbivores by producing a temporal variation in host quality. Insect herbivores often adapt to this temporal variation in resources by synchronizing their life cycles with the phenologies of the host plants. Dawn et al. (1999) reported that modification of planting dates has been widely used to disrupt synchrony between crops and pests. The insect pests cause the greatest yield reductions when their damaging stages are feeding during the most susceptible stage of crop growth. Although the population fluctuations and biology of bean leaf beetle on grain soybean has been studied (Lam et al. 2001), there is lack of information about their behavior and possible management practices on edamame soybeans. The modification of crop phenology can result in reduction of insect numbers and injury per insect. Such changes include planting alternative varieties, changing planting dates, or both. The objective of this study was to describe the population dynamics of bean leaf beetle adults on Butterbeans and Envy edamame soybean cultivars and relate insect densities to the different growth stages of these two cultivars. The study also determines the differences in pod damage caused by bean leaf beetle between the two cultivars at different planting dates.
Materials and Methods
Location and Experimental Design. A field experiment was conducted at the University of Nebraska-Lincoln's Agricultural Research and Development Centre near Ithaca, in 2004 and 2005. Two edamame soybean cultivars, Envy (64 days to harvest maturing) and Butterbeans (75 days to harvest maturing) were planted at two different planting dates to investigate the seasonal occurrence of bean leaf beetle. A 5850 square foot field area was divided into two parts corresponding to two planting dates for 2004 and 2005. Plots with four rows, measuring 18 meters by 3 meters, were planted with Edamame soybean at the rate of 150, 000 seeds per acre at 76 cm row spacing. Planting was done on 20 May and 3 June in 2004 and on 18 and 31 May in 2005. Sweet corn was planted as buffer at the margins of the field to reduce field border effects. The two planting dates were separated by a 40ft strip which was planted to sweet corn. Plots were hand weeded twice each year.
Bean leaf Beetle Populations. All the experimental plots were naturally infested with bean leaf beetle. The Bean leaf beetle adult counts per plant were counted twice weekly from the day they started to appear (V3 growth stage for early planted soybean and V1 growth stage for late planted soybean) up to maturity (R6 growth stage) of the soybean. Sampling was done between 1000 and 1500 h on every recording date. Soybean developmental stages were recorded on each sampling date. Data collection was done during the process of plant growth by in-situ observation (visual counts on the plant) of insects on the plant. The bean leaf beetle numbers were determined by counting those present on ten randomly selected plants per row in a plot. Counts were taken from two inner-rows in a plot. The first 20 and last 20 plants in each row chosen were not sampled to avoid field margin effects.
Bean leaf beetle damage on pods was assessed at the end of the R6 stage for each variety when pods started to change to a yellowish color. This was done by randomly picking five plants per row in a plot. Ten pods from each selected plant were randomly chosen in 2004 and 20 pods in 2005. Each pod was then assessed and evaluated visually for bean leaf beetle damage on a 0 to 5 score (Aruna et al. 2005) (0 = plants with no pod damage, 1 = ≤5% pod damage, 2 = ≤10% pod damage, 3 = 11 to ≤20% pod damage, 4= 21 to ≤ 40% pod damage and 5 = ≥41% pod damage).
All the experimental data were analyzed using SAS (SAS Institute 2004). The analysis of infestation levels of bean leaf beetle between and within cultivars was done by using PROC GLM to test for the significant differences. Least-significant differences were calculated for mean separation when the F-test showed significant differences among treatments at the 0.05 level of probability. The pod damage between varieties was also analyzed using PROC GLM.
Results and Discussion
Cerotoma trifurcata began to be observed at the same time (mid-June) for both Butterbeans and Envy and also for both planting dates even though the varieties were at different growth stages.
Butterbeans and Envy Comparison in C. trifurcata Densities for 2004/2005 First Planting Date. The average bean leaf beetle counts per plant between the two cultivars were significantly different in 2004 (F = 22.96; df = 1, 56; P = 0.0001) and 2005 (F = 48.29; df = 1; P = 0.0001) when compared up to R6 growth stage. The C. trifurcata beetles for both seasons were abundant on Butterbeans than Envy (Table 1).
Plant stage comparisons within and between cultivars in C. trifurcata Densities for 2004/2005 First Planting Date. C. trifurcata were first observed on the Butterbeans plants during the V3 growth stage in both years (Table 2). The beetle infestation levels among the Butterbeans growth stages were significantly different in 2004 and 2005 first planting period. Butterbeans growth stages were significantly different in C. trifurcata populations (Table 2) during this planting period of 2004 (F = 21.56; df = 8, 31; P = 0.0001) and 2005 seasons (F = 24.20; df = 8, 71; P = 0.0001). C. trifurcata were observed to exhibit one peak during the growth of Butterbeans which occurred from the V3 growth stage (Mid June) which was not significantly different from V4 to R1 growth stages in average bean leaf beetle counts per plant (Table 2). The population of C. trifurcata was observed to distinctly increase at the R7 growth stage at the end of August (results not shown here). R6 is a growth stage at which the harvesting of these green soybeans is done. This implies that timely harvesting can reduce or avoid damage potential by this second population peak. The first increase in C. trifurcata population for the first generation is associated with foliar feeding and the second generation with seed and pod feeding (Hammond et al. 1991).
C. trifurcata first appeared on Envy cultivar during first planting period at the V4 growth stage (Table 2). There was a significant difference in average populations of C. trifurcata on Envy growth stages during the first planting period of the 2004 (F = 29.14; df = 8, 25; P = 0.0001) and 2005 (F = 51.73; df = 8, 59; P = 0.0001) (Table2). The beetle population increased and reached their highest levels around R1 growth stage (blooming stage) in 2004 which was not significantly different from that of V3 to V6 growth stages (blooming stage). In 2005, the beetles reached their population peak at R1 growth stage which was not significantly different from V5 growth stage. Envy plants experienced one bean leaf beetle population peak because of their early maturity. By the time the resurgence of C. trifurcata occurred late in the season, Envy plant leaves were maturing showing yellowing and reddish coloration. There were no C. trifurcata found on Envy plants after R6 stage. The plant quality was deteriorating and also less attractive for bean leaf beetle.
Comparison of the two cultivars revealed that Butterbeans were significantly higher in bean leaf beetle counts per plant than Envy at most growth stages in 2004 (F = 4.31; df = 8, 56; P = 0.0004) and 2005 (F = 8.65; df = 8, 56; P = 0.0001) (Table 2). Therefore, yield or quality of Butterbeans is likely to be affected by the C. trifurcata infestation more than that for Envy because it is highly infested at certain growth stages such as in 2004 (V3 growth stage) and in 2005 (V3, V4, R5 and R6 growth stages).
Butterbeans and Envy Comparison in C. trifurcata Densities for 2004/2005 Second Planting Date. C. trifurcata mean numbers were significantly different between Butterbeans and Envy cultivars in 2004 (F = 31.40; df = 1, 62; P = 0.0001) but it was not significantly different in 2005 (F = 25.50; df = 1, 146; P = 0.97) for this planting period. In 2004, more beetles were seen on Butterbeans than Envy (Table 3).
Plant stage comparisons within and between cultivars in C. trifurcata Densities for second Planting Date. In the 2004 second planting period the Butterbeans cultivar growth stages were significantly different in bean leaf beetle counts per plant (F = 17.32; df = 10, 31; P = 0.0001). These plants experienced two peak populations of bean leaf beetle. The first peak was around V3 and V4 growth stages which were not significantly different from the V1, V2, V5 and V6 growth stages (Table 4). The bean leaf beetle populations declined from R1 to R4. The second peak population occurred at the R6 growth stage. Similarly at the second planting of 2005, the Butterbeans growth stages showed significant difference (F = 36.93; df = 10, 77; P = 0.0001) in terms of the average bean leaf beetle counts per plant (Table 4). Two peaks of bean leaf beetle populations occurred on this cultivar with the first observed around V3 and V4 growth stages and the second at R5 and R6 growth stages. This second peak population on Butterbeans is likely to affect the beans if timely harvesting is not done.
Similarly, Envy growth stages significantly differed from each other in C. trifurcata infestation per plant in 2004 (F = 16.19; df = 10, 31; P = 0.0001) and also in 2005 (F = 51.80; df = 10, 69; P = 0.0001). However, unlike the Butterbeans, Envy was not infested by the second peak population in 2004 second planting period (Table 4) but was only infested in 2005 at the R6 growth stage. Envy only had a single peak population which in 2004 occurred around V2 and V3 growth stages which were not significantly different from V4, V5 and V6 growth stages. In 2005 the first population peak occurred at V5 growth stage which was not significantly different to V3, V4 andV6 growth stages (Table 4) and the second peak population was observed at the R6 growth stage. This second peak population increase in 2005 was observed to continue to increase beyond R6 growth stage (Data not shown). The beetle populations per plant were very low especially starting from R1 up to R6 growth stages. This is the time when the vegetative growth of Envy had changed to yellowish and reddish color hence presumably less attractive to the beetles.
When comparing the two cultivars in terms of the average bean leaf counts per plant, it was revealed that some Butterbeans and Envy growth stages were significantly different in 2004 (F = 5.12; df = 10, 62; P = 0.0001) and in 2005 (F = 25.93; df = 10, 146; P = 0.0001) (Table 4). The Butterbeans infestations by C. trifurcata at V1, R3, R5 and R6 growth stages in 2004 second planting period were found to be significantly higher than on Envy. In 2005 second planting period, Butterbeans V4, R4, R5 and R6 growth stages were also observed to be significantly higher in bean leaf beetle counts per plant compared to Envy (Table 2) even though Envy showed higher beetle infestations than Butterbeans at V5, V6 and R1growth stages. Butterbeans cultivar had some green leaves and pods later in the season which were a likely source of attraction to C. trifurcata even at the R6 growth stage. Baur et al. (2000) reported that maturity group of a soybean plant can influence the abundance of insect pests. When cultivars of differing maturities are present, insects will often prefer one cultivar over another.
The first peak experienced by both cultivars is not likely to affect the soybean growth and development as it has been reported that vegetative soybeans are capable of withstanding extensive defoliation before they suffer serious economic injury (Hammack et al. 2010). Soybeans can also compensate for defoliation by producing excess leaves (Higley and Boethel, 1994). The increase in C. trifurcata populations around the R4 growth stage (full pod), R5 growth stage (seed filling) and R6 growth stage is likely to affect yield and quality by damaging the pods. Damage through pod feeding that occurs during R4 to R6 growth stages is most likely to cause yield losses (Hammack et al. 2010). Therefore, Butterbeans are more likely to be affected by C. trifurcata infestations than Envy.
Pod Damage by Cerotoma trifurcata. There was no significant difference in pod damage between Butterbeans and Envy both in 2004 (F = 2.70; df = 1, 18; P = 0.12) (Fig. 1) and 2005 (F = 3.75; df = 1, 38; P = 0.06) (Fig. 2) during the first planting period. This could be the result of both cultivars not experiencing the second peak population which is the one that affect the seed number and quality. However, during the second planting period Butterbeans cultivar was significantly more damaged compared to Envy in 2004 (F = 6.43; df = 1, 18; P = 0.02) (Fig. 1) and in 2005 season (F = 36.69; df = 1, 38; P = 0.0001) (Fig. 2). The second peak population is likely cause of the higher damage on pods of Butterbeans. More so the Butterbeans continued to have green vegetative growth even at the R6 growth stage and this could have attracted more beetles to Butterbeans plants. The within Butterbeans cultivar comparison of the early and late planted soybeans pod damage show that late planted soybeans are significantly more damaged than early planted soybeans (F = 9.56; df = 1, 18; P = 0.006). A similar pattern was shown by Envy cultivar (F = 4.93; df = 1, 18; P = 0.04). Therefore, soybean pod damage was significantly higher in late than early planting dates. The population pattern between the early and late planted plants for both cultivars is similar for 2004 and 2005.
It has been observed in this study that time of planting can be utilized to avoid impacts of bean leaf beetle populations on these cultivars. Both cultivars planted during the first the second planting period matured on time before the occurrence of second peak. However, only Envy during the second planting period was able to avoid attack by the second peak of the bean leaf beetle population. This study also shows that choice of cultivar can help to reduce the impacts of C. trifurcata infestations. Envy was not attacked by C. trifurcata at the critical growth stages in both the first and second planting dates. Butterbeans was infested when planted at the second. This means that Envy can be planted at both times (first and second planting periods) without severe problems due to C. trifurcata populations. However, Butterbeans can do better if planted during the first planting period. Timely harvesting of these cultivars can also help reduce the destruction likely to be caused by C. trifurcata populations.
In conclusion, this study has revealed that planting date can affect the infestation levels of C. trifurcata on Butterbeans and Envy. The pods of the first planting period Butterbeans and Envy were less damaged by C. trifurcata than the second planting period because they did not experience the second C. trifurcata population peak. It is also evident that the choice of soybean cultivar planted can help reduce the population density of C. trifurcata on plants. Butterbeans which is late maturing attracted more bean leaf beetle than Envy (early maturing).
References Cited
Aruna, R., D. M. Rao, L. J. Reddy, H. D. Upadhyaya and H. C. Sharma. 2005. Inheritance of trichomes and resistance to pod borer (Helicoverpa armigera) and their association in interspecific crosses between cultivated pigeon pea (Cajanus cajan) and its wild relative C. scarabaeoides. Euphytica 145: 247-257.
Baur, M. E., D. J. Boethel, M. L. Boyd, G. R. Bowers, M. O. Way, L. G. Heatherly, J. Rabb, L. Ashlock. 2000. Arthropod Populations in Early Soybean Production Systems in the Mid-South. Environ. Entomol. 29: 312-328.
Carrillo, M. A., R. L. Koch, E. C. Burkness, K. Bennett, D. W. Ragsdale, and W. D. Hutchison. 2005. Supercooling Point of Bean Leaf Beetle (Coleoptera: Chrysomelidae) in Minnesota and a Revised Predictive Model for Survival at Low Temperatures. Environ. Entomol. 34: 1395-1401.
Dawn, H. G., O. W. Michael., K. Allen., C. Greg., P. Carl. 1999. Managing Soybean Insects. Texas Agricultural Extension Service. B1501.
Hammack, L., J. L. Pikul, Jr. and M. S. West. 2010. Phenology and Abundance of Bean Leaf Beetle (Coleoptera: Chrysomelidae) in Eastern South Dakota on Alfalfa and Soybean Relative to Tillage, Fertilization, and Yield. Environ. Entomol. 39: 727-737.
Hammond, R. B., R. A., Higgins, T. P., Mack, L. P., Pedigo, and E. J., Bechinski. 1991. Soybean Pest Management, pp.341-472. In D. Pimentel (eds.), CRC Hand Book of Pest Management in Agriculture, 2nd ed. Boca Raton, Florida.
Higley, L. G., and D. J. Boethel. 1994. Handbook of soybean insect pests. Entomological Society of America, Lanham, MD.
Krell, R. K., L. P. Pedigo, J. H. Hill and M. E. Rice. 2004. Bean leaf beetle (Coleoptera: Chrysomelidae) management for reduction of bean pod mottle virus. J. Econ. Entomol. 97: 192-202.
Lam, W.-K. F., L. P. Pedigo & P. N. Hinz. 2001. Population dynamics of bean leaf beetles (Coleoptera: Chrysomelidae) in central Iowa. Environ. Entomol. 30: 562-567.
SAS Institute. 2004. SAS/STAT User's Guide, Version 9.1.4. SAS Institute, Cary, NC.
Smelser, R. B. and L. P. Pedigo. 1991. Phenology of Cerotoma trifurcata on soybean and alfalfa in central Iowa. Environ. Entomol. 20: 514-519.
Zheng, C., P. Chen, T. Hymowitz, S.Wickizer, and R. Gergerich. 2005. Evaluation of Glycine species for resistance to bean pod mottle virus. Crop Prot. 24:49-56.
List of tables
Table 1. Mean number of C. trifurcata per plant in 2004 and 2005 first planting dates
Table 2. Mean number of C. trifurcata per plant for 2004 and 2005 first planting date growth stages
Table 3. Mean number of C. trifurcata per plant in 2004 and 2005 second planting dates
Table 4. Mean number of C. trifurcata per plant for 2004 and 2005 second planting date growth stages
List of Figures
Fig. 1: Mean pod damage ratings for C. trifurcate in 2004
Fig. 2: Mean pod damage ratings for C. trifurcate in 2005
Table 1. Mean number of C. trifurcata per plant in 2004 and 2005 first planting dates
Planting Date
Season
Mean ± SE
Butterbeans
Envy
June 3
2004
1.08 ± 0.06 a
0.71 ± 0.15 a
May 31
2005
0.87 ± 0.03 a
0.53 ± 0.04 b
* Means for each date within a row followed by the same letter are not significantly different (P ≤ 0.05)
Table 2. Mean number of C. trifurcata per plant for 2004 and 2005 first planting date growth stages
Plant stage
2004 Mean ± SE
2005 Mean ± SE
Butterbeans
Envy
Butterbeans
Envy
V3
1.70 ± 0.20 a A
0.00 ± 0.20 b C
0.66 ± 0.09 a B
0.00 ± 0.13 b D
V4
1.80 ± 0.14 a A
1.30 ± 0.20 a A
1.20 ± 0.09 a A
0.60 ± 0.09 b C
V5
1.40 ± 0.20 a A
1.20 ± 0.14 a A
1.45 ± 0.13 a A
1.00 ± 0.09 a B
V6
1.80 ± 0.20 a A
1.30 ± 0.20 a A
1.31 ± 0.13 a A
1.33 ± 0.13 a A
R1
1.75 ± 0.14 a A
1.37 ± 0.12 a A
1.33 ± 0.09 a A
1.16 ± 0.08 a AB
R3
0.65 ± 0.14 a B
0.70 ± 0.14 a B
0.41 ± 0.08 b B
0.73 ± 0.09 a C
R4
0.43 ± 0.10 a B
0.50 ± 0.14 a B
0.00 ± 0.08 a C
0.00 ± 0.09 a D
R5
0.17 ± 0.08 a B
0.00 ± 0.12 a C
0.38 ± 0.07 a B
0.00 ± 0.08 b D
R6
0.00 ± 0.20 a B
0.00 ± 0.14 a C
1.08 ± 0.09 a A
0.00 ± 0.13 b D
*Means within a column of the same year followed by the same capital letter are not significantly different (P ≤ 0.05).
*Means for each plant stage within a row followed by the same small letter are not significantly different (P ≤ 0.05).
Table 3. Mean number of C. trifurcata per plant in 2004 and 2005 second planting dates
Planting Date
Season
Mean ± SE
Butterbeans
Envy
June 3
2004
1.34 ± 0.05 a
0.91 ± 0.07 b
May 31
2005
0.76 ± 0.04 a
0.62 ± 0.04 a
* Means for each date within a row followed by the same letter are not significantly different (P ≤ 0.05)
Table 4. Mean number of C. trifurcata per plant for 2004 and 2005 second planting date growth stages
Plant stage
2004 Mean ± SE
2005 Mean ± SE
Butterbeans
Envy
Butterbeans
Envy
V1
1.30 ± 0.22 a AB
0.00 ± 0.22 b D
0.69 ± 0.08 a D
0.70 ± 0.11 a B
V2
1.75 ± 0.16 a AB
1.70 ± 0.22 a A
1.15 ± 0.08 a BC
0.73 ± 0.11 a B
V3
2.05 ± 0.16 a A
1.60 ± 0.16 a A
1.38 ± 0.08 a AB
1.11 ± 0.08 a A
V4
2.12 ± 0.16 a A
1.70 ± 0.16 a A
1.50 ± 0.08 a A
1.28 ± 0.08 b A
V5
1.90 ± 0.16 a AB
1.48 ± 0.22 a A
0.90 ± 0.08 b CD
1.38 ± 0.11 a A
V6
1.80 ± 0.22 a AB
1.30 ± 0.22 a AB
0.00 ± 0.11 b E
1.20 ± 0.11 a A
R1
0.65 ± 0.16 a C
0.85 ± 0.16 a BC
0.00 ± 0.08 b E
0.78 ± 0.08 a B
R3
0.00 ± 0.22 b D
0.60 ± 0.16 a CD
0.00 ± 0.08 a E
0.00 ± 0.08 a C
R4
0.40 ± 0.13 a CD
0.15 ± 0.16 a CD
0.25 ± 0.08 a E
0.00 ± 0.08 b C
R5
1.18 ± 0.12 a B
0.32 ± 0.10 b CD
1.04 ± 0.06 a CD
0.05 ± 0.07 b C
R6
1.63 ± 0.22 a AB
0.30 ± 0.16 b CD
0.95 ± 0.11 a CD
0.64 ± 0.07 b B
*Means within a column of the same year followed by the same capital letter are not significantly different (P ≤ 0.05).
*Means for each plant stage within a row followed by the same small letter are not significantly different (P ≤ 0.05).
Fig. 1. Mean pod damage ratings for C. trifurcate in 2004. Pod damage rating of 0 referred to plants with no pod damage, 1 those with ≤5% pod damage, 2 those with ≤10% pod damage, 3 those with 11 to ≤20% pod damage, 4 those with 21 to ≤ 40% pod damage and 5 those with ≥41% pod damage.
Fig. 2. Mean pod damage ratings for C. trifurcate in 2005. Pod damage rating of 0 referred to plants with no pod damage, 1 those with ≤5% pod damage, 2 those with ≤10% pod damage, 3 those with 11 to ≤20% pod damage, 4 those with 21 to ≤ 40% pod damage and 5 those with ≥41% pod damage.
