Site and application information. 1 of 10 8/5/2010 8:46 AM. Trial: SBAseedearl, SBAseedlate

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1 1 of 10 8/5/2010 8:46 AM myu One Stop Directories Search U of M Print this page using Acrobat Reader Trial: SBAseedearl, SBAseedlate Seed applied insecticide control of the Soybean Aphid (Aphis glycines) and Bean Beetle (Cerotoma trifurcata). Bruce Potter, University of Minnesota Background The soybean aphid (SBA) is a new pest of Minnesota soybeans. It was first identified during the late summer of 2000 in SE MN. SBA subsequently spread throughout the soybean growing areas of MN during the 2001 growing season. The bean leaf beetle (BLB) is a native soybean insect pest whose populations have increased over the past few years in response to favorable wintering conditions. This insect is capable of transmitting bean pod mottle virus (BPMV). This virus has previously been documented but at low levels in Southwest Minnesota soybeans. A trial was conducted during the 2002 growing season to: 1) Evaluate rates of two neonicotinoid insecticide seed treatments, Cruiser (thiamethoxam, Syngenta Crop Protection, Inc.) and Gaucho (imadochloprid, Gustafson LLC), on SBA and BLB 2) Compare seed treatment SBA control to foliar applications of pyrethroid (Warior, l-cyhalothrin, Syngenta Crop Protection, Inc.) and organophosphate (Lorsban 4E, chlorpyrifos, Dow AgroSciences, LLC) insecticides. Site and application information The trial was placed near a grove with a high density of buckthorn, Rhamnus spp. on the University of Minnesota Southwest Research and Outreach Center (U of M SWROC). This area was also presumed to provide an overwintering site for SBA. Additionally, the trial was near an area where SBA hotspots occurred in August The wooded area was expected to produce a high level of overwintering BLB adults. This field has a history of iron deficiency chlorosis and soybean cyst nematode and an attempt to chose a soybean cultivar tolerant to these conditions was made. Two planting dates for the insecticide seed treatments were used to: 1) evaluate the duration of control and 2) try to minimize expected overwintered bean leaf beetle pressure in the late planting. Cooperator: University of Minnesota Southwest Research and Outreach Center County: Redwood County Nearest town: Lamberton, MN Soil type: Canisteo clay loam and Knoke silty clay loam Cultivar: NK S18-U9 Row spacing: 30

2 2 of 10 8/5/2010 8:46 AM Seeding rate: 175,000 with cones on JD Maxemerge units. Planting date: May 2, 2002(early planting) and May 23,2002 late planting Experimental design: randomized complete block, 4 replications Plot size: 10 X30 Application date: Post emerge insecticide application date (July 23,2002 late planting) Crop stage at post application: R3 Soybean aphid population at application: 0.8 aphids/upper expanded trifoliate Insecticide treatments were applied with a CO2 pressurized backpack sprayer, 8002XR flat fan nozzles on 18-inch spacing, 20 gallons/acre, 35 PSI. Results and discussion In spite of variety selection, soil property and soybean cyst nematode induced chlorosis and stunting of soybeans occurred in this trial. Two of the 4 replications of both the early (south rep 3 and 4) and late (north rep 1 and 2) planting dates were affected by chlorosis and the reduced growth doubtless increased the variability in this study. Plant growth: No phytotoxicity was observed for any of the seed treatments. Soybean emergence was very slow in the early-planted portion of the trial, beginning 12 days after planting with emergence continuing over the next 21 days. Faster emergence was observed for both compounds at all rates at the early planting date. Although a trend still existed for greater stand where seed applied insecticides were used, statistical differences no longer present at the VC stage after additional plants emerged. Stand differences were not observed with the late planting date (Table 1, Figure 1). The basis for these stand differences is unclear but may be due to seed corn maggot as damage from these insects was observed in the trial. In the early planting date portion of the trial differences in height were observed between untreated and insecticide treated plots. The higher rate of Gaucho and Cruiser were taller than lower rates (Table 1, Figure 2). Bean pod mottle virus was detected in these trials and is discussed under bean leaf beetle. Height differences were fairly uniform within plots and are likely related to a factor(s) other than BPMV. Height differences were not observed in the later planted portion of the trial and were not rated in those plots. Soybean aphid Soybean aphid was first detected in these plots on June 19, one of the first 2002 SBA observations in Minnesota. Soybean aphid populations, however, were slow to increase in this study. Aphid populations were estimated by examining trifoliate leaves from 20 randomly selected plants in the center two rows of each plot. Leaves were selected from the portion of the soybean canopy with the greatest SBA density. Aphid densities were greatest on the uppermost expanded trifoliate for early sampling dates. The exception was the August 16 sampling date in the late-planted trial. In this case mid canopy leaves (terminal 5 to 7 nodes) were sampled. Aphid populations in the early-planted portion of the trial were very low throughout the growing season (maximum of 1.3 aphids/ trifoliate leaf July 23-31). SBA populations in the untreated checks at insecticide application averaged < 1 aphid/ upper trifoliate and populations increased late in the season (14 aphids/mid trifoliate August 16) in the late planted portion of the trial.

3 3 of 10 8/5/2010 8:46 AM Aggregated, low SBA population densities make interpreting this SBA control data difficult. To facilitate analysis of variance (ANOVA), aphid counts were transformed to a 1-10 scale. Differences (p<0.1) in aphid populations between seed treatments and the untreated check were observed on July 23 (R3 stage) of the early planting date. With the possible exception of the low (20g) rate of Cruiser all seed treatments preformed similarly with 54 to 76 percent less aphids than untreated. These differences had disappeared by July 31(R4 stage) and were not significant at either date when the data were transformed (Table 2a). In the late planting date, the same trend for seed treatments to have lower SBA populations than the untreated plots was observed (Table 2b). Foliar insecticide treatments were applied July 23 to this planting date. No differences between seed treatments were observed on that date and SBA populations were extremely low (1.0 aphids / trifoliate). Percent control is based on the number of aphids/leaf data (not shown). Plots with insecticide seed treatments had 20-77% and 0-30% fewer aphids than untreated plots on August 1 and August 16 respectively. Seed treatments allowed more aphids than Warrior at and lbs AI/acre or Lorsban 4E at 0.05 lbs AI/acre as foliar insecticide treatments. Foliar treatments had 92 to > 99% l and 75 to 95% less SBA than untreated plots at 9 and 24 DAT. Aphid populations in untreated plots had declined after 24 DAT. The 20g rate of Cruiser and 62 gram rate of Gaucho had equal or higher levels of SBA aphid than untreated in late season samples of both planting dates. This indicates that insecticide concentrations in soybean plants had declined. Early season aphid control may have led to lower populations of SBA aphid predators and parasites in these plots. In turn, lower natural enemy populations and low in-plant insecticides might allow rapid SBA increases during late season. Bean leaf beetle Overwintering BLB adults were assessed by examining 20 plants (2 sets of 5 plants) and the adjacent soil surface from each of the center two plot rows. Defoliation caused by bean leaf beetle was estimated visually on the unifoliate and first trifoliate leaves of these same 20 plants. Bean leaf beetle populations were higher in the early (May 2 nd ) planted portion of the trial as expected. At no time were beetle populations greater than 0.3 BLB/plant. Bean leaf beetle populations were higher in untreated plots (p<0.05), which averaged 0.32 BLB/plant on May 29 and declined thereafter. Dead BLB were observed in the insecticide treated plots, which ranged from 0.03 to 0.10 BLB/plant on the May 29 sampling. Evidence of a rate response within products was observed (Table 3,Table 4) with the higher rates of both products allowing less defoliation. BLB control lessened by the second trifoliate where only the 30g and 50g rate of Cruiser had less defoliation then untreated (Table 3). No significant differences between insecticide treated and untreated were detected by the R3 stage. Bean leaf beetle populations were at much lower levels in the May 23 planting date. All insecticide seed treatments had significantly less defoliation than the untreated (Table 4). Defoliation injury levels and BLB populations were well below levels that are assumed to cause economic injury. However, at flowering, a high incidence of virus symptoms (Figure 3) appeared in the May 2 nd planted portions of this trial. Fifty randomly selected plants from the center two plot rows were used to assess virus incidence. Only those plants with obvious symptoms (mottling and distortion of leaves) were counted as positive. Incidence of virus symptoms was significantly higher greater in the untreated plots (Table 4). Expression of virus symptoms was lower and symptoms were subtler in the chlorotic 1 st and 2 nd replication and accounts in part for the high CV values. Differences between insecticide treatments and rates were not observed. The virus was suspected to be bean pod mottle virus (BPMV) because of its association with bean leaf beetle activity. Serology tests, subsequent to the visual ratings, confirmed the presence of BPMV in symptomatic plants. Virus symptoms were at very low levels in the May 23 planted portion of this trial and as a result not rated. The differences in BPMV incidence by planting date combined with low levels BPMV symptoms in insecticide treated plots indicates that transmission probably occurred soon after the beetles moved to the early planted plots. Yield

4 4 of 10 8/5/2010 8:46 AM Iron deficiency chlorosis impacted yields in these trials and caused obvious yield reduction in two of the four replications. Yield data for the May 2 planting date is presented in table 5 for the entire trial Additionally yields/treatment using only the two non-chlorosis affected replications are shown. With the former, differences occurred at the 10% but not the 5% level. As a result of few degrees of freedom, no differences were detected in the latter. Yields in general, corresponded to the plant height differences observed between insecticide treated and untreated plots and indicates that early season beetle feeding and/or bean pod mottle virus reduced yield. Analysis of viral symptoms (seed discoloration) by treatment has not yet been completed. Aphid pressure was low in this planting date and probably did not reduce yield Chlorosis was more severe in the late planting date. Stepwise regression indicates that chlorosis scores explain more than 85% of yield. Likewise aphid populations were negatively correlated with chlorosis scores. Yields are presented in table 6 for the two non-chlorotic replications only. For comparison, the SBA per leaf data at 9 and 16 DAT in the same plots is included. Yields tend to correspond to the 9 DAT aphid populations. Foliar treatments provide the best aphid control and tended to yield higher. The trend for higher yield in may also be due to in part to late season bean leaf beetle control. The high C.V. values are stark indicators of the problems in accurately sampling SBA populations. Chlorosis and low, non-uniform aphid populations caused tremendous variability in this trial. Therefore, in spite of trends that may appear in these data, it is best to refrain from drawing many conclusions. Acknowledgments: Thanks to Scott Anderson, Derek Erickson, and Tim Lendt, for aphid counting and application assistance.

5 5 of 10 8/5/2010 8:46 AM Table 1. Soybean seed applied insecticides effect on stand. U of M SWROC, Lamberton, MN 2002 Means within columns followed by the same letter(s) are not significantly different (p <0.05), least significant difference (LSD), 1 Soybean plant population was estimated from the center two rows of each plot. 3 Heights based on 5 plants from center 2 rows of each plot Figure 1. Figure 2. Height differential between untreated (left) and insecticide treated (right) soybeans, May 23 planting date. U of M SWROC, Lamberton, MN 2002.

6 6 of 10 8/5/2010 8:46 AM Table 2a. Insecticide seed treatment control of Soybean aphid (05/02/02 planting) - Lamberton, MN 2002 Means within columns followed by the same letter(s) are not significantly different (p <0.10), least significant difference (LSD), Table 2b. Insecticide seed treatment control of Soybean aphid (05/23/02 planting) - Lamberton, MN 2002 Means within columns followed by the same letter(s) are not significantly different (p <0.05), least significant difference (LSD),

7 7 of 10 8/5/2010 8:46 AM 1 Soybean aphid density was estimated by counting the number of aphids /trifoliate leaf from 20 randomly selected plants in the center two rows of each/plot. Leaves were selected from the portion of the canopy most heavily colonized at each sampling date. Aphid density was greatest on uppermost-expanded trifoliate leaves for all sampling dates but the August 16 sampling of the May 23 planting date. In this case, the mid canopy (terminal 5-7 nodes) was sampled. 3 Aphid counts/trifoliate leaf were transformed to the following rating scale for analysis of variance: 1) no SBA, 2) >0-1 SBA, 3) >1-2 SBA, 4)>2-4 SBA, 5)>4-8 SBA, 6)>8-16 SBA, 7)>16-32 SBA, 8)>32-64 SBA, 9)> SBA, 10) >128 SBA. Table 3.Insecticide seed treatments effect on overwintering and F1 BLB (05/02/02) planting. Lamberton, MN Means within columns followed by the same letter(s) are not significantly different (p <0.05), least significant difference (LSD), * Non - homozygous variances, LSD values and non-transformed data shown for illustrative purposes only. 1 Bean leaf beetle damage was visually estimated on the unifoliate, 1st trifoliate, and 2nd trifoliate leaves (where expanded) leaves of 20 randomly selected plants in the center two rows of each/plot ( 2 sets of 5 consecutive plants in each row). F1 beetle damage based on 20 randomly selected plants in the center two rows of each plot. Upper expanded trifoliates were recorded as positive when BLB occurred. Table 4. Insecticide seed treatment effect on over wintering bean leaf beetles and bean pod mottle virus. Lamberton, MN 2002.

8 8 of 10 8/5/2010 8:46 AM Means within columns followed by the same letter(s) are not significantly different (p <0.05), least significant difference (LSD), * Mean separations based on transformed data (arcsine sqrt percent). 1 Bean leaf beetle damage was visually estimated on leaves of 20 plants in the center two rows of each plot (2 sets of 5 consecutive plants in each row). Data shown are average of unifoliate and 1 st trifoliate defoliation. 3 Bean pod mottle virus was assessed visually on 25 plants from each of center two plot rows. Only plants with obvious virus symptoms (leaf mottling and distortion) were counted as positive, Figure 4. Soybean expressing Bean pod mottle virus symptoms on upper foliage. U of M SWROC 2002 Table 5. Soybean seed applied insecticides effect on yield. U of M SWROC, Lamberton, MN 2002

9 9 of 10 8/5/2010 8:46 AM Means within columns followed by the same letter(s) are not significantly different (p <0.10), least significant difference (LSD), Table 6. Soybean seed applied insecticides effect on yield. May 23 planting date. Means calculated from the two non-chlorotic replications only. U of M SWROC, Lamberton, MN 2002 Means within columns followed by the same letter(s) are not significantly different (p <0.10), least significant difference (LSD), 1 Data transformed (arcsine sqrt percent). Means reported as non-transformed, LSD transformed. Return to the SWROC Pest Management Page Return to the SWROC Home Page This page was created 12/9/03 by B. Potter with technical assistance from M. Werner Regents of the University of Minnesota. All rights reserved.

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