Efficacy of Thiamethoxam to Suppress Soybean Aphid Populations in Minnesota Soybean

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Brian P. McCornack, Research Fellow, and David W. Ragsdale, Professor, 219 Hodson Hall, 1980 Folwell Ave, Department of Entomology, University of Minnesota, St. Paul 55108

Abstract

Soybean aphid poses a serious threat to soybean production in the United States and Canada by reducing plant height, pod number, and yield. Since its introduction, foliar insecticides have been the most common method to control soybean aphid. Treatment of soybean seed with a systemic insecticide may provide producers with an alternative to foliar applied insecticides. Therefore, our objective was to evaluate the efficacy of a seed treatment to suppress soybean aphid populations in soybean. In 2003, 2004, and 2005 we conducted field studies and supportive laboratory and in-field bioassays (2003 and 2004 only for bioassays) to assess mortality under controlled conditions. In the field, soybean aphid populations were assessed weekly by counting the total number of aphids per plant. In an excised-leaf bioassay, aphid mortality persisted 23 to 35 days after planting. In an in-field bioassay (2004 only), intact plants showed longer persistence of thiomethoxam and aphid mortality persisted 49 days after planting and mortality was significantly higher in the older leaves than in newly-expanded leaves. In all years and under various aphid densities, thiamethoxam significantly reduced season-long aphid pressure by 45.0 to 66.7%. However, thiamethoxam treatment did not significantly increase yield in three of four location-years, which coincided with low aphid density in untreated control plots.

Introduction

Soybean aphid (Aphis glycines Matsumura) poses a serious threat to soybean [Glycine max (L.) Merrill] production in the United States and Canada. High soybean aphid densities (i.e., > 1000 aphids per plant prior to flowering) damage soybean by reducing plant height, pod number, and yield (3,6,11). Soybean aphid also causes indirect damage through feeding and honeydew excretion (i.e., virus transmission and sooty mold formation) (2,8). Even at low densities, feeding by soybean aphid may impair photosynthetic processes vital to pod formation and development (7). Since its confirmed occurrence in the United States in 2000, foliar insecticides have been the most reliable method for control (9,12).

Insecticidal seed treatments may provide an alternative to foliar sprays and may reduce the negative impact on some beneficial insects associated with foliar application of insecticide. Generally, seed-applied insecticides are used in situations where insect damage and the potential for economic loss are likely. To date, seed treatments registered for soybean aphid control are all neonicotinyl-based insecticides. Here we used thiamethoxam (Crusier Maxx, Syngenta Crop Protection, Greensboro, NC) as a representative of the neonicotinyl-based insecticides. However, the effect of thiamethoxam on soybean aphid population dynamics is not fully understood. Therefore, the objective of this study was to evaluate effectiveness and length of control of thiamethoxam when applied to soybean seed in suppressing soybean aphid populations in Minnesota.

Soybean Aphid Colony Management and Experimental Design

Laboratory colonies of soybean aphid were started in July 2003 with field-collected apterae (i.e., wingless aphids) from soybean fields at the University of Minnesota Outreach Research and Education (UMore) Park, Rosemount, MN. Approximately 200 to 300 field-collected apterae were added to the colonies in July of 2004 and again in July of 2005. Colonies were maintained on soybean seedlings (NK Brand, ‘S19-V2’) at 25 ± 1°C, 44 to 48% RH, and a photoperiod of 16:8 (L:D) h.

In 2003, 2004, and 2005, plots (10 × 60 ft) were planted to soybean (NK, ‘S19-V2’) in a randomized complete block design at UMore Park, Rosemount, MN. The planting density in all study location-years was approximately 150,000 seeds per acre using 30-inch row spacing. In addition, soybean vegetative (i.e., V followed by the number of expanded trifoliolates) and reproductive growth stages (i.e., R stage) were determined at weekly intervals using methods described by Fehr and Caviness (4). This was a single-factor experiment with four replicates and three treatment levels including an untreated control, thiamethoxam-treated seed (1.28 oz /100 lb of seed), and an aphid-free, i.e., foliar application of lambda-cyhalothrin (Warrior, Syngenta Crop Protection, Greensboro, NC) at 1.92 oz/acre as needed. In 2003 there were two planting dates with the early planting on 21 May and the late planting on 3 June. In 2004 and 2005 there was a single planting date on 28 and 24 May, respectively. Overall, four location-years were used to assess the field efficacy of thiamethoxam.

Excised-Leaf Bioassay

In 2003 and 2004, sub-samples of newly-expanded soybean leaflets were randomly selected (five leaflets per plot) from all control and thiamethoxam-treated plots and returned to the laboratory where individual leaflets were placed in petri dishes (15 × 90 mm). During vegetative growth through the early reproductive soybean stages, soybean aphid is observed to feed on newly-expanded leaves near the growing point of the soybean. All field-collected leaflets were inspected for soybean aphid and all aphids were removed prior to placing leaflets into petri dishes. Soybean leaflet moisture was maintained by placing the cut end of each petiole into a water-saturated cube (1 cm³) of Oasis floral foam (Smithers-Oasis, Kent, OH). Next, apterous adult aphids from laboratory colonies were placed on each of the treated and untreated excised leaflets at a density of five aphids per leaflet. Soybean aphids and leaflets were maintained in a growth chamber at 25 ± 1°C, 70 ± 5% RH, and a 16:8 (L:D) h photoperiod for 48 h. Total nymph production and percent adult mortality were recorded after 24 and 48 h exposure. Results after 24 h were similar to the 48-h data so only results from the 48-h exposure are reported for clarity. Abbot’s formula (1) was used to adjust for mortality observed on leaflets from control plots to account for natural and handling mortality. The excised-leaf bioassay was started immediately after soybean emergence and before unifoliolates were fully expanded (i.e., prior to VC) and was repeated weekly until no observable differences were present among treatments.

Count data from nymph production and percent mortality curves were analyzed using repeated measures (PROC MIXED, SAS Institute Inc., Cary, NC). We used an autoregressive correlation model with plot as our repeated variable to model the covariance structure between sample dates within a treatment. Estimate and contrast statements were used to test differences in mortality between thiamethoxam-treated plots and control plots within a sample date; level of significance was set at α = 0.05.

Mortality patterns in the excised-leaf bioassay were similar among all three location-years. Direct toxicity occurred between 14 to 35 days after planting with peak mortality at 71.3, 48.3, and 75.3% in 2003 (early and late plantings) and 2004, respectively (Fig. 1). In 2003 aphid mortality was no longer different from the control at 35 (V3) and 30 (V2) days after planting (P > 0.05) (Fig. 1A,B). In 2004, by 42 days after planting (V4) mortality was no different from the untreated control (Fig. 1C). For aphids that survived in the excised-leaf bioassay long enough to produce nymphs, thiamethoxam significantly reduced aphid reproduction. There were no differences in rate of nymph production between untreated and thiamethoxam treatment by 35 and 42 days after planting in the 2003 (early only) and 2004 plantings, respectively (P < 0.05) (Fig. 2A,C). Nymph production for the thiamethoxam treatment in the 2003 late planting was significantly lower than the control for all sample dates (Fig. 2B). Based on the excised-leaf bioassay, thiamethoxam was only effective at causing aphid mortality and reducing aphid reproduction during the early vegetative stages of soybean development and by V2 to V5, depending upon the location-year, thiamethoxam was no longer present at toxic levels.

Fig. 1. Adjusted percent mortality of soybean aphids in a bioassay using field-collected, excised leaflets from thiamethoxam-treated and untreated control plots in (A-B) 2003 and (C) 2004. Asterisk denotes a significant difference (P < 0.05) between exposure time (48 h) and the control or 0% mortality (dashed line) within a sample date. Soybean vegetative stages (V#) are noted where thiamethoxam treatment was no longer different from the control.

Fig. 2. Soybean aphid nymph production in an excised-leaf bioassay for adult apterae exposed to excised leaflets from thiamethoxam-treated soybean in (A-B) 2003 and (C) 2004. Asterisk denotes a significant difference (P < 0.05) between the thiamethoxam treatment and the control within a sample date. Soybean vegetative stages (V#) are noted where the thiamethoxam treatment was no longer different from the control.

In-Field Bioassay

In 2004, apterous adults were placed in clip-cages (five per cage) and clipped to intact, randomly-selected soybean leaflets. Clip cages were made from PVC pipe (0.5-inch diameter × 0.5-inch height) with no-see-um netting (Balson-Hercules, Pawtucket, RI) glued to the bottom of each cage for air circulation and a large, hinged hair clip was adjusted to secure the cage to the leaf surface. Clip-cages were placed at two leaf positions within the canopy, i.e., on a fully expanded leaf (older leaf) in the mid canopy and on the newest open trifoliate (newer leaf) on the same plant. Nymph production and percent adult mortality were recorded at 24 and 48 h, but only the 48-h data are reported here for clarity. All treated and untreated plots were sampled weekly and different soybean plants were randomly selected for the in-field bioassay each week. Mortality was adjusted and data were analyzed as described in the excised-leaf bioassay and the Ryan-Einot-Gabriel-Welsch (REGWQ) test was used to separate means between leaf ages within a sample date.

For all sample dates, percent mortality was significantly higher in the older leaves than in newly-expanded trifolioates (P < 0.05) (Fig. 3). Significantly higher peak aphid mortality occurred in older leaves (90.3%) compared to newer leaves (61.1%) (Fig. 3). Overall, fewer nymphs were produced on older leaves in thiamethoxam-treated plots up to 49 days after planting compared to the untreated control (Fig. 4). There was no significant effect of thiamethoxam on nymph production in the newly-expanded leaves from the untreated control after 35 days (Fig. 4). Older leaves appear to sequester thiamethoxam and maintain higher levels of toxicity compared to younger leaves.

Fig. 3. Adjusted percent mortality for an in-field bioassay of soybean aphids after 48-h exposure to leaflets in thiamethoxam treated soybean in 2004. Percent mortalities followed by different letters within a sample date are significantly different from each other (P < 0.05); the baseline level or 0% mortality (dashed line) was given the significance letter "a". Soybean vegetative stages (V#) and reproductive stages (R#) were noted where thiamethoxam treatment was no longer different from the control.

Fig. 4. Soybean aphid nymph production for an in-field bioassay after 48-h exposure to leaflets in thiamethoxam treated soybean in 2004. Percent mortalities followed by different letters within a sample date are significantly different from each other (P < 0.05). OL denotes "older leaf" and NL denotes "newer leaf."

Mortality for the in-field bioassay persisted 49 days after planting (Fig. 3) compared to 35 days observed in the excised-leaf bioassay (Fig. 1C). When comparing peak mortality with the in-field bioassay of new leaves with the peak mortality observed in the excised-leaf bioassay (75.3%), this difference was not significant (t = 1.574, df = 6, P > 0.05). Thiamethoxam significantly reduced aphid reproduction under field conditions (P < 0.05) when compared to the untreated control (Fig. 4). In addition, mortality persisted to later plant growth stages (V7-R2) (Fig. 3) when compared to the excised-leaf bioassay in 2004 (V4) (Fig. 1C). Intact plants would continue to take up thiamthoxam from the soil leading to longer persistence. Therefore, results from the excised-leaf and in-field bioassays adequately describe the effectiveness and persistence of the thiamethoxam treatment in the field.

Field Efficacy

In 2003, 2004, and 2005 soybean aphids were counted in replicated field plots. Weekly counts of aphids per plant were made from time of soybean emergence (VE) until complete leaf senescence (R8). Whole-plant aphid counts ranged from 20 plants per plot during early-season aphid colonization to 5 plants per plot once aphid densities exceeded 100 aphids per plant and 100% of the plants were infested. Cumulative aphid-days (CAD) were used to describe season-long aphid pressure (i.e., 1 aphid/plant/day = 1 aphid-day) (5). Yield (bu/acre) was measured from the entire center 2 rows of each plot using a small-plot combine and was adjusted to 13% moisture. Analysis of variance was performed for CAD and yield data. The level of significance was set at α = 0.05 and the Ryan-Einot-Gabriel-Welsch (REGWQ) test was used to separate means.

In both years and under various aphid densities, thiamethoxam significantly reduced CAD (Fig. 5). In addition, nearly all aphid-days accumulated after thiamethoxam was no longer detectable in the excised-leaf or field bioassays (i.e., 30 to 56 days after planting). During the 2003 growing season when aphid pressure was the highest, thiamethoxam-treated soybean had a significantly higher yield than untreated soybean in the 2003 late planting (Fig. 5B). However, thiamethoxam treatment did not result in a significant increase in yield in any of the low aphid density location-years (P < 0.05) (Fig. 5A,C,D).

Fig. 5. Comparison of soybean yield and cumulative aphid-days (CAD) between control (i.e., no insecticide), thiamethoxam, and aphid-free (i.e., lambda-cyhalothrin applied to aphid densities at or below 100 aphids per plant) field plots in (A-B) 2003, (C) 2004, and (D) 2005. Cumulative aphid-days followed by different lowercase letters are significantly different from each other (P < 0.05). Yields followed by different capital letters are significantly different from each other (P < 0.05).

Conclusions

In terms of yield, there was no advantage using a seed treatment over a foliar applied insecticide in any location-year. In three of four location-years, a single application of lambda-cyhalothrin was needed to keep plots essentially aphid-free (i.e., densities were kept below 100 aphids per plant) and only in the 2003 late planting was a second application deemed necessary. Comparing CAD in untreated control plots to thiamethoxam, the latter reduced CAD by 46.0 to 66.7% while one or two foliar applications of lambda-cyhalothrin reduced CAD by 90.7 to 96.2%. Use of thiamethoxam as a seed treatment resulted in a significant yield increase in only one of four location-years (late-planting 2003). However, this yield increase was not significantly different from the yield in plots treated with foliar-applied insecticides (Fig. 5B). Since there was no yield advantage to applying thiamethoxam in three of the four location-years, the results would have been less net income in those three location-years.

Successful management of soybean aphid requires repeated scouting and sampling of populations throughout the season. A soybean field should only be treated when aphid densities reach the current, accepted economic threshold of 250 aphids per plant with > 80% of the plants infested with aphids (9). In this study, toxicity did not persist past the V7, R2 plant stage. Soybean aphids that colonize after R2 can cause yield reductions (Tom Hunt, personal communication) and would require treatment with a foliar insecticide. Therefore, treatment of seed with thiamethoxam at planting will not provide adequate control of these late aphid infestations. In addition, Smith and Krischik (10) demonstrated negative effects on nontarget insects with the use of systemic neonicotinoids. Future research should address potential negative impacts of thiamethoxam on predators and parasitoids of soybean aphid.

Since predicting soybean aphid outbreaks that will translate into yield-reducing densities is not possible at planting and late-season aphid colonization after R2 can result in significant yield loss, an at-planting application of thiamethoxam for soybean aphid control provides little consistent benefit to the grower.

Acknowledgments

Research was funded by Syngenta Crop Protection, the University of Minnesota Experiment Station, the Minnesota Soybean Research and Promotion Council, and the North Central Soybean Research Program. Thank you to A. Pereira, A. Jensen, A. Hughland, E. Hodgson, K. Siitari, R. Mendenhall, K. Koch, V. Verma, and S. Rostampour for help with data collection.

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