Optimizing Early-season Pest Control in Corn: Untangling the Contributions of Neonicotinoid Seed Treatments, In-furrow Pyrethroids, and Bt Hybrids

Progress report for GNE20-230

Project Type: Graduate Student
Funds awarded in 2020: $14,961.00
Projected End Date: 07/31/2022
Grant Recipient: University of Maryland
Region: Northeast
State: Maryland
Graduate Student:
Faculty Advisor:
Dr. Kelly Hamby
University of Maryland College Park
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Project Information

Summary:

Corn growers have several options for at-planting pest control, including neonicotinoid seed treatments (NSTs), pyrethroids that can be applied in the planting furrow, and corn hybrids with plant incorporated protectants sourced from Bacillus thuringiensis bacteria (Bt); growers may also use all three together. Because these treatments target similar pests, using multiple products can be redundant. Targeting insect control and using the most effective product could reduce insecticide use, thereby increasing environmental sustainability and decreasing input costs. Insecticide treatments can also disrupt biological control. For example, NSTs can reduce slug predator populations in soybeans, causing economically damaging outbreaks of slugs. Because slugs are a concern in the Mid-Atlantic and difficult to control, it is important to determine if at-planting insecticides similarly impact biological control of slugs in corn. Lower corn prices may increase adoption of less expensive non-Bt corn, which has a unique pest complex for which at-planting applications must also be optimized. To improve pest management decision making in corn, we will conduct separate field studies using Bt and non-Bt hybrids and evaluate the impacts of NSTs and in-furrow pyrethroids on pest control by measuring pest pressure and damage to seedlings. Because treatments may disrupt slug biological control, we will also measure slug abundance, slug predator abundance, and slug predator activity. Finally, we will compare yield and quantify ear pest pressure. Results will be disseminated through extension presentations and publications, enabling growers to plan targeted early-season pest control programs for more financially and environmentally sustainable corn production.

Project Objectives:

My goal is to provide information to help growers to tailor early season pest control tactics to their needs, increasing sustainability and profitability. In order to achieve this, my specific objectives are to:

(1) Determine how NSTs and in-furrow pyrethroids affect pest pressure and damage when used against pest complexes in three Maryland growing regions in separate Bt and non-Bt corn fields.

(2) Determine the impact of NSTs and in-furrow pyrethroids on biological control of slugs by measuring slug damage, slug predator abundance, and predation rates on slugs in three Maryland growing regions

(3) Determine the impacts of NSTs and in-furrow pyrethroids on yield and evaluate late season ear and stand damage in three Maryland growing regions using separate in Bt and non-Bt corn fields.

Introduction:

The purpose of this project is to increase the sustainability and efficiency of early-season pest control in Mid-Atlantic corn production by comparing at-planting management options in Bt and non-Bt hybrids. Growers can use multiple insect management products, including neonicotinoid insecticide seed treatments (NSTs),  in-furrow pyrethroid insecticides, and Bt corn hybrids [1–3]. NSTs and in-furrow pyrethroids control assorted soil and seedling pests [4, 5], with significant overlap in their spectrum of control. In the Mid-Atlantic, these applications primarily target soil pests such as white grubs and wireworms [4, 7].  NSTs, pyrethroids, and Bt traits all target multiple lepidopteran pests, including black cutworm and armyworm [4, 6, 7]. Because these products were developed for corn belt pest complexes and target similar pests, comparing efficacy against Maryland pest complexes will allow growers to choose the most effective treatment and eliminate redundant insecticide applications.

In addition to their inherent redundancy, insecticides do not provide yield benefits if target insect pest pressure is too low [9–13]. Many of the pests targeted by NSTs and in-furrow pyrethroids are sporadic and do not occur in economically important numbers in every field every year [13]. Preliminary data suggest that Maryland corn experiences relatively low pest pressure (Fig. 1). Lepidopteran pests especially have declined following wide-spread Bt corn adoption [6]. However, Maryland is a geographically diverse state, so regional pest pressure varies (Fig. 1). Additionally, soil pests are difficult to scout and impossible to control with rescue treatments [2], so at-planting treatments can be valuable in fields where soil pest pressure is anticipated. By measuring how treatments affect pests in different locations in Maryland we will determine the conditions and regions where applications are warranted and where they can be avoided, further reducing insecticide use.

At-planting insecticides may also disrupt biological control and negatively impact non-target organisms. Biodiversity in agricultural systems provides pest suppression [14]. Non-target impacts to biocontrol organisms can cause secondary pest outbreaks, complicating pest management and potentially reducing yields [24, 15]. Impacts to non-target organisms have been associated with NSTs [9, 16–19] and foliar pyrethroid applications [20–23], although there are few studies on pyrethroids applied in the planting furrow. In soybeans, NSTs disrupt biological control of slugs [28], a key pest in Maryland corn (Table 1).  Measuring the impact of at-planting insecticides on slugs and slug predators will help clarify how the treatments impact biological control of slugs in corn. This information will allow growers to conserve slug predators and better control these difficult to control pests.

Better understanding of how NSTs and in-furrow pyrethroids contribute to pest management will significantly improve on-farm decisions. Because falling grain prices may also incentivize planting less expensive corn lacking Bt traits, which experience a different pest complex [24], we will evaluate at-planting insect pest control in both Bt and non-Bt hybrids. Our results will allow growers to optimize profitability and environmental sustainability by selecting the treatments most suited to their pest pressure and production system, reducing unneeded insecticide applications, and enhancing biodiversity and biocontrol.

Research

Materials and methods:

To account for variability in weather and pest pressure, research will be replicated across three growing seasons (2020-2022). To investigate the pest and non-target impacts of insecticides applied at-planting in corn, we are evaluating three different treatments: (1) bare, untreated seed with no insecticide applied in-furrow, (2) seed treated with the NST Poncho 250 (active ingredient clothianidin, 0.25 mg/seed), and (3) bare, untreated seed with the pyrethroid Capture LFR (active ingredient bifenthrin, 13.6 fl oz/acre) applied in-furrow.

The three treatments are arranged in a Latin square with three replicate plots a minimum of 200 ft in length consisting of a minimum of 24 rows at 30 inch spacing (60 ft) (Fig. 2). We planted fields following this design of the Bt hybrid LC1196 VT2P (Local Seed, Memphis, TN) in 2020, and P1197YHR (Pioneer Hi-bred International, Inc. Johnston, IA) in 2021. We used the conventional non-Bt hybrid, P1197LR (Pioneer Hi-bred International, Inc. Johnston, IA) both years. All hybrids are considered high yielding and have similar maturity dates. Because the Bt and non-Bt varieties are planted in different fields and are not always isolines, they are treated as separate studies and analyzed independently. We plan to use the same methods and the same hybrids from 2021 in 2022, the final year.

Fig. 2. Plot plan

To test these treatments under differing pest pressure across the region, we replicated the study at three UMD research farms: Central Maryland Research and Education Center (Beltsville, MD), Wye Research and Education Center (Queenstown, MD), and Western Maryland Research and Education Center (Keedysville, MD). Planting, seedling sampling, and harvest dates for the farms in both years are shown in table 2.

Table 2. 2020 and 2021 planting, sampling, and harvest dates at UMD research farms (both Bt and non-Bt corn plots).

Year

Location

Planting date

Seedling sampling date

Harvest date

2020

Keedysville

May 18

June 8

Oct 9

Beltsville

May 21

June 10

Oct 22

Queenstown

May 13

June 3 + 4

Oct 5

2021

Keedysville

May 14

June 1 + 4

Oct 7

Beltsville

May 17

June 2

Oct 8

Queenstown

May 4

May 25 + 26

Sept 21

Objective 1. Determine how NSTs and in-furrow pyrethroids affect pest pressure and damage when used against pest complexes in three Maryland growing regions using separate in Bt and non-Bt corn fields.

    To evaluate the impact that NSTs and in-furrow pyrethroids have on soil pests at planting, we sampled seedlings in the V2-V4 growth stage 3-4 weeks after planting (Table 2). Data were collected in three subsamples per plot (50 ft sections of row in three different locations). We counted the number of healthy and stunted plants in each 50 ft section. Plants estimated to be less than half the size of the average healthy plants, but without signs of serious pest damage, were considered stunted. We excavated plants with signs of soil pest damage (yellowing or wilting of center leaves or entire plant) to confirm presence of soil pests (Fig. 3a). If identifiable feeding damage or the pest itself was present, plants were categorized as damaged by soil pests. We classified foliar pests feeding by pest category: lepidopteran chewing damage which included signs of cutworm (holes in rows or plants cut off at the soil) or armyworm (leaf-margin feeding), slugs (rasping that removes leaf tissue in this stripes), and non-lepidopteran foliar feeding (abnormal growth from piercing-sucking damage, small feeding holes from leaf beetles) (Figs. 3b-f). Only one type of damage, whatever affected the greatest area, was recorded for each plant. In 2020 we did not record the area of damage for individual plants. In 2021 we recorded damage severity (data not shown). In both years we measured plant height to determine whether overall plant vigor was affected by the treatments (there were no differences, data not shown).

Fig. 3a-f. Diagnostic signs of pest damage. a) soil pest, b) cutworm, c) armyworm, d) slug, e) stinkbug, and f) flea beetle

            We performed separate analyses for Bt and non-Bt experiments. Stand counts were summed across subsamples and then were converted to the number of plants per acre and analyzed using JMP Pro 15. Treatment, location, year, and all interactions were included in the model. The interaction terms were dropped when not significant (p>0.05). Pest damage data were summed across subsamples and converted to the percentage of plants damaged by replicate plot. These were also analyzed using JMP Pro 15. Assumptions of normality and homogeneity of variances were checked, and models weighted by the variance of the most unequal variable were run when necessary. Non-normal data were typically zero-inflated (e.g. the pest percent data), and these were analyzed by GLM with a Poisson distribution. Differences in means were determined with post-hoc tests (Tukey’s HSD or contrasts).

Objective 2. Determine the impact of NSTs and in-furrow pyrethroids on biological control of slugs by measuring slug damage, slug predator abundance, and predation rates on slugs in three Maryland growing regions.

To determine whether NSTs and pyrethroids impact slug pressure in corn, we sampled slug activity-abundance, slug predator activity-abundance, and predation by slug predators. Rates of slug feeding on seedlings was measured during foliar sampling for Objective 1. The majority of our sampling occurred within 4 weeks of planting when slugs impact corn yields [35]. We measured slug activity abundance following the procedure described by Raudenbush et al. [36] who found that the addition of a pitfall trap to the traditional shingle trap increased captures. Traps consisted of nested 12 oz cups (WebstrauntStore, Lancaster, PA), sunk into the ground, filled with soapy water. These were covered with a 1 ft2 roof shingle secured with a heavy stone. We placed two slug traps at two locations per plot and counted slugs once a week for eight weeks following planting in 2020 and 6 weeks in 2021. We checked slug traps in the morning when temperatures were lower, roughly between 7:00 and 11:00 AM on every date except for July 15th 2020 in the non-Bt plots at Queenstown, which were sampled between 11:20 and 12:10 PM. We inspected the area under the shingle footprint, within the pitfall trap, and under the pitfall trap for slugs and slug eggs, and the number of each recorded. The soapy water solution was replaced after sampling each week. No slug eggs were found in any treatment throughout the sampling period in 2020, but eggs were found at Queenstown in early June 2021.

            We measured slug predator activity abundance using dry pitfall traps at Queenstown twice starting one week after planting in 2020. Traps consisted of nested 12 oz cups (WebstrauntStore, Lancaster, PA), sunk into the ground and covered with 1 ft2 of stiff plastic. Three carriage bolts were attached to each plastic piece, allowing them to be secured in the soil and slanted for water runoff. We placed two pitfall traps per plot. Pitfall traps were deployed for 12 hours overnight, corresponding with the two deployments of sentinel prey (described below) at Queenstown. Trap contents were collected in the morning and immediately be frozen and stored at -80° C. After the two dates at Queenstown in 2020, we used antifreeze-filled traps for all dates and locations in order to increase captures. These pitfall traps were collected weekly, filtered, and contents stored in 70% ethanol. We counted the taxa credited with slug control [30, 36, 37] and pinned carabid beetle specimens. These will be identified to species to differentiate between feeding guilds [38].

            We measured the activity of slug predators using sentinel prey. We used waxworm caterpillars, Galleria mellonella L., as a stand-in for slugs [38], because of difficulty in restraining slugs and as predation on G. mellonella correlates with large carabids that are slug predators [28]. We secured five healthy waxworm larvae (Grubco, Fairfield, OH) to each index card (four cards per plot) using double-sided hem tape (Jo-Ann Stores LLC, Hudson, OH) and clear scotch tape. We prepared index cards the day of deployment and transported them to the field in a cooler with ice packs. To exclude vertebrate predators, we placed index cards in circular cages made from ½ inch mesh, 19-gauge hardwire cloth (The Home Depot Inc, Atlanta, GA) secured to the ground with two ground staples [38]. We covered cages with clear 6 inch by ½ inch petri dishes (Fig. 4). We deployed waxworms in the field for approximately 12 hours overnight, collected them in the morning, and immediately evaluated or froze them for later evaluation. We counted larvae showing signs of damage, and recorded the number of intact, predated, and missing larvae per card.

Fig. 4. Sentinel prey cage

We performed separate analyses for Bt and non-Bt experiments. The activity density of slugs was compared by averaging the captures between the two traps in each plot, then comparing the average number of slugs counted per week. Because data were zero-inflated, we analyzed them by GLM with a Poisson distribution. Slug seedling damage was analyzed by the same method as described for seedling damage in Objective 1. Sentinel prey data will be analyzed with logistic regression. Slug predator abundance will be analyzed with a linear mixed model.

Objective 3. Determine the impacts of NSTs and in-furrow pyrethroids on yield and evaluate late season ear and stand damage in three Maryland growing regions using separate in Bt and non-Bt corn fields.

            While NSTs and pyrethroids applied at-planting will not affect ear damage, Bt controls lepidopteran stalk and ear pests. To better understand stalk and whorl pest pressure and ear pest pressure and damage given regional pest suppression [32, 33], we sampled whorls and ears for pest damage. Sampling dates are presented in Table 3.

Table 3. 2020 and 2021 whorl and ear sampling dates at UMD research farms (both Bt and non-Bt corn plots).

Year

Location

Whorl/stalk sampling date

Ear sampling date

2020

Keedysville

July 20

August 14

Beltsville

July 23

August 10

Queenstown

July 22

August 7

2021

Keedysville

July 9

August 5

Beltsville

July 6

August 12

Queenstown

July 7

August 3

To assess late-season pests that might attack the stalk, we sampled 10 plants in 4 different locations per plot. In 2020 we assessed one eye-level leaf from R2 plants for damage from boring lepidopteran larvae, Japanese beetles, grasshoppers, and stinkbugs, and measured any area damaged in cm2 to the nearest ½ center. In 2021 we looked for the same types of pest damage in the whorl stage and used a rating system from 1-5 instead to assess degree of damage. 1= unblemished plant, 2= minor feeding, 3= significant feeding, but stalk intact, 4= feeding that damaged the stalk or tassels, and 5= completely defoliated plant. In both years we field-identified any pests present, and collected or photographed them for later identification.

For ear sampling in both years, we removed ears from the stalk, husked them, and counted the area of kernels damaged in cm2 using a transparent grid (Fig. 5). Damage was classified as (1) caterpillar damage, (2) stink bug damage, and (3) sap beetle damage. Any pests present were identified and recorded. If the caterpillar was present, it was identified.

Fig. 5. Measuring ear damage.

Stand count was also recorded shortly before harvest to determine if stand was lost to stalk pests over the course of the season. The number of stunted (those too small to make an ear) and non-stunted corn plants in 50 ft of row were counted in three locations in each plot, and stand per acre was calculated. The whole plot was harvested by a combine harvester. Yield was taken at harvest using a calibrated combine, weigh wagon, or truck scale, depending upon the equipment available at each farm. We converted the weight to bushels per acre corrected to 15.5% moisture.

All data were analyzed separately in Bt and non-Bt experiments. Subsamples of ear damage and stand were summed at the plot level, and the percent of ears damaged was calculated for incidence, then analyzed by ANOVA in JMP. Yield in bushels per acre was analyzed by ANOVA. Seedling and final stand count were compared with paired t-tests to determine if stand changed significantly over the season.

Research results and discussion:

Results and discussion

Objective 1: Determine how NSTs and in-furrow pyrethroids affect pest pressure and damage when used against pest complexes in three Maryland growing regions in separate Bt and non-Bt corn fields. 

Non-Bt seedling stand and pest damage. In non-Bt corn, seedling stand was not affected by treatment in either year (Fig. 6). Across 2020 and 2021, the percent of plants damaged by both soil pests and lepidopteran pests was significantly higher in the control treatment than either of the insecticide treatments (Table 3). Despite significant differences, damage was fairly low overall. On average, less than 2% of plants were damaged by soil pests, and it was not enough to effect stand count (Fig. 6). The incidence of lepidopteran pests was generally below the treatment threshold for both types (cutworm=10% plants damaged, armyworm=25%), and the severity of lepidopteran pest damage was low, with feeding never damaging the growing point and rarely cutting seedlings.

Fig. 6. Mean number of plants per acre measured visually in non-Bt plots in V2-V4. 150 ft of row was sampled for each plot. Stand count did not statistically differ between treatments. n=27.

Table 3. Mean ± standard error percent of pest-damaged seedlings per 150 row feet in conventional non-Bt corn over 2 years. Means within columns followed by different letters are significantly different (p<0.05).

Treatment

Mean % seedlings damaged by soil pests ± SE

Mean % seedlings damaged by lepidopteran pests ± SE

Untreated

1.6 ± 0.6 A

7.5 ± 2.1 A

Capture LFR

0.5 ± 0.2 B

2.8 ± 0.3 B

Poncho 250

0.1 ± 0.1 B

2.9 ± 0.5 B

Bt seedling stand and pest damage. In contrast to non-Bt corn, the stand count for Bt corn was impacted by treatments (Fig. 7). Poncho 250 had higher stand compared to the control treatment. Capture LFR was not significantly different from the other two treatments.

Fig. 7. Mean number of plants per acre measured visually in Bt plots in V2-V4. 150 ft of row was sampled for each plot. Treatments with different letters are significantly different (p<0.05) from other treatments. n=27.

In Bt corn, the control treatment had a significantly higher percentage of plants damage by soil pests and lepidopteran pests than the Poncho 250 treatment, but Capture LFR did not differ from either the control or Poncho treatment (Table 4). As with the non-Bt corn, lepidopteran incidence was well below the treatment threshold, and the severity of the damage was low.

Table 4. Mean ± standard error percent of pest-damaged seedlings per 150 row feet in Bt corn over 2 years. Means within columns followed by different letters are significantly different (p<0.05).

Treatment

Mean % seedlings damaged by soil pests ± SE

Mean % seedlings damaged by lepidopteran pests ± SE

Untreated

0.8 ± 0.5 A

5.4 ± 0.8 A

Capture LFR

0.4 ± 0.1 AB

4.8 ± 0.9 AB

Poncho 250

0.1 ± 0.0 B

2.3 ± 0.4 B

Objective 2. Determine the impact of NSTs and in-furrow pyrethroids on biological control of slugs by measuring slug activity abundance, slug damage, slug predator activity abundance, and predation rates on slugs in three Maryland growing regions.

Slug activity abundance. The number of slugs counted per week did not differ significantly by treatment in either non-Bt (Fig. 8) or Bt corn (Fig. 9). Slug counts were significantly higher in 2021 which agreed with growers’ experiences across the state of bad slug damage. They also varied by location, and Keedysville had significantly more slugs than the other locations both years.

Fig. 8. The mean number of slugs counted per week in non-Bt corn at three University of Maryland Research farms. There were no differences between treatments.
Fig. 9. The mean total of slugs counted per week in Bt corn at three University of Maryland Research farms. There were no differences between treatments.

Slug damage. The percent of plants damaged by slugs was not different between treatments in non-Bt corn in either year (Fig. 10), or Bt corn in 2020 (Fig. 11). However, in 2021 in Bt corn, Poncho 250 did have significantly higher damage compared to the Capture LFR or control treatments, although the difference was small (25% of plants in the Poncho 250 treatments across locations were damaged versus 16% and 18% in the Capture LFR and control treatments, respectively).

Fig. 10. Mean percent of pest-damaged seedlings per 150 row feet in conventional non-Bt corn in 2020 and 2021. There were no significant differences between treatments. Keedysville had significantly higher damage than the other locations (p<0.05).
Fig. 11. Mean percent of pest-damaged seedlings per 150 row feet in Bt corn in 2020 and 2021. There were no significant differences between treatments or locations in 2020. In 2021 Keedysville had significantly higher damage than the other locations and Poncho 250 had significantly more damage than Control or Capture LFR (p<0.05).

Predation. Consistent predation was observed both years across most dates and locations. Preliminary data visualizations do not show an obvious trend of differences between the treatments, but data have not been analyzed yet. We have observed both predatory carabid beetles and wolf spiders predating on sentinel prey cards in the field (Fig. 12a & b).

Fig. 12. Slug predators feeding on sentinel prey. a) Carabid beetle, b) wolf spider.

Slug predator abundance. We have pinned beetles for identification, which is still ongoing. Data analysis will follow the identification of all of the beetles collected.

 Objective 3. Determine the impacts of NSTs and in-furrow pyrethroids on yield and evaluate late season ear and stand damage in three Maryland growing regions using separate Bt and non-Bt corn fields.

Non-Bt yield. At-planting insecticides that target pests in the first several weeks of corn growth did not impact yield in non-Bt corn (Fig. 13). Given our results in Objective 1, this is probably because there was not economically important seedling pest pressure.

Fig. 13. Non-Bt corn yield (bu/ac) corrected to 15.5% moisture. There were no significant differences between treatments.

Non-Bt late season stand. The difference between early and late season stand was not affected by treatment. While final stand was somewhat lower that seedling stand in 2020, in 2021 there was no difference between stand counts. This suggests that mid to late season boring pests were not very abundant or damaging overall, which was backed up by the whorl damage data.

Non-Bt stalk pests. Despite the non-Bt hybrid having no in-plant protectant against caterpillars, there was almost no damage during R2 in 2020 or the whorl stage in 2021. In 2020 fewer than 6% of plants had any caterpillar feeding, and the area never exceeded 1 cm2. In 2021 less than 0.5% of plants were damaged by caterpillars, and the amount of damage was minor. We found both corn earworm and European corn borer caterpillars in the whorl stage in 2021. Damage from pests like grasshoppers, Japanese beetles, and leaf beetles was also very low.

Non-Bt ear pests. In the ear stage all of the caterpillars causing damage to the ears were corn earworm (CEW). Across both years, fewer than 21% of the ears had CEW damage (Table 5). Beltsville had more ears damaged by CEW and sap beetle than the other farms both years. However, at all locations the area damaged was small, with a maximum of 1.3 cm2 damaged by CEW. The proportion of ears with stinkbug damage was higher in Queenstown than at other farms. At Queenstown in 2021 the mean area damaged by stinkbugs was also high at 7.1 cm2. At other locations and in 2020, it was less that 3 cm2 per ear.

Table 5. Mean ± standard error corn earworm (CEW) and stinkbug ear damage in the conventional non-Bt hybrid P1197LR over 2020 and 2021. 40 ears of corn were assessed per plot.

Year

Site

Mean % ears damaged by CEW

Mean area (cm2) damaged by CEW

Mean % ears damaged by stinkbug

Mean area (cm2) damaged by stinkbug

2020

Keedysville

10.3 ± 2.2

0.6 ± 0.1

40.3 ± 3.3

1.9 ± 0.2

Beltsville

18.1 ± 2.3

1.3 ± 0.4

43.6 ± 3.9

2.0 ± 0.3

Queenstown

4.2 ± 0.9

0.4 ± 0.1

60.6 ± 3.3

2.8 ± 0.4

2021

Keedysville

9.4 ± 2.3

0.2 ± 0.1

21.7 ± 2.7

0.3 ± 0.1

Beltsville

20.3 ± 2.8

0.7 ± 0.1

26.9 ± 3.9

0.3 ± 0.1

Queenstown

8.3 ± 1.6

0.5 ± 0.1

98.3 ± 0.9

7.1 ± 0.4

Bt yield. In Bt corn there were no yield differences between at-planting treatments in Bt corn across 2020 and 2021 (Fig. 14). Again, this agreed with a lack of economically important early season pests, and the short window of efficacy that the at-planting treatments have.

Fig. 14. Bt corn yield (bu/ac) corrected to 15.5% moisture across 2020 and 2021 and three research farms. There were no significant differences between treatments.

Bt late season stand. Early and late season stand were not significantly different across treatments, but as in the non-Bt corn, some stand was lost over the season in 2020 while there were no differences in 2021. Again, this was probably due to a lack of stalk pests.

Bt Stalk pests. Although CEW has developed resistance to multiple Cry toxins, caterpillar damage was low both years. In 2020, fewer than 3% of plants were damaged by caterpillar pests, and damage did not exceed 0.5 cm2. In 2021 there was almost no damage during the whorl stage (less that 0.5% of plants were damaged by caterpillars for all treatments in all locations). As with the Non-Bt corn, severity was extremely minor, with only a few holes made in leaves and never significant whorl-feeding or stalk-boring. We also saw very little damage from other pests in this stage, including grasshoppers and Japanese beetles.

Bt ear pests. Unsurprisingly, treatments did not differ in the proportion of ears damaged or the amount of damage. In the ear stage, fewer than 17% of ears had any CEW feeding in 2020 or 2021. As with the non-Bt corn, Beltsville had higher numbers of CEW-damaged ears (Table 6). However, the area damaged tended to be low, with a mean of 2 cm2 in 2020 and 0.5 cm2 in 2021.

Queenstown had higher stink bug damage than the other locations, with about 41% of ears damaged in 2020 and 72% in 2021. Despite the high percentage of ears with damage, the severity was low, however, with never more than 2.5 cm2 of kernels with signs of feeding. 

Table 6. Mean ± standard error corn earworm (CEW) and stinkbug ear damage in hybrids LC1196 VT2P and P1197YHR over 2020 and 2021. 40 ears of corn were assessed per plot.

Year

Site

Mean % ears damaged by CEW

Mean area (cm2) damaged by CEW

Mean % ears damaged by stinkbug

Mean area (cm2) damaged by stinkbug

2020

Keedysville

6.1 ± 1.2

0.3 ± 0.1

23.1 ± 3.8

0.6 ± 0.2

Beltsville

16.9 ± 1.9

2.1 ± 0.2

10.0 ± 2.4

0.3 ± 0.1

Queenstown

7.5 ± 1.6

0.6 ± 0.2

41.4 ± 3.4

1.3 ± 0.2

2021

Keedysville

3.3 ± 1.0

0.1 ± 0.0

2.8 ± 0.9

0.0 ± 0.0

Beltsville

16.1 ± 1.8

0.5 ± 0.1

3.6 ± 0.7

0.0 ± 0.0

Queenstown

5.0 ± 1.4

0.2 ± 0.1

71.9 ± 4.9

2.4 ± 0.4

 

Research conclusions:

Objective 1 conclusions. Our 2020 and 2021 results in non-Bt and Bt corn showed that the NST Poncho 250® consistently reduced the number of seedlings damaged by early season pests, while the in-furrow pyrethroid Capture LFR® only reduced the number of pests damaged in the non-Bt corn. Despite significant differences, pest pressure was low, and correspondingly we did not see yield differences between treatments in Objective 3. Even where seedling stand was increased by using the NST treatment in Bt corn this did not translate to yield gains.

Plants were most frequently damaged by lepidopteran pests like armyworm and cutworm. These are easily scouted for and can be controlled with sprays if they reach treatment thresholds, providing an alternative to prophylactic management. Soil pests require preventative treatments, and although we did not have damaging soil pest pressure our results suggest that the NST may be more consistently effective than the in-furrow pyrethroid, at least against wireworm, which was our primary soil pest. In general, if a field does not have a history of soil pests, our preliminary results suggests that prophylactic NST or in-furrow pyrethroid applications are not necessary for pest control.

 Objective 2 conclusions. Sentinel prey data are still being analyzed, and we are still identifying predatory ground beetles. With the slug count data currently analyzed, there is no evidence that at-planting insecticides are indirectly causing an increase in slug abundance. In one case (the Bt corn in 2021) the NST plots had a small increase in the percent of plants damaged by slugs compared to the control or in-furrow pyrethroid treatment, but we did not see this effect in 2020, or in 2021 in the non-Bt corn.
            While we do not have clear evidence that prophylactic insecticides contribute to outbreaks of slugs in Maryland corn fields, NSTs are still widely documented to have non-target impacts. Without the analysis of all of our data, non-target effects of NSTs and in-furrow pyrethroids in this context should not be ruled out.

Objective 3 conclusions. At-planting insecticides did not result in yield gains compared to untreated corn with no in-furrow application. This was true for both non-Bt and Bt corn and suggests that NSTs and in-furrow pyrethroids could be eliminated in many cases without yield loss.

The at-planting insecticides also did not impact whorl or ear pest damage, which is expected from their window of efficacy. Final stand counts did not show evidence for serious late-season pests destroying stand in either corn. Bt toxins are very effective against European corn borer, the main pest target of Bt hybrids and a key whorl, stalk, and ear pest, so widespread Bt corn adoption is likely keeping pressure low.  The incidence and severity of ear pests varied by location, and CEW and stinkbugs were the main ear pests. CEW damage was at similar levels in both Bt and non-Bt corn, which may have been the result of area-wide Bt adoption, or more likely, widespread resistance of CEW to the Bt toxins in our Bt corn variety. These results suggest that at the moment, given pest suppression from Bt use, Bt and non-Bt hybrids have similar pest complexes and non-Bt corn may be grown with relatively few pest issues. It is important for growers to remember that pest problems do sporadically occur, and monitoring is important for avoiding unexpected losses. Finally, all of our results reaffirm that in the absence of known pest issues, most fields do not benefit from prophylactic insecticides.

Participation Summary

Education & Outreach Activities and Participation Summary

1 Published press articles, newsletters
7 Webinars / talks / presentations

Participation Summary:

300 Farmers
30 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

The results from this project will benefit Mid-Atlantic grain growers, extension specialists, and researchers by giving more information for decision-making. Our research will give growers regionally-specific, science-based evidence to help choose at-planting insect management products. The results will benefit both growers producing high-input genetically engineered corn hybrids with Bt traits and growers producing low-input non-Bt corn. In order to communicate information with each stakeholder group we will share results through a number of different channels.

            Due to Covid-19, many of the usual avenues for communicating this research have not been available over the past 2 years. In 2020 and 2021 we presented results virtually at the Maryland Grain Producer’s Utilization Board January meeting. In July 2021 we presented a poster summarizing results at the Commodity Classic, a large grower’s meeting for grain producers. Results from this study have also been incorporated into three extension talks on pest management in grain crops, reaching a total audience of about 250 people. Results from this project were included in the 2021 CMREC Upper Marlboro Roots in Research, an extension publication updating stakeholders on Agricultural research at the University of Maryland. Results will be also published in Agronomy News, a popular newsletter published by UMD extension. This free publication reaches a readership of approximately 3,000 and serves as an easy-to-access reference for stakeholders.

            We have also been sharing the preliminary results of this study with scientific audiences, including researchers and extension specialists, through presentations at scientific conferences such as the annual Eastern Branch meeting of the Entomological Society of America (ESA) and the annual national ESA meeting. In November 2020 we presented preliminary results in a student research talk at the virtual national ESA meeting, and in November 2021 we presented a virtual poster at the hybrid national ESA meeting. In spring 2022 we will also present results in a talk at the Eastern Branch ESA meeting. Presenting to scientific audiences adds to the growing body of research on how prophylactic insecticides impact insect pests and non-targets. It also fosters collaboration and innovation with other researchers working on related projects.

Project Outcomes

Project outcomes:

This area will be evaluated at the end of the project. 

Knowledge Gained:

This area will be evaluated at the end of the project. 

Assessment of Project Approach and Areas of Further Study:

This area will be evaluated at the end of the project. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.