Helicoverpa zea Boddie (Lepidoptera: Noctuidae), the corn earworm, is a key pest of sweet corn (Poales: Poaceae) in many parts of the United States. Integrated pest management (IPM) practices for H. zea in fresh and processing sweet corn use pheromone trap counts of male moths for management decisions. In this study, we examined whether sweet corn could be protected more effectively if insecticides were applied to target the most attractive silking periods for female H. zea oviposition instead of current IPM practices using pheromone trap catches alone. Specifically, we investigated the relationship between insecticide application timing from tassel through silk stages and marketable yield at harvest. We also evaluated the effectiveness of 3 registered insecticide products with different active ingredients (methomyl, chlorantraniliprole, and lambda-cyhalothrin), under various timing scenarios. Results were compared with yields obtained using current IPM recommendations for the northeastern United States. Reduction of H. zea damage in sweet corn among insecticides and timing treatments varied within and between years. In year 1, only interaction effects between insecticide and timing were significant, but in year 2, only main effects of insecticide and timing were significant. Chlorantraniliprole produced inconsistent results in year 1 but had significantly higher percentages of clean sweet corn ears compared with lambda-cyhalothrin in year 2.
Helicoverpa zea Boddie (Lepidoptera: Noctuidae), the corn earworm, is a key pest of sweet corn (Poales: Poaceae) in many parts of the United States (Barber 1943; Phillips & Whitcomb 1962; Coop et al. 1992, 1993; Shelton et al. 2013). Helicoverpa zea is restricted to the western hemisphere (Cohen et al. 1988; CABI 2014) and, if left unmanaged, can cause severe yield reduction and subsequent economic losses (Horner et al. 2003; Hutchison & Storer 2010; Shelton et al. 2013).
Helicoverpa zea infestation occurs when larvae enter the tip of sweet corn ears to feed (Hardwick 1965; Coop et al. 1992). This process begins when adult females are attracted to plant volatiles emitted by fresh corn silks (Flath et al. 1978; Cantelo & Jacobson 1979; Raina et al. 1992). After a suitable plant host has been located, one or more eggs are deposited directly on fresh silk and occasionally on other plant parts (Barber 1943). A single female can lay from 800 to 1,100 eggs in her lifetime (Akkawi & Scott 1984).
Sweet corn is a preferred host of H. zea, and infestation on this host results in higher rates of successful development than on other plant hosts (Johnson et al. 1975; Hayes 1988). The presence of a single H. zea larva or its damage renders a sweet corn ear unmarketable for the high-value fresh market. Typically, H. zea only infests the tip of the sweet corn ear, and the ear tip can be removed mechanically, allowing what is left of the cob to be used for processing (Shelton 1986).
Current integrated pest management (IPM) practices for freshmarket and processing sweet corn in the northeastern United States use pheromone traps to monitor male H. zea moth activity, from which treatment decisions are made (Boucher et al. 2014; Shelton et al. 2014). Treatment decisions are based on the number of moths captured over a pre-determined period of time during ear development, starting when the ear (female flower) produces silk (the stigma and style) to be pollinated (Clemson University Cooperative Extension 2015). Treatments continue in this manner until harvest. This approach is based on research showing that male moths captured in pheromone traps correspond well to female moth populations in the same area (Chowdhury et al. 1987a, 1987b). Growers may use multiple applications of pyrethroid insecticides to successfully manage H. zea infestations in sweet corn (Shelton et al. 2013). However, there is concern about pyrethroid resistance as well as interest for more efficacious, safer, and longer residual products to reduce damage (Jacobson et al. 2009).
A new insecticide class, the anthranilic diamides, includes products that are longer lasting, especially against Lepidoptera, and have a safer environmental profile than previously used insecticides (Hannig et al. 2009; Lai & Su 2011). This chemical class has not extensively been tested against H. zea under field conditions, but the lack of consistent H. zea damage reduction using pyrethroid insecticides makes evaluation of chlorantraniliprole as a reduced spray treatment a worthwhile pursuit (Shelton et al. 2013).
The 1st objective of this project was to evaluate reduction in H. zea damage in sweet corn by initiating an insecticide spray program earlier than the green silk stage as recommended in current IPM guidelines, starting instead at the late tassel/early silk stages of corn development. We hypothesized that targeting late tassel/first green silk with an insecticide would decrease H. zea damage compared with the traditional application timing that begins at the mid-green silk stage. The 2nd objective was to compare differences in reduction of H. zea feeding damage using various insecticides. We hypothesized that chlorantraniliprole would significantly reduce H. zea damage compared with products containing lambda-cyhalothrin and methomyl. Finally, we hypothesized that the greatest reduction in H. zea damage to sweet corn would be obtained by targeting late tassel/first green silk stages with chlorantraniliprole.
Materials and Methods
PLOT ESTABLISHMENT
Experimental plots were established on 14 Jun 2012 and 8 Jul 2014 at the Cornell University Agricultural Experiment Station Fruit and Vegetable Research Farm located in Geneva, New York (42.872692°N, 77.019242°W). Plots were established in 2013, but H. zea moth densities were unusually low and precluded efficacy testing. ‘Obsession’ and ‘EX08767143’ conventional sweet corn varieties were planted in 2012 and 2014, respectively (Seminis™ Vegetable Seeds, St. Louis, Missouri). Fields were seeded on 76 cm centers and 20 cm in-row plant spacing using a Monasem™ vacuum seeder (Edwardsville, Kansas). Nitrogen was added at a rate of 57 kg/ha in the furrow with seed at planting time. An additional 57 kg of N per ha was side-dressed when plants reached the 7-leaf stage.
INSECTICIDE TREATMENTS
Methomyl (Lannate® LV, DuPont, Wilmington, Delaware), chlorantraniliprole (Coragen® SC, DuPont™, Wilmington, Delaware), and lambda-cyhalothrin (Warrior®, Syngenta, Greensboro, North Carolina [2012] and Lambda-T®, Helena Chemical™, Collierville, Tennessee [2014]) were selected as insecticide treatments. All 3 insecticides were applied using maximum labeled rates of 504.3 g active ingredient (AI) methomyl, 73.2 g AI chlorantraniliprole, and 33.6 g AI lambda-cyhalothrin per ha.
Insecticide treatments were made using a 3 row CO2 pressurized Hagie 200 High-Boy tractor (Hagie Equipment Company, Clarion, Iowa) equipped with 3 Tee-Jet flat fan 11003 nozzle tips per row (1 over the top and 1 drop nozzle on each side aimed at the ear zone), delivering 137 L H2O per ha at 2.8 kg/cm2 pressure and a speed of 5.1 kph. The adjuvant Dyne-Amic (Helena Chemical™, Collierville, Tennessee), a modified vegetable oil and organosilicone surfactant blend, was added to all treatments at a 0.1% v/v ratio.
INSECTICIDE TIMING
Insecticides were applied using either an assigned timing schedule based on plant reproductive phase or according to current IPM guidelines (Boucher et al. 2014; Shelton et al. 2014). Timing schedules were implemented to examine efficacy of a given insecticide during several phases of ear development (Clemson University Cooperative Extension 2015). Five timing schedules were used in this study. In timing schedule 1, insecticides were applied 3 times between first green silk and 25% dry silk stage. In timing schedule 2, insecticides were applied once at first green silk. In timing schedule 3, insecticides were applied at a frequency determined by IPM guidelines between first green silk and harvest. In timing schedule 4, insecticides were applied once at 50% tassel. In timing schedule 5, insecticides were applied 4 times between 50% tassel and 25% dry silk stages. In 2012, timing schedules 1, 2, and 3 were evaluated, but timing schedules 4 and 5 were not. In 2014, timing schedules 1 though 5 were evaluated.
Primary ears, those that develop first and highest on the plant, were evaluated. First green silk was defined as the date of first observed silk, of any length, emerging from any of 25 randomly sampled ear tips. Fifty percent green silk was defined as the day on which >50% of 25 randomly sampled ears reached the silk stage. Fifty percent tassel was defined as the date on which >50% of 25 randomly sampled plants displayed a tassel.
PEST PRESSURE
Treatments that followed timing schedule 3, the IPM guidelines, required an estimate of adult pest pressure based on pheromone trap catch values. Three Scentry™ Heliothis traps (Great Lakes IPM Inc., Vestaburg, Michigan) were placed around the perimeter of each field in 2012 and 2014. Traps were checked for adult male moths at 3 d intervals beginning when corn plants reached the final vegetative stages of development. Pheromone trap counts were then used to determine insecticide application frequency for plots assigned to timing schedule 3 (Boucher et al. 2014; Shelton et al. 2014) (Table 1).
EXPERIMENTAL DESIGN
Treatment plots consisted of 3 rows, 8 m in length. A randomized complete block design was implemented in 2012 and 2014 with each treatment replicated 4 times. In 2012, methomyl, chlorantraniliprole, and lambda-cyhalothrin were evaluated in combination with timing schedules 1, 2, and 3. In 2014, chlorantraniliprole and lambda-cyhalothrin were evaluated combined with timing schedules 1, 2, 3, 4, and 5.
Table 1.
Insecticide application dates and respective Helicoverpa zea pheromone trap catches upon which spray decisions were made for plots following timing schedule 3, IPM guidelines from 50% silk to harvest, in 2012 and 2014.
FIELD EVALUATION
Treatments were evaluated at harvest, 21 d after first green silk in each year. Twenty-five randomly selected primary ears of corn were harvested from the 3 center rows of treatment plots. Ears without damage or larvae in the silk or on the kernels inside the husk were classified as clean. Ears with larvae in the silk or on the ear were classified as damaged.
STATISTICAL ANALYSES
JMP 11.0 for Macintosh (SAS Institute, Cary, South Carolina) was used for statistical analyses. Pest pressure was much higher in 2012 compared with 2014. In 2012, methomyl, lambda-cyhalothrin, and chlorantraniliprole were evaluated using timing schedules 1, 2, and 3. In 2014, lambda-cyhalothrin and chlorantraniliprole were evaluated using timing schedules 1, 2, 3, 4, and 5, and methomyl was excluded from the analysis. For these reasons, 2012 and 2014 datasets were not combined. Instead, each year was evaluated separately using linear mixed model regression. Insecticide and timing were assigned as main effects, insecticide*timing interaction effects were measured, and replicate was assigned as a random effect. Tukey's honest significant difference (HSD) test (P = 0.05) was used to separate treatment means when appropriate. The untreated control was not included in the analyses, but results are presented. We already know from prior studies that without prophylactic insecticide treatment, sweet corn is all but certain to become infested even at low population densities (Shelton et al. 2013). We felt it was more appropriate to evaluate the nuanced similarities and differences between treatments based on frequency and timing of insecticide applications rather than presence or absence.
Results
In 2012, the main effects of insecticide (F = 1.8521; df = 2; P = 0.1763) and timing (F = 1.8723; df = 2; P = 0.1732) were not significant. In contrast, the interaction effect between insecticide and timing was significant (F = 6.1220; df = 4; P = 0.0012) (Table 2). Similar levels of H. zea damage reduction were achieved regardless of application timings for lambda-cyhalothrin and chlorantraniliprole. In contrast, methomyl treatments applied 3 times from first green silk to 25% dry silk (3 sprays) resulted in significantly less damage than methomyl applied using current IPM guidelines, applied from 50% green silk to harvest (as needed).
In 2014, the main effects of insecticide (F = 7.8148; df = 1; P = 0.0090) and timing (F = 4.7464; df = 4; P = 0.0044) were significant, but the interaction between insecticide and timing was not significant (F = 0.6458; df = 4; P = 0.6342) (Tables 3 and 4). In 2014, lambda-cyhalothrin treatments had a significantly lower percentage (57.8 ± 3.4%) (± SE) of clean ears compared with chlorantraniliprole treatments (69.8 ± 3.7%). Among the 2014 timing treatments, insecticides applied 4 times from 50% tassel to 25% dry silk had significantly higher percentages of clean ears (76.5 ± 3.2%) than insecticides applied once at 50% tassel (47.5 ± 5.5%). Other treatments were not significantly different from each other.
Discussion
Chlorantraniliprole is an efficacious insecticide with systemic activity and long-lasting protection against arthropod pests (Hannig et al. 2009). However, our results from 2012 showed superior ear protection to other treatments in only 1 case, namely, when chlorantraniliprole had been applied using timing schedule 3 (IPM guidelines) compared with methomyl applied using timing schedule 3 (Table 2). Although 9 insecticide timing treatments were evaluated in 2012, chlorantraniliprole significantly reduced H. zea damage in only 1 instance and there was no evidence of superior ear protection compared with other chemistries. In 2014, insecticide was significant as a main effect and chlorantraniliprole resulted in significantly greater numbers of undamaged ears compared with lambda-cyhalothrin treatments (Table 3). However, in 2014 H. zea density was lower compared with 2012 (Table 4).
The effects of insecticide-timing treatment combinations were not consistent. In 2012, timing schedule made no difference when lambda-cyhalothrin was used, nor was it significant for chlorantraniliprole (Table 2). However, ear damage was significantly reduced when methomyl was applied using timing schedule 1, from first green silk to 25% dry silk, and timing schedule 3, IPM guidelines from first green silk to harvest. In 2014, the main effect of timing significantly increased the percentage of undamaged ears when insecticides were applied from 50% tassel to 25% dry silk, compared with 1 spray at 50% tassel (Table 4). Insecticide applications made using current IPM guidelines did not significantly differ from insecticide applications made with any other timing schedule evaluated in this study.
The 2014 analysis of application timing schedule as a main effect showed a numerical advantage in ear damage reduction when applications were made using timing schedule 5, from 50% tassel to 25% dry silk (76.5 ± 3.2% undamaged ears), compared with timing schedule 3, IPM guidelines from first green silk to harvest (64.0 ± 6.5% undamaged ears) (Table 4). However, we could not provide statistical support for these differences. The 12.5% difference is notable and could arguably serve as justification for further research. A larger sample size with more replications would increase statistical power and reduce variance. Effects of application timing schedules within the reproductive phase of sweet corn development should be researched further.
The ability of chlorantraniliprole to consistently reduce H. zea ear damage is unclear despite our research. In 2012, a significant difference was detected between chlorantraniliprole (75.8 ± 3.7% undamaged ears) and methomyl (45.5 ± 3.1% undamaged ears) using timing schedule 3, IPM guidelines from first green silk to harvest (Table 2). In 2014, however, chlorantraniliprole (69.8 ± 3.7%) resulted in a significantly higher percentage of undamaged ears than lambda-cyhalothrin (57.8 ± 3.4%) (Table 3). Based on these findings, the suitability of chlorantraniliprole as a consistent and effective insecticide for protecting sweet corn from H. zea is unclear.
Table 2.
Percentages of undamaged sweet corn ears by Helicoverpa zea based on interaction of insecticide and application timing in 2012.
Table 3.
Percentages of sweet corn ears undamaged by Helicoverpa zea with insecticide as a main effect in 2014.
The 2014 results showed not only that chlorantraniliprole provided significantly better ear protection from H. zea than lambda-cyhalothrin (Table 3) but also that insecticides applied using timing schedule 5, from 50% tassel to 25% dry silk, produced numerically greater yields than other timing schedules (Table 4). These results are mixed. Because we could not find strong, consistent statistical support, we rejected our hypothesis that reductions in H. zea ear damage can be achieved by targeting the late tassel/first green silk stages with chlorantraniliprole. Additional studies with more replications and samples to increase statistical power and reduce variance are warranted.
Table 4.
Percentages of sweet corn ears undamaged by Helicoverpa zea with application timing as a main effect in 2014.
Pyrethroid insecticides are the most commonly used insecticides in sweet corn production because they are inexpensive and have been effective against the European corn borer, Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), the traditional main pest of sweet corn prior to the emergence of H. zea. However, significant yield improvements using lambda-cyhalothrin with standard IPM guidelines (timing schedule 3), or even modified timing to target very early silk stages (schedule 1) were not achieved in this study. The reasons are unclear. Pyrethroid resistance has been reported in H. zea populations from the southern United States (Pietrantonio et al. 2007; Hopkins & Pietrantonio 2009, 2010)Helicoverpa zea (Boddie, and in the Midwest (Jacobson et al. 2009)“plainCitation”:“(Jacobson et al. 2009. There is no published evidence of resistance in New York. However, it is possible that resistance contributed to the failure of lambda-cyhalothrin to protect sweet corn ears from H. zea feeding damage in our experiments. Laboratory screening assays of New York field-collected adults in 2010 and 2011 have been conducted and suggested that a low level of resistance was present (Olmstead & Shelton unpublished).
Chlorantraniliprole represents a relatively new insecticide class. Anthranilic diamides have a very specific mode of action (Cordova et al. 2006), have few non-target effects in the field (Preetha et al. 2009; Brugger et al. 2010; Gradish et al. 2011; Huang et al. 2011), have longlasting plant systemic activity, and have anti-feeding effects on target pest insects (Hannig et al. 2009). However, results of this study suggest that the suitability of chlorantraniliprole for use in sweet corn to reduce H. zea infestation and ear damage is variable. When the 2014 results are considered alone, chlorantraniliprole was more efficacious compared with lambda-cyhalothrin (Table 3).
In 2014, timing schedule 5, from 50% tassel to 25% dry silk, used 4 insecticide sprays. A comparison of environmental impact quotient values (EIQ) (Kovach et al. 1992) demonstrated similar ecological benefits of using chlorantraniliprole or lambda-cyhalothrin. At the rates used in our study, chlorantraniliprole and lambda-cyhalothrin had per application EIQ values of 3.1 and 2.4, respectively (NYSIPM, www.nysipm.cornell.edu/EIQCalc/input.php), with the smaller number being more environmentally favorable. The total EIQ for all applications (n = 4) based on current IPM guidelines were 12.4 for lambda-cyhalothrin and 9.6 for chlorantraniprole.
This research demonstrated that H. zea management in sweet corn is influenced by both insecticide chemistry and application timing. The results also showed that successful reduction in ear damage caused by H. zea varied between years. In 2012, only the interaction effects between insecticide and timing were significant, whereas in 2014 only main effects of insecticide and timing were significant. Chlorantraniliprole provided inconsistent results in 2012 but had significantly higher percentages of undamaged sweet corn ears among treatments compared with lambda-cyhalothrin in 2014.
Acknowledgments
We thank Brian Nault and Hilda Collins at Cornell University for their comments and feedback.
