Pyrethroid insecticide effects on pests and beneficials in field corn

Maria Cramer, Edwin Afful, Galen Dively, and Kelly Hamby
Department of Entomology, University of Maryland

Overview

Background: Due to their low cost, pyrethroid insecticides are often applied when other chemical applications are made. For example, they may be included in tank mixes with herbicides in early whorl corn and with fungicides during tasseling. These pyrethroid sprays often target stink bugs; however, the timing of these treatments is not ideal for stink bug management. Pyrethoid insecticides may harm beneficial insects that help keep pest populations in check and repeated use of pyrethroids can contribute to insecticide resistance.

Methods: In this study, we examined the effect of Bifenture EC® (pyrethroid active ingredient: bifenthrin) applied with herbicides in V6 corn and with fungicides in tasseling corn. We evaluated impacts on pests and beneficials at both application timings. Yield was measured at harvest.

Preliminary Results:  At both application timings, Bifenture EC® did not improve insect pest management because pests were not present at economic levels. We did not find evidence for flare-ups of aphids or spider mites, but a rainy late summer made it unlikely that we would see many of these pests. There were no yield differences between the treatments.

Background

As a result of the low cost of pyrethroid insecticides, preventative applications are common, especially in tank mixes with other routine chemical inputs, such as herbicides and fungicides. However, lower grain prices and low insect pest pressure make it less likely that pyrethroid applications will provide economic returns. Bt hybrids1 and neonicotinoid seed treatments control many of the pests targeted by pyrethroid insecticides. Because they have broad spectrum activity, pyrethroids can negatively impact natural enemies2 which can result in flare-ups of secondary pests3. Tank mix timings may be less effective than applying when insect populations reach threshold. For example, when pyrethroids are combined with herbicide applications, they are too late to control early-season stink bugs and other seedling pests. When pyrethroids are combined with fungicide sprays at tasseling, few insect pests are present at damaging levels. Stink bugs may feed on the developing ear at this time, causing deformed “cowhorned” ears; however, this is rarely a problem in Maryland and stink bug damage is generally not economic throughout a field because feeding is primarily concentrated at the field edge4. Insecticide applications at tasseling have a high potential to affect beneficial insects, especially pollinators and natural enemies that are attracted to corn pollen.

Objectives: Our objectives were to determine the effect of pyrethroids applied preventatively in tank-mixes on corn pests, beneficials, and yield.

Methods: This study was conducted in 2018 and 2019 at the University of Maryland research farm in Beltsville, MD. For each application timing, we planted four replicate plots of a standard Bt field corn hybrid, DeKalb 55-84 RIB (SmartStax RIB complete Bt insect control in addition to fungicide and insecticide seed treatments) at 29,999 seeds per acre. Standard agronomic practices for the region were used.

The herbicide timing compared two treatments:

  1. Herbicide alone (22 oz/acre Roundup WeatherMAX®, 0.5 oz/acre Cadet®, 3 lb/acre ammonium sulfate
  2. Herbicide (same as above) + Insecticide (Bifenture EC® 6.4 oz/acre)

Treatments were applied at V6/V7. We visually surveyed corn plants for pest and beneficial insects before and after application. We also placed sentinel European corn borer (ECB) egg masses in the field to assess predation rates before and after treatment.

The fungicide timing compared two treatments:

  1. Fungicide alone (Trivapro® 13.7 oz/acre)
  2. Fungicide (same as above) + Insecticide (Bifenture EC® 6.4 oz/acre)

Treatments were applied at green silk. We inspected the ear zone and silks for pests and beneficial insects before application. After application, we recorded the number of ears with pest damage and the kernel area damaged. We also counted stink bug adults and cowhorned ears. Six weeks after application, we visually assessed plants for spider mite and aphid colonies.

Sampling for pests and beneficials (left) and; sentinel European corn borer egg mass (right).
Sampling for pests and beneficials (left) and; sentinel European corn borer egg mass (right).

Results

In the herbicide-timing study in 2019 we observed no effect on beneficial insects from the treatments (Figure 1). The most abundant beneficial species were minute pirate bugs and pink spotted lady beetles, which are very mobile and may have recolonized treated plots after treatment. Similarly, treatments did not affect predation on the sentinel egg masses, suggesting that the pyrethroid application may not have affected predators’ ability to locate and consume eggs. Across the treatments, 30-50% of egg masses were consumed by predators.

Minute pirate bug on European corn borer egg mass.
Minute pirate bug on European corn borer egg mass.

The treatments did not impact the number of beneficials at the herbicide timing (N.S.). The pyrethroid insecticide significantly reduced the number of plant hoppers and plant bugs from less than 4 per plant on average to less than 2 per plant (significantly different p<0.05, *), though these insects are not economic pests at this stage. There were never more than 2 stink bugs per 90 plants, well below the treatment threshold of 13 per 100 plants4.

In the fungicide-timing study in 2019, beneficials, especially minute pirate bugs, were abundant at the time of application (3 in every 10 plants), while stink bugs, the presumed target pest, were very rare (1 stink bug in every 68 plants). In 2018, stink bugs were similarly scarce. Overall pest abundance was low (1 in every 35 plants). After application, there was no difference in the incidence or amount of the corn ear damaged by worms, stink bugs, or sap beetles between treatments. Average stink bug and earworm incidence was roughly 1 in 10 ears, while sap beetle was even less frequent. Cowhorned ears and adult stink bugs were almost non-existent in both treatments.

Six weeks after application we found no differences in aphid or spider mite populations between the treatments, suggesting that pyrethroid applications at tasseling did not cause secondary pest outbreaks. We sampled after a period of dry weather; however, the late summer was rainy at Beltsville, which likely suppressed spider mite and aphid populations. Under drought-stress, reductions in the natural enemy population from pyrethroid use might contribute to flare-ups of aphids and spider mites.

Figure 1. Herbicide timing. July 3, 2019, Beltsville MD. Mean number of insects per 10 plants in V7 corn after treatment. N.S.=not significant. H=herbicide; P=pyrethroid.

Yield

For the herbicide timing and fungicide-timing (Figure 2) studies, treatments did not affect yields in either 2018 or 2019.

Conclusions

Figure 2. Herbicide timing (left) and fungicide timing (right), 2018 and 2019, Beltsville MD. Mean yield per acre under two treatments. Yields were not significantly different by treatment in either study. For the fungicide-timing study, 2019 yields were significantly higher than in 2018. N.S.=Not Significant. H=Herbicide; F=Fungicide; P=Pyrethroid.

Results from the 2018 and 2019 studies suggest that pyrethroid applications do not provide yield benefits in corn when tank-mixed with herbicides or fungicides, likely due to the lack of insect pest pressure at these spray timings. Beneficial insects were abundant in the crop at each of these timings and did not appear to be affected by the pyrethroids in the herbicide plots. Repeated preventative use of pyrethroids in the same field could potentially hinder the natural biocontrol of corn pests.

Lady beetle larva (a predatory insect) in silks.
Lady beetle larva (a predatory insect) in silks.

Sources

1 DiFonzo, C. 2017. Handy Bt Trait Table for U.S. Corn Production, http://msuent.com/assets/pdf/BtTraitTable15March2017.pdf

2Croft, B.A., M.E. Whalon. 1982. Selective toxicity of pyrethroid insecticides to arthropod natural enemies and pests of agricultural crops. Entomophaga. 27(1): 3-21.

3Reisig, D.C., J.S. Bacheler, D.A. Herbert, T. Kuhar, S. Malone, C. Philips, R. Weisz. 2012.Efficacy and value of prophylactic vs. integrated pest management approaches for management of cereal leaf beetle (Coleoptera: Chrysomelidae) in wheat and ramifications for adoption by growers. J. Econ. Entomol. 105(5): 1612-1619

4Reisig, D.C. 2018. New stink bug thresholds in corn, https://entomology.ces.ncsu.edu/2018/04/new-stink-bug-thresholds-in-corn/

 

MDA Implements New Pesticide Sensitive Crop and Beehive Locator

Maryland Department of Agriculture press release

Beekeepers, Specialty Crop Growers, and Pesticide Applicators Encouraged to Sign Up for FieldWatch Registries

The Maryland Department of Agriculture’s Pesticide Regulation Section has announced the implementation of FieldWatch, an online registry that connects beekeepers, specialty crop growers, and pesticide applicators around the state. This registry uses precision mapping tools to help commercial applicators know where beehives and specialty crops are located, thus showing which areas are more sensitive to pesticides.

“We are thrilled to have this new system in place so we can promote better communication between beekeepers, specialty crop growers, and pesticide applicators,” said the department’s Pesticide Regulation Section Program Manager Dennis Howard. “This stream-lined system will help create awareness among these groups and improve stewardship in Maryland. Since switching over to FieldWatch early this year, the department has received excellent feedback from registrants on the user-friendly interface and the timeliness of the data.”

The department has maintained a sensitive crop registry for several years. In January of this year, the department entered into an agreement with FieldWatch to start providing registry services. The FieldWatch mapping tools allow users to update their own information in real-time, so pesticide applicators can make the most informed decision before spraying.

FieldWatch offers three free voluntary mapping tools called DriftWatch, BeeCheck, and FieldCheck. All Maryland beekeepers, specialty crop growers, and pesticide applicators are encouraged to sign up.

To register, go to FieldWatch.com and choose the type of account you would like to create. Beekeepers should register for BeeCheck, specialty crop growers should register for DriftWatch, and applicators should register for FieldCheck. For more information about FieldWatch, read this helpful resource or call the department’s Pesticide Regulation Section at 410-841-5710.

Guess the Pest! Week #24 Answer: European Corn Borer

Guess The Pest Logo

 

 

 

 

 

Bill Cissel, Extension Agent – Integrated Pest Management, University of Delawarebcissel@udel.edu

Congratulations to Grier Stayton for correctly identifying the insect as a European corn borer and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #24 Answer: European Corn Borer

It’s hard to believe that a pest that once caused an estimated annual economic loss of $1 billion dollars in the United States is now a rare occurrence. The European corn borer (ECB), as the name implies, is actually native to Europe and was introduced into North American in the early 1900s. In addition to being a pest of corn (field corn and sweet corn), it is also considered a pest of many vegetable and field crops. Since the adoption of transgenic corn hybrids in the mid-1990s, losses due to ECB have been virtually eliminated in Bt crops and significantly reduced in other vegetable and non-Bt field crops. This is one of the pests that the UD Insect Trapping Program monitors with black light traps. The reason we continue to monitor ECB populations throughout the state is because even though generally speaking, populations have been low, there are still local pockets where ECB is causing damage. The photo above of the ECB larva was taken on the Eastern Shore of VA by Helene Doughty from a non-BT sweet corn plot that was 100% infested with ECB.

For information on the benefits of Bt adoption, read this article: Regional pest suppression associated with widespread Bt maize adoption benefits vegetable growershttp://www.pnas.org/content/early/2018/03/06/1720692115