Corn Rootworm Management Strategies for 2018

Elson Shields, Entomology Department, Cornell University

The excessive wet soil conditions during the 2017 corn rootworm (CRW) hatching period during late May – early June caused a major reduction in corn rootworm adult populations during the 2017 growing season.  Adult surveys in most fields during early August showed a scarcity of adult beetles during the egg-laying period.  As a result, most fields in NY will have a reduced risk for CRW damage during the 2018 growing season.  In these lower risk fields, CRW management costs can be reduced by growing non-Bt-CRW corn and using either a reduced rate of soil insecticide or the 1250 rate of seed treatment.  First year corn is never at risk from CRW and therefore Bt-CRW corn, a soil insecticide or the 1250 rate of seed treatment is an unnecessary expense.  This includes any application of Capture in the pop up fertilizer.    Well drained fields which did not experience the typical periods of water logged soils during late May – early June 2017 will be at higher risk from CRW injury in 2018 and should be managed accordingly.  These higher risk fields may benefit from planting Bt-CRW corn varieties.  In “normal” years, the risk of economic damage from CRW is 0% – 1st year corn, 25%-35% – 2nd year corn, 50%-70% – 3rd year corn and 80%-100% for 4th year and longer continuous corn.

Status of Bt-CRW resistance in the US:

CRW Bt resistance continues to build across the corn growing regions of the US with multiple localized resistant populations identified for each of the Bt-CRW traits.  Cross resistance has been identified within the Cry3 family (Cry3Bb1-Yieldgard Rootworm, eCry3.1Ab-Duracade, mCry3A-Agrisure RW) and if one of the Cry3 traits are failing in your field, the planting of another toxin within the Cry3 family may lead to disappointing CRW management results.  Resistance has also been reported in several states to Cry 34Ab1/Cry35Ab1. There has been no reported cross resistance between the Cry3 family of toxins and Cry34Ab1/Cry35Ab1 toxin combination.

The rootworm Bt-toxin pyramids consist of two different Bt-RW toxins in the same plant.  Some seed companies have included two different toxins from the Cry3 family where cross resistance has been reported where other seed companies utilize the pyramid mix of a toxin from the Cry3 family and Cry34Ab1/Cry35Ab1 where no cross resistance has been reported.  If control failures have been reported in your fields/region to any one of the Cry3 family of toxins, planting a pyramid composed of two different Cry3 toxins is not recommended.  Instead, it is a better CRW resistance choice to plant a pyramid consisting of a Cry3 toxin with the Cry34Ab1/Cry35Ab1 toxin.

A very handy resource to identify the Bt traits in your corn varieties is the annually updated Bt trait table.  The 2018 Handy Bt Trait Table for US Corn Production is made available by Dr. Chris Difonzo, MSU, Dr. Pat Porter, Texas A&M and Dr. Kelley Tilmon, OSU can be found at the following URL:

https://lubbock.tamu.edu/files/2018/01/BtTraitTableJan2018.pdf

As Bt –CRW traits are failing to resistance by corn rootworm, the promise of the next effective trait is ever appealing.  The development of the RNAi technology against CRW has been touted as the next effective plant incorporated toxin with a very slim chance of resistance development by CRW.  However, it only took about 20 million individuals from a single Illinois continuous corn field and a few generations to generate an RNAi resistant laboratory population. In addition, field results with RNAi containing corn varieties suffer a noticeable amount of root feeding damage before the slow-killing toxin kills the insect larvae.  As a result, the new RNAi technology will not be the “silver bullet” everybody has hoped for.  Stewardship of the Bt technology has become increasingly important in areas where Bt resistance has not been reported because the next technology needs effective Bt toxins to help it out.

Bt Trait Stewardship Suggestions:

A few simple management adjustments can go a long way in preserving the efficacy of the Bt-CRW traits in NY.

  • Long-term corn fields need to be rotated to a non-corn crop on a regular basis.  Continuous corn matched with a long-term use of same Bt-CRW trait promotes the development of a resistant population.
  • Rotate toxins between the Cry3 family and Cry34Ab1/Cry35Ab1 toxins. There is no recorded cross resistance between these two groups of toxins.
  • Use the Bt-CRW technology only in fields of 3rd and longer continuous corn fields. Rotate the toxin groups and rotate the long-term corn to at least 1 year away from corn to break the CRW cycle.
  • Plant some fields to non-Bt-CRW varieties and use either a granular soil insecticide or the 1250 rate of seed treatment. Liquid insecticides in the popup fertilizer are not effective and not recommended.
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What’s Cropping Up? – Volume 27, No. 5 – September/October 2017

The full version of What’s Cropping Up? Volume 27 No. 5 is available as a downloadable PDF and on issuu.  Individual articles are available below:

 

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Managing Corn Rootworm in Non-GMO Corn

Elson J. Shields, Entomology, Cornell University

Aerial view of lodged corn from extensive corn rootworm larval feeding.

An increasing number of dairy producers are being asked by their milk processors to seriously consider producing milk from dairy cows fed non-GMO forages and grains.  Many milk producers feel the pressure to comply with the request in order to preserve their milk market.  The decision to grow non-GMO corn impacts both the weed control program and management of corn rootworm.

Biology:

A review of the biology of corn rootworm is a good starting point for this discussion. Adult corn rootworm emerge from existing corn fields around the first of August, where the larvae have been feeding on corn roots. Adults begin emerging around corn pollen shed and start feeding on corn pollen. After about 3-weeks, the females begin to lay eggs in existing corn fields. The eggs overwinter with the larvae hatching the following May. If the field is planted to corn, then the larvae start feeding on corn roots, but if the field has been rotated to another crop, the newly hatched larvae die. Since eggs are laid in existing corn fields, first year corn has zero risk of corn rootworm damage in NYS. In terms of risk, second year corn fields have a 25-35% chance of risk for rootworm damage, third year corn a 50-70% risk of losses from corn rootworm larval feeding and fourth year corn risk for rootworm losses is between 80-100%.

Adult rootworm scouting procedures are available and help to decide if the field is medium-high risk for larval damage the following growing season. Adult beetle scouting occurs around pollination and is conducted for three subsequent weeks. If beetle counts average 1 beetle per plant and the females have mature eggs, the field is at risk for larval feeding damage.

(https://fieldcrops.cals.cornell.edu/sites/fieldcrops.cals.cornell.edu/files/shared/documents/fcorn_scouting.pdf).

Adult beetle scouting does not account for the subsequent larval mortality when the soils are waterlogged during the hatching period, so using adult counts to estimate risk usually overestimates risk. Assessment of the rootworm larval population after hatch is difficult and very labor intensive.

Management options:

Since first year corn has zero risk from rootworm larval damage, the standard seed treatment (Poncho, Cruiser 250) is all that is needed for protection from germinating-seedling damaging insects. Second through fourth year corn need some protection for potential corn rootworm larval feeding.

Shortened Rotation:

Risk of rootworm feeding damage increases with the duration of continuous corn within a field. Since first year corn has zero risk from rootworm and second year corn has reduced risk (25-35%), a shortened corn rotation reduces the need (and cost) of rootworm management. Producers who can only grow two years of corn before rotating to a non-corn crop can frequently grow corn without any extra rootworm management expense.

High Rate of Seed Treatments:

The high rate of seed treatment (Poncho 1250, Cruiser 1.25) offered for seed corn has activity on corn rootworm larvae. In growing seasons with adequate rain fall and moderate corn rootworm pressure, the high rate of seed treatment will provide adequate protection for the crop. In situations where the field has high rootworm pressure, the insects often overrun the insecticide, resulting in economic root feeding damage. High rates of seed treatments are also challenged to provide adequate control during times of limited rain fall during June-July or in times of excessive rain fall during the same time period. Use of the high rate of seed treatment is best matched to the second and third year of continuous corn when rootworm pressure is lower.

Liquids Soil Insecticides:

The use of liquid soil insecticides (Capture, Force) mixed with the liquid starter fertilizer and applied in-furrow has become a popular option for growers without granular insecticide boxes on their corn planter. Past research in NY has consistently shown that either of these two insecticides applied in this manner are highly variable in control. In talking with farmers who are avid supporters of the use of liquids in this manner, they frequently admit to control failures consistent with the research results. The major issue with this application method is not the efficacy of the insecticide on the rootworm larvae, but the timing of the application with a liquid formulation. Application of a soil insecticide at planting is introducing the soil insecticide into the soil environment three to four weeks before corn rootworm larvae begin to hatch. It has always been a challenge for soil insecticides to still be in the root zone 3-5 weeks after application so the insecticide can be present to kill the newly hatched larvae. Granular insecticides bridge this time period with the slow-release properties of the granule. High-rate seed treatments bridge this time period with the slow-release properties of the seed coating. Liquid insecticides mixed with liquid fertilizer and applied in the seed furrow at planting does not have any slow release properties. Heavy rainfall events in the 3-5 weeks between planting and rootworm hatch flush the liquid insecticide out of the root zone along with the starter fertilizer. Heavy rain fall events during May-early June is not an unusual event in NY. Highly variable efficacy of liquid insecticides applied at planting are more directly linked to the lack of a slow release formulation and being flushed out of the root zone than the efficacy of the insecticide against rootworm. In their best years in research plots, liquid insecticides were also challenged to suppress heavy populations of rootworm larvae. Liquid insecticides applied in-furrow with the starter fertilizer is not recommended for rootworm control in NYS.

Granular Soil Insecticides:

Granular soil insecticides applied at planting were the primary management strategy before the introduction of seed treatments and rootworm-active GMO trait in the corn. They remain a very effective tool to manage corn rootworm and were left behind due to the convenience of the newer technologies. For producers who have insecticide boxes for their planters, granular soil insecticides provide a more reliable management tool than either high-rates of seed treatments or liquid insecticide applied in-furrow with the liquid fertilizer. Each different granular insecticide has its strengths and weaknesses and I will try to summarize them below.

Force 3G:

Force 3G is a widely used soil insecticide in corn production. It performs best when soil moisture is adequate to in excess due to its low solubility in water. If a producer calibrates accurately, use rates can be reduced to 75% of the label rates for moderate rootworm populations. In dry years, even a full label rate faces challenges controlling rootworm populations due to the insecticide’s low solubility in water. Force is effective across all soil PH ranges.

Counter 15G, 20G:

Counter 15G, 20G is an effective soil insecticide against corn rootworm larval populations. This insecticide performed best in dry to moderately wet soils, but was challenged to perform adequately under conditions of excessive rainfall. Counter has a higher solubility in water than Force and effective across all soil PH ranges. With the introduction of ALS inhibitor herbicide, Counter was shown to have a serious interaction leading to plant injury. An introduction of a 20 CR (controlled release) granular was attempted with variable results. Due to the herbicide interactions, the use of Counter was significantly reduced by producers.

Lorsban 15G:

Lorsban 15G has been an effective soil insecticide against low to moderate populations of corn rootworm larvae. However, this insecticide is PH sensitive and cannot be used in soil PH above 7.8. In the high PH soils at the Cornell Musgrave Farm, Lorsban was deactivated by the high PH before rootworm hatch. In soils with PH of 7 or lower, this material can provide effective control against low to moderate rootworm population levels.

Summary:

If producers are serious about growing non-GMO corn, they need to invest in granular insecticide boxes for their planter if they do not have them. Granular insecticides are the only reliable way to control corn rootworm larvae across corn rootworm population levels and weather conditions. Liquid insecticides in liquid fertilizer applied at planting demonstrates highly variable control and is a poor investment. High rate of seed treatments are effective against low to moderate population levels of rootworm larvae and are sensitive to soil moisture levels. Granular insecticides provide the most consistent control of corn rootworm. However, in most situations, the corn planter operator needs to have a Pesticide Applicator License.

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What’s Cropping Up? – Volume 26 No. 6 – November/December Edition

Emerging Corn Rootworm Resistance to BT: Strategies to preserve the technology

Elson Shields
Department of Entomology, Cornell University

In 2016, corn rootworm control failures have been once again reported in central NY.  As reported in 2013, the control failure was in fields planted to corn containing a single rootworm BT toxin, Cry 3Bb1 (Figure 1).  Severe rootworm injury is usually easy to spot during the growing season by the goose-necked corn plants.  The plant tilts over due to the loss of roots and then the stem straighten up, leaving a curved plant stem.  Many times during fall harvest, lodged plants are blamed on corn rootworm damage.  However, during the fall, many of the lodged plants have straight stems and have fallen over due to their poor rooting, top heavy condition and wet soils in the fall.  The key diagnostic characteristic for rootworm damage in the fall is the curved stem.

Root damage and plant lodging from corn rootworm larval feeding. The curve stem of the plant is characteristic of this type of feeding, which occurs in July. The plant tips over from loss of roots and then turn back up. These plants contain CRW-BT Cry 3Bb1 protein and the larvae feeding on the roots were not killed by the toxin.
Root damage and plant lodging from corn rootworm larval feeding. The curve stem of the plant is characteristic of this type of feeding, which occurs in July. The plant tips over from loss of roots and then turn back up. These plants contain CRW-BT Cry 3Bb1 protein and the larvae feeding on the roots were not killed by the toxin.

Keeping the various BT traits straight and which corn varieties they occur in is an ever changing and difficult task.  One very helpful tool is the “Handy BT Trait Table” published by Dr. Chris Difonzo at Michigan State University.  The 2016 version of the table can be located at www.msuent.com/assets/pdf/28BtTraitTable2016.pdf.  The 2017 trait table is not available until March 2017 and can be located by conducting a Google search using “Handy BT Trait Table” as the search words.

For corn rootworm control with plant incorporated BT, we have only three groups of BT proteins in our toolbox.  The first protein Cry 3Bb1 was first incorporated into corn by Monsanto and was the first BT-CRW protein marketed.  The second protein is Cry 34/35, a two protein/toxin mixture, incorporated into corn by Dow and the third group is the Syngenta proteins mCry3A and eCry3.1Ab.  Some corn varieties have two groups of CRW proteins incorporated called a pyramids.  For example, a common pyramid is Cry 3Bb1 and Cry 34/35.

As predicted, corn rootworm is developing resistance to these widely deployed corn incorporated proteins across the corn belt with resistance to Cry 3Bb1 the most wide spread.  Multiple field failures have been reported to mCry 3A in the Midwest and fields with poor control have been reported planted to corn with Cry 34/35.  In the Northeast, reports of resistance to Cry 3Bb1 occurred in NY in 2013 and 2016 with reports of resistance to mCry 3A in Pennsylvania in 2014.  With new technologies on the distant horizon, producers should make a focused effort on preserving the current BT-CRW technology as long as possible.

Strategies to preserve the technology:
If a farm is experiencing a loss of control or performance from the BT-CRW toxin, there are several effective strategies to use which has both the benefit of reducing losses in future years and helping to preserve the technology from increased insect resistance.

  • Rotate the field experiencing damage to a non-corn crop:

Since corn rootworm eggs overwinter in corn fields and the newly hatched larvae must find a corn root to survive, crop rotation remains an effective CRW management tool.  Simply rotate the field to another crop for a season and then the field can be rotated back to corn the following year without the need for any CRW management measures.

  • If the field cannot be rotated, use a soil insecticide with conventional corn variety:

If growers still have insecticide boxes on their planter, excellent control can still be achieved using an application of soil insecticide in a 7” band in front of the press wheel.  In furrow applications have a reduced efficacy compared to the 7” band.  The use of a liquid insecticide such as Capture either in furrow or in the fertilizer band is widely considered as ineffective against CRW larvae.

  • If the field cannot be rotated, use a corn variety with a different BT-CRW toxin:

If the field has experienced loss of performance with a corn variety containing Cry 3Bb1, then plant a corn variety using Cry 34/35.  Substituting a corn variety with mCry 3A or eCry3.1Ab CRW toxin is not the best choice because of potential cross resistance (observed in the laboratory) with Cry 3Bb1.  If the loss of performance is due to a corn variety containing Cry 34/35, then any of the Cry 3 choices would be appropriate.

  • If the field cannot be rotated, it is a poor choice to plant a corn variety containing two CRW-BT toxins, with one of the toxins identical to the toxin failing in the field:

For example, if Cry 3Bb1 is giving poor control in the field, planting a variety which contains both a Cry 3 toxin (Cry 3Bb1, mCry3A or eCry3.1Ab) and Cry 34/35 with only a 5% refuge in the bag puts a lot of selection pressure on the CRW population to develop resistance against Cry 34/35.  The dual CRW-BT toxin containing corn varieties were allowed to be marketed with only a 5% “refuge in the bag” under the conditions both toxins where effective on the insect population.

  • Resist the temptation to layer a soil insecticide over a failing CRW-BT event:

When a soil insecticide is used in addition to a CRW-BT trait with a history of poor CRW control, this combination increases the development of CRW resistance to the event.  A soil insecticide applied either in furrow or banded in a 7” band protects the area surrounding the base of the plant from larval feeding.  However, this insecticide application has no impact on the CRW larvae feeding on corn roots outside the insecticide treated area surround the plant base.  CRW larvae feeding outside the insecticide protected zone are still being exposed to the failing BT toxin and larvae continuing to survive feeding on the toxin will continue to interbreed with other survivors, resulting in an increased level of resistance in the next generation of rootworms.

 

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Developing a CUCE / High School Research Partnership

Aaron Gabriel, Senior Extension Educator, Cornell Cooperative Extension, Capital Area Agriculture & Horticulture Program

Over the last three years, I have been developing a partnership with high schools to have their students participate in my research projects.  It started out with my interest in black cutworm.  As I scouted corn fields, many of the skips were not due to rocks or soil conditions, as farmers often assume.  Black cutworm frequently was the culprit (as well as birds).  To collect enough data to support my observation, would take a lot of time for one person.  So, I developed a protocol for determining the cause of skips in corn and contacted several high schools to see if they wanted their students to participate in field research.  I found interest at Berne/Knox/Westerlo (BKW), Greenwich, Salem, New Lebanon, and Taconic Hills High Schools.  I also found interest from a 4-H club in Columbia County and one Master Gardener.

It just so happens that there is a nation-wide effort to engage students in STEM (science, technology, engineering, and math).  The desire to engage students in a research experience and the need for Cooperative Extensions to do research is coming together into a successful partnership.  To obtain financial support, grant writing has taken a new perspective.  I received two small grants from regional foundations, not to study the pedestrian black cutworm, or nematodes (which have been studied for many years), but to give students a research experience by involving them in relevant local Cooperative Extension research.  The program objectives focus on the students and on helping the local agricultural community, not on solving a specific agricultural problem.

Program Objectives:

  • Develop a 4-session high school curriculum to give students a real-world research experience that will:
    • Teach students how to conduct and interpret research.
    • Help students recognize their interest and potential in pursuing careers in science and research.
    • Teach students how to critically evaluate research that is broadcast through news media.
  • Conduct agricultural research that will:
    • Provide useful information that CUCE can extend to farmers for positive impacts.
    • Give students an experience to help them better understand agricultural.
    • Teach students the impact that research has on the community.

My first collaboration was with a class of BKW Advanced Placement Biology students in late May, 2012.  Having taken their last exam in mid-May, like all AP Biology students in New York, they needed some projects until the end of the year in mid-June.  First, I gave them a presentation in class to explain the dilemma of missing corn plants and my interest in the black cutworm (BCW).  I armed them with a data collection sheet, tools, and pictures of the insects, bugs, and diseases they might find digging in a corn field looking for the culprits that cause skips in corn.  The first field we sampled was an early-planted corn following sod.  The seed had a low dose of seed-applied insecticide.  The sod and weeds had not yet been sprayed with herbicide.  There were many missing corn plants.  To my surprise, at most of the skips they were finding seedcorn maggot pupae.  We learned that at high pressure, the low-dose of seed-applied insecticide did not provide protection.  The corn population was reduced by 23% to 21,017 plants/acre, with 30% of the skips due to seedcorn maggot.  Seedcorn maggot was not as severe in other fields.

Berne/Knox/Westerlo students tallying the causes of missing corn plants.  Seedcorn maggot was the most prominent culprit.  The low-dose of seed-applied insecticide could not fully protect this early-planted field after sod.
Berne/Knox/Westerlo students tallying the causes of missing corn plants. Seedcorn maggot was the most prominent culprit. The low-dose of seed-applied insecticide could not fully protect this early-planted field after sod.

Currently, I am doing research to learn how to use beneficial nematodes to control insect pests in corn (grubs, black cutworm, and corn rootworm).  Two students helped me by doing a bioassay in the lab to confirm that our nematodes would infect BCW.  They entitled their project, “The Farmer, the Field, and the Nematode”, entered it into the Greater Capital Region Science and Engineering Fair and won the environmental award.

Two Junior High School students received an environmental award at the Greater Capital Region Science and Engineering Fair for a doing a bioassay that showed the nematodes were lethal to black cutworm.
Two Junior High School students received an environmental award at the Greater Capital Region Science and Engineering Fair for a doing a bioassay that showed the nematodes were lethal to black cutworm.

With confidence that these nematodes (from the lab of Dr. Eslon Shields, Cornell Univ.) will infect black cutworm, plots were set up to evaluate their effectiveness on BCW in the field.  Plots 5’ X 7” were treated with either nematodes or water, as a control, and then infested with purchased BCW.  Fields were located in Salem and Berne.  AP Biology students from the local schools helped infest the plots and collect data to compare the damage from the two treatments.  Nematodes did kill some of the BCW, but damage between the two treatments was similar.

Students help set up field plots of nematodes and black cutworm, and then collected the data to compare cutworm damage in the two treatments.
Students help set up field plots of nematodes and black cutworm, and then collected the data to compare cutworm damage in the two treatments.

As I was trying to figure a new way to get the nematodes established in a field before the corn crop, Donna McGovern, BKW teacher, asked if I had a project for her entire class of ninth-grade biology students.  We developed a plan to apply the nematodes to a white grub-infested hay field before it would be planted to corn.  So, 52 biology students sampled 160 locations and collected and tallied up the white grubs in two fields.  On my own, I could never sample the grub population like that.  Nematodes were applied and corn will be planted in the spring.  This time, some plots will be infested with BCW, and others with corn rootworm.  The students will be there to collect the data on BCW damage.  Since rootworm damage is evaluated in July, I will need to find a 4-H club to help, or students that want an education outside of the school year.

Fifty-two biology student sample 160 locations to determine the white grub population.  Nematodes will establish themselves on the grubs before the corn is planted, and then lay in wait next spring to protect the corn seedlings.
Fifty-two biology student sample 160 locations to determine the white grub population. Nematodes will establish themselves on the grubs before the corn is planted, and then lay in wait next spring to protect the corn seedlings.

Students have also performed lab bioassay experiments and determined that nematodes will not survive in pop-up fertilizer, unless it is diluted with 50% water.  The purpose is to evaluate other methods of applying nematodes (which are suspended in water).  New Lebanon students determined that our nematodes do not infect fly larvae, like the seedcorn maggot.  These fairly simple experiments give students a real research experience and help me generate the information I need as a CCE educator to help local farmers.

The next step of the CCE / School Research Partnership is to complete development of a 4-session curriculum that can be used by any pair of Extension Educator and school teacher.  This is currently underway where students will learn how to research a topic, formulate a hypothesis, design an experiment, do the experiment, analyze the data and make conclusions.  The finale of the curriculum will be to visit a local farm and learn how research has shaped agriculture and how it impacts farmers.

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