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.
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.
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.
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.
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.
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.
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.
