Cutworms challenge success of organic no-till corn

Rodale Institute research shows biodiversity, trees may be best defense

Natural features, such as lines of trees that block wind-borne black cutworm moths, are as effective as the commercial controls available to organic growers for this pest, research shows.

By Christine Ziegler Ulsh and Alison Grantham

Bold innovation sometimes creates new challenges. Such was the case when Rodale Institute staff saw their corn seedlings disappear in 2007 after being well established by the promising organic no-till planting approach using the Institute’s roller-crimper.  Research checking out cutworms as the culprit shows the critical role of seasonal conditions, the lay of the field and natural barriers in how serious cutworm damage will be.

Black cutworm moths, left , are carried by winds in spring to farm fields. They quickly lay eggs, which become larvae, at right , the stage at which they attack corn seedlings. (Moth photo: University of Missouri; Larva photo: Courtesy John L. Capinera, University of Florida)

Background: In 2005 and 2006, as Rodale Institute (RI) researchers worked to refine organic no-till crop production practices, they discovered that black cutworms presented a serious challenge to corn establishment in rolled hairy vetch.  While the rolled vetch mat offers great benefits in terms of erosion prevention, weed suppression, cultivation reduction and savings of time and energy, it also provides the perfect incubating environment for black cutworm larvae. After a few days, they also have an excellent food source: small, untreated organic corn seedlings.  

Teaming up: To investigate the black cutworm pest management challenge, RI researchers teamed up with the Pennsylvania State University (PSU) entomology group led by Mary Barbercheck and pioneering organic no-till farmer Kirby Reichert. They obtained an Organic Farming Research Foundation (OFRF) grant to investigate the mechanics of cutworm infestations and evaluate several cutworm control tactics in organic no-till corn.  Our specific objectives were to:

1. Identify effective management strategies for cutworm in organic no-till corn through trials both on organic farms and the Institute’s ag research land.
2. Share successful strategies with farmers, ag researchers, and educators.

During the 2008 and 2009 field seasons, RI research staff led field trials that measured cutworm pest pressure and compared organically approved cutworm control treatments to untreated control plots at the RI research farm and at Kirby Reichert’s “Seek No Further Farm” in Grantville, Pennsylvania, on land  Reichert leases from the Caspar Kohler family.

Trapping is tricky

Corn seedlings from a no-till organic planting into rolled hairy vetch, at a stage where the plants are vulnerable to being severed at the base by cutworms.

From early April through early June, researchers monitored two types of cutworm traps (one expensive research version, and one inexpensive farmer-accessible version) placed in three locations at the Institute farm for cutworm moths. The purpose of the trapping was to identify the time at which the black cutworm (Agrotis ipsilon) moths arrived, carried by early spring storm fronts, to lay their eggs in lush cover crops, crop residue, or weeds.  

In 2008 cone traps (expensive pheromone lure traps) and sticky wing traps (inexpensive general traps with no lures) were used at each location, and in 2009 cone traps and uni-traps (inexpensive pheromone lure traps) were used. The inexpensive trap type was changed between 2008 and 2009 because the sticky wing traps were unspecific and less effective in capturing black cutworm moths than the uni-traps with pheromone lures.  The pheromone lures in all the lure traps were replaced every 14 days to ensure potency. (Sidebar: “Pheromone traps beat the stickies.”)

From year to year, the timing of the peak cutworm moth influx, or biofix, varied with the timing of weather front arrivals. The location of their landing depended upon local topography.  Most moths arrived on hilltops (59 percent) in May (82 percent) as opposed to valley locations.  Since the moths arrived on storm fronts, landscape features that physically protect production fields, such as rows of trees and buffer strips, appeared to block cutworm influxes and provide important habitat for cutworm predators, such as birds that were frequently observed catching insects or eating insect larva from the ground.

This is the result when black cutworm larvae encircle a corn seedling, then sever it close to the base.

Tracking the damage

As the incoming moths were tallied, field plots were laid out in each location (at RI and at Seek No Further Farm [SNFF]) to test the influence of three different factors on cutworm predation of corn: field location, planting time, and cutworm control treatment.  

In 2008, both farms had an “early” and a “late” planting date (June 6 and 16 at RI, and June 12 and 19 at SNFF) to capture two hairy vetch bloom stages, “early” bloom and “full” bloom. Unfortunately, in 2009, rain fell on all but five days in June, and therefore corn was established on only one planting date at each location, at “full” bloom stage (June 10 at SNF and June 16 at RI).

Based on 2008 growing degree units (GDUs), the time of greatest cutworm damage to corn plants that year would have ranged from June 6 or 7 through June 11 or 13, 30-38 days after the biofix (moth arrival).  These dates are calculated based on the fact that cutworm eggs, laid by the moths at the time of arrival, begin to mature when temperatures exceed 50° F (10° C) and require an accumulation of 312-430 GDUs to reach physiological maturity (and their most damaging stage, instars 3-5).  

Of course, varied weather each year causes GDUs to accumulate, and cutworm larvae to mature, at different rates (faster in warm years, slower in cool ones). In our part of Pennsylvania, this process takes usually takes about 30 days. As such, the 2008 planting dates caused the corn to reach its most cutworm-vulnerable growth phase after June 16, several days after the cutworms moved beyond their most damaging stages (June 6-13). Some states, including Pennsylvania present online summaries of statewide environmental data and pest outbreak reports.

Buying in cutworms

Due to the extremely low cutworm biofix in 2009, researchers ordered 10,000 cutworm eggs and applied them to the field on June 1 (when more than 90 percent of larva appeared to have hatched) in order to simulate the biofix egg deposit. The two-fold goal of this effort was to:

1. Create a significant enough infestation, and subsequent pest pressure, to allow proper testing of the various cutworm control treatments
2. Time the infestation to synchronize cutworm destructive maturity and corn vulnerability as much as possible.

After each planting date, each field plot was treated with one of the six organically approved cutworm management treatments, or left untreated as a control. (Sidebar: Cutworm materials evaluated.) In 2008, treatments were applied at prescribed times ranging from planting to post-emergence, whereas in 2009, since cutworms were artificially seeded, treatments were applied one week after the cutworm larvae were seeded into the plots, and prior to corn planting at both locations. Since some treatments needed to be applied in an aqueous solution, all plots received 2 gallons of water or solution at treatment time.

Pheromone traps, installed in early spring, can be useful in monitoring for pest pressure. The large cone trap, right, was effective in trapping the black cutworm moths, while the unitraps were not. Tracking moth flights allows farmers to calculate when pest pressure will peak to allow changes in planting dates and to avoid cutworm damage to corn.

Researchers then surveyed the soil for larval cutworms. In 2008, technicians checked random row sections weekly after planting in 6-inch bands in 1-meter of row per treatment plot. In 2009, surface transect monitoring was conducted weekly for cutworm moths, eggs, larvae at both locations in three 1-meter transects per plot. In both years, larvae were counted, packed in an ethanol-glycerin solution, and sent to Penn State for identification and analysis.

Corn plant emergence and survival was also tabulated in 1-meter row sections weekly after planting, noting the number of corn plants, their growth stage, cutworm or other foliar damage. The final corn population (stand establishment) was calculated when the corn plants reached growth stage V5 and were no longer vulnerable to cutworm predations (about 4 weeks after planting), representing the final population as a percent of the initial seeding rate.  At season’s end, plot yields were assessed via hand harvests of 8-foot-long sections from the two center rows in each plot.

Cutworms not the problem

On average, stand establishment was about 75 percent of the seeding rate, resulting in slightly lower than ideal populations in most treatments (range: 17,500-35,000 plants/A; grand mean: 26,034). Establishment did not correlate with cutworm pest pressure or cutworm treatment (control plot mean population was 27,500, or 80 percent stand establishment). Bird grazing in 2008 and flooding (leading to seed molding and failed germination) in 2009 were also anecdotally noted to contribute to sub-optimal corn populations.

These data suggest that the purchased treatments were not as effective as the native soil pathogen in overcoming cutworm larva.  What’s more, though researchers found significant differences in the amount of corn plant foliar damage between locations and planting times (p<0.001, for both), this damage did not appear to be caused by cutwoms.  Early season Japanese beetle infestation were responsible for some foliar damage, as were slugs, birds, and hail at different times at each location. However, few instances of clear cutworm damage were observed in either location.

Despite these challenges, yields at both sites in 2008 averaged over the local yield goal of 130 bu/A for all treatments.  In 2009, the wet weather and the weed growth it supported appeared to reduce yields at both locations, as plots were overtaken by thick growth of annual and perennial weeds within a month of planting.   

Natural defenses

The most interesting finding of this study was the distribution of cutworm pest pressure and its implications for pest management in organic systems. Because cutworm moths move in on storm fronts and tend to land on unprotected hilltops (as opposed to lower elevations and locations protected by buffers), land management strategies that physically protect production fields, such as rows of trees, may offer cutworm management services. In addition to physically blocking pest influxes, tree lines and other buffer strips provided important habitat for birds that were frequently observed catching insects or eating insect larva from the ground at both sites.

Though these natural pest managers frustrated the objectives of this project, they may provide better pest management for this production system than the economically prohibitive commercial biocontrol treatments that were tested.

Christine Ulsh Ziegler is research agroecologist and Alison Grantham is research manager at the Rodale Institute.