Fresh, local sweet corn is a big-time vegetable crop, drawing customers to markets across the Northeast and generating significant income for growers. In 2008, this crop generated cash receipts of $47 million in New England, $74 million in New York and $44 million in Pennsylvania. However, there’s not a lot of money to be made on wormy corn.
Three major pests
Corn worms are actually caterpillars, which are the larvae, or immature stage, of three moths: European corn borer (ECB), corn earworm (CEW) and fall armyworm (FAW). As adults, the moths don’t damage plants, but their larvae may feed on the leaves, stalks and ears of the corn plants.
These three pests are similar in many ways, but different enough to require different management strategies. For example, ECB can overwinter in most of the Northeast, but CEW and FAW moths have to fly north from southern states in the summer. Thus, the arrival of these pests, and the damage they do, happens at different times.
Using an IPM approach
Integrated pest management (IPM) is a set of strategies aimed at minimizing pest damage while also minimizing pesticide use as much as practical. There are three components of an IPM program: first, monitoring insect pest populations in the field; second, using pest population information to make decisions about when to apply a pesticide; and third, using the least toxic pesticide option to control the pest population.
There are two well-established ways to monitor for sweet corn worms: field scouting and trapping.
Scouting for feeding damage and live caterpillars is a simple way to get accurate information about the level of pest damage—if it’s done correctly. To get an estimate of pest pressure that represents the whole field, plants from at least five locations per variety or block should be scouted. Examine plants in an X pattern, so that sampling locations are near each corner of the block, plus one near the middle. Each sample should include 20 corn plants next to each other in a row. Examining a total of 100 plants makes it easy to estimate the percentage of the crop infested.
It’s important to select plants for examination randomly, and to stay away from plants at field edges since they tend to differ from those in the rest of the field. Walk into the field to the area you intend to scout, then take a few steps and select a plant without looking right at it. Start there and examine the next 20 plants closely for holes, insect frass (excrement) and larvae. It’s critical that you can recognize the signs of damage and the larvae of the pest you are scouting for.
| These two Heliothis traps have been set up early in the season in the grassy border of a sweet corn field to capture European corn borers (ECB). Note there are two traps, about 100 feet apart, one for the Iowa strain of ECB and another for the New York strain. Pictured is Ray Pestle, former Windham County, Vt., agricultural extension agent, an early proponent of IPM and the author’s mentor when he first joined UVM Extension.
Photos by Vern Grubinger.
While scouting gives you an estimate of a pest population in a field, trapping gives you an estimate of the extent to which moths are arriving to lay their eggs in the corn crop. That’s a pretty good predictor of damage that will follow once the eggs hatch, so you can be prepared to take action if the moth population is high enough to warrant an insecticide application, or if scouting finds the number of damaged plants to be high enough to warrant an application. The population level or damage level that warrants applying a control is known as the action threshold. Action thresholds may differ for different pests and different stages of crop growth.
Trapping the moths
This is done using pheromone traps. The traps are plastic nets or containers that are baited with a lure (the pheromone). The lures are small pieces of rubber or plastic that contain the pheromone, and are placed inside the trap. Pheromones are the chemicals that female moths use to attract their mates, and are very specific to the pest species. Only male moths get caught in the traps, which are good for monitoring arriving pest populations, but can’t be used to actually control the pest.
European corn borer
To monitor for ECB in a sweet corn field, you will need two Heliothis traps and two different kinds of lures, because there are two different strains of ECB-the New York and the Iowa strain--that respond to different pheromone lures. The different lures must be used to bait separate traps so as not to confuse the insects. Place the two traps 100 feet apart in grassy areas at the edges of the field. Baited traps should be set up before ECB typically emerges, which is about mid-June in northern New England.
ECB traps are used to give you a heads-up that the moths have emerged and you need to start scouting the field for damage and larvae. Scouting for ECB should begin at the whorl stage of corn (about eight leaves) and through the pre-tassel stage. Look for shot-hole injury in the leaves at the whorl stage, and later larvae in the pre-tassels.
| Scouting for European corn borer includes close examination of sweet corn tassels for the presence of larvae or their frass, a polite word for caterpillar poop.
Unlike ECB, CEW does not emerge from the soil early in the growing season, but flies in later in the season, often on storm fronts. The adult moths arrive loaded with eggs and lay them directly on fresh silk, where the larvae hatch and crawl down into the ears. Therefore, in this case, field scouting cannot be used to monitor for CEW and its population levels to make control recommendations. Instead, spray recommendations are based on the number of corn earworm moths caught in the traps, either on a nightly or weekly basis.
To prepare for the arrival of CEW, baited traps should be placed in the middle of cornfields just as green silks begin to appear. The base of the trap should be at the same height as the silks. Check the trap at least once a week until the first CEW moth is caught; check it two or three times a week after that.
Like CEW moths, FAW moths arrive via air, usually in late summer. FAW traps are also used to tell you when the moths are in the area and it will soon be time to scout your corn for their larvae. Look for large, ragged holes in the leaves and sawdust-like waste, then see if you can find live larvae to be sure the damage is fresh. FAW can be monitored with a green plastic Universal Moth Trap baited with FAW lures. You’ll also need a vaportape killing strip for each FAW trap to keep the moths from getting out once caught.
|Pheromone traps are baited with lures that attract male moths of the target pest species. The lures should be changed every two weeks, and they come in packages that can be stored in ziplock bags in the freezer until needed. Take care not to handle one type of lure and then another without cleansing your hands or changing disposable gloves, as that may cause cross-contamination.
Sources of traps and lures include Great Lakes IPM (800-235-0285, www.greatlakesipm.com) and Gemplers (800-382-8473, www.gemplers.com). Lures have a field life of about three weeks and you can order all that you need for the season, storing them in a ziplock bag in your freezer until needed.
Many states maintain a sweet corn IPM monitoring and reporting network that provides scouting reports, trap counts and related spray recommendations. Before ordering pheromone lures, you should consult with your state’s vegetable extension specialist to see which brands of lure they recommend that best match up with those used by the monitoring network. Lures should be replaced in your traps every two to three weeks.
While IPM networks are helpful, keep in mind that insect pressure can vary even over a relatively small distance, so the best insect pest information is the information you can get right from your own fields.
IPM extension resources
For more information on sweet corn insect pest IPM, including specific action thresholds for different pests, visit http://pmo.umext.maine.edu/swetcorn/cew.htm, www.umassvegetable.org/SweetCornIPMScoutingGuide.html and www.hort.uconn.edu/ipm/veg/htms/cornpest.htm.
The author is Vegetable and Berry Specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at email@example.com. Comment or question? Visit www.farmingforumsite.com and join in the discussions.