Mid- to late-August is the time of year when we usually see a large increase in the populations of corn earworm, the pest that is also called the tomato fruitworm. As of last week, this trend has been seen in some parts of Ohio but not in others, which is unusual; we usually see an increase at all sites at this time of year. Our pheromone trap in Clark County jumped to 555 moths last week, up from 36 moths the previous week. Our pheromone trap in Franklin County showed an increase to 43 moths last week, up from 12 moths the previous week. The current moth population is likely composed of some recently immigrating moths from the southern USA as well as moths that emerged locally as the later generation of moths that migrated into Ohio back in early June.
Trap reports for corn earworm at several Ohio locations can be viewed using this link: https://docs.google.com/spreadsheets/d/10gh3rHahdxLKkXQapGyEPxWsjHYRmgsezOoFHnwtyEo/edit?usp=sharing
Fresh-silking corn is the preferred host of the corn earworm, but tomato is another common host. It can also attack a variety of other crops, including bell peppers, lettuce, beans, potatoes, cole crops, cucurbit crops, as well as many weed species.
Infestation of the tomato fruitworm on tomatoes in Ohio is most likely in late August and September, but can sometimes occur much earlier, as has been seen this year. It prefers green tomatoes over ripening red tomatoes. Larvae often feed on one tomato fruit for a short time then move to another fruit. Damage in fruit appears as deep wet cavities. Eggs are usually laid on a leaf below the highest flower cluster.
Each female moth of corn earworm can lay 500 to 3000 eggs. Eggs usually hatch in 3-4 days but can be faster when weather is very hot. As the larvae feed, they progress through six instars or sub-stages, with each instar lasting 2-3 days. The larval stage lasts about 15 days at 86 degrees F. Once larvae are fully grown, they drop to the ground, where they tunnel 2-4 inches deep to pupate. The pupal stage lasts about 13 days. New moths start to lay eggs about 3 days after emerging from the pupal stage. The moths are active mostly at night, and hide in vegetation during the day. The moths feed on nectar in flowers of various trees and shrubs and weeds. The moths usually have a 5-15 day lifespan, but can live up to 30 days.
When corn is in the fresh-silk stage, it is attractive to corn earworm. During the time that Ohio’s large acreage of field corn is silking, our relatively small acreage of sweet corn and tomatoes is usually not attacked much by this pest. Once the field corn in any area begins to mature and dry, it is no longer as attractive to the earworms as late sweet corn and tomatoes. This year, much of Ohio’s field corn was planted later than normal due to frequent rains, so this protective effect of nearby silking corn has been happening later than usual this year but is now likely ending in most locations.
One of the most effective ways to monitor this pest is to use a pheromone trap to catch adult moths. As soon as the target moth is found in traps, fields of sweet corn and tomatoes and bell peppers should be scouted for signs of larval damage so that control measures can be taken in a timely and preventive manner.
In addition to the challenge of knowing when the corn earworm arrives, another challenge is its susceptibility to insecticides and transgenic crops. Observations over the past 12 years in the Midwest have shown that pyrethroid insecticides (Warrior, Brigade, and others) are not as effective at controlling corn earworm on sweet corn as they were previously. In years when the corn earworm population density is low, we have seen that pyrethroids can provide very good control, but in years when their density is high, pyrethroids are not very effective. Alternatives to pyrethroids for sweet corn are Coragen, Radiant, Blackhawk, Lannate, and Sevin. Alternatives to pyrethroids for tomato are Avaunt, Coragen, Exirel, Intrepid, Lannate, Radiant, Rimon, and Sevin. Among the transgenic sweet corn hybrids, we are seeing that the old Attribute hybrids are no longer very effective for caterpillar control, but the Attribute-II hybrids are very effective. Some growers are reporting that some of the Performance Series hybrids are not providing adequate control. We have a field trial in progress to determine how well the transgenic hybrids are currently working under Ohio conditions.
-by Celeste Welty, Extension Entomologist