Corn earworm moths counts in pheromone traps have been increasing over the past few days in some areas in Maryland and Delaware. Sweet corn growers should keep an eye out and consider shortening spray intervals to a 2 to 3-day spray schedule while others could still be around a 3 to 4-day spray schedule.
Corn earworm (CEW), Helicoverpa zea, also known as tomato fruitworm, sorghum head worm, and podworm, is a common pest on sweet corn and vegetable crops during the summer and early fall in the Mid-Atlantic region.
Corn earworm moths are nocturnal, living for about 2 weeks. They can be found flying from mid-July and continuing into September. Corn earworm adult moths are tan-colored. They have wingspan of ~1.5 inches, with dark central spots that are easily visible even from the underside. A single female moth can lay over 3000 eggs. Corn earworm eggs are cream to pale green colored, are ~1/2 mm in size, and have a ribbed dome appearance. Eggs hatch ~3-4 days with reddish brown bands appearing prior to hatching.
Photo by Whitney Cranshaw, Colorado State University, Bugwood.org
Corn earworm larvae range in color from green, brown, pink to yellow or sometimes black, with dark lengthwise stripes. Larvae have four pairs of abdominal prolegs, with a light brown- orange head. They undergo 6 instars in 2-3 weeks depending on temperature. Upon completion of development, larvae drop to the ground from the infested crop, where they then burrow ~2-4 inches within the soil to pupate and overwinter. Corn earworm pupae are ~0.5-1 inch in length and dark brown.
Corn earworms are strongly attracted to and prefer fresh corn silks for egg laying. Outbreaks in other crops often follow a midsummer drought, which causes the corn to ripen earlier and become less attractive to the moths. As early planted corn fields dry down, moths will move into other vegetable and grain crops.
Sweet Corn Control:
Direct sampling for caterpillar pests during silking is not practical. Monitoring pressure using on-farm traps provides the most accurate information for making management decisions. We recommend using two pheromone traps and replacing the lures frequently, especially during periods of hot weather. Trapping can indicate the severity of pressure and can inform the timing and frequency of insecticide applications.
Moths are attracted to ears with fresh green silks, where most eggs are laid. Complete silk emergence from all ears generally takes 4 to 8 days after the first silks appear. After spraying at the early silking stage, new fresh silks emerge and are not protected. This means spray intervals should be tightest during fresh silk. In addition, when weather is in the 90s (°F), it takes 2 days for eggs to hatch and 2 hours for small larvae to move down the silk channel and begin feeding on the ear tip, where they are protected from foliar sprays.
Generally, an insecticide spray is applied at early green silking as soon as the first moth is captured on the farm, and applications are repeated at 3 to 6 day intervals based on moth pressure, corn growth stage, weather, Bt trait being used, and tolerance for ear damage. When CEW populations are heavy (> 10 moths captured per night) it may be necessary to treat on a 2-3 day schedule. Sprays may be required up until 5 days from the final harvest date. For best control during heavy infestations, maximize the gallonage of water per acre and/ or use a wetting agent if possible.
It is important to rotate insecticide classes within a season. The cheaper pyrethroid (Group 3A) products have been the popular choice but their control efficacy has significantly declined due to resistance in corn earworm populations. When first introduced, pyrethroids provided greater than 95% control of corn earworm, but currently control efficacy has declined to around 50% due to resistance development. The reality is that pyrethroids no longer provide enough ear protection on many farms, so growers need to consider incorporating other modes of action into spray programs. ALWAYS read and follow instructions on the pesticide label; the information presented here does not substitute for label instructions.
As an alternative, the most potent bioinsecticide for sweet corn insect control is provided by transgenic hybrids expressing one or more insect-active toxins from the bacterium, Bacillus thuringiensis (Bt sweet corn). Bt hybrids that express single or multiple Cry proteins no longer control earworms due to the development of resistance; only hybrids expressing the Vip3a Bt protein have been shown to provide good ear protection. Although these hybrids provide 100% control of the European corn borer, they do not provide enough control of corn earworm and other lepidopteran pests depending on the expressed toxins and thus supplemental insecticide sprays may be needed to ensure quality ears, especially during high moth activity. Stink Bugs are not controlled by Bt.
Other vegetables:
Corn earworm is a pest of tomato, pepper, snap bean, lima bean, and many other vegetable crops. Sampling plans and thresholds have not been developed for many of these crops, but a general rule of thumb based on trap captures is that treatments may be warranted at nightly captures of 20 moths or more. Visual inspection of the crop to detect eggs and small larvae will help select insecticide materials and determine pressure. Beneficials will feed on the eggs and small larvae. Consider looking for beneficials when scouting and using materials that are less harmful for these natural enemies.
Lima beans: Monitoring should begin at full bloom and samples should be taken in at least 10 locations in the field. Sample by placing a 3 foot drop cloth between two rows and shake the plants on both sides over the cloth (sampling 6 total feet of row). Count the fallen larvae and estimate the average size. If you are using a product that is effective on larger larvae, treatment should be delayed until at least 1/3 of the larvae have reached approximately ½ inch in size. The treatment threshold is 1 or more larvae per 6 foot of row. Since the 2008 season, numerous reports of control failures with pyrethroids used for earworm control have been reported from the Mid-Atlantic region and states to our south. This insect has developed moderate to high levels of resistance to this class of insecticides, so growers need to consider other modes of action. If a pyrethroid (e.g., Asana, Bifenthrin, Hero, Mustang Maxx, Warrior) is used, the highest labeled rate timed for small to medium rather than large worms is recommended. Combination products such as Besiege that include a pyrethroid as well as a diamide can be useful to simultaneously manage multiple insect pests. Coragen and Intrepid are effective and are less harmful to natural enemies. Intrepid is an insect growth regulator so applications should target small larvae only. Spinosyns (e.g. Blackhawk, Radiant) and carbamates (Lannate) are also effective. It is important to rotate insecticide classes within a season. ALWAYS read pesticide labels carefully and follow all instructions; the information presented here does not substitute for label instructions.
Snap beans: A standard drop cloth can be used to detect small larvae as described above, and larval size is an important consideration for selecting spray materials. Treatments may be warranted when captures exceed 20 moths per night and local corn crops are mature. A 5-7 day spray interval may be necessary thereafter. Treatment recommendations are as described above (see lima beans).
Tomatoes: Eggs can be detected on the leaves directly below the flower clusters, typically on the highest clusters on the plant. For a reduced spray approach, inspect 20-30 plants for signs of eggs, and consider initiating sprays if ~10% of the plants have at least 1 egg, with subsequent sprays at 3 damaged fruit per 100 unripe. Given the number of insect pests (armyworms, hornworms, stinkbugs, etc.) that occur in tomatoes, a 7-10 day interval once fruit begins to set is often used for insect management. Pyrethroids offer poor to moderate control of corn earworm in the Mid-Atlantic, and will not control heavy infestations or large worms. In addition to the products mentioned above, several other effective insecticide options are labeled for tomato, including Avaunt, Proclaim, Rimon, and Exirel. It is important to rotate insecticide classes within a season. ALWAYS read pesticide labels carefully and follow all instructions; the information presented here does not substitute for label instructions.
Article By Alan Leslie , Dr. Kelly Hamby , Dr. Galen P. Dively II , Andrew Kness , Kelly Nichols , Emily Zobel , Maria Cramer, and Sarah Hirsh
