Corn Earworm Pheromone Trapping

Table 1. Most recent corn earworm pheromone trap captures in Maryland

Table 2. Recommended insecticide spray intervals for corn earworm in non-Bt sweet corn from green silk through just prior to harvest for moth activity measured using nylon mesh traps and pheromone lures

VEGETABLE CROPS

Sweet corn: Generally, an insecticide spray is applied at early silking as soon as the first corn earworm moth is captured on the farm, and applications are repeated at 2 to 6 day intervals based on moth pressure and corn hybrid (See Table 2). Transgenic hybrids expressing one or more insect active toxin from the Bacillus thuringiensis (Bt) bacterium provide 100% control of European corn borer. However, control of other caterpillar pests varies, and corn earworm has developed resistance to multiple Bt toxins including those expressed in Attribute® and Performance SeriesTM hybrids. While supplemental sprays may be needed in these hybrids, the timing is less critical and wider spray intervals can be used than for non-Bt sweet corn under the same insect pressure. Attribute® II sweet corn provides excellent control of all foliage feeding and ear invading worms, thus no insecticide sprays are required, except for secondary pests such as sap beetles. However, the absence of worm damage that attracts beetles significantly reduces the infestation risk of this pest. Stink bugs are not controlled by Bt.

Timely sprays of non-Bt hybrids help minimize ear damage by corn earworm and other ear invading insects. Moths are attracted to ears with fresh green silks, where most eggs are laid. The first spray should be timed for early silking, after which new fresh silks that are unprotected will emerge. 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. Thus, it is especially critical to maintain protection on silk tissue when both temperatures and moth pressure are high.

Other vegetables: Corn earworm is a major 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 (Kuhar et al. 2006). 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.

AGRONOMIC CROPS

Soybeans: Female corn earworm moths prefer to lay eggs in open-canopied, late-blooming fields, and are most attracted to soybeans for egg laying from flowering to early pod-set. Drought conditions can delay soybean maturity and prevent normal canopy growth, so peak moth activity is more likely to coincide with blooming of open-canopied fields. In irrigated fields, activity may be greater along pivot tracks and dry corners. Corn earworm larvae can damage flowers. However, because soybeans produce more flowers than needed, flowering sprays are rarely necessary. Podworms cause the most damage when large larvae are feeding on full seed pods with large seeds.

When scouting bean fields, pay special attention to those fields with a more open canopy in areas where the nearby maturing corn is no longer attractive to earworm moths. Sampling should start during mid-August and be repeated at least weekly in each field until a spray decision is made or the pods reach full maturity. Most fields are planted as narrow-row beans, so a 15-inch sweep net is the most practical way to sample for earworms. Walk along the rows, swinging the sweep net so that the opening passes through the foliage. The net is turned 180° after each sweep as you advance with each step to swing the net through the foliage in the opposite direction. Each stroke is counted as one sweep. A series of 25 sweeps should be taken at each of five sites for every 40 acres.

Treatment is recommended when counts exceed 3 medium to large podworms per 25 sweeps in narrow row fields, or 5 podworms per 25 sweeps in wide row fields (20 inches or greater). The timing strategy is to wait until most of the larvae are 3/8 inch or more in length, and then treat when pod damage is first evident. This allows for most egg laying and hatching to occur before treatment and reduces the chances of a second spray being needed later. These static thresholds are based on long-term averages for control costs and soybean prices. North Carolina State extension has developed a dynamic online threshold calculator for corn earworm in soybeans that takes into account the sampling method (uses a 15 sweep rather than a 25 sweep sample), row spacing, cost for control, and the value of soybeans, which can be found at:

https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html

Since the 2008 season, numerous reports of control failures with pyrethroids (Group 3A) 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., Brigade, Warrior, Mustang Maxx, Hero, Baythroid, Tombstone) is used, the highest labeled rate timed for small to medium, rather than large worms, is recommended. Alternative classes such as diamides (Group 28; e.g., Coragen, Prevathon), oxadiazines (Group 22A; e.g., Steward) and spinosyns (Group 5; e.g., Blackhawk, Radiant) will be most effective. These materials are also generally softer on beneficial insects which prey upon other late season soybean pests, such as soybean looper and stink bug. ALWAYS read and follow instruction on the pesticide label; the information presented here does not substitute for label instructions.

Sorghum

Headworms (corn earworm, fall armyworm, and sorghum webworm) are caterpillar pests that infest grain heads. Flowering or heading sorghum is attractive to corn earworm females for egg laying. Headworms feed on the flowers and developing kernels and large larvae can cause significant yield loss.

Scouting in sorghum fields should go from the end of flowering until hard dough. Sample heads by bending them into a clean white 5 gallon bucket and beating them to dislodge the headworms. Sample 10 heads per location and sample multiple locations per field. If most larvae are small (up to ¼ inch) sample the field again in 3 to 4 days. Thresholds vary by the size and species of larvae and sorghum value. In general, 2 corn earworm larvae per head would warrant treatment, and Texas A&M has developed a dynamic online threshold calculator that incorporates cost of control, grain value, anticipated yield (heads/acre), and larval size, which can be found at:

https://agrilife.org/extensionento/sorghum-headworm-calculator/

As mentioned above, pyrethroids (Group 3A) offer poor to moderate control of corn earworm in the Mid-Atlantic, and will not control heavy infestations or large worms. If a pyrethroid (e.g., Brigade, Warrior, Mustang Maxx) is used, the highest labeled rate is recommended. Alternative classes such as diamides (Group 28; e.g., Prevathon), spinosyns (Group 5; e.g., Blackhawk, Tracer), or carbamates (Group 1A; e.g., Sevin, Lannate) will be most effective. Selective insecticides that are less damaging to beneficials are recommended, such as Prevathon (most recommended) or Blackhawk. ALWAYS read and follow instruction on the pesticide label; the information presented here does not substitute for label instructions.

While scouting for headworms, growers are encouraged to look for sugarcane aphid in sorghum as well. If sugarcane aphids are also present, we strongly advise using selective insecticides to preserve the natural enemies that slow sugar cane aphid population growth.

Acknowledgements: Corn earworm trapping efforts in were supported by the Crop Protection and Pest Management Program [grant numbers 2017-70006-27171 and 2017-70006-27286] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.