Sweet Corn Insect Update

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu and David Owens, Extension Entomologist, owensd@udel.edu and

A whorl-stage sweet corn field on station had an economic infestation of fall armyworms, whorl thresholds are 15%. Be wary of fall armyworm in tassel-push corn, worms dislodged by the emerging tassel may go to the developing ear. Pyrethroids will not give complete worm control, scout fields soon after treatment. Other alternative mode of actions that are softer on beneficials include diamides (Coragen) methoxyfenozide (Intrepid), indoxacarb (Avaunt) and spinetoram (Radiant). Be sure to read the labels for use restrictions (indoxacarb cannot be used after tassel-push) and restrictions on the number of applications. A commonly used earworm product is Besiege which has chlorantraniliprole (Coragen) in it; earlier use of chlorantraniliprole may limit later use.

Corn earworm populations are higher than last week. Drier evening weather favors moth flight, and worms that developed in field corn are starting to emerge as adults. I expect moth flight activity to continue increasing state-wide until early-September. You may notice some trap locations that had been catching a lot of moths are now catching fewer; in some locations traps were adjacent to sweet corn that has since been harvested. However, other traps have been catching many more moths than they had been, especially in the Monday Laurel data. Blacklight trap captures are also increasing. Focus more on the state-wide trends. Monday trap capture can be found at (http://agdev.anr.udel.edu/trap/trap.php), and Monday trap captures were much higher from nearly all sites. As a reminder, what is reported on the website is on a per night basis, the table below is cumulative over Monday, Tuesday, and Wednesday night.

Trap Location BLT – CEW Pheromone CEW
3 nights total catch
Dover 0 7
Harrington 1 4
Milford 5 11
Rising Sun 7 5
Wyoming 7 25
Bridgeville 2 23
Concord 4 20
Georgetown 2 14
Greenwood 6
Laurel 0 78
Seaford 1 21

Guess the Pest! Week #15 Answer: Stink Bug

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations Chris Leon for correctly identifying the damage in the photo as stink bug damage and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #15 Answer: Stink Bug

The damage on the corn stalk is stink bug feeding injury. Stink bugs will use their piercing-sucking mouthparts to probe into the stalk of the plant, removing plant fluids. If the stink bug hits the ear at this stage, the ear will often fail to develop kernels at the feeding site. This causes the ear to develop into the classic “C”-shaped or boomerang-shaped ear. This is why the greatest damage and yield loss potential due to stink bug feeding is prior to pollination. This is also why waiting until after tasseling (pollination), to control a stink bug infestation in field corn is too late. Here is a link to last week’s article discussing stink bug management in field corn: http://extension.udel.edu/weeklycropupdate/?p=12194

Guess the Pest! Week #14 Answer: Corn Rootworm

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to Kathleen Heldreth for correctly identifying the damage in the photo as corn rootworm damage and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #14 Answer: Corn Rootworms

The corn plants in the photo are damaged by corn rootworm larvae. As you can see, the larvae feed on the roots and root tissue of the plants causing the plant roots to be “pruned”. Older larvae will tunnel into the roots leaving visible entrance holes and blackened root tips. Plants with excessive root pruning will usually lodge and in reaching for the sun, become “goosenecked”. Corn rootworm infestations are unusual for Delaware and not something we typically have to manage for. Crop rotation is the preferred method of control in regions with sporadic populations. Corn rootworm females prefer to lay eggs in corn fields in August and September. The eggs do not hatch until the following spring. If the field is rotated out of corn, the larvae will starve to death in the absence of a suitable host plant.

Vegetable Insect Update – July 6, 2018

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sweet Corn
by Bill Cissel and David Owens
Sweet corn trapping data is updated by Tuesday and Friday mornings and can be accessed here: http://agdev.anr.udel.edu/trap/trap.php. If there is one thing insects are really good at, it is making liars out of people. That said, I expect low earworm numbers until mid-July. Trap catches are as follows:

Trap Location BLT – CEW Pheromone CEW Corn spray schedule
3 nights total catch
Dover 3 0 4 day
Harrington 0 0 No spray
Milford 0 0 No spray
Rising Sun 1 0 6 day
Wyoming 1 0 6 day
Bridgeville 0 0 No spray
Concord 0 0 No spray
Georgetown 0 1 6 day – no spray
Greenwood 0 0 No spray
Laurel 0 6 4 day
Seaford 0 1 6 day – no spray

 Cucurbits
by David Owens
With the warm weather, it is not surprising that low levels of spider mites can be found in most fields. Check field edges and near wood lines. The action threshold we use as a benchmark is 1-2 mites per crown leaf, 20 – 30% of the crown leaves infested. During hot dry spells, try to limit mowing as much as possible, spider mites feeding on the grasses will be forced to look elsewhere for food once the plant they are on has been cut. I have also noticed a slight uptick in cucumber beetle activity.

Sweet Corn Insect Management

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sweet corn trapping data is updated by Tuesday and Friday mornings and can be accessed here: http://agdev.anr.udel.edu/trap/trap.php. As we suspected last week, trap captures have plummeted in part, I am sure, because we are trying to capture sizeable earworm moths for pyrethroid resistance monitoring. We typically experience a lull in moth activity until mid-July. Trap catches are as follows:

Trap Location BLT – CEW Pheromone CEW Corn spray schedule
(3 nights total catch)
Dover 1 3 5 day
Harrington 1 0 No spray
Milford 0 0 No spray
Rising Sun 0 15 4 day
Wyoming 0 1 6 day – No spray
Bridgeville 0 1 6 day – No spray
Concord 0 0 No spray
Georgetown 0 0 No spray
Greenwood 0 0 No spray
Laurel 1 1 6 day – No spray
Seaford 0 1 6 day – No spray

Sweet Corn Insect Management

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sweet corn trapping data is updated by Tuesday and Friday mornings and can be accessed here: http://agdev.anr.udel.edu/trap/trap.php . I expect earworm counts to start decreasing; you can look at our historical trapping data

Trap Location BLT – CEW (3 nights total catch) Pheromone CEW

(3 nights total catch)

Corn Spray Schedule
Dover 1 8 4 day
Harrington 0 4 4-5 day
Milford 0 3 5 day
Rising Sun 0 22 4 day
Wyoming 0 0 No spray
Bridgeville 0 12 4 day
Concord 0 3 5 day
Georgetown 0 0 No spray
Greenwood 0 0 No spray
Laurel 0 7 4 day
Seaford 0 0 No spray

The spray schedule listed in the table is based off our sweet corn action thresholds which can be found here: http://extension.udel.edu/ag/insect-management/insect-trapping-program/action-thresholds-for-silk-stage-sweet-corn/ .

Sulfur and Vegetable Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

With the recent heavy, leaching rains, we are seeing signs of sulfur deficiency in some vegetable crops. Sulfur is considered one of the secondary macronutrients that vegetable crops require for growth. Sulfur is a component of four amino acids and is therefore critical for protein formation. It is also a component of certain glycosides that give pungency to mustard family crops (greens, cole crops) and Allium crops (onions, garlic).

In the last 25 years, as industrial air pollution has been reduced (especially pollution from coal fired power plants) we have had less sulfur deposition from rainfall. Sulfur deficiencies are more common and sulfur additions in fertilizers or manures is being required for many crops to produce high yields.

Most of the sulfur in the upper part of the soil is held in organic matter. Upon mineralization, sulfur is found in the soil as the sulfate ion (SO42-) which has two negative charges. The sulfate ion is subject to leaching, especially in sandy textured soils (loamy sands, sandy loams). It does accumulate in the subsoil but may not be available for shallow rooted vegetables.

Sulfur can be added by using sulfate containing fertilizers such as ammonium sulfate, potassium sulfate, and K-mag (sulfate of potassium and magnesium). It is also a component of gypsum (calcium sulfate). In liquid solutions, ammonium thiosulfate is often used as the sulfur source. Sulfur is also found in manures and composts. For example, broiler litter has about 12-15 lbs of sulfur per ton.

In vegetable crops, sulfur removal is generally in the 10-20 lb/A range. Mustard family crops (cole crops such as cabbage and broccoli, mustard and turnip greens, radishes) remove between 30 and 40 lbs/A of sulfur. Research in our region has shown response to added sulfur for sweet corn and for watermelons. In Florida research it was shown that adding 25 pounds of sulfur per acre boosted yields by 1.7 tons per acre in tomatoes. Similar results were found with strawberries.

Our general recommendations are to apply 20-30 lbs of sulfur per acre on sandy soils for most vegetable crops. Remember to take credit for any sulfur being added with fertilizer sources such as ammonium sulfate (24% sulfur).

One vegetable where we want to limit sulfur is with sweet onions. Because sulfur increases onion pungency, and sweet onions are sold based on their low pungency, we limit sulfur applications to this crop.

Poor Stands and Plant Vigor in Early Planted Fresh Market Sweet Corn

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Growers are reporting issues with stands and vigor in sweet corn fields in 2018, especially in early planted fields. There can be many causes for stand loss and weak seedlings: surface compaction and crusting, birds, soil insects, slugs, cold soils that delay emergence, soil diseases affecting seeds or seedlings, wet soils, fertilizer injury, deep planting, and herbicide injury are just a few examples.

When checking sweet corn fields with vigor and stand problems, it is important to dig up seeds and affected plants and examine the seed remnants, roots, and mesocotyl (stem that pushes the seed leaf to emerge above the ground). Corn seedling survival and early vigor is directly tied to a healthy seed kernel and mesocotyl from planting through the six-leaf stage. Any damage to the seed or mesocotyl during this period can lead to stunted or weak seedlings, and in severe cases, seedling death. This is because the corn seedling depends on the seed for food to grow for several weeks after emergence until sufficient leaf area has been produced and nodal roots have become established. The seed kernel provides the means for early roots to grow and these food reserves are also mobilized and transported through the mesocotyl to grow the first stalk and leaf tissue. The mesocotyl also serves to transport water and mineral nutrients from the seedling roots.

Sweet corn is more susceptible to stand loss and poor vigor problems than field corn because the seed has less food reserves. Shrunken types (supersweet, sugary enhanced, augmented shrunken, synergistic varieties) have even less stored food than “normal” types and therefore are more susceptible to stand problems.

I have looked at sweet corn fields with stand loss and vigor problems (uneven growth) over the years. Often, when digging up the seedlings and examining the seed remnants and mesocotyls, the kernels will be disintegrated and there will be darkening at the mesocotyl attachment. This means that the seeds deteriorated prematurely and the full content of the food reserves in the seed were not available for seedling development, leading to the stand and vigor issues. Premature seed deterioration and/or poor vigor seedlings can be due to diseases that cause seed rots, seedling blights and/or root rots. Soil insects can cause seed deterioration by feeding on seed contents or creating entrance wounds for disease organisms. In addition, certain soil insects and slugs can feed on the mesocotyl causing seedlings to collapse. Sweet corn that takes more than 10 days to emerge is at great risk of injury due to insects and diseases as seed treatments dissipate.

Cold stress and cold soils are common stress factors leading to poor stands. Often growers are pushing the limits and are planting sweet corn very early. In 2018 we had a cool April which further stressed early sweet corn. While field corn will start to germinate at 50°F, many types of sweet corn need much warmer soils. This is especially true of supersweet varieties and other shrunken types, which perform best at higher soil temperatures (above 60°F). When soil temperatures are below 55°F, germination is greatly extended. Food nutrients are mobilized in the seed but are not being utilized rapidly by the plant. The seed then becomes a perfect food source for many soil microorganisms. On a positive note, many of the newer sweet corn varieties have much more cold tolerance and emerge more rapidly in cold soils.

Stand issues are often related to the inherent poor vigor of sweet corn. Work with seed suppliers to obtain their best lots for early plantings with the largest seed sizes. Obtain varieties that perform better under cold stress. When possible, obtain reports from early planted sweet corn trials to assess which varieties are the most cold tolerant. Request seed treatment information and select treatments with the best protection potential for early plantings. There are in-furrow fungicide options; however, research is limited with sweet corn in our region.

Growers often face the decision on whether or not to keep plantings with poor stands. This is most often a marketing decision based on the need for and value of early sweet corn for that farm. An estimate of potential marketable ears will be based on stand counts of full vigor plants from 20-40 sites throughout the field. This stand count information then can be used to estimate the value of the field as is versus the value of a later planted full stand crop.

Guess the Pest! Week #5 Answer: Cutworm

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to John Comegys for correctly identifying damage as cutworm and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Cutworm
By David Owens, Extension Entomologist

Corn planting is at full speed, and early season insect pest pressure is not far behind. The above photo is an example of small larval cutworm feeding injury to field corn. Small larvae cannot cut plants, they will chew holes through the whorl that, as the leaves unfold, leave a characteristic, symmetrical shot hole pattern. Cutworms can be a significant corn pest. Older larvae can bore through larger plants causing dead heart and cutting plants off at the ground level.

In general, there has been a trend towards decreasing cutworm activity, though not as strong as the trend seen with European corn borer. How can we manage cutworm, and what do the holes mean? First, many of the Bt varieties have suppressive efficacy on early instar cutworms. Please reference the Handy Bt trait table here: https://lubbock.tamu.edu/files/2018/01/BtTraitTableJan2018.pdf. There are traits that do not have cutworm efficacy, so know what you have! DoublePro, TriplePro, and Yieldgard corn do not have traits that are effective on cutworm. If you see this type of injury in fields planted without an effective trait, pay special attention to the field and be ready to treat it with a pyrethroid if you see 5% damaged plants and you find larvae. But what about those fields that do have one of the traits that does have suppression activity (Cry1F or Vip3A)? The Bt traits are stomach active toxins, and specific to target insects or groups, meaning they need to be ingested and there is specificity. Rootworm traits only work on rootworms, not on wireworms, caterpillars, or grubs. Larvae may feed on plants for short period of time before dying. If you see this type of injury in a field with Cry1F and Vip3A corn, monitor, preferably within a few days. It also means that traits are most effective on small larvae that require less of a dose. We are seeing large dingy and black cutworms out right now in some cover crops. These will be done with their life cycle by the time the field is planted and seed germinating. However, cover crops are attractive to ovipositing moths, so if a field is planted soon after burndown or into a green cover, pay attention to stand. A few holes in leaves will alert you to a population, but if that is all you see in consecutive field visits, the traits have probably done their work. My colleagues in western states will caution that if initial populations are REALLY high, the traits may need to be supplemented with additional control. It also pays to scout a field. There are seedling corn pests that are not managed by any Bt trait. If you see feeding injury, try to find the culprit as best as possible.

Supplemental control can be in the form of a non-neonic seed treatment. Right now, there is chlorantraniliprole, trade name Lumivia (same active ingredient as Prevathon and Coragen). It is rated by Auburn and University of Tennessee entomologists Dr. Kathy Flanders and Dr. Scott Steward as having good efficacy for cutworm. Neonics do not. You can find their efficacy charts here: http://blogs.ext.vt.edu/ag-pest-advisory/files/2014/10/Field-corn-insecticide-seed-treatment-chart.pdf and here: http://www.utcrops.com/cotton/cotton_insects/pubs/PB1768-Corn.pdf.

Additional control can be achieved with a pyrethroid. What scenarios are at greater risk for cutworm? LATE planted fields that have living weeds/cover within a week of or two of planting. Pay special attention to these fields and to fields planted green. As we wrote recently, there is a cost/benefit trade off with high seed treatment rates and a pyrethroid in furrow/over the soil. The closer you are to planting, the more likely a pyrethroid at burndown will hit an insect pest. However, if you are not planting right away, pyrethroids break down over time. Should another seedling pest move in, the original spray will not be effective. If your concern is soil pests such as wireworm, put the pyrethroid in the furrow. The pyrethroid should still have efficacy on cutworm.

Bird Repellent for Corn Seed

David Owens, Extension Entomologist, owensd@udel.edu

Some folks have recently asked about products to discourage birds from feeding on field and sweet corn seed. Avipel (9,10-Antraquinone), manufactured by Arkion Life Sciences LLC has a 24(c) special local needs label for use on corn seed in Delaware. Approved Delaware formulations are dry hopper box treatment and a liquid formulation for treating seed ONLY in commercial seed treatment equipment. It should also be used with a dye and cannot be used for other purposes than planting – standard language for seed treatments. Both registrations expire July 1, 2022. Registration was originally requested because of incidences of severe bird feeding damage in fields east of Route 1. Large flocks of blackbirds can quickly pull up a stand as they migrate north. You might want to consider this if you have had significant bird damage in the past and are seeing large numbers of blackbirds in your field. Should you use Avipel or have it applied to your seed using seed treatment equipment, you MUST have a copy of the 24(c) label in your possession when handling and planting. As always, consult the label for additional application guidance. The labels for Delaware can be found here: http://arkionls.com/av/states/delaware.html. Maryland and Virginia ONLY have the hopper box treatment approved, labels can be found here: http://www.arkionls.com/av/states/maryland.html, and http://www.arkionls.com/av/states/virginia.html.