1999 - Fresh Market Sweet Corn IPM
Scouting Procedures

Editors: C. Petzoldt and M. Hoffmann

Contributors: R. Bellinder, N. Call, H. Dillard, D. Gilrein, M. Hoffmann, J. Grant, J. Mishanec, D. Moyer, C. Petzoldt, D. Riggs, E. Rutkowski, A. Seaman, R. Straub, F. Wiles

IPM Bulletin No. 111FM

A.General Scouting Information

It is necessary to scout sweet corn fields at least once per week. Fields should be scouted in a systematic manner that places sample sites throughout most of the field. A V-shaped sampling pattern has been found to be practical for many sweet corn fields. This pattern ensures that the scout will visit both border and central areas of the field and minimizes the number of sweet corn rows that must be crossed. Each time the field is inspected make sure to initiate the v-pattern from a different point. Sampling sites should be chosen without bias, except in those cases where specific sites may be designated. Remember, you are attempting to sample representative sections of the whole field.

Although the scouting procedures outlined here are rather rigidly structured, remain alert to possible pest problems that may not be detected by the systematic sampling plan and are noted as you walk from the field.

Spray Record:Always check the grower's spray record before entering the field. It is important to check whether or not a pesticide has been applied recently, and if it is safe to enter the field.

B.The Scouting Unit

Fresh market sweet corn presents some unique challenges when choosing a unit to scout. Growers frequently plant small acreages of sweet corn on an almost daily basis to ensure a constant supply for market. The amount planted on any one day is often smaller than what can be economically scouted. Therefore plantings will likely need to be grouped for scouting purposes.

When grouping plantings for scouting the most important characteristics to keep in mind are crop phenology and variety. A scouting unit should be grouped as uniformly as possible for these two characteristics. Phenology may vary with weather conditions but a rule of thumb for grouping is not to group plantings with more than one week difference in planting date. If varieties are to be grouped, it is desirable to group those varieties with similar susceptibilities to corn pests. In general the goal should be to have any scouting unit be as uniform as possible for variety and phenology.

C. Corn Plant Growth Stages

The few studies on corn plant growth stages have been done on field corn. The following is a general scheme which can be used for sweet corn:

Seedling - up to 3 leaves.

Early to Mid-Whorl - 4 to 10 leaves; emerging tassel not yet able to be seen or felt within the whorl.

Late-Whorl - 11 to 14 leaves; tassel, which is still rolled up in youngest leaves, can be felt within the whorl.

Early Tassel - tassel can be seen emerging from top of plant, but has not completely unfolded.

Tassel - tassel has unfolded and pollen-shed has begun.

Silk - beginning silk to dry silk.

Knowledge of these growth stages is important to the proper timing of scouting procedures and treatments.

D. Insect Characteristics

Many insect species have been recorded as corn pests in the US., but specifically in the Northeast, there are five which cause primary or direct damage to sweet corn: European corn borer (ECB), fall armyworm (FAW), corn earworm (CEW), corn flea beetle (CFB), and to a lesser degree the corn leaf aphid (CLA).

The major sweet corn insects can be categorized according to residency (indigent or migratory) and/or feeding habit and damage (seedling, foliage feeder, or silk/ear feeder):

Insects that over winter here (indigent) are predictable to a high degree, but the seasonal occurrence of migratory species is difficult to forecast because their northward movement is dependent upon the weather conditions to the south of New York. Several of these species will infest silks and ears and must be controlled, but whorl feeders in some instances must also be controlled.

E. Insect Identification

The major insects encountered in New York, as well as other corn insects that affect corn here are described in the Fact Sheets. Larval stages are often difficult to distinguish - particularly the younger instars. The following descriptions and illustrations are provided to aid in field identification:

Eggs

1. ECB - Laid in masses of 15 to 35 on the undersides of leaves of early season corn (may be found on upper leaves, ears or flag leaves of late season corn). Eggs overlap each other much like fish scales. Eggs hatch in 5 to 6 days.

2. CEW - Almost all laid on silks. The yellowish, hemispherical eggs are deposited singly, although several may be laid on the silks of one ear. No other species lays eggs in this fashion.

3. FAW - Spherical eggs are laid in clusters or masses of 100 or more eggs, at random locations on the plant. Egg masses are covered with body scales and hairs, giving them a fuzzy, gray to green appearance

Larvae

The following guide should help you distinguish between larvae of the European corn borer, corn earworm and fall armyworm. Use this information along with the type of feeding damage observed. Clearness of characters will vary with age of worm.

Fig. 1. Head of Fall armyworm

Fig. 2. Microspines on Corn Earworm

Fig. 3. Hooks on prolegs of European corn borer.

F. Insect Scouting Procedures

1. European Corn Borer

2. Fall Armyworm

3. Corn Earworm

European corn borer (ECB):

Recent research has shown that there is generally no benefit to be gained from treating corn for worm pests at whorl stage or earlier. Disease pests and other insect pests may justify scouting sweet corn earlier than tassel.

Scouting for European corn borer should be done weekly, or twice a week when daytime temperatures are consistently above 80°F, starting shortly before tassels appear and continue until about 5 days before harvest. Different sampling techniques are required depending on the growth stage of the corn.

Early tassel and tassel: Use the sampling procedures described below. Pay particular attention to the tassel as it emerges because larvae of different ages may be present here. Frass or newly broken tassels (flags) indicate the presence of larvae. For the second brood of ECB it is important to begin sampling fields at first tassel emergence.

Silk stage: Silk stage to harvest is a critical time for scouting fresh market sweet corn. Larvae of European corn borer may infest the silk and cause damage to the ear.

Using the sequential sampling procedure described below, inspect the ear and silk for the presence of larvae, frass or damage. Check between the stalk and ear. One method for inspection of silks is to carefully tease apart the silks down to the tip of the ear, looking closely for any discoloration or other evidence of infestation. When finished, squeeze the silks back into their original position. This will not harm the ear or reduce pollination. Scouting silk stage corn will take longer than the whorl or tassel procedures. In most fields, 2 to 3 visits for inspection of silks will be required before harvest. If any larvae or infestation is found, count that plant as infested.

An alternative method for scouting silk stage corn for European corn borer is to look for egg masses on corn plants using the sequential sampling method described below. A corn plant infested with one or more egg masses should count as one infested plant in the sequential sampling plan.

Fall armyworm and corn earworm: Fall armyworm and corn earworm are not always present in all areas of New York. Pheromone traps or black light traps can be useful guides to determine if fall armyworm or corn earworm are present. If information from traps is available and indicates that either of these pests are present, fields should be scouted in a manner similar to that used for European corn borer. It is very important to carefully examine the silks and tip of the ear for corn earworm infestation. If trapping information is not available then scouting for these pests should begin approximately July 15th.

Researchers at the Univ. of Mass. have developed a method to use CEW pheromone trap catches as a guide to making treatment decisions. This technique is now being evaluated in New York. It has not been thoroughly tested. It is provided here as a possible optional method for making treatment decisions. When selecting a scouting method and decision-making procedure for CEW, consider the potential for infestation based on past experience, level of control expected with the available spray equipment and tolerance for crop damage.

Action thresholds based on larval infestation:

European corn borer, corn earworm and fall armyworm thresholds are based on plant infestation. The thresholds for the pests are the same and when all are present they are additive.

European corn borer, corn earworm and fall armyworm thresholds

Action thresholds based on CEW trap catches - Silk Stage

The University of Massachusetts technique for using CEW trap catches as action thresholds is currently being evaluated in New York. It should only be used as a guideline until testing is complete. (See fact sheet: "Managing Sweet Corn Pests in Massachusetts" for full details). Requires use of Scentry Heliothis trap and Hercon CEW pheromone lure placed with the base of the trap at silk height in fresh silking corn. Check traps at least once per week until first CEW moth is caught, and thereafter, check traps 2-3 times each week, especially after storms.

Average CEW Pheromone Trap Catches During Silk Stage

Spray intervals should be lengthened by a day if daily maximum temperatures were less than 80°F for the previous 2-3 days.

* Spray fresh silk once, then wait until trap is checked again before determining further action. Sprays for CEW should be discontinued in fields when silk turns completely dry and brown or 5-7 days prior to final harvest.

It is important to maintain records of which species are present since choice of insecticide may vary according to pest incidence. When the treatment threshold is reached, foliar applications of insecticides (high-gallonage targeted to the ear zone preferable) should commence and be repeated as needed.

Sampling procedures:

Sampling frequency: At a minimum, sample fields at least once per week, twice a week when daytime temperatures are consistently above 80°F. During flights of fall armyworm it is also advisable to sample twice per week.

Field size and sampling: Use the sampling protocol in fields of 10 acres or less. If fields are larger, divide them into 10 acre blocks and sample each separately. Smaller units of differing planting dates can be treated as sampling blocks if the planting dates are spread over no more than one week.

To sample fresh market sweet corn requires that a representative sample be taken from the field. This can be accomplished by walking the field in a "V" or "U" pattern. At several sites stop and sample five consecutive plants. During the "pre-silk" stages sample each consecutive plant. During the "silk to harvest stage", only sample plants that have an ear in the silk stage or more mature. Avoid barren plants and plants with unharvestable ears (nubbins). The total number of sites that need to be sampled to reach a decision will be determined as you sample the field. Because a decision may sometimes be made by sampling as few as five sites, it is important that these encompass about 50 percent of the field. For example, sample five sites on the first leg of the "V".

Using Table 1: For each site record the number of infested or damaged plants (by pest) in the "damage or infestation" column. A minimum of 5 sites (25 plants) must be sampled before a decision can be made. At each increment of 5 plants total the number of damaged plants across the three pest columns and enter it in the "Running Total" column. Add damage recorded in each set of 5 plants to that previously recorded to give a constant running total. Whether or not a plant is infested with one or several worms, it is recorded as one infested plant.

Decisions: If the running total is greater than (>) the number in the "Treat" column an insecticide application is warranted. If the number is less than or equal to (²) the value in the "No treat" column, an application is not warranted. You will have to sample more than 25 plants before a "No treat" decision can be made. Keep sampling until a decision is made or you take all 105 samples. A decision is forced at the maximum sample size.

This sampling procedure is relatively fast, especially when infestations are very low or high. For example, small fields can be sampled in less than 10 minutes.

Frequency of sprays and temperature: If daytime temperatures are consistently above 80°F, European corn borer eggs will hatch in four days or less. Under these conditions of high temperatures, and if sampling indicates that the field is over threshold, treatments should be applied at least twice per week.

Table 1. Field form for classifying worm infestations in fresh market sweet corn using sequential sampling.

a Accumulate the number of damaged plants at each increment of five plants.

For example, if there was one infested plant in the first set of 5 plants, two in the second and one in the third, then the running total would equal four after 15 plants had been examined.

Damage is additive (European corn borer + fall armyworm + corn earworm).

This table is based on action thresholds of 15% for pre-silk stages and 5% for the silk stage through harvest.

ECB: European corn borer

FAW: Fall armyworm

CEW: Corn earworm

nd: No decision

≤: Less than or equal to

>: Greater than

4) Flea beetles and Stewart's wilt

Flea beetles are the vectors of Stewart's wilt disease and varieties having some resistance or tolerance to this disease should be used whenever possible. A list of resistant varieties is available on the CENET computer system or from your Cooperative Extension agent. If varieties not resistant to Stewart's wilt are used, fields should be sampled for flea beetles and treatment(s) with insecticide considered. When flea beetle populations are very high, the use of systemic insecticides and/or repeated foliar applications of insecticide for control of flea beetle infestations are probably not cost effective. The decision to treat or not treat is subjective and should consider the susceptibility of the variety and past incidence of Stewart's wilt in the area and the field. Fields with Stewart's wilt problems the previous year should be planted to resistant varieties or avoided if possible.

Until an improved method is developed, the sampling protocol and action threshold recommended by New Jersey will be used. Sampling for flea beetles should take place from plant emergence through mid whorl, the most susceptible stages of sweet corn. Count the number of flea beetles present on 10 plants at 10 sites.

Action threshold for flea beetles on varieties susceptible to Stewart’s Wilt:6 flea beetles on 100 plants

5) Slugs:

Slugs can be a problem in some upstate sweet corn fields. Although no action threshold is known, their presence should be noted during the time of plant emergence. Note their presence when you are making the first weed map of the season (Section H).

6) Sap beetles and corn leaf aphids:

Although these are generally not serious pests in New York sweet corn, they can be problems under certain conditions. As the scout is walking through the field note any heavy infestations of these insects. Corn leaf aphid can become a problem for shipping corn if pyrethroids are used exclusively in the insecticide program. This insect (and the sooty mold fungus which grows on honeydew secreted by the aphid), if not controlled in the early stages, can reduce quality in fresh market sweet corn. Aphids can also infest husk leaves and contaminate the ear. Sap beetles have been observed as a problem occasionally in the Hudson Valley. Generally corn leaf aphids are not a serious problem if colonies are confined to leaves. Aphid colonies in tassels and silks can require treatment. If corn leaf aphids are present, inspect 100 plants and record the number of plants with colonies of at least 50 aphids.

Treatment Guideline for Corn Leaf Aphid: If 50% of plants have a colony with 50 or more aphids treatment may be necessary.

7) Corn rootworm: Although the western corn rootworm is considered a key pest of field corn in New York, it is typically not a problem for sweet corn producers. Recently however, there have been reports of large numbers of adult western corn rootworm in sweet corn, feeding on tassels and silks. Generally, feeding by adult western corn rootworms will not result in yield reductions. According to recommendations used in field corn, a yield loss may occur when tassels are shedding pollen and green silks are continuously being clipped back to 1/2 inch or less. If there are less than 10 beetles per plant it is unlikely that a yield loss will occur. There have also been reports of damage to the roots of processing sweet corn, resulting in lodging. This type of damage is caused by the larval stage and could be more significant than silk feeding damage.

Rootworm life cycle: Adult western corn rootworms emerge from the soil about midsummer and migrate to corn where they feed on pollen and silks. Later maturing corn fields can attract large numbers of adults since neighboring and more mature corn may have stopped producing pollen. The females lay eggs in the soil near the base of the corn plant. They continue laying eggs until killed by frosts. The eggs overwinter in the soil and hatch the following year during May. The larvae that hatch from these eggs then feed on corn roots, mature, and emerge as adults in mid-summer, starting the cycle over again. Extensive feeding by the larval stage on corn roots can result in yield losses and lodging of corn stalks. This is well documented for field corn, but has not previously been considered a problem in sweet corn.

Management: Rotating away from corn, and the overwintered rootworm eggs, is a very effective method of control. In most cases, rootworm larvae cannot complete their development on crops other than corn. Therefore, larvae emerging in the spring do not have a satisfactory food supply and starve to death. In comparison, continuous corn can result in the buildup of rootworm infestations. Insecticides, applied at planting, will also provide control. In field corn, the decision to use an insecticide at planting is based on counts of adult rootworms taken the previous year. It is recommended that 100 plants be sampled (avoid field edges) for adult corn rootworms. This should be done late in the summer. If 100 or more beetles are recorded, indicating that large numbers of eggs are being laid, then corn should not be planted in that field next year, or if rotation is not possible, to treat with an insecticide at planting the next year.

Many factors can influence the severity of western corn rootworm infestations, including, frequency of sprays for worm pests, soil type, and date of sweet corn harvest the previous season. Given what has been observed recently, growers should, at a minimum, inspect their mid- and late-planted sweet corn fields for adult western corn rootworms. Those fields with large numbers of rootworms may be at risk next year if planted again to corn.

G. Sampling for diseases:

NOTE: The following disease recommendations were devloped for processing sweet corn. For fresh market corn growers may want to make appropriate adjustments.

1) Common Rust:

Sampling for common rust should begin at the early whorl stage and continue through to the tassel stage. After tassel, any actions taken to control this disease will not benefit yield, therefore scouting for this disease is not necessary at that time. Inspect 100 plants for common rust.

a) For each plant record the number of leaves with any common rust pustules.

b) Make a 10 plant sample of the field and count the number of leaves on each plant. Calculate an average number of leaves per plant for the field.

c) Calculate the percent leaves infected and report this number to the grower.

% leaves infected = total # leaves with rust on plants inspected x 100

Avg. leaves per plant x # plants inspected

Action Threshold: 80% leaves infected for all varieties which are equal to 'Jubilee' in susceptibility to rust. 'Jubilee' is moderately susceptible.

NOTE: This threshold is not valid for highly susceptible varieties such as Silver Queen, Sweet Sue, or Florida Staysweet. Further research is underway on highly susceptible varieties. This threshold was designed for processing sweet corn. Fresh market growers may need additional sprays for cosmetic quality.

2) Northern Corn Leaf Blight:

Northern Corn Leaf blight can be a problem in some areas of the state. The sampling procedure for this disease is identical to that for common rust. Report the results as percent leaves infected. There is currently no threshold known for making control decisions.

3) Stewart's Wilt (see Flea Beetle and Stewart's wilt Section F.4.)

H. Sampling for weeds:

Two weed maps should be prepared during the season. Timing should be as follows:

1. Early - within 2 weeks after planting. Purpose: to evaluate the success of the current season program.

2. Approximately at harvest. Purpose: to evaluate the next crop's weed control needs.

Preparation of a weed map:

Weeds or weed species may not be evenly distributed over a field. Where localized areas of severe infestations are found or atypical conditions exist (poorly drained area, high spots, field edges), they may be recorded on a weed map. This shows growers where problem areas exist and monitors their movement and changes over the years. Areas of severe infestations can be targeted for specific control practices, rather than treating the larger area needlessly or failing to control problems at all.

The scout should first obtain copies of the crop field maps from the growers or make a rough sketch, including landmarks, boundaries, crop row direction, compass, roads, a numbering or naming system consistent with the grower's, planting date, and any other important details. Then the following information should be indicated on the map:

--Weed species, or if this is unknown, at least some effort should be made to distinguish annuals from perennials, and broadleaf species from grasses and from yellow nutsedge. Common names can be abbreviated, e. g. YNS for yellow nutsedge, LCG for large crab grass, etc. Be sure to make a key to the map, explaining it.

--Abundance of each species estimated according to the following system:

0 = None

1 = Scattered, few weeds

2 = Slight; 1 weed /6 row feet

3 = Moderate: 1 weed/3 row feet

4 = Severe; > 1 weed/3 row feet

--Distribution of weeds in the field is important and can be rated as follows:

SPOTTY - found in a few places around the field

LOCAL - found in a small portion of the field

GENERAL - found throughout the field

Distribution can be indicated on the grower report form and specific areas of severe infestation drawn on the weed map.

--Weed size

The following size ratings can be used, but it is important that the grower understands their meaning:

WHITE SPROUTS - seeds are just germinating or emerging.

TINY - weeds show only cotyledons or first true leaf

SMALL - weeds less than 1" tall or less than the diameter of a quarter.

LARGE - weeds more than 1" tall or more than the diameter of a quarter.

I. Sweet Corn Fact Sheets

The following sweet corn pest fact sheets are available from Media Services Resource Center, Cornell University, 7 Business and Technology Park, Ithaca, NY 14850; phone (607) 255-2080:

Reference:

Ferro, D. N. and D. C. Weber. 1990. Managing Sweet Corn Pests in Massachusetts. Sweet Corn Integrated Pest Managment Program, Department of Entomology, University of Massachusetts at Amherst. Publication AG-335:8/88-2M.

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