IPM Events Calendar

1999 - Processing Sweet Corn IPM

Scouting Procedures

Editor: C. Petzoldt

Contributors: R. Bellinder, N. Call, H. Dillard, J. Knodel, M. Hoffmann, L. Pedersen, C. Petzoldt, A. Shelton

IPM Bulletin No. 111PR

A. General Scouting Information

It is necessary to scout sweet corn fields at least once per week during critical stages of development. Fields should be scouted in a systematic manner that places sample sites throughout most of the field. A U-shaped or staircase shaped sampling pattern has been found to be most practical for 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 pattern from a different point. Sampling sites should be chosen without bias, except in those cases where specific sites may be designated. Most insects and diseases will require inspecting between 40 and 120 plants 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 note them as you walk the field. Keep your eyes on the crop at all times!

Spray Record: Always check the grower/processor'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. Make sure your growers keep these records up to date.

B. 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.

C. Insect Characteristics

Many insect species have been recorded as corn pests in the U.S. 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 (indigenous or migratory) and/or feeding habit and damage (seedling, foliage feeder, or silk/ear feeder):

	Vectors Stewart’s Wilt	Native	Migratory	Seedling problem	Whorl or foliar feeder	Silk/ear damage
ECB		+			+	+
FAW			+		+	+
CEW			+			+
CLA		+			+	+
CFB	+	+		+

The time of occurrence of insects that over winter here (indigenous) is very predictable, but the occurrence of migratory species is difficult to forecast because their northward movement depends on the movement of weather fronts south of New York. Migratory and indigenous species must often be controlled.

D. Insect Identification

The major insect pests encountered on sweet corn in New York, as well as other insects that attack sweet corn are described in detail 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-late season corn (occasionally found on the underside of 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 - Laid only 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.

Character	Corn Earworm	Fall armyworm	European corn borer
Body Color	Variable; green, brown, pink, stripes apparent but do not extend the length of the body	Variable; green, brown, pink, stripes apparent; extend entire length of body	Pale, whitish to pinkish series of brown spots, no distinct stripes
Head Color	Light brown, no distinct markings	Sides dark, center with light brown triangle, inverted whitish "Y" (see figure 1)	Dark Brown to black
Body Shape	Plump	Plump	Slender
Skin Surface	Small larvae with black bumps, short spines (whiskers) all over body (see figure 2)	Relatively smooth, no short spines	Relatively smooth
Fleshy Prolegs (soft legs on rear half of insect)	Half circle of hooks on bottom of "feet"	Half circle of hooks on bottom of "feet"	Almost full circle of hooks (see figure 3)

Fig. 1. Head of Fall armyworm

Fig. 2. Microspines on Corn Earworm

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

E. Insect Scouting Procedures

There are several tools that an IPM scout can make use of to monitor sweet corn insect pests. Among the most important are field sampling or scouting and using pheromone traps to monitor populations of adult male moths. These tools should be viewed as complementary rather than interchangeable. A scout should not make the choice to use one or the other. Efficient sweet corn monitoring will result from utilizing trap catches to better time when to make scouting visits. The information gained from both scouting and trap monitoring will be essential and are the basis for pest management decisions. See Appendix II for a flow charts representing the key elements of monitoring for European corn borer, fall armyworm, and corn earworm.

Role of Pheromone Traps in IPM

Pheromone trapping systems for ECB, CEW, and FAW indicate the flight activities of adult male moths. The male moth is attracted to the synthetic lure which imitates the female’s sex pheromone of that particular species. There are numerous advantages in using pheromone traps for pest monitoring. Pheromone traps are easy to maintain, relatively inexpensive, and specific to the insect of interest. Trap catch provides a picture of local pest pressures from field to field. Trap catch can also serve as a sensitive early warning system of migratory moths, and as a relative indication of larval infestations in the field. This information helps determine when to initiate field scouting and/or feasible control tactics. With the use of degree day models which describe insect development, increases in pheromone trap catch ("peaks") can also be used to initiate forecasts for the different pest life stages in a field. Thus, effective control decisions can be timed for the "most" susceptible life stage – early larval instars and critical ear development stages based on pheromone trap catch and degree day accumulation. Additional information on the operation of pheromone traps can be found in the Appendix I of this document.

Where to Locate Traps

Pheromone traps should be placed near the grassy edges of sweet corn fields and out of the way of equipment. Traps should also be spaced at least 40 m apart to avoid any possible pheromone interference, and placed on windward side of field so pheromone is blown into field. Wooded edges are not good trap sites because the pheromone will not dissipate over the field. All critical trap openings should be free of weeds or any other obstruction which could prevent moths from easily entering the trap.

When to Check Traps

Traps should be serviced as often as possible and on the same day of week, such as every Monday. Frequent checking will prevent trap overloading and will facilitate identification of trapped specimens. It is suggested that traps be checked at least weekly, preferably twice a week. When moth numbers begin to increase, more frequent checking at least 2-3 times a week can be desirable. For example, eggs of Corn earworms can hatch in only 48 hrs. if the temperatures are high, traps would then need to be checked 3 times a week for effective monitoring and management.

Insect Pests and their Pheromone Trapping Protocol

A minimum of one trap per field (preferably 2) is recommended for monitoring the Lepidopterous insect pest complex of processing sweet corn. The migratory pests, CEW and FAW, usually arrive around mid-late July depending on the region of New York and weather patterns, and can heavily infest mid-late planted sweet corn. However, the endemic ECB occurs during the entire growing season (late May through September) with three different races having three or more flights annually in New York State. The three races of ECB include an univoltine Z-race, bivoltine Z-race and bivoltine E-race. Although not all races of ECB are present in all sweet corn growing regions, one trap for each race (Z- and E-race) is recommended at each field. The univoltine population has only one generation a year, and the peak flight usually occurs around mid-July depending upon temperature. The bivoltine populations have two generations a year, and the two peak flights occur during mid-June and mid-August depending on temperature. For the bivoltine populations, a possible third peak flight can occur during mid-September if higher than average heat units have accumulated. The trap and lure recommendations and critical growth stage(s) are summarized in Table 1. The critical growth stage is the preferred stages of crop development for pest infestation.

How to Record Pheromone Trap Results

A form for recording pheromone trap catch is included in the back. Information that is important to understand "trap catch" include check date, number of moths trapped, if lure was replaced, and crop growth stage (Section B). For graphing insect flight patterns, the number of moths captured per day can be easily calculated by dividing total number of moths captured by the number of trapping days, and plotted over time (check dates). Increases in trap catches (peak flights) can be easily seen when graphed. Please remember that pheromone traps measure the activity of adult male moths – not adult female egg laying!

Table 1. Pheromone trap and lure recommendations for monitoring insect pests of processing sweet corn.
Insect Pest	Trap Design	Lure	Lure Field Life	Corn Growth Stage
European corn borer (E-race & Z-race)	Scentry™ Heliothis net	Trécé™ rubber septum	2-3 weeks	Mid-whorl to silking
Corn earworm	Scentry™ Heliothis net	Hercon™ laminated luretape	2 weeks	Green silks to silking
Fall armyworm	IPS™ Universal Moth trap (Unitrap)	Scentry™ Four-component rubber septum	3 weeks	Late whorl to silking

1) European Corn Borer Trapping and Egg Sampling

European corn borer should be monitored using the pheromone trap specifications and protocol described in Section E-1 and Table 1. Since ECB are active from mid-May through September, traps should be placed in fields before the late whorl stage. The Scentry™ Heliothis net trap needs to be suspended from a 6-7 foot fence post using the three tie downs on the back side of the trap and a tent stake. The lure is suspended on a wire across the bottom opening of trap and held in place with a binder clip. Please refer to the factsheets on ECB for identification of adult male moths.

The time periods critical for the management of ECB are defined by the simultaneous occurrence of two events:

1) the sweet corn field in question is in late whorl/tassel/silk stage

AND

2) pheromone trap catches of either ECB race (E- or Z-race)are increasing.

Observing this guideline will result in scouting most sweet corn fields between 1 and 5 times. If pheromone trap catch is increasing during a susceptible crop growth stage, it is necessary to begin field scouting for ECB. The following procedure should be used for making a control decision based on economic thresholds in the field.

Monitoring ECB Egg Masses - The 20 Minute Scouting Plan:

Research and experience indicate that egg masses are the best life stage to sample for ECB pest management decisions. Correlations between egg mass counts and eventual ear damage have been noted in the field; in contrast, research has not been documented a good correlation between larval counts and eventual ear damage. Since treatments can also be timed to coincide with egg hatch, control can be expected to be more effective.

Sample 40 plants (5 plants at each of 8 sites) for the presence of ECB egg masses. Inspect all parts of each sampled plant. Count and record the number of egg masses found. The eight sites should be chosen based on the following guidelines: a) no 2 sites should be chosen in the same row, b) the area of the field the sites are chosen from should represent at least 25% of the entire field, c) half of the sites should be in areas close to the border of the field and half in the interior, and d) different areas of the field should be scouted on each visit. A U-shape or "stair step" pattern may be the most efficient way to walk through the field to minimize the number of times a scout needs to cross rows.

Action Thresholds:

When 2 or more egg masses in "black head" stage are found in forty plants, a treatment should be applied 2 days later.
When 2 or more egg masses without the "black head" stage are found in forty plants, a treatment should be applied 3 days later.

2) Fall Armyworm Trapping and Damage Sampling

During late whorl, damage from FAW should be noted while scouting for eggs of ECB. It is important to distinguish between damage from the two insect pests (see factsheets 794.00 and 790.00 listed at the end of this document). Egg masses can also be found on leaves when FAW is present.

Trapping is most useful for FAW from silking through harvest when larvae can infect ears but are not easily detected. Fall armyworms should be monitored for using the pheromone specifications and protocol described in Section E-1 and Table 1. Traps should be placed in fields by around July 15. The Unitrap buckets is suspended on a 6x6 inch bracket from a 4-5 foot fence post. A No-pest strip™ (containing vapona) is taped to the inside of bucket side to kill moths as they become entrapped. The lure is clipped onto the batmin-like cup of the top saucer-like cover. Unfortunately, the 4-component pheromone lure of FAW also attracts a non-target moth – the common armyworm. It is essential that the FAW not be confused with the common armyworm since the trapped moths can appear similar from battering in the traps. Refer to the factsheet on FAW for identification of adult male moths.

Action Threshold:

In many cases FAW is not a concern for sweet corn processors. In other cases, FAW cannot be effectively controlled with insecticide applications.

If fall armyworm is a marketing concern, a spray program should be initiated when traps show a dramatic increase in catch during tasselling through harvest for a given field. If excessive damage or a high number of egg masses are observed during scouting visits then traps should be checked on a more frequent basis

3) Corn Earworm Trapping and Action Threshold:

This pest cannot be easily monitored by looking for the tiny eggs on the silks or the larvae secretly feeding on the silk inside the ears. Therefore, pheromone traps are the primary means of monitoring for CEW. Traps should be placed in fields by approximately July 15, and near silking sweet corn fields, preferably green silks. The specific trap specifications and protocols for CEW are described in Section E-1 and Table 1. The Scentry™ Heliothis net trap needs to be suspended from a 6-7 foot fence post using the three tie downs on the back side of the trap and a tent stake. The lure is suspended on a wire across the bottom opening of trap and held in place with a binder clip. Please refer to the factsheet on CEW for identification of adult male moths.

Action Threshold:

If corn earworm damage is a marketing concern, a spray program should be initiated when traps show a dramatic increase in catch during the susceptible period for a given field. Susceptible period: green silking through brown silking. A five day schedule (which could vary depending on temperature) is advisable until the susceptible stage is past or trap catch decreases.

4) Flea beetles and Stewart's wilt

Flea beetles are the vectors of Stewart's wilt disease and varieties resistant to this disease should be used whenever possible. A list of resistant varieties is available from your Cooperative Extension agent. If varieties not resistant to Stewart's wilt are used, fields should be sampled for flea beetles and insecticide treatment(s) considered. The decision to treat or not treat should be based on the threshold (6 or more beetles/100 plants) and stage of growth (smaller corn is more susceptible to Stewart’s wilt, knee high corn is generally considered large enough to outgrow the disease).

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: 6 flea beetles on 100 plants

5) 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.

6) 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 G).

F. Sampling for diseases:

1) Common Rust:

Many sweet corn varieties show some tolerance or resistance to common rust. Processors and growers should choose resistant varieties whenever possible.

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 be of benefit, therefore scouting for this disease is not necessary at that time. Common rust should be sampled using a 120 plant sample (10 sites with 12 plants per site).

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 120 plants x 100

Avg. leaves per plant x 120

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.

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 E.4.)

G. 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.

H. Sweet Corn Factsheets

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

Item Number	Title	Year	Price
VFSCORN	Sweet Corn factsheet set (8 factsheets)		$4.00
139VCFS790.00	Fall Armyworm	1983	$1.00
139VCFS791.00	Corn Earworm	1983	$1.00
139VCFS792.00	Potato Stem Borer	1983	$1.00
139VCFS793.00	Stalk Borer	1983	$1.00
153VCFS727.10	Stewart's Disease of Corn	1979	$1.00
153VCFS727.20	Smut of Sweet Corn	1979	$1.00
153VCFS727.30	Virus Diseases of Sweet Corn	1984	$1.00
153VCFS727.40	Common Rust of Sweet Corn	1987	$1.00
102GFS794.00	European Corn Borer	1983	$1.00
139NC327	European Corn Borer: Development and Management	1989	$3.95
795.00	Pheromone Traps - Effecive Tools for Monitoring Lepidopterous Insect Pests of Sweet Corn	1995	$1.20

APPENDIX I. HOW TO MAINTAIN PHEROMONE TRAPS

Where to Store Lures

Lures should be stored in tightly sealed glass containers or foil pouches. All lures should be stored in a freezer (preferred) or refrigerator to enhance longevity. Again, care should be exercised so that different moth species lures are not mixed in the same container, which would result in cross contamination. Lures should not be exposed to strong light or high temperatures for long periods of time, because some pheromone components are photo / heat sensitive and will degrade rapidly if left in bright light or high temperatures. A small cooler filled with ice packs is recommended for transporting lures from office / laboratory to field or vice versa.

When and How to Change Lures

Volatility and degradation rate vary between pheromone components among the various moth species, and release characteristics are different for the different types of dispensers. For these reasons, no generalization can be made about field life of lures. The expected field life and recommended intervals of lure replacement are provided by pheromone manufactures for each individual species (Table 1).

Lures of different species need to be handled carefully to prevent contamination of the trap or cross contamination with other lures. Forceps or plastic gloves are recommended for replacing lures or switching lures of a different moth species. Forceps should be cleaned with acetone or nail polish remover and one forceps should be used for only one moth species. Gloves should be also changed when switching lures for different moth species.

Where to Purchase Pheromone Supplies

GREAT LAKES IPM	PEST MANAGEMENT SUPPLY, INC.
Jim Hansel	Thomas Green
10220 Church Road NE	311 River Drive
Vestaburg, MI 48891	Hadley, MA 01035
517-268-5693	413 549-7246 or 800 272-7672
FAX 517 268-5311	FAX 413 549-3930

Trece Corporation	Scentry Monitoring Products
P.O. Box 5267	26405 West Highway 85
635 South Sanborn Road, Suite 17	P.O. Box 426
Salinas, CA 93915	Buckeye, AZ 85326
408-758-0204	602-233-2772

Hercon Laboratories Corp.
200B Corporate Court
Middlesex Business Center
South Plainfield, NJ 07080
201-755-7730

This is not a complete listing of sources of pheromone trapping systems. There may be other suppliers of pheromone trapping systems in your area. Mention of a product or private organization does not constitute an endorsement.