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7/13/2009 - 7/19/2009

Scout for Corn Leaf Aphids Before Tasseling

By Erin Hodgson, Department of Entomology

Corn leaf aphids can be common in Iowa corn, but they rarely cause significant economic losses. They prefer sorghum as a host plant, but will also feed on corn, barley, millet and many grasses. Although infestations become apparent when tassels emerge, colonies can start to develop in late June.

Description and life cycle. Corn leaf aphids are bluish-green in color with dark legs and cornicles (tailpipes at the end of the abdomen), and usually less than one-eighth inch in length. Like all aphids, they have a piercing-sucking mouthpart that feeds on plant phloem. Corn leaf aphids excrete a sugar-rich honeydew that can cover corn leaves, tassels and silks.

corn leaf aphids and moltings

Corn leaf aphids and cast skins from molting.

 

The corn leaf aphid is not known to overwinter in Iowa, but migrates on airstreams each season. Females give live birth to nymphs via asexual reproduction during the summer. Wingless forms are more common, however, migratory winged forms are produced during crowded conditions.

corn leaf aphid

Heavy corn leaf aphid infestation. 

 

Damage. Feeding by large aphid colonies can reduce yields significantly. Aphids feed on upper leaves, in the whorl or on unemerged tassels. Heavy corn leaf aphid infestations may cause leaves to curl, wilt and become yellow. Tassels and silks can become covered with honeydew. Aphid colonies can slowly kill the tassel and top leaves of a corn plant. The plant responds by shunting nutrients to the feeding area and not to the developing ear, resulting in stunted ears or even barren plants. The most severe damage occurs between the late-whorl and pollination stages.

Scouting and management.  Scouting for corn leaf aphids must be done before tasseling - ideally three weeks before. Developing colonies are most commonly found deep within the whorl. Stop at five locations and examine twenty plants. Carefully pull the whorl leaves away from the stalk and unroll the leaves to estimate the number of aphids. Closer to tasseling, aphids move from within the whorl and are very exposed to weather and natural enemies. Take note of beneficial insects (e.g., ladybeetles and lacewings) and/or parasitized mummies; consider not treating aphids if more than 20 percent are parasitized.

Spraying for corn leaf aphids while they are protected deep in the whorl is not effective. Treating for corn leaf aphids after tasseling is usually not a cost-effective management decision because aphid populations will decline naturally. A well-timed spray while aphids are outside the whorl or on the tassel is recommended when they exceed thresholds. Corn fields can be particularly sensitive to corn leaf aphid feeding if they are drought-stressed. If 50 percent of the plants have more than 100 aphids per plant, tassels are coated in honeydew and plants are under drought stress, treatment may be justified. There are several products registered in Iowa for corn leaf aphid (Table 1). Follow label directions and pay attention to spray guidelines.

corn leaf aphid spraying table

 

Portions of this article were originally appeared here: http://www.ipm.iastate.edu/ipm/icm/1998/8-10-1998/cornaphids.html

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847.

Concerns about Herbicide Resistant Weeds in Iowa

By Mike Owen, Department of Agronomy

While the evolution of herbicide resistance in weeds is not a new issue, and Iowa has long had weeds with resistance to herbicides, there have not been any field-scale problems with resistance to glyphosate or PPO inhibitor herbicides identified. Iowa State University Weed Science has always recommended that growers utilize a diverse weed management program and not focus on one herbicide that is applied repeatedly.

Despite these recommendations, ISU Weed Scientists have received numerous calls during the past few years about difficulties controlling weeds with specific herbicides. In order to investigate these problems, approximately 15 demonstrations were established this spring in grower fields to determine the responses of common waterhemp and giant ragweed to commonly used herbicides. The fields were selected because of grower complaints and alleged failure of herbicides to control the specific weeds in these fields. 

Early results from these in-field demonstrations are now available and it is clear that the herbicide resistance weed situation in Iowa has changed. Our preliminary results suggest that Iowa has populations of common waterhemp that are resistant to PPO inhibitor herbicides. We have also identified populations of giant ragweed that appear to have evolved resistance to glyphosate. At this time, we have not documented how widely spread these problems have become or the specific details about the alleged resistance. Research to better describe the weed resistance is underway.

It is important to reinforce that these data are preliminary and further research must be done to fully understand the evolved herbicide resistance in these weed populations. However, the preliminary data are sufficiently robust to indicate that Iowa has a building problem of evolved resistance to herbicides in several important weed species. Recognize that the herbicide does not cause the evolution of resistance in weeds, but rather the management decision to use a specific herbicide and how that herbicide is used facilitates the selection for the resistant weeds. As ISU Weed Science has suggested for many years, it is not if herbicide resistant weeds will evolve in Iowa; it is when the resistant weed populations will be recognized. The when is now and growers must change the strategies that have been used historically to control weed, or face the inevitability that herbicide resistant weeds will become a more serious economic issue. 

More information about weed management, protecting crop yields, and herbicide resistance on the ISU Weed Science Webpage (www.weeds.iastate.edu) .

 

 

Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at mdowen@iastate.edu or by phone at (515) 294-5936.

Corn Quality Issues Continue

By Charles R. Hurburgh, Department of Department Agricultural and Biosystems Engineering

The saga of the 2008 corn crop continues as mold-damaged corn comes out of storage and onto the market. Present market conditions are reducing movement which will cause longer storage times. Elevators report that they are still receiving 15-17 percent moisture corn in deteriorating condition, which will add to last winter’s problems with temporary piles and flat storages.

Last fall and again this spring, ISU Extension put out several bulletins on the quality and handling of the 2008 corn crop.  At that time, corn was coming out of the field very wet, with soft texture, low test weight and low protein content. These were consequences of the cool wet growing season; much of Iowa corn never did reach full maturity. This type of corn has roughly half the storage life of normal corn under the same moisture and temperature conditions.

End users (corn wet mills, feed mills, ethanol plants) are reporting that the increase in damaged kernel levels is sufficient to require closer grading of inbound grain, and in many cases increased discount scales for damage, above the typical 2 to 3 cents per bushel per point over 5 percent. When many types of damage are present, from total discoloration to internal germ or blue-eye damage, the visual grading process becomes quite time consuming. No less than 125 grams of corn, mechanically divided not hand scooped, from an original sample of at least 1 to 2 pounds should be picked for damage. USDA graders start with approximately five pounds before subdivision. Any sample that creates a price discount for damage should be retained until settlement, because damage grading can be subjective and retesting or appeal is sometimes requested. Either party to a trade has the right to request an Official USDA-GIPSA analysis in cases of dispute, although there will be costs and time delays for this process.

The numbers are simple; No. 2 Yellow Corn allows 5 percent damage.  It takes about 20 bushels of normal corn from the field (3 percent or less damage) to absorb 1 bushel of 30 percent damaged corn; more for higher damage.  Damage reduces product yields and performance for every user industry, which is why the market retains the No. 2 levels as benchmarks.


What to do now?
Very wet corn (15 percent or higher) should be dried immediately. Any corn that has had any signs at all of heating or mold should be moved as soon as possible; the situation will only get worse. Test weight is a reasonable indicator of future problems; if you have test weights over 56 lb/bu and no spoilage has happened yet, you can probably keep the corn a while longer. 

Expect closer grading of corn. The apparently increased supply of corn will allow buyers to become more restrictive about quality. This may require additional marketing actions to sell the physical grain and still retain market participation.
 
It is possible to estimate when corn has been heavily blended with severely damaged corn as opposed to gradually going out of condition; the damaged kernels will not look like (size, shape, color, etc.) the overall average of the sample.  Sensitive users such as poultry or corn processing (wet or dry) will be more affected by high damage inclusions. Expect larger feed users to be more stringent in auditing the quality of the corn going into the mill.


Going Forward to 2009
There will be significant carryover of corn into 2009. This will present challenges because 2008 corn will be very hard to keep into next year. Rotation of stock will be critical. Never mix crop years in a storage structure, and especially not this year! 

Move the 2008 corn and replace it with (hopefully) better quality 2009 crop. If this cannot be done, use great selectivity about which 2008 corn to keep. Advance planning will be necessary, both in terms of which corn to keep and store, and in terms of which storage structure to use for long term storage. 

The common practice of blending wet corn from the field with dry old crop corn will be hard to work with this year; spoilage will start very rapidly. This corn would need to be used immediately, not stored. Expect the export market to be on the lookout for moisture blends because it has storage-in-transit issues.

Complete cleanout and sanitation of storage structures will be important this fall.  Remove all residual corn, fines, moldy grain, and be sure that aeration ducts are clear with the perforations all open. Sometimes dry corn is carried over flat bottomed bins; this practice is not advisable this year. Insects feed on molds; there are several insecticides that can be used on empty bins as a pretreatment. Go into the harvest season with all the equipment clean and functional.

Recognize that the corn quality problems with 2008 crop were large enough that they will not go away immediately at the 2009 harvest. It will take a year or more to move all the 2008 corn within the market limits for No. 2 Yellow Corn. We need a good quality crop in 2009, and close attention to good storage practices to preserve it.

 


Charles Hurburgh is a professor of Agricultural and Biosystems . He can be contacted at (515) 294- or by email at tatry@iastate.edu.

Eyespot and Gray Leaf Spot Occurring in Corn

By Alison Robertson, Department of Plant Pathology

Corn across the state is beginning to tassel and many of you, like me, have been out in fields scouting for foliar diseases of corn. 

Eyespot
The disease of this year, thus far, seems to be eyespot, particularly in central and northern Iowa.  Eyespot loves wet and cool weather so it is probably not surprising that the disease is especially prevalent this year. If such weather continues, we can expect the disease to spread.  

Eyespot tends to be more severe in corn following corn fields, especially if residue is present on the surface. Older corn leaves appear to be more susceptible to infection. The disease usually starts on the lower leaves of the plant and moves up the plant; however, random lesions may also occur in the upper leaves. Hybrids do vary in their susceptibility to the disease.

Initial symptoms are small (one-sixteenth inch in diameter), water-soaked, circular lesions. The lesions enlarge to about one-eighth inch in diameter, and develop a light tan center, dark brown margin and yellow “halo” (photo below). If you hold the leaf up to the light, the halo becomes more apparent, and the center of the spot looks translucent.  Time from infection to symptom development is about 9-10 days.

Early and severe eyespot can result in yield loss on susceptible hybrids.  Furthermore, increased stalk rot severity is associated with severe eyespot disease. 

Gray leaf spot
Gray leaf spot (GLS) is starting to show up on the lower leaves of susceptible hybrids across the state. Tamra Jackson, Extension Plant Pathologist at University of Nebraska – Lincoln, reported GLS is highly prevalent at this time in Nebraska, occurring slightly earlier than normal in that state, and the severity is increasing.  Here in Iowa, the disease is perhaps a little ahead of schedule.  We usually see a few lesions of the GLS on the lower leaves of the plant around mid to late July.  GLS infections in June and early July were associated with the epidemics of GLS in Iowa in the mid 1990s.

GLS disease development is favored by is very warm, humid weather (“Iowa State Fair” weather), and the disease is usually more severe when the previous crop was corn and residue is present on the surface. Like eyespot, GLS usually starts on the lower leaves of the plants and progresses up the plant. 

Lesions of GLS are rectangular in shape (photo below) since the veins of the leaf stop the lesions expanding sideways.  The lesions may develop up to 4 inches in length.  Lesions are initially tan to brown but become silvery-gray when they produce spores. Time from infection to symptom development is about 14-21 days, depending on weather conditions and hybrid susceptibility.

GLS can result in considerable yield loss particularly when the disease spreads to leaves above the ear soon after tasseling. High GLS severity may also increase the risk of stalk rot.

Common rust can also be found in Iowa corn fields this year but at extremely low incidence. Common rust does not survive the winter in Iowa and spores blow into the state from the south each year. This time last year, common rust could be easily found in every corn field. 

Management
The cornerstone of eyespot and GLS management is resistance. Rotation can also reduce risk of disease. Fungicides are also effective at reducing disease and protecting yield.

To determine if a fungicide application is necessary, the following factors should be considered:
• disease pressure in the field
• hybrid susceptibility
• predicted weather conditions during grain fill
• price of corn and cost of fungicide plus application
• previous crop
• field history of disease

eyespot

Small, circular lesions of eyespot.  Note the yellow halo, dark brown ring and light tan center.

 

 

gray leaf spot

Characteristic rectangular gray leaf spot lesions.

 

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu.

July 13 Crop and Weather Report

By Doug Cooper, Extension Communications

The July 13 weekly crop and weather report includes interviews with Iowa State University Extension climatologist Elwynn Taylor, integrated pest management specialist Rich Pope, and soybean agronomist Palle Pedersen.

Taylor says El Niño may be about to return, but for it to have significant impact on Iowa agriculture the southern oscillation index will have to be at least minus 0.8, right now it is minus 0.1. He says El Niño is generally positive to Iowa crop production.

Pope reports severe storms caused some hail problems in northwest and northeast Iowa last week. Damage ended the growing season for some corn and soybean crops.

Pedersen tells us weeds (especially volunteer corn) remain a problem for Iowa soybean producers. The cool and wet weather has made it difficult to slow the growth of weeds.

Degree Days - A Bit Wet and a Bit Wild

By Rich Pope, Department of Plant Pathology

Iowa weather continued to track cool and wet as degree days fell just a bit further behind long-term averages.  With the cooler weather and precipitation since June 1 registering 168 percent of normal, many fields have remained damp.

The wettest part of the state was crop reporting district 1, northwest, where an average of almost 4 inches of rain have fallen since July 1.  Storms with hail and high winds have caused damage in scattered areas, and a few fields are suffering from ponding. These problem areas will hopefully recover as the season progresses. 

 

Degree day accumulation map for July 12

Many cornfields are starting to tassel, and the bulk of pollen shed and pollination should happen in the next week or so.  

Soybean aphids are being seen occasionally in the northern half and in central Iowa, but remain in general at moderate to low populations. Fields should continue to be monitored for the next few weeks.  

 

Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at ropope@iastate.edu or by calling (515)294-5899.

Should Hail Damaged Crops be Sprayed With a Fungicide?

By Alison Robertson and Daren Mueller, Department of Plant Pathology

Last week severe storms hit several counties in Iowa with hail ranging from softball to pea size causing significant crop damage. Hail damage to corn may negatively affect yield potential (see Elmore and Abendroth, 2009) but may have little effect on soybean yield potential (see Pedersen, 2008). 

There have been reports from previous years of hail-damaged crops benefiting from an application of fungicide. To date, we know of only one statistically sound study to test the effect of fungicide on damaged corn that has been done. To compare the effect of a fungicide on hail-damaged corn, we need a non-hail damaged check to compare the effect of a fungicide on both hail damaged and non-hail damaged corn at the same site.

Carl Bradley, Extension Plant Pathologist at University of Illinois, conducted such a study in 2007 near Champaign, IL. In this replicated study, a weed-eater was used to simulate hail damage. Treatments included “hail-damaged” and “non-hail damaged corn” that was either sprayed or not sprayed with a fungicide. Fungicide applications did not statistically increase yield when applied on tasseling corn that was “hail-damaged” the previous day (Table 1).

 

Table 1. Effect of simulated hail damage and foliar fungicides applied at tassel emergence on gray leaf spot severity and yield of a susceptible corn hybrid near Champaign, Illinois, in 2007 (Bradley and Ames, 2008)

hail damage table

In the limited number of replicated studies conducted in Iowa that were established on hail-damaged crops, no significant yield responses have been seen from fungicides. For example, last year a trial was established on hail-damaged soybeans in western Iowa. Severe hail damage occurred in late June when soybeans were at the beginning of the R1 growth stage and fungicide was applied on July 28, when soybeans were at the R2 growth stage. There was no statistical difference between the fungicide-treated (38.6 bu/ac) and the non-treated control (39.9 bu/ac).


Disease risks associated with hail damage
It is important to remember that a fungicide application can not recover yield potential lost due to hail damage. Fungicides protect yield potential by reducing disease. There are some diseases of corn that are favored by wounding, e.g., Goss’s wilt, common smut and stalk rot. Similarly bacterial blight and bacterial pustule on soybeans are favored by wounding. Fungicides are not effective against the pathogens that cause these diseases. The foliar diseases that are managed by fungicides (e.g., gray leaf spot, northern corn leaf blight, eye spot, and common rust on corn, and brown spot and frog eye on soybeans) are caused by pathogens that do not require wounds for infection. These foliar diseases will influence the yield response to fungicides more so than hail damage.

 

 

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu; Mueller at (515) 460-8000 or by emailing dsmuelle@iastate.edu.

Considerations for Soybean Insecticides and Fungicides

By Alison Robertson, Daren Mueller and Nate Bestor, Department of Plant Pathology Matt O’Neal and Rebekah Ritson, Department of Entomology

Several Iowa agribusinesses are offering soybean growers pest management plans that include applications of fungicide and insecticide. Although combining an insecticide and fungicide may be convenient, the results from our 2008 Pesticide Stewardship trials suggest this is a convenience that may not pay off.

2008 Statewide Pesticide Stewardship Trials
In 2008, we conducted trials at six locations in Iowa to compare calendar based applications of pesticides with applications of pesticides based on insect or disease thresholds. Treatments evaluated were fungicide alone, insecticide alone, or a tank mix of fungicide plus insecticide at either R1 or R3. Data collected from the trials included seasonal exposure of soybean to aphids (measured in cumulative aphid days; CAD), foliar and stem disease severity, yield and much more.

Insecticides. Aphid pressure varied across locations from very high at the ISU Northwest Research Farm (Peak aphids = 4073 per plant on August 17) to low at the ISU Southeast Research farm (Peak aphids = 114 aphids per plant on August 20).   The best yield-protection was observed when insecticides were applied at or near the 250 per plant threshold.

When applied on a calendar basis, the highest yields came from plots treated with a foliar insecticide applied at R3 than R1. However, we did not see yield reduction at every location as aphid populations did not always reach damaging levels. Using insecticides when they are not needed is not an efficient use of this tool and may, under some circumstances, result in greater pest problems later in the season.

Last growing season there were numerous reports of growers who had applied an insecticide at R1, when there was no insect pressure, but still had to apply a second dose of insecticide later in the season when soybean aphid reached threshold.

Fungicides. In general, severity of brown spot and anthracnose stem blight were reduced by an application of fungicide at R3. Greater yields were achieved with fungicides applied at R3 compared to R1 applications at all locations. This was no surprise to us, since others have reported similar findings with regards to fungicide timing on soybean. Furthermore, disease pressure within the soybean canopy is usually very low at R1, and increases from growth stage R3 onwards. Therefore, an application of fungicide as a disease management tool makes more sense at R3.

We will continue this research in 2009, but so far we can conclude that applications of either a fungicide alone, an insecticide alone or a tank mix of fungicide plus insecticide at R1 has not been beneficial. Applications of both fungicide and/or insecticide may cause aphid populations to be higher than with no application.

Growers are advised to scout for soybean aphid and apply insecticide when thresholds are reached. Similarly, to improve chances of a significant yield response, fungicide should not be applied before growth stage R3, and fields should be scouted to determine the level of disease pressure before application.

Fungicide Product Update
There were no major changes in product availability over the past year. Two Section 18 labels remain for soybean rust – Punch and Topguard. Remember that the ONLY legal use of these fungicides in Iowa should be targeting soybean rust. Punch will not be available for use on soybean in 2010 so check with local agriculture dealers if product is still remaining at the end of the season. A decision by EPA on Topguard should come sometime in late 2009.


 

 

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. Matt O’Neal is a professor of entomology with extension and research responsibilities. Nate Bestor is a research associate in plant pathology; Rebekah Ritson is a graduate assistant in sustainable agriculture. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu; Mueller at (515) 460-8000 or by emailing dsmuelle@iastate.edu; O’Neal at (515) 294-8622 or by emailing oneal@iastate.edu.

July Workshops Offer Comprehensive Corn Nematode and SCN Training

By Greg Tylka, Department of Plant Pathologist

Plant-parasitic nematodes can be a major yield-limiting factor in Midwest corn and soybean production.  Iowa State University will be conducting two all-day workshops on the biology, scouting, and management of corn nematodes and the soybean cyst nematode (SCN).

The workshops are being held July 29 and July 30 and will be taught by ISU scientists Thomas Baum, Palle Pedersen and Greg Tylka.

workshop group

Workshop participants will earn 7.0 Certified Crop Adviser pest management CEUs in each workshop and will receive printed course notes, other print publications, and computer training modules on CDs.

The registration fee for each workshop is $150, and there is a 10 percent discount for individuals registering for both workshops. Enrollment is limited, but there are spots still available in each workshop. Additional workshop details and for registration forms are available online.

 

 

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at gltylka@iastate.edu or by calling (515) 294-3021.

It is Time to Sample for Corn Nematodes

By Greg Tylka, Department of Plant Pathology

There is much discussion about corn nematodes this season, and interest likely will continue or increase in the next few years. Many articles have been published in the agricultural press about changes in corn production practices that may increase the occurrence of these microscopic worms that live in the soil and can damage corn. Numerous quick facts about corn nematodes were reviewed in the Integrated Crop Management News, April 28, 2009

Nematode feeding on corn can stunt plants and cause leaves to turn pale green or yellow and wilt. Damage is rarely field wide, more often occurring in patches that have non-discrete edges that fade into healthy-looking plants. 

There are many production problems that stunt corn plants and affect the foliage. To determine if plant-parasitic nematodes are causing observed damage to corn, a soil sample must be collected from around damaged plants to identify the nematode species present and their population densities (numbers). A root sample also needs to be collected because two species of nematodes that can damage corn exist almost exclusively in roots during the growing season. These endoparasitic corn nematodes will be detected only in low numbers in the soil during the growing season and damage could be misdiagnosed if only a soil sample was collected.

Sampling
Sampling mid season, when damage symptoms are apparent and nematode numbers are greatest, is necessary because corn nematode population densities increase through the growing season and the different nematode species vary in the number needed to cause damage. 

To check for corn nematodes, collect 20 or more 12-inch-deep soil cores from plants showing symptoms that might be due to corn nematode feeding damage. The soil cores should be mixed well and placed in a moisture-proof bag and submitted for processing as soon as possible. Also collect root balls (see image) from 2 or 3 plants that have damage symptoms. The tops of the plants can be discarded. The root balls can be included in the bag with the soil or placed in a separate moisture-proof bag.

collecting core samples

Collect twenty 12-inch-deep soil cores from the root zone.   

 

collect root balls

Collect 2 or 3 root balls from plants with symptoms
of damage.

 

Shipping and additional information
Avoid shipping samples near the end of the work week if hot temperatures are expected; this helps prevent having samples become overheated while in an uncooled delivery truck over the weekend. Excess heat will damage the nematodes within the sample and can affect the sample results, especially for the endoparasitic nematode species that exist in the roots.

Samples can be sent to the ISU Plant and Insect Diagnostic Clinic, 327 Bessey Hall, Iowa State University, Ames, IA 50011. The test for corn nematodes is called the complete nematode count. Samples should be accompanied by a completed Plant Nematode Sample Submission Form,  PD 32 and a check for the $30 per sample processing fee.

The ISU Extension publication Nematodes That Attack Corn in Iowa, PM1027, contains more information about corn nematodes and is also available for download from the extension online store.

stunt nematode

Stunt nematode feeding from outside of a corn root.

 

 

lance nematode

Endoparasitic lance nematodes (stained purple) inside roots.

 

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at gltylka@iastate.edu or by calling (515) 294-3021. 

Don't Let Potato Leafhoppers Burn Hay

By Erin Hodgson, Department of Entomology

There have been recent reports of potato leafhopper in Iowa alfalfa, and it's time to think about sampling again. Potato leafhoppers do not overwinter in Iowa, but adults migrate here every spring.

Females deposit 2-3 eggs per day in plant stems. Pale, green nymphs emerge in 7-10 days depending on the temperature; the fastest development occurs at 86 degrees F. Nymphs look similar to adults except they are smaller and are wingless. They go through five instars in about 2 weeks. Adults are lime green, one-eighth inch long, and have a broad, wedge-like head. Adults live for 4-7 weeks.

As a result of the extended egg-laying period, at least two overlapping generations occur in Iowa every year. Potato leafhoppers are very active and easily disturbed – adults jump or fly away while nymphs quickly move sideways and backwards.

plh nymph and adult

Nymph and adult potato leafhopper.
 

Damage
Potato leafhoppers have a piercing-sucking stylet mouthpart. They cause physical damage when probing to feed and also inject saliva that plugs vascular tissue. Initially, alfalfa leaf tips will turn yellow, commonly referred to as "hopperburn." Heavily infested plants will be stunted, particularly new stands and regrowth after cutting. In some cases, large leafhopper populations can significantly reduce tonnage of the current crop, as well as the following crop.
 
Scouting 
Potato leafhoppers do not typically build up to damaging levels during the first crop in Iowa. Fields should be monitored weekly after the first cutting until the end of the season. A sweep net is the most effective way to sample for nymphs and adults.

Fields should be sampled when dry and in calm conditions. Sweep vigorously through foliage, using a 180-degree motion for one sweep. For each field, stop at 4-5 locations and take 25 sweeps per location. Count the number of nymphs and adults at each location and estimate the number of potato leafhoppers per sweep for each field. Keep in mind nymphs will be near the sweep net ring and adults will be at the bottom of the net.
 
Management
Remember, healthy and vigorous stands are able to tolerate potato leafhopper (and other insects) feeding. Heat or drought-stress can make alfalfa more susceptible to insect feeding. Protecting alfalfa from potato leafhopper usually involves a three-pronged approach.

1. The use of glandular-haired, potato leafhopper-resistant alfalfa varieties can significantly reduce yield losses. Using resistant varieties does not mean fields will be hopper-free, but plants should be able to tolerate moderate populations compared to conventional varieties. Newly-planted resistant fields may not show tolerance immediately, but should express tolerance after becoming established. Consider using potato leafhopper-resistant varieties if the local area is consistently infested.

2. The cultural control tactic of cutting management can disrupt potato leafhopper populations as they develop in alfalfa. Delaying harvest will allow nymphs enough time to become adults and start reproducing. Young nymphs will be destroyed or starve before regrowth occurs. Timely cutting will force adults to move to nearby crops, but they often move back into a field after regrowth. It is important to start scouting 7-10 days after each cutting to monitor for reinfestations.

3. Insecticide applications can protect alfalfa yield from potato leafhoppers, and are economically justified with regular scouting and the use of action thresholds. The fluctuating values of hay and control costs are important considerations for making a treatment decision. Tables 1 and 2 offer a dynamic action threshold for potato leafhopper based on conventional and tolerant alfalfa. There are several products registered in Iowa for potato leafhopper control in alfalfa (Table 3). Follow label directions and pay attention to preharvest interval guidelines. 
 

Table 1. Action thresholds for potato leafhopper (# per 10 sweeps) on conventional alfalfa.

plh table 1

 


Table 2. Action thresholds for potato leafhopper  (# per 10 sweeps) on tolerant alfalfa.

phl table 2

 

 

Table 3.  Insecticides registered for potato leafhopper control in Iowa.

phl table 3

 

 

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847.



This article was published originally on 7/20/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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