SOYBEAN APHIDS ARE POPPING UP IN NORTHEAST NEBRASKA

Soybean aphids have been found in northeast Nebraska near Crofton in Knox County, south of Decatur in Burt County, and south of Pilger on the Cumming-Stanton county line.  In at least one location (Pilger) the populations reached the economic threshold of 200/plant in part of the field.  This is the time they started popping up last year, so we expect to be getting more aphid reports.  Refer to the article “The Soybean Aphid in Nebraska” in last week’s issue (July 23, 2003) of the Integrated Pest Management and Crop Production Newsletter for more information on aphid biology and management, or go directly to http://entomology.unl.edu/newsflash/soyaphid03.html  on the UNL Entomology Department Webpage (entomology.unl.edu). (TH)

EUROPEAN CORN BORER MOTHS ARE FLYING

The second European corn borer flight is occurring in much of Nebraska and just beginning in northeast Nebraska.  If you have not begun yet, start scouting non-Bt cornfields for European corn borer egg masses.  Current information on light trap catches at Concord, NE at the NEREC Website http://nerec.unl.edu/agriculture.htm . Trap counts for other regions can be found through the Entomology Department Website (http://entomology.unl.edu/) or directly at http://entomology.unl.edu/fldcrops/fldcrops.htm

Timely and accurate scouting is the key to managing ECB in standard (non-Bt) corn hybrids.  Remember that conditions are localized and fields must be scouted on an individual basis to make accurate decisions.  We have had it easy the last few years - flights have been relatively low; however, this cannot last forever.  Last year's flights were up a little from the previous few years.

Fields with green silks during the peak moth flight period are most susceptible to second-generation egg laying. The white, flat eggs overlap each other like fish scales and are laid in masses of five to 40 eggs. Eggs are most likely found on the underside of leaves, near the mid-rib, on the ear leaf and the three leaves above or below the ear leaf.  Approximately 90 percent of the egg masses will be found on these middle seven leaves.  A black spot is visible on the eggs for about 24 hours before they hatch. The spot is the head of the developing corn borer; this stage is often referred to as the black head stage.

To determine whether control would be profitable, examine 25 plants at four sites per field (100 plants total). Record the number of egg masses and the number of plants sampled. If you sampled only the middle seven leaves, multiply the number of egg masses by 1.1 to estimate the total present over the whole plant. Use this adjusted mean in the worksheet below. Go through the calculations outlined in the worksheet (also available from NebFact 98-365, Second generation European corn borer scouting and treatment decisions http://www.ianr.unl.edu/pubs/insects/nf365.htm ) to determine if an economic infestation is present. You will also need to know:

     crop stage

     expected yield

     expected market price for corn

     percent control with insecticide

     cost of control (insecticide plus application costs)

An interactive version of this worksheet is available at http://www.ianr.unl.edu/forms/forms.skp/ecb_2nd.html

This worksheet will help you better evaluate the factors influencing the cost/benefit relationship for second-generation European corn borer treatments. Average values are suggested in the worksheet but may be modified for local conditions.

1. Borer survival is suggested to be 15%. Larval survival varies with weather conditions and irrigation. In irrigated corn, larval survival may be 20% or more, while in dryland corn with no significant rainfall, it may be 10% or less. Survival of eggs and small larvae decreases greatly in hot, dry weather or with extended periods of heavy rain.

2. Yield loss will be about 4% per borer for infestations occurring before silks turn brown and 3% per borer after silks turn brown but before blister stage. These averages are based on published research but only account for physiological yield losses (reduced grain production) and do not consider yield loss from stalk breakage or ear drop. These factors are difficult to predict and vary with hybrid, cultural practices and weather.

3. Percent control with insecticides is suggested to be 70%; change this value if you think that control will be different in your situation.

Infestations are most damaging when corn borers enter the stalk early in corn’s reproductive cycle. There is a short time between first egg hatch and significant stalk tunneling when corn borers are best controlled. Concentrate scouting efforts in this early egg laying period and repeat every three to five days. Often, second generation egg laying may extend to 21 days or more. Although later hatching corn borers do not directly reduce grain yield as much, they may still cause stalk breakage or ear drop. Early harvest of fields damaged by corn borers and selecting varieties with good stalk strength and resistance to stalk rot can reduce this loss.

If treatment is needed, time insecticide applications to coincide with the beginning of egg hatch to achieve acceptable control. Generally, liquid and granular formulations of the same insecticide are equally effective against corn borer larvae. However, in considering other pests that may need to be controlled at this time of year (western bean cutworms, rootworm beetles, grasshoppers, spider mites), liquids may be preferred. Rates and restrictions of registered insecticides for European corn borer control can be found on the label or at the UNL Entomology home page at http://www.ianr.unl.edu/ianr/entomol/fldcrops/fldcrops.htm    (TH)

 

COWPEA APHID FOUND ON ALFALFA IN CENTRAL NEBRASKA

Cowpea aphids have been found on alfalfa in central Nebraska this week (July 29). Last year (2002) the aphid was found throughout Nebraska alfalfa fields, resulting in treatment at some locations.  Heavy rains significantly reduced aphid numbers in parts of Northeast Nebraska (yes, some of us did get rain last year).

The insect was first reported in Nebraska in Knox Co. in 1999.  The following information has been largely taken from University of Texas, University of Arizona, and Oklahoma State University websites. During the past few years, this insect, Aphis craccivora Koch, has been extremely abundant in alfalfa fields throughout arid parts of the Southwest, including Arizona, California, and Texas. Commonly referred to as the “black aphid”, it has been around for many years in the south, usually present in low numbers on cotton, alfalfa, and weeds.  In addition, outbreaks of cowpea aphid have been reported sporadically throughout Kansas and Oklahoma, all within the past year. Infestation levels in these areas were reported to be from 50-125 aphids per stem and there was noticeable yellowing and stunting. In Nebraska in 2002 the aphids were noticeable but were not quite at economic levels in most fields.  Since many growers were in the process of harvesting the third cutting when the aphids popped up, we advised them to take the cutting and watch the regrowth. 

Description - The cowpea aphid is easily distinguished from other aphids in alfalfa largely because it is the only black aphid found infesting the crop. In general, it is a relatively small aphid, less than 2 mm long.  Non-winged and winged adults are usually shiny black while the smaller nymphs may appear to be a dull gray to black. The first half of the antennae is white, and the legs are usually a creamy white color with blackish tips. In alfalfa, these aphids obviously feed on young terminal growth, but can be found infesting leaves, blooms, and stems.  Damage symptoms include yellowing, wilting, and dieback. In general, legumes can be seriously damaged, either by direct insect feeding or by the transmission of virus diseases.

Distribution - The cowpea aphid is generally distributed across North America and has been reported in at least 28 states and in three Canadian provinces. This aphid species also has an extensive host range with a marked preference for legumes. Other known host plants are apple, carrot, cotton, cowpea, dandelion, dock, goldenrod, kidney bean, lambsquarters, lettuce, lima bean, pinto bean, peanut, pepperweed, pigweed, red clover, shepherdspurse, vetch, wheat, white sweet clover, and yellow sweet clover. The aphid lives throughout the year without producing sexual forms and they are always parthenogenetic viviparous females (ready to produce offspring at birth).  

Monitoring and Treatment Guidelines - Because the cowpea aphid has only recently become a problem in alfalfa, no monitoring guidelines or economic thresholds have been developed for this aphid.  An Oklahoma State University Entomologist provided the following information. “Normally, we do not worry much about cowpea aphid, and if temperatures increase, predators will feast heavily on them; however, if damage (yellowing and stunting) is evident, then insecticide treatment may be appropriate.” Based on his observations, cowpea aphids damage alfalfa and feed on the plant similar to the pea aphid; therefore, thresholds are likely similar. OSU Cooperative Extension recommends that on alfalfa less than 10 inches tall, 50 aphids/stem should be used as a threshold. On alfalfa taller than 10 inches, 100 aphids/stem may be used. On the other hand, the Texas website suggests a threshold near or below that of blue alfalfa aphid: Height less than 10 inches: 10-12 aphids per stem or 50 per sweep. Height greater than 10 inches: 40 - 50 aphids per stem or 200 per sweep.  These thresholds have not been verified locally, but may be helpful in making treatment decisions.

Control - Very little information is available on insecticide efficacy against cowpea aphids. We suggest you use pea aphid management guidelines until more cowpea-specific information is available.  A glance at a University of Arizona insecticide trial showed that Warrior at 2.6 oz/acre provided the best overall control.  We established an insecticide trial at the Haskell Ag Lab (Concord, NE) in 2002, but heavy rains caused the aphid populations to crash right after insecticide treatment.  Rain and possibly an abundance of lady beetles, a major aphid predator, appeared to have solved the aphid problem for us in our production fields. Early harvest is also suggested as a control method.  Time will tell if this aphid will become a more common pest of alfalfa in Nebraska.

More information on the cowpea aphid can be found through the UNL Entomology Department Website at:  entomology.unl.edu    (KJ&TH)