ndsucpr_L_sm.jpg (11690 bytes)


ISSUE 13   July 25, 2002

 

SOYBEAN APHID NUMBERS INCREASING, TREATING UNDERWAY

Soybean aphid numbers continue to climb. Fields in the Fergus Falls, MN area had treatable levels of aphids. Colonies were found on upper leaves, lower stems and pods. There have been reports from Richland, Cass, and southern Traill counties from people who are treating or are considering treating at this time. Farther north and west, aphids have been found in Grand Forks, Barnes, Sargent, and Ransom counties, but at lower levels.

The field infestations are often greatest near tree lines, forested areas, and farmsteads. This may be due to buckthorn, the overwintering host plant, being nearby. More likely, it could be that the early winged aphids settled out of the wind passing over these obstructions. Whatever the reason, it appears to be a common trend based on many peoples observations . With our large fields, these heavily infested areas represent only a portion of the field. Some decisions have been made to treat only those areas where the aphids are abundant and leave the remainder of the field go for now. If this decision is made, monitor populations after treatment to determine if the remainder of the field is reaching problem levels, or if there is a rapid rebound of the population within the treated area.

Critical Decision Period

(source: Managing Soybean Aphid by Ken Ostlie, Dept of Entomology, U of Minnesota,. July, 2002)

Timing is critical. Insecticides only suppress soybean aphid populations for 7 to 14 days. Soybean aphids have their biggest impact on pod set. Applications during the initiation of flowering (Stage R1) and into full flower (Stage R2) had the greatest benefit in 2001. Earlier applications risk rebound of aphid numbers just as pod set is occurring. Later applications may achieve less than desired benefits: damage to canopy development and pod set has already occurred, and aphid populations will soon decline as a result of migration or fungal disease outbreaks. The critical period for insecticide decisions is from initiation of bloom to early pod set, when aphid populations are building rapidly, but there is still time to protect pod set.

Yield benefits of insecticide for soybean aphid
Yield benefits of insecticide applications against
soybean aphid in 2001 (K. Ostlie, U of MN)

Within this region, soybeans in counties where aphids have been found are at this critical period. Decisions to treat are probably best made within the next week.

 

U OF MINNESOTA APHID ALERT:

Aphid Situation in Week Ending July 19

(the following is from Dr. Ted Radcliffe, Entomologist, U of Minnesota and is available on the Internet at:

http://ipmworld.umn.edu/alert.htm

Aphid flight activity increased this week. Bird cherry-oat aphid captures doubled during the sampling week ending on 19th of July with numbers 2--old higher than in the same week in 2001. The increase in winged aphids corresponds with the buildup of aphids in small grains fields across the region. No winged green peach aphids have been captured in the traps and apterae are rare in potato fields. We have one report of green peach aphid detection on potato from Tappen, North Dakota. Green peach aphids are found on canola but their numbers are low for this time of the year. The high temperatures observed this week in the region approached the upper developmental threshold of green peach aphid. Above 90F (32 C), reproduction diminishes and mortality rapidly increases in green peach aphid populations. However, growers should be cautious in interpreting this observation because aphids have a very high reproductive potential. A single green peach aphid can give birth to ~80 nymphs during its reproductive life of approximately 3 weeks. Under favorable conditions these newly born nymphs can reach reproductive maturity within 7 days. Thus, when conditions become favorable aphid populations can increase explosively. Vigilant monitoring of seed potato fields is strongly recommended.

 

CEREAL APHIDS SHOWING UP IN LATER CROP

In the northern ND counties or later planted fields, the Bird cherry-oat aphid has been turning up in increasing numbers. Most fields are advanced enough that significant injury is not likely to occur. Treating headed wheat for aphids has not been recommended based on previous research. What might be seen is some incidence of Barley yellow dwarf virus (BYDV). The BCO aphid is the most efficient vector of this virus. The later an infection of BYDV enters a plant, the less the impact on yield, though. Infections after heading should have minimal consequences.

 

EUROPEAN CORN BORER DECISIONS

Egg laying wrapped up last week and larvae have been feeding. Scouting activities should provide a good assessment of potential problems and whether treatment is needed at this time. The flight was a little later this year, pushing the timing of our treatments about a week later than recent years.

We apologize for poor black light records on corn borer this year. A change to different lights for added flexibility resulted in some unexpected mech. problems. In some cases, when lights were working, the capture of numerous other insects resulted in degradation of the moths and limited our ability to correctly identify ECB.

 

GRASSHOPPERS ON THE MOVE

Adult grasshoppers are moving in search of green plant material. Small grains are ripening in the southern half of the state, so they are moving out of these sites. Some grasshopper nymphs were late risers, and are putting a lot of pressure on some field margins where numbers were lower, earlier.

In general, adult grasshoppers at numbers of 8 or more per square yard are capable of causing enough feeding injury to impact yield of field crops. When monitoring movement into fields, it is important to assess feeding injury before making a treatment decision. Field crops often require defoliation levels of 20% or more before effects on yield are realized. Direct feeding on pods in bean or ears in corn crops should also be considered.

Treatment guidelines for grasshoppers on corn are not well established. Information on beans, specifically soybeans is a little better. Many of these infestations will be the heaviest on the field margins. Treating these areas may lessen the total numbers of grasshoppers successfully entering a field.

Soybeans are most sensitive to defoliation during pod development (Growth stages R4 to R6). During this time, plants can tolerate up to 20% defoliation. Of greater concern, would be direct feeding damage to pods and seeds. Grasshoppers are able to chew directly through the pod walls and damage seed directly. If more than 5 to 10% of the pods are injured by grasshoppers, an insecticide application may be warranted.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 

TIME TO BEGIN SCOUTING SUGARBEETS FOR LYGUS BUGS

Small numbers of tarnished plant ("Lygus") bugs are beginning to appear in North Dakota and Minnesota sugarbeet fields. Areas where Lygus have been observed have included Page, ND, and Crookston and Glyndon, MN. Currently, these infestations are not sufficient to warrant insecticide applications yet; however, they have the potential for causing significant problems under optimal developmental conditions. Growers, agriculturists, and crop scouts should closely monitor beet fields during the next few weeks as other crops and small-seeded broadleaf weeds begin to mature and dry down to determine whether treatment will be necessary.

Lygus bugs use piercing and sucking mouthparts to feed on plant tissue. They inject a salivary toxin as they feed that liquifies and kills plant tissue. Feeding injury in sugarbeets is usually concentrated in new leaves and petioles. Symptoms of Lygus feeding injury include curling and wilting leaves, swollen tumor-like feeding scars on petioles, a black oil-like exudate, and frequently a blackened sooty appearance of new growth near the plant crown (see Fig. 1.). Injury often causes the plant to respond by using carbohydrate reserves to produce new leaves and stems. Unfortunately, this occurs at a time of the season when these reserves should be building up and can result in a reduction of sugar produced by the beet. Both adult and immature (nymphal) stages of the Lygus bug are capable of causing this injury to the plant.

black exudate from lygus
Fig. 1a. Black Exudate seeping
from Lygus feeding site.

Healed scar from lygus injury

Fig. 1b. Healed scar from
Lygus feeding injury.

Adults are about 1/4 inch in length, 1/8 inch wide, and their color can range from dark greenish yellow to a dingy, mottled brown (Fig. 2). Older adults will usually have the distinctive mottled coloration with lightened wing tips and a pale yellow V-shaped mark near the middle of their back.

lygus adult
Fig. 2. Lygus lineolaris adult
(courtesy, USDA)

The Lygus bug passes through five nymphal stages (instars) before reaching adulthood, with each progressive instar increasing in size. First-instar nymphs are very small (1/25 inch long), wingless, and look like a robust, bright green aphid. Also, they have a faint black spot in on the center of their abdomen. Later-instar nymphs will have a total of 5 spots on their backs (Fig. 3).

lygus nymphs
Fig. 3. Lygus nymphs
(courtesy, U. of California Davis)

Lygus adults and nymphs are very elusive. Adults will often fly away and nymphs will usually hide or drop off of the plant as soon as the beet canopy is disturbed. Therefore sampling should be done with care to ensure accurate population estimates. Currently there is no established economic threshold for Lygus bug control in sugarbeets. However, after checking 30 to 50 plants in a field and finding that at least 1/3 of plants are infested with one or more adult or nymph, treatment may be justified. This insect has usually infested beets around mid-August. Thus, consideration of pre-harvest interval may be a critical factor in choosing an insecticide (refer to Table 1 for treatment options). Also, border treatments may be effective if highest Lygus densities are along the edges of a given field. If fields are within two weeks of harvest, injury is not likely to be significant when infestations are at the 1/3 level.

Table 1. Insecticide options for control of tarnished plant bugs in sugarbeet during 2002. 

INSECTICIDE

DOSAGE IN
LB AI/ACRE

PRODUCT
PER ACRE

RESTRICTIONS ON USE

Dibrom 8

0.94

1 pt

Do not apply within 2 days of harvest. Apply by air or ground equipment. Apply in 1 to 5 gallons of finished spray per acre when treating by air. Do not apply more than 5 pts per acre per season.

Lorsban 4E

RUP

0.5

1 pt

Do not apply within 30 days of harvest. Do not apply more than 8 pt/acre per season or make more than 4 applications per season. Field re-entry interval is 24 hours. Fields must be posted.

Mustang 1.5EW

 

RUP

0.028 0.05

2.4 4.3 oz

Do not apply within 50 days of harvest (tops or roots). Do not apply more than 0.15 pound active ingredient per acre per season. Apply by air or ground equipment using sufficient water to obtain full coverage of foliage (minimum of 2 gallons/ac by air and 10 gallons/ac by ground). Field re-entry interval is 12 hours. Fields must be posted.

RUP - Restricted use pesticide

A number of insecticides that are approved for use on sugarbeets have tarnished plant bug or the generic term Lygus as a target pest in their labels for other crops; however, the tarnished plant bug is not listed as a target pest in the sugarbeet portion of those labels. These insecticides include Asana, carbaryl (Sevin), Lannate, Lorsban, and malathion. It is legal to apply an insecticide if it is labeled for use in the crop; however, if the target pest is not listed for that crop, efficacy is not implied by the manufacturer and growers that choose to use the product assume their own liability for any unsatisfactory performance.

Mark A. Boetel
Research & Extension Entomologist
Department of Entomology
mboetel@ndsuext.nodak.edu


cprhome.jpg (3929 bytes)topofpage.jpg (3455 bytes)tableofcontents.jpg (4563 bytes)previous.jpg (2814 bytes)next.jpg (1962 bytes)