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ISSUE 16  August 23, 2001

 

SOYBEAN APHIDS FOUND IN NORTH DAKOTA

On Friday, August 17, soybeans in the Mapleton, ND (Cass County) area were checked for soybean aphid. Soybean aphids were found at that time. Since that original announcement, several consultants in the valley region have reported finding soybean aphids in fields, as well.

The aphids were present in very low numbers. We were sampling by looking at the undersides of upper leaves, often looking at plants one right after the other. Our goal at the time was just to find them if present. During 40 minutes of checking one field, a single alate (winged) aphid was found. In addition, we found young nymphs, ranging from 1 to 3 aphids per leaf when present.

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These nymphs were small, less than or equal to 1mm, therefore it required some close scrutiny. Often, it was only a small yellow spot on a leaf that would catch your eye. With closer scrutiny, the aphid could be seen. Their size suggests the migration of winged aphids and their bearing of young has been recent and the stay is rather transient for the winged individuals. In other words, the female lands, bears one or two babies and then moves on to another plant.

These young nymphs that we found a week ago are beginning to have young of their own. This means some small colonies are going to start forming. Thes colonies will consist of a larger, mother aphid surrounded by her smaller offspring

I would be surprised if soybean aphid causes any problems with this years crop. The arrival is late, numbers are low, and plants are generally in good health. It does mean that next year we are likely to see soybean aphid much earlier in the season due to the likelihood of overwintering in the region.

Surveys this week will be much more extensive. I suggest others take a look where you work and see if you can find the small yellow aphids on soybean plants. If you find an indication of soybean aphid, we would be interested in knowing the location. It would be nice to determine the northen and western extent of their dispersal.

For more information on soybean aphid, visit the North Dakota Insect Updates page at:

http://www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates/index.htm

There are links under soybean to info on soybean aphid. Pictures are included in these links.

 

VARIEGATED AND BLACK CUTWORM DAMAGING SUGARBEETS

There have been many fields where one or both of these cutworms are feeding on beets. The Variegated cutworm is a climbing cutworm so feeding injury is on the leaves, perhaps some feeding on petioles as well. This cutworm is most easily recognized by the 4+ yellowish-white spots down the middle of the back. Treatment guidelines are sketchy since we lack a lot of experience with this pest at this time of year. Treatment may be warranted when there are 3 to 5 larvae per square foot and larvae are are still small (3/4 inch or less).

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In addition, Black cutworms are feeding in fields, as well. In some cases the black cutworms are more abundant than variegated, or the reverse. This cutworm is more of a charcoal color. The Black cutworms typically feed at or below the soil line and up to a few inches deep depending on soil moisture conditions. They will move up in the soil profile with increasing soil moisture. This poses a much bigger problem since larvae will be difficult to control with insecticide sprays that may not reach the feeding site. Many of these larvae are also large (1 inches) and should be about finished feeding.

Phillip Glogoza
Extension Entomologist
 
pglogoza@ndsuext.nodak.edu
 

 

TARNISHED PLANT BUGS INFESTING RED RIVER VALLEY SUGARBEETS

Tarnished plant bugs (TPB) are again causing problems for Red River Valley sugarbeet producers. Moderate to heavy infestations have been observed in a few fields near Crookston, MN, in the central portion of the Valley.

The tarnished plant bug is often referred to as the "Lygus bug." This term is derived from its scientific name, Lygus lineolaris. However, Lygus actually refers to a complex of a few plant-feeding insect species in the Lygus genus. These insects use piercing and sucking mouthparts to feed on plant tissue. As TPB feed, they inject a toxin with their saliva that liquifies and ultimately kills plant tissue. Feeding injury in sugarbeets is usually restricted to new leaves and petioles. Symptoms of TPB feeding injury include curling and wilting leaves, tumor-like feeding scars on petioles, and blackening of the new growth near the center of the crown. 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 TPB are capable of causing injury to the plant.

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. 1). 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.

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Figure 1. Adult tarnished plant bug (courtesy, USDA).

The TPB 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. 2).

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Figure 2. Fifth-instar TPB nymph

The Both adults and nymphs are very elusive. Adults will often 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 TPB control. 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 TPB adult or nymph, treatment may be justified. This insect has usually infested beets in August. Therefore, 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 the majority 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.

Growers, agriculturists, and crop scouts should promptly assess their beet fields and keep a close watch on them 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. Also, beets in areas where other crops such as soy- and dry beans are stressed due to iron chlorosis and injury from severe weather events may be more at risk than others because the stressed plants may mature earlier than normal causing adult TPB to move out to search for more attractive food sources.

Table 1. 2001 Insecticide Options for Tarnished Plant Bug Management in Sugarbeets

Insecticide

Dosage in Lb/AI/Acre

Product Per Acre

Restrictions on Use

Sevin XLR

1.0 - 1.5 lb/acre

2 - 3 pts

Do not apply within 28 days of harvest

Lannate LV
RUP

0.22 - 0.9 lb/acre

0.75 - 3.0 pts

Do not apply within 7 days of harvest. Do not feed tops to livestock within 30 days after application. Field re-entry interval is 48 hours. Fields must be posted

Lannate SP
RUP

0.22 - 0.9 lb/acre

0.25 - 1.0 lbs

Do not apply within 7 days of harvest. Do not feed tops to livestock within 30 days after application. Field re-entry interval is 48 hours. Fields must be posted

Lorsban 4E

0.25 - 0.5 lb/acre

0.5 - 1.0 pts

Do not apply within 30 days of harvest. Do not apply more than 8 pints/acre (broadcast basis) per season, or make more than 4 applications per season. Do not feed tops to livestock within 30 days after app.

Malathion 57 EC

1.0 - 1.25 lb/acre

1.5 - 2.0 pts

Do not apply within 3 days of harvest

RUP - Restricted Use Pesticide

Mark Boetel
Research & Extension Entomologist
NDSU Department of Entomology



LYGUS IN SUGARBEETS - SPRAY TRIAL

On Sunday Aug 19, we applied a spray trial against Lygus bug on sugarbeets at the U. Minnesota
Northwest Research & Outreach Center in Crookston. 

Application treatments were: 
Lorsban 0.5 pt/ac (low label rate) 
Lorsban 1.0 pt/ac (high label rate) 
Malathion 1.5 pt/ac (low label rate) 
Malathion 2.0 pt/ac (high label rate) 
Untreated Control 

We evaluated low and high label rates for both chemicals. Asana was included for comparison only. It is important to remember that Asana is NOT registered against lygus in sugarbeets.  Although Asana was included in this trial, it's performance was below that of either Lorsban or Malathion. There were 4 replications of each treatment. We assessed the impact of the insecticides by calculating the percentage of the lygus population killed in each treatment (comparing pre-counts of lygus in each plot with counts taken 48 hours after treatment and calculating the percentage of lygus killed by the treatment).

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The graph above shows the results. The data indicate that Lorsban is probably the most effective insecticide to use against lygus in subargeets. It is Interesting that there was ~50% mortality shown in the control plots. We had a heavy rain on Friday, after the precounts and before the application and it is probable that the mortality in the control plots is associated with this weather. Heavy rain can knock insects off plants to the ground where they are exposed to greater levels of predation. Both
rates of Malathion performed well, and given this products short pre-harvest interval (3 days) it may be a good fit for late season lygus control.

Ian MacRae
UMN Dept of Entomology, NWROC
imacrae@tc.umn.edu

Mark Boetel
Dept of Entomology, NDSU

mboetel@ndsuext.nodak.edu


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