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ISSUE 15    August 11, 2005

SUNFLOWER INSECTS

Reports of red sunflower seed weevil continue to come in and populations of this insect are more widespread than in the last several years. Numbers of the red sunflower seed weevil are at treatable levels in some locations.

The banded sunflower moth is present throughout our sunflower grower region. Carefully monitor for this insect as the adult moths are sometimes difficulty to detect. The moths will often stage in broadleaf weeds and crops such as soybean around the margins of sunflower fields during the day. If the moths are easily seen on surrounding plants, be sure to check your fields.

Numbers of Lygus bugs also seem to be higher than last year. If you are growing confection sunflower, watch for this insect.

Check past issues of the Crop and Pest Report for sampling techniques and economic thresholds for all the sunflower insects.

Gary Brewer
Research Entomologist
North Dakota State University

 

SOYBEAN APHID

Many soybean fields are at their most susceptible stage at this time and the soybean aphid is present in most North Dakota fields. A few fields exceeded the 250 aphids per plant threshold during the past week, and some in the southeastern region of the state were treated with insecticides. Aphid numbers are also nearing treatable levels in the following counties: Grand Forks, Walsh, and Traill. Luckily, the infestations in most fields are not at economic levels at this time. The recent period of warm temperatures has accelerated soybean development and many fields are well into the reproductive stages, so we'll soon be out of the window of economic concern for needing to treat in southern growing areas of the state. Vigilance in scouting efforts will be needed during the next two weeks to determine the need for action.

There is some confusion on what is the actual economic threshold for the soybean aphid. If the field_wide average is 250 aphids per plant, this is an INDICATION that you may be approaching an economic infestation. It is NOT a treatment threshold. At the 250 aphid level, one must determine if aphid numbers are continuing to increase. To do this, the field should be assessed again in 2 to 4 days. If numbers increase, then an insecticide application is probably needed. If the aphid numbers are declining, let nature take its course and remove the aphids for you.

Although aphid numbers can increase quickly, they also often decline because of biological control from predators or disease, or because of weather conditions. Another cause for decline is that the aphids will soon start to naturally convert to winged forms and leave soybean for their overwintering host, buckthorn.

So again, the actual threshold to treat for soybean aphid is an average of 250 aphids per plant for the field AND if the aphid numbers are increasing. Previous issues of the Crop and Pest Report give more details of how to sample.

As the season progresses, it is also going to become more important to pay attention to the Pre-harvest Interval (PHI) for insecticides used to control soybean aphid. A couple of the pyrethroids have 45 day PHIs, and several other insecticides are at 28 days required after the application before the crop can be harvested. Consult the North Dakota Field Crop Insect Management Guide (Extension Circular #E-1143) for more information on soybean aphid control. The document is located online at: http://www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm

Gary Brewer
Research Entomologist

Mark Boetel
Research & Extension Entomologist
North Dakota State University

 

LYGUS BUGS INFESTING AREA SUGARBEET FIELDS

Lygus bugs have been detected in many Red River Valley sugarbeet fields during the past 10 days. The heaviest infestations are in southeastern North Dakota (Richland, Cass, Traill, and Grand Forks counties) and adjacent growing areas of Minnesota (Wilkin, Clay, Norman, and Polk counties). Some fields have already received an insecticide treatment while infestations in other fields remain low. Fields should be monitored during the next three weeks to determine if an insecticide application will be needed.

Description. Lygus adults are about 1/4 inch long and 1/8 inch wide, and their color can range from dark greenish yellow to a dingy, mottled brown. Most have a pale yellow V-shaped mark near the middle of their back and two faint light yellow patches near their hind end (Fig. 1).

Lygus lineolaris adult
Figure 1. Lygus lineolaris adult.

Immature Lygus bugs are called nymphs. They pass through five nymphal growth stages (called instars) before reaching adulthood. First-instar nymphs are very small (1/25 inch long), wingless, and look like a large, bright green aphid. Also, they have a faint black spot (actually a scent gland) on the center of their back. Later-instar nymphs have 4 additional true spots (Fig. 2).

Lygus nymphs
Figure 2. Lygus nymphs
(courtesy J.K. Clark, Univ. Cal-Davis)

Damage. Lygus adults and nymphs use piercing and sucking mouthparts to feed on the plant. They pierce the plant and inject a salivary toxin which liquifies and kills plant tissue. The bugs then suck up the resulting liquid. Feeding injury in sugarbeet is usually concentrated in new leaves and petioles. Symptoms of Lygus feeding injury include curling and wilting leaves, seepage of a black oil-like exudate (Fig. 3.a.; sometimes referred to as "tarring"), swollen tumor-like feeding scars on petioles (Fig. 3.b.), and often a blackened sooty appearance on new growth near the plant crown.

Black exudate seeping from Lygus feeding site.
Figure 3a. Black exudate seeping from
Lygus feeding site.

Healed scar from Lygus feeding injury
Figure 3b. Healed scar from Lygus feeding injury.

Lygus injury is believed to cause plants to respond by using carbohydrate reserves to produce new leaves and stems. This can lead to major sugar yield losses if it occurs later in the season when the reserves should be building up in the root.

Sampling. Sampling for Lygus bugs should be done with care because adults often fly away and nymphs usually hide or drop from the plant as soon as the canopy is disturbed. Also, young nymphs blend in well with the beet canopy due to their green color and can be difficult to detect.

Threshold: an insecticide treatment is probably needed if harvest is at least 3 weeks away and if the infestation exceeds one Lygus bug per plant (adults and nymphs combined). An insecticide application is probably not needed if harvest will occur in 3 weeks or less. For more information on control strategies, consult the Insects section of the 2005 Sugarbeet Production Guide. The online version is at:

http://www.sbreb.org/Production/production.htm

Mark Boetel
NDSU Entomologist
mboetel@ndsuext.nodak.edu

Ian MacRae
University of Minnesota Entomologist
imacrae@umn.edu

 

WHAT ARE THE WHITE OR YELLOW COLORED BUTTERFLIES FLYING AROUND IN ROADSIDES, GARDENS AND AGRICULTURAL CROPS?

These beautiful butterflies belong to the Family Pieridae called Whites or Sulphurs. Their light coloration is a result of the production of uric acid wastes during metamorphosis. The most common Pieridae that is present now is the cabbage butterfly (Pieris rapae), which is widely distributed across North Dakota and found in many environments. Adult butterflies feed on nectar of flowering plants including mustards, dandelion, red clover, asters, and mints. The larvae (caterpillar) called imported cabbageworm feeds on many Brassicaceae hosts, such as cabbage, broccoli. It can become a problem for the home gardener. Imported cabbageworms are also found on canola, but are not considered an economically significant crop pest in North Dakota. There are usually multiple broods reaching "swarm" levels by late summer, coating the radiators and bug screens of automobiles statewide.

cabbage butterfly (male)
Cabbage butterfly (male)

Janet Knodel
Area Extension Specialist
North Central Research Ext. Center
jknodel@ndsuext.nodak.edu


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