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ISSUE 15    August 10, 2000

QUESTIONS ABOUT CORN LEAF APHIDS

    Corn leaf aphids reached noticeable numbers in fields in areas east of the Red River Valley. In most cases, plants had smaller colonies of aphids on tassels or developing ears, with larger colonies confined to only isolated plants within the field. In some fields though, large colonies covering tassels were reported on 50% or more of the plants.

    Normally, it is relatively easy to find small colonies of this blue-green aphid feeding on the leaves of most plants. The severely infested plants will have large numbers of the aphids colonizing the emerging tassel and upper most leaves.

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    The following information appeared in the NDSU Crop and Pest Report in July, 1997, the last time large numbers of corn leaf aphid caused some concern in the region.

    What kind of impact can corn leaf aphids have on the plant ? In 1996, University of Illinois Extension entomologists, Kevin Steffey and Mike Gray, provided information through their newsletter that reported on a "quick-and-dirty" study from 1980 conducted to assess the impact aphids had on field corn yields. They tagged plants with no aphids, moderate numbers (100 or more but no leaves covered), and heavy numbers (tassel and the top 2 to 4 leaves with aphids). The plants were tagged after pollination was complete. The ears from tagged plants were hand harvested and kernel weight was determined. They found a significant decrease in kernel weight as the density of aphids increased. In their "high-management" plots, moderate infestations reduced kernel weight by 11%, heavy infestations by 37% when compared to non-infested plants. They also found more barren plants in the samples from heavily infested plots than in the moderately and non-infested plots.

    Corn leaf aphids cause the greatest injury to corn before pollination is completed. Results of a heavy infestations are stunted plants, barren stalks, and shrunken ears. Aphids can continue to stress plants after pollination, but general moisture stress plays a larger role in the extent of this impact.

    So, what is the treatment threshold ? Throughout the corn belt, the accepted threshold is when 50% of the plants during the late-whorl to early tassel stages have light to moderate infestations (50 to 400 aphids per plant) AND plants are under drought stress, a treatment may be warranted. If pollination is complete and plants are not drought stressed, corn leaf aphids pose little threat to corn yield.

    When assessing an aphid situation in corn, determine the average infestation level in the field, not just the worst plants. Be sure to take in account beneficial insects present, whether parasitized aphids are present ("mummies"), and the presence of diseased aphids. All of these things would indicate conditions that are likely to lead to a natural decline of the population. Currently, lacewing larvae, lady beetle larvae, and predatory fly larvae are throughout corn fields. Many diseased aphids can currently be found as a result of heavy dews and sporadic showers which favor aphid disease outbreaks.

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EUROPEAN CORN BORER UPDATE

    As mentioned last week, there was an increase in numbers of ECB moths being caught in blacklight traps in ND. It looks like those captures are continuing this week. Reports from central and southern Minnesota indicate that the second generation flight is well underway . Though the impact of this second generation infestation on corn in ND is not well understood, it would be advisable to watch how the larval populations develop. The direct impact on yield during these later infestations is less (6% yield loss per borer in pretassel versus 3% loss per borer after kernels are initiated). The big unknown is the ear shank tunneling and lodging potential that occurs prior to harvest. If larval infestations do develop, fields at risk to lodging should be identified and harvested in a timely manner, exposing them to fewer weather events that can break the stalk.

 

EVALUATE WHEAT MIDGE LARVAL INFESTATIONS

    In northwestern counties, it is time to evaluate wheat midge levels in wheat heads before larvae drop to the ground. An infested wheat head does not change its appearance so it is necessary remove the glume and expose the kernel or rub drier heads. We would suggest that random samples of wheat heads (approx. 20 heads per site, 4 to 5 locations) be used for this type of evaluation. This needs to be determined before midge drop from the heads after rainfall or heavy dews. So do not delay too long to look for the larvae.

    If you evaluate a field for potential yield reduction due to this insect, you want to establish the percent infestation level for the field, not just selecting some of the worst infested wheat heads.

    Research has shown that the percentage yield loss is closely related to the percentage of kernels infested with wheat midge. Entomologists estimate that an average of 13 larvae per head will reduce wheat yields to a point where control becomes profitable. For example, an infestation level of 2% reflects a larval population of 1-2 larvae per head (or 0.033 larva per kernel). The relationship between yield loss and the number of midge larvae and infestation level is summarized in the table below.

 

Larvae per kernel

% infested kernels

Estimated % Yield Loss

1

30-38%

40%

2

58-60%

65%

3

78%

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4

90-96%

79%

 

RED SEED WEEVIL UPDATE

    As more fields begin to flower, reports are coming in from around the state of moderate seed weevil activity. Many of the oilseed fields are staying below treatable levels at this time. Common reports indicate numbers ranging from 2 to 5 seed weevils per flower, just below the current threshold of 6 to 8 weevils per head. Many confection sunflowers have been or are being treated. The situation changes quickly as more buds flower in fields and seed weevils migrate to those sites.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 

TIME TO MONITOR SUGARBEET FIELDS FOR LYGUS BUGS

    Lygus Bug (a.k.a., Tarnished Plant Bug) populations are increasing in a variety of plant habitats throughout the Red River Valley. These insects have piercing and sucking mouthparts and inject a toxin with their saliva that kills plant tissue. Feeding injury in sugarbeets is usually restricted to new leaves and stems. Symptoms of Lygus feeding injury include curling and wilting leaves, tumor-like feeding scars on stems, 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.

    Adults are about 1/4 inch in length, 1/8 inch wide, and their color can range from dark greenish yellow. Older adults will usually have a distinctive mottled coloration with lightened wing tips and a pale yellow "V-shaped" mark near the middle of their back. There are five immature (nymphal) stages (all green in color) before adulthood, with each progressive stage increasing in size. First-stage nymphs are very small (1/25 inch long), wingless, and are bright green in color. Both adults and nymphs are very active elusive, and usually hide or drop off of the plant as soon as the beet canopy is disturbed.

    Currently, Lygus numbers are not at economically significant levels in sugarbeets yet. However, growers, agriculturists, and crop scouts should be keeping a close eye on fields during the next few weeks as other crops and small-seeded broadleaf weeds begin to dry down to determine whether treatment will be necessary. Significant infestations were observed in Pembina and Grand Forks Counties of North Dakota, and Polk, Red Lake, and Kittson Counties of Minnesota during the 1999 growing season. Fields in those counties may be at risk this year since overwintering conditions were very mild and conducive to Lygus Bug survival. Also, beets in areas where other crops such as sunflowers, soy-, and dry beans are stressed and dying due to June flooding events may be more at risk than others because the stressed plants may mature earlier than normal causing Lygus adults to move out to search for more palatable food sources.

    There is no established economic threshold for Lygus Bug control. However, after checking 30 to 50 plants in a field and finding that at least 33% of plants are infested with one or more 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. 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 expected to be significant when infestation levels are at the 33% level.

 

Insecticide Options for Lygus Bug Management in Sugarbeets

Insecticide

Dosage in lb
ai/acre

Product
Per Acre

Post Harvest Interval

Asana XL

RUP

0.03 - 0.05 lb/acre

5.8 - 9.6 fl oz

21 days

Sevin XLR

1.0 - 1.5 lb/acre

2 - 3 pts

28 days

Lannate LV

RUP

0.22 - 0.9 lb/acre

0.75 - 3.0 pts

7 days

Lannate SP

RUP

0.22 - 0.9 lb/acre

0.25 - 1.0 lbs

7 days

Lorsban 4E

0.25 - 0.5 lb/acre

0.5 - 1.0 pts

30 days

Malathion 57 EC

1.0 - 1.25 lb/acre

1.5 - 2.0 pts

3 days

RUP - Restricted use pesticide

 

Mark Boetel
NDSU Entomologist
mboetel@ndsuext.nodak.edu


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