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ISSUE 9   June 29, 2006

WATCH FOR INCREASING APHID POPULATIONS IN SMALL GRAINS

Increasing populations of aphids were detected this past week in isolated spots within and near the Red River Valley. Aphids cause injury to grain crops by sucking plant sap and vectoring barley yellow dwarf virus. Late-planted small grains will be at a higher risk for aphid infestations. The treatment threshold is 85% stems with at least one or more aphid present, prior to heading. The crop’s growth stage also affects its susceptibility to aphids. The vegetative to boot stages are the most susceptible stages to aphid feeding and subsequent yield loss. For example, in the 4-6 leaf stage, aphid injury results in stunting, decreased number of kernels per head, and decreased kernel weight. In the boot stage, only kernel size and weight are affected by aphid feeding (not number of kernels per head). By the time heading occurs, only kernel weight is affected. After flowering, small grain crops are less susceptible and producers are discouraged from spraying. In addition to the incidence of aphids (percent of infested stems), pest managers should take into account how many aphids are on a plant and how long of a time period aphids have been on a plant. Research on aphids in small grains indicates that yield loss occurs when at 0.6 bu for 100 aphid days (MacRae, Univ MN). So, if a field averaged 10 aphids per stem and was 12 days from heading, it would have 120 aphid days (10 aphids x 12 days). This is above the 100 aphid-day threshold and an insecticide should be applied to prevent yield loss. On the other hand, if the field averaged only one aphid per stem and had 12 days to reach heading, it would only have 12 aphid days (1 aphids x 12 days). Thus, no control action would be necessary. Be sure to check for natural enemies, such as lady beetles and lacewings, when scouting fields. They often are very effective at keeping aphid infestation levels in check later in the summer.

 

WATCH FOR INCREASING SOYBEAN APHIDS POPULATIONS

Continue to scout for soybean aphids (see photo). The critical growth stage for making most soybean aphid management decisions is in the late vegetative to reproductive (Vn to R4). Assessing aphid populations at this time is critical.

Recent pest management research has led to the following economic thresholds based on crop growth stages:

- R1(beginning flower) to R4 (full pod) = 250 aphids/plant when populations are actively increasing

- R5 (beginning seed) = 250 aphids/plant when populations are actively increasing (Note: Positive yield response from treating at this stage is less predictable.)

- R6 (full seed) = No treatment necessary. Research trials throughout the north central states have not demonstrated a yield benefit to treating soybean for soybean aphid management at R6 or later stages.

Stay tuned for more scouting updates and insecticide efficacy information.

Soybean aphids
Soybean aphids

 

BERTHA ARMYWORM EMERGENCE UNDERWAY IN CANOLA!

Bertha armyworm is out in northeast and north central regions of North Dakota. Pheromone trap catches ranged from 0 to 109 moths per trap week. Locations with high trap catches (>50 moths per trap week) were in McLean and Bottineau counties. Adult moths have been flying for probably two weeks. Adults are greyish moths with a wingspan of about 1½ inch. Each forewing has a white kidney shaped cell and a border of white fringe (see photo). Bertha armyworm eggs will hatch in about a week and develop into a larvae (caterpillars). Emerging larvae (1/10th of a inch) are usually green in color and hide underneath leaf litter and clumps of soil during the day. This makes it difficult to find small larvae! Mature larvae are about 1½ inch long (see photo) and vary in color from green to brown or velvety black. Larvae take six weeks to complete development. Crop injury results from larval feeding on canola pods.

Adult bertha armyworm
Adult bertha armyworm (Canola Coucnil of Canada)

Generally, when the accumulated trap numbers exceed 500, there is a good chance that crop injury will occur. So far, all accumulated trap catches are below 500 moths. Economic threshold levels range from 18 to 22 larvae per square yard.

Mature bertha armyworm larvae
Mature bertha armyworm larvae

 

UPDATE ON WHEAT MIDGE EMERGENCE

Continue to be vigilant in scouting wheat / durum crops for wheat midge infestations in susceptible stages - heading to early flowering! Midges in the northern part of state are near 10% female emergence while those in the southern region are near 100%. Remember, adults will continue to live about 3-7 days after emergence, depending on latitude and environmental conditions. Higher temperatures will accelerate emergence and crop development, which should shorten the interval of crop susceptibility to wheat midge injury.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu

 

LEAFMINERS INFESTING AREA SUGARBEET FIELDS

Leafminers were detected in Red River Valley sugarbeet fields during the past week. Infestations in most sites have been below economically significant levels. Most infestations are believed to be in central and southern portions of the Valley.

Description & damage: Female flies lay groups of 3 to 10 white, oval eggs on the undersides of beet leaves. Eggs hatch within 3 to 10 ten days, depending on air temperatures. After hatching, the larvae (pale-green to whitish colored worms; tapered from front to back [Fig. 1]) tunnel into and feed between the upper and lower surface of the leaf, creating "mines".

Full-grown leafminer
Fig. 1. Full-grown leafminer larva inside "mine."

The mines expand and run together as feeding progresses, and the injury leads to necrotic leaf blotches (Fig. 2).

Necrotic blotch on sugarbeet leaf
Fig 2. Necrotic blotch on sugarbeet leaf from leafminer injury.

Although major problems are not expected to develop later in the season, fields should be monitored for the next two weeks to determine if treatment is needed.

Scouting: Field scouting and early detection are crucial for effective control. Scouting is important because leafminers may only be a major problem isolated fields. Early detection will allow for more effective use of insecticides because they work best when applied just before or at egg hatch; however, very good control has been achieved after larvae have already tunneled into leaves. To scout a field, sample several sets of 10 random plants in several representative areas within a field. The more samples taken, the more reliable the estimate will be.

Economic threshold: Fields should be treated if the combined number of eggs and live larvae exceeds square of the average number of leaves on plants. For example: beets in the 8-leaf stage need to have at least 64 eggs + larvae per plant. Thus, it is unlikely that any fields will require treatment at this time. Refer to Table 1 for treatment options.

Insecticides not specifically labeled for leafminer control can be used as long as they are registered for foliar application to sugarbeet; however, the manufacturers are not liable for losses or unsatisfactory performance associated with control failures if the label does not explicitly offer leafminer control.

Table 1. Treatment options for controlling leafminers in sugarbeet.

Product

Use Rate / Acre

(formulated product)

Re-entry Interval
(hours)

Asana XL 0.66 EC*

9.6

12

D Z N diazinon 50W RUP

0.75 – 1.0 lb

24

D Z N diazinon AG500
RUP

0.75-1.0 pt

24

D Z N diazinon AG500 WBC (water-based concentrate)
RUP

10.0 - 13.5 fl.oz

24

D Z N diazinon 500AG
RUP

0.75-1.0 pt

24

Lannate SP*
RUP

0.25 - 1.0 lb

48

Lannate 2.4LV*
RUP

0.75 - 3.0 pt

48

Lorsban 4E

1 pt (2/3 pt banded)

24

Mustang Max 0.8EC*

2.24 – 4.0 fl oz

12

RUP – Restricted Use Pesticide

*Labeled and legal for use in sugarbeet, but manufacturer does not include leafminer control on label and satisfactory control is therefore not warranted.

Always read, understand, and follow all label instructions and precautions!

For more specific information on leafminer biology, identification, and management, please refer to NDSU Extension Circular #E-1288. An online, printable version of this publication is located at:

http://www.ext.nodak.edu/extpubs/plantsci/pests/e1288w.htm

Mark Boetel
Research & Extension Entomologist
mark.boetel@ndsu.edu


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