NDSU Crop and Pest Report

Entomology


ISSUE 11  July 10, 2003

SOYBEAN APHID TIME

Soybean aphid reports are now starting to trickle in from around the region. The NDSU IPM Pest Survey has been checking fields for two weeks now, reports are coming in this week of finding fields with aphid colonies present. Counties where aphids have been found include: Richland, LaMoure, Dickey, Steele, Traill, and Grand Forks. By the end of the week we will probably see some of the gaps filled in, such as Cass and Barnes.

Therefore, it is time to start scouting for soybean aphid. Based on patterns of field colonization observed last year, and already this year, focus on surveying field margins, particularly near the shelterbelts and other structures. It appears these areas are where the aphids settle in after their migration to fields. Eventually, the fields have aphids throughout, but last year it took about five weeks for fields to become generally infested after first detection.

Infestations last year were always greatest near these shelterbelts/structures where initial colonies formed. Often these were the only areas that required treatment. The large field size and general openness of the landscape appears to have an influence on how the aphids establish in fields. Areas of the field that are quite open, without trees, the aphid numbers remained low until the end of August, after the critical growth stages believed to be at risk.

Soybean Aphid Sampling Strategies

The following sampling procedure has been suggested by DiFonzo and Hines (2002) based on their field experience from the 2001 production year. As populations increased in 2001, they found the easiest way to sample for soybean aphid was to evaluate individual leaflets on a 0 to 4 scale. This sampling method proved useful when evaluating fields before and after spraying. Using this scale, leaflets can be quickly rated without counting individual aphids. The leaflet in the example is clearly a "4" on the scale. When the average leaf rating is 3 or greater, and other conditions are met, treatment would be advised.

Soybean aphid leaflet rating

Treatment Threshold and Spray Timing

The treatment threshold is still being better defined. Currently, the guidelines for making soybean aphid treatment decisions are:

Begin scouting soybean fields at the V3 to V4 stage to determine if soybean aphids are present in fields. No treatment is recommended at this time and is discouraged so insecticides do not reduce the presence of predators and parasites.

The critical growth stage for making most soybean aphid treatment decisions appears to be the late vegetative to early reproductive (Vn to R2). Assessing aphid populations at this time is critical. Conclusions from 2001 management programs in Wisconsin, Minnesota and Michigan find that the best results from an aphid treatment occurred from mid July to early August.

Treatment to manage soybean aphid would be recommended at early flowering (R1 to R2) when aphids are abundant on most plants (guideline: aphids number 25 or more per sampled leaflet - see rating scale). University of Wisconsin research trials during 2001 found that a population of 200 aphids/plant during susceptible growth stages (R2 to R4) resulted in a yield loss of about 6 bushels/acre, a yield loss near or above the break-even point for the cost of an insecticide application.

Reference: DiFonzo, C, and R. Hines. 2002. Soybean Aphid in Michigan: Update from the 2001 season. Michigan State University Extension Bulletin E-2748.

Soybean Aphid Insecticide Options

In the next few weeks, decisions to control soybean aphid infestations are going to be made. Several insecticide efficacy trials were conducted in 2001when large populations were present in neighboring states. The following table and graph provides a summary of two trials conducted by University of Minnesota researchers. The rates correspond with different label rates but, the objective was to achieve well over 85% control, minimizing the numbers of aphids surviving the treatment in order to delay the anticipated increase of the aphid population, and give the soybean plant a chance to progress to a growth stage where aphid feeding will have minimal impact on yield.

Though many will not like the idea of using the mid to high label rates, the best results are expected to be achieved with treatments that are in the upper range of the label rates. Using low rates will increase the risk of leaving enough aphid survivors to result in a rapid increase in the population, especially in the absence of predators and parasites that will be killed by the insecticide treatments. A rapid recovery of the population could result in the need to reapply treatments for adequate protection over the risk window of R1 to R4 growth stages.

Insecticide efficacy against soybean aphid based on mean # of aphids per plant, 4 days following treatment (Source: Ostlie, Ragsdale, and Hodgson.   University of Minnesota, 2001)

Treatment

Rate
(oz /acre)

Mean #
Aphids/Plant

% Control

Untreated control

-----

393.2 A

-----

Dimethoate 4 EC

16 (1 pint)

58.2 B

85.1

Pounce 3.2 EC

8.0 (˝ pint)

40.1 B

89.8

Fulfill 2 50 WG *

2.75

13.3 BC

96.6

Asana 0.66 EC

9.6

3.7 CD

99.1

Warrior T 1EC

3.2

2.0 D

99.5

Provado 1.6F *

3.75

1.6 D

99.6

Actara 25 WG *

3.0

1.2 D

99.7

Leverage 2.7 L *

3.75

1.1 D

99.7

Furadan 4F

8.0 (˝ pint)

0.03 D

99.9

Penncap-M 2FM

48 (3 pints)

0.03 D

99.9

Lorsban 4EC

32 (2 pints)

0.00 D

100.0

Means followed by the same letter are not significantly different
* Insecticide not labeled on soybean

 

Aphid densities graph

Aphid densities after treatment with labeled
insecticides for the control of soybean Aphid. 
2003.

 

Soybean Aphid vs Potato Leafhopper Nymphs

While scouting soybeans for aphids, be sure to recognize the difference between these two insects. The key differences are summarized below.

Soybean aphid vs. Potato Leafhopper image

Soybean Aphid

Potato Leafhopper

yellow

pale green

pear-shaped

longer than wide

stationary

moves sideways rapidly

found in colonies

found separately

Be sure to recognize the difference between the two. Typically, we haven’t seen problems with potato leafhopper in soybean. One published threshold for potato leafhopper in soybean suggests "... 9 leafhoppers per plant for R1-R2 (first bloom) stage soybean." Leafhopper concerns in soybean are most often associated with dry conditions.

 

POTATO LEAFHOPPER IN OTHER CROPS

Dry beans, alfalfa, and potato are affected more seriously by potato leafhopper. Numerous reports of large populations of potato leafhopper in dry beans and potato are coming in from around the region. Nymphs are hatching in these crops, and should be monitored. The adults are quite mobile and pose less of a threat to plants. Nymphs on the other hand are not mobile and will feed on leaves where they hatch. Most control efforts are directed at nymphs because of the greater threat of plant injury that they pose.

 

SECTION 18 FOR MUSTANG MAX ON FLAX

On July 3, the ND Department of Agriculture issued a section 18 allowing the use of Mustang Max on flax to control grasshoppers. A copy of the label must be in possession of the user, and is available on line at:

http://www.ag.ndsu.nodak.edu/aginfo/pesticid/sec18_2003.htm

Use rates are 2.8 to 4.0 fl oz per acre. The post harvest interval is 30 days. The exemption expires September 30, 2003. Refer to the label for additional use restrictions.

 

PEA APHID IN FIELD PEA

Pea aphids have been observed in field peas growing in the central part of the state. Pea aphid is a common aphid found in legumes, particularly alfalfa. However, populations have rarely reached treatable levels in North Dakota fields.

Pea aphid image

Adult pea aphids are soft_bodied, slow_moving, and range in color from light to dark_green. They are pear shaped, about 1/8 in. long and 1/16 in wide, with long slim legs. The antennae of the pea aphid show narrow dark bands at the tip of each segment. The nymphs are smaller but closely resemble the adults.

Economic Thresholds

Sampling to determine aphid density should be done when 50_75% of the pea plants are in flower. The threshold in Century peas is 2_3 aphids per 8 in of plant tip at flowering. Trapper peas can tolerate higher levels. Plants infested before the flowers open recover without loss of yield.

Insecticides labeled in field peas include: Asana, carbaryl, dimethoate, Mustang, and Warrior.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 

NEW CANOLA SEED TREATMENT RECEIVES FEDERAL REGISTRATION APPROVAL

PROSPER™ FL, Flowable insecticide and fungicide seed treatment, from Gustafson LLC has just received federal registration approval. Prosper FL is a systemic insecticide (clothianidin) and fungicide (Vitavax, Thiram, Metalaxyl) seed treatment for use on canola and rapeseed to control certain insect pests and diseases. These pests include: flea beetles, seed borne Blackleg, seed rot, damping off, seedling blight, and early season root rots caused by Pythium, Rhizoctonia, Fusarium, and Alternaria. Prosper FL is for protection of seeds and emerging seedling only. Research data will be available this fall on Prosper FL and flea beetle control (J. Knodel).

Janet J. Knodel
Area Extension Specialist Crop Protection
North Central Research and Extension Center
Minot, ND


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