Aphid Alert 1998, No. 4, August 20

Features of this issue

  • Insecticidal control of green peach aphid
  • Aphid flight activity to date
  • Aphid flight activity 3-13 August

In the last issue of Aphid Alert, we stated there are few insecticides that reliably control green peach aphids. Currently, there are only three registered products that give consistent results, Monitor®, Provado®, and Admire®. In this issue we present the results of an insecticide trial for green peach aphid control conducted at the Rosemount Agricultural Experiment Station. The purpose of this trial is to evaluate performance of currently registered products side by side with unregistered products that are in the process of obtaining full EPA registration. We also wanted to compare efficacy of the registered products at lower than label rates.

Biological Control

Green peach aphid populations can often be held below damaging levels by a variety of predators, parasites, and diseases. These natural enemies unfortunately develop more slowly and are quite susceptible to insecticides and fungicides, thus limiting their effectiveness. We often see dramatic increases in aphid populations following an insecticide application to control Colorado potato beetles and potato leafhoppers. This occurs because predators and parasites are killed by these broad spectrum insecticides. Recently, we have observed that frequent (weekly) application of fungicides is harmful to a group of beneficial fungi that are important in keeping the green peach aphid populations in check. In general, these crop protection chemicals (insecticides and fungicides) used to control a variety of pests in potato, reduce the effectiveness of the natural enemies and allow aphid populations to increase rapidly. As a result, aphid management in potatoes is primarily by insecticides.

Insecticide options

Provado and Admire have the same active ingredient, imidacloprid. If you used Admire at planting (soil applied), you should not use Provado later in the season as a foliar applied material. This will accelerate the development of resistance in aphids and Colorado potato beetles. Fortunately in tests conducted at the University of Minnesota, Admire provides season-long aphid control (Table 1), especially when applied at the maximum label rate (13.1 fluid ounces per 1000 row feet or 18.9 oz/acre with 36" row spacing).  

Description of Insecticide Trial

A green peach aphid insecticide trial was conducted between 28 July and 17 August, 1998. Russet Burbank, New Leaf potatoes were planted on 18 May at the Rosemount Experiment Station. Plots were four rows wide, 50 ft long, with 36 inches between rows and 12 inches between plants within a row. Plots were separated on all sides by 10 ft of cultivated soil. Applications were made using a tractor-mounted Brittonya boom sprayer with drop nozzles. Foliar sprays were applied in 50 gal of water/acre at 80 psi on 28 July and 11 August. All plots received a weekly application of fungicide (Polyram) and three applications of Asana to control potato leafhopper and uniformly increase the green peach aphid population throughout the experiment. Admire was applied at planting (18 May) using an electric powered "dribble" applicator attached to the planter placing the insecticide in-furrow ahead of the seed piece. Data reported in Table 1 are the number of green peach aphids per 35 midplant leaves per plot taken 3 and 6 days after the second application of foliar insecticide.

Interpretation of results

In general, all recommended insecticides (Monitor, Provado and Admire) when used at labeled rates gave adequate control of green peach aphid. The lowest rates of Monitor and Admire, both below the recommended labeled rate, failed to give satisfactory control. Cutting the rate to save on insecticide costs would compromise control.

The experimental products Fulfill, Legend, and GCA-293343 (Actara®) gave good control by 6 days after application. Fulfill has a unique mode of action that needs some explanation. This product paralyzes the muscles associated with the aphids sucking mouthparts. Death is essentially by starvation. Soon after applying Fulfill reproduction ceases, but death takes several days.

Some insecticides such as Asana and Penncap-M cause aphid populations to increase dramatically. Most of the older compounds often used to control potato leafhopper and Colorado potato beetle will flare green peach aphids. This is especially evident if the products are used more than once.

At the present time insecticide options for aphid control are limited. Hopefully by next year one or two of the experimental compounds will be registered, expanding the options available to growers for green peach aphid control.

Table 1. Average number of green peach aphid per 35 mid-plant leaves, Rosemount, MN 1998.

Treatment Rate
(product / acre)
Pre-treatment
count
3 days 6 days 10 days
  (Product/Acre) count       
Untreated ¾ 2301 6266 6922 6,090 ab
Registered Products – Foliar Application
Monitor 0.66 pt (1) 1181 593 510 93 d
Monitor 1.33 pt (1) 650 115 191 9 e
Monitor 2.0 pt 918 83 132 6 e
Provado 3.75 oz 578 189 125 3 e
Registered Products - Applied in-furrow at Planting
           
Admire (2) 6.3 oz (1) 506 1034 1424 1,706 c
Admire (2) 13.1 oz 92 74 171 51 d
Admire (2) 18.9 oz 81 23 148 41 d
Experimental Products – Foliar Application
           
Fulfill 2.75 oz 494 479 77 11 d
Legend 3.75 oz 498 71 39 5 d
CGA 293343 1.42 oz 597 273 52 1 d
Pirimor 0.38 lbs 2609 4478 7641 14,101 ab
Pirimor 0.50 lbs 2639 3005 4684 4,399 b
Pirimor 0.66 lbs 2679 3940 5465 6,590 ab
Registered insecticides which flare aphid populations
           
Asana 9.6 oz 3165 9731 14427 17,158 a
Penncap-M 3.0 pts 2765 9573 16309 17,990 a
LSD – 5%   797 1976 3939 4473

(1) Below label rate applied for comparison with low and high labeled rates
(2) 18 August is 90 days after planting

Melon aphid

The melon or cotton aphid, Aphis gossypii (Figure 1), was first found in Minnesota potato fields last year in Dakota county. We have found a few melon aphids on the Rosemount Agricultural Experiment Station this year, but currently the population remains low. To date, no melon aphid has been captured in the traps we have throughout the Valley. The melon aphid is a serious threat to potato production because it is resistant to Monitor. The only effective insecticide is imidacloprid (Admire or Provado). A section 18 request for Fulfill was denied by the EPA in June there may not be an effective control available for this aphid.

Refer to the aphid key presented in last week’s Aphid Alert. The melon aphid would key out to the buckthorn aphid which is a close relative. The way to distinguish between the buckthorn aphid and the melon aphid is by looking at the cornicles (tube-like structures arising out of the aphid abdomen). The melon aphid has short black cornicles (Figure 1). It will take a 10X hand lens to see these structures. The color of the aphid is variable. The body may be green, bright yellow, black or pink. Failure to recognize when you have this aphid could result in control failure.

Figure 1. Melon or cotton Aphid. Note the black cornicles. (Figure from University of California Statewide IPM Project, photo by Jack Kelly Clark).

photo of the melon (or cotton) aphid, highlighting their distinctive black cornicles

Aphid Trapping

During the week of August 3 through 11, aphid flight activity has declined in the southern part of the Red River Valley (Figure 2, Table 2). Aphid capture data from an additional trapping site near Little Falls, MN was also added to Figure 2. Unfortunately, logistics and federal regulations hindered our ability to process samples from Little Falls in a more timely manner. Trapping sites in Karlstad, Hoople, Cando, and Rolette showed a dramatic increase in the numbers of aphids captured. An increase of green peach aphid captures has also been observed throughout the Red River Valley indicating the increased flight activity of summer migrants (please refer to the previous issue of Aphid Alert for information on the aphid life cycle). Inspectors of the Minnesota Department of Agriculture Potato Seed Certification Division have also reported an increase in seed potato fields colonized by green peach aphids. Remember once the threshold of between 3 and 10 wingless aphids per 100 leaves is reached, insecticides should be applied to prevent further spread of PLRV by wingless aphids. However, the most effective way to reduce late season spread of potato viruses is killing the vines as early as possible!!

Figure 2. Aphid captures on the first five sampling dates, 1998.

graph showing aphid captures on the first five sampling dates, 1998.

Table 2. Aphid species (% of total capture, 08/03-08/13)
* in traps with >50 aphids / trap a sub-sample of 50 aphids

  Little Falls Baker Climax seed Climax process Karlstad Williams Hoople seed Hoople process Cando Rolette
green peach aphid 3.2 0.6 1.1 1 0.7   4   3 6.5
PVY-vectors                    
birdcherry-oat aphid   0.6     0.7 2.4 0.5   1 0.5
corn leaf aphid 6.3 20.9 3.2 1 4.7 19     2.5 1.5
English grain aphid                    
green bug                    
pea aphid       2         1  
potato aphid             1.5     0.5
sunflower aphid 63.5 63.2 89.4 81 75 33.3 76 80 63 42.5
thistle aphid   1.8       4.8 2.5     0.5
turnip aphid 15.9 1.2 2.1 3 3.4 2.4 1.5 1 16 33.5
Identified non-vector species 0 2.5 0.5 0 1.3 14.3 0 1 0.5 0
Unidentified 11.1 9.2 3.7 12 14.2 23.8 14 18 13 14.5
Total # captured 63 244 1531 451 1559 42 5931 1517 460 1905