Aphid Alert 2000, No. 7, July 23

Update: Summary of virus vector situation through 23 July

Captures of winged aphids in the Aphid Alert network traps increased substantially this week. Most of the aphids captured this week are those associated with small grains but green peach aphids were found at most locations. As small grains mature throughout the Valley a greater proportion of aphids will develop wings. Winged aphids leave small grains in search of a suitable host. None of the grain aphids can colonize potato, but they are capable of spreading potato virus Y (PVY). Minnesota and North Dakota locations averaged 8.45 aphids per trap of all species and 0.17 green peach aphids per trap. Total aphid captures in the sixth week of 2000 are 1/2 to 1/3 that of the same week of 1998 and 1999, green peach aphid captures are below that of 1999 but more than we were capturing on the same date in 1998.

We continue to find green peach aphids on wild mustard. Wild mustard is growing profusely in many drowned out row crops. Matt Carroll (entomology Ph.D. student) and his crew found over half the mustard plants in two separate fields had green peach aphids. Wild mustard is not a host of PLRV or PVY but the aphids that we have seen on this plant (green peach and turnip aphid) are good vectors of leafroll (PLRV) and PVY. If these wild mustard infested fields persist, we would expect to capture green peach aphids throughout the month of August which is not good news for seed potato growers.

PVY spread cannot be stopped with insecticides. Reducing inoculum in seed fields by roguing and isolating seed fields (about 150 yards) from a known source of virus and aphid vectors will help. It is too early in most areas to consider top killing as tubers are just beginning to form. However, seed growers should consider killing the earliest generation fields as soon as possible to keep these basic seed lots as free of virus as possible.

You should immediately begin scouting potato fields for green peach aphids. These aphids will colonize at field margins so concentrate your scouting efforts on the outer 20 rows. Pick 20 leaves from 5 locations selecting only lower leaves as green peach aphids can only be found in the lower canopy. An economic threshold for green peach aphid in seed potato fields is essentially "at detection" or 3-10 wingless green peach aphids per 100 leaves. The same threshold should be used for processing potatoes known to be susceptible to net necrosis, e.g, Russet Burbank. All other processing and chip varieties can use a threshold of 30-50 green peach aphids per 100 leaves.

There are only three insecticides that provide reliable green peach aphid control: Monitor® (methamidophos); Provado® (imidacloprid); and Fulfill® (pymetrozine). In our research plots these three compounds have consistently provided 90-99% control. Because green peach aphids are found on the lower canopy, high pressure, high gallonage sprays provide the best control. Fulfill has the added advantage of being transported across the leaf membrane and is mobile in the vascular tissues. Fulfill kills more slowly than Provado or Monitor and aphids may be found in the crop 3-4 days after application. However, populations invariably drop by 1-week after application. Fulfill must also be used with a non-ionic surfactant so read the label before using this product.

Comparison of green peach aphid flight activity 1992-1994 and 1998-2000:

graph showing a comparison of green peach aphid flight activity 1992-1994 and 1998-2000

Green peach aphid captures per trap (above) for week ending 23 July. If the green peach aphid flight activity patterns observed in 1998 and 1999 are repeated in 2000 we will see a marked increase in green peach aphid flight activity within the next two weeks. There do not appear to be large populations of green peach aphid on canola perhaps because much of the canola is more advanced than at the same time last year, but the large populations supported on wild mustard are of concern.

Aphid captures reported by location for week ending 23 July.

Aphid captures per trap by location for week ending 23 July

Minnesota

Minnesota Baker
26 July
Climax
25 July
Crookston
24 July
Gully
July
Hollandale
July
Karlstad
24 July
  Aphids per trap per week
Species PLRV/PVY vectors  
green peach aphid   2 (2.6)       1 (3.0)
potato aphid            
  PVY vectors
bird cherry-oat aphid 8 (19.5) 8 (10.5) 2 (2.6) 2 (6.5)   1 (3.0)
corn leaf aphid 12 (29.3) 5 (6.6) 20 (25.6) 13 (41.9) 13 (65.0) 9 (27.3)
English grain aphid 1 (2.4) 4 (5.3) 3 (3.8)     1 (3.0)
greenbug     1 (1.3)      
sunflower aphid   1 (1.3) 2 (2.6)      
thistle aphid 7 (17.1) 22 (29.0) 34 (43.6) 2 (6.5)   12 (36.4)
turnip aphid 5 (12.2) 14 (18.4) 7 (9.0)   2 (10.0) 3 (9.1)
Other PVY vectors 2 (4.9) 2 (2.6) 1 (1.3) 2 (6.5) 1 (5.0) 1 (3.0)
  Non-vectors and unidentified
non-vectors 2 (4.9) 5 (6.6) 4 (5.1) 2 (6.5) 2 (10.0) 3 (9.1)
unidentified 4 (9.8) 13 (17.1) 4 (5.1) 8 (25.8) 2 (10.0) 2 (6.1)
Totals 41 76 78 31 20 33

Minnesota

Minnesota Little Falls
24 July
Morris Rice
24 July
Rosemount
24 July
Thief River
24 July
Williams
24 July
  Aphids per trap per week
Aphid species PLRV/PVY vectors
green peach aphid 1 (5.0)     2 (2.4) 2 (2.2)  
potato aphid 2 (10.0)     1 (1.2) 3 (3.2) 3 (5.2)
  PVY vectors
bird cherry-oat aphid 1 (5.0)   1 (16.7)   9 (9.7) 3 (5.2)
corn leaf aphid       71 (84.5) 37 (39.8) 22 (37.9)
English grain aphid           11 (19.0)
greenbug         4 (4.3)  
sunflower aphid         1 (1.1) 1 (1.7)
thistle aphid       1 (1.2) 5 (5.4) 3 (5.2)
turnip aphid 2 (10.0)   3 (50.0) 2 (2.4) 24 (25.8) 1 (1.7)
Other PVY vectors 1 (5.0)     1 (1.2) 1 (1.1) 5 (8.6)
  Non-vectors and unidentified
non-vectors 6 (30.0) 1 (16.7)   2 (2.4) 3 (3.2) 2 (3.5)
unidentified 4 (20.0)   1 (16.7) 4 (4.8) 4 (4.3) 7 (12.1)
Totals 20 No data 6 84 93 58

North Dakota

North Dakota Cando
20 July
Hoople
20 July
Linton
25 July
Mandan
25 July
Minot
24 July
Rolette
20 July
Walhalla
20 July
  Aphids per trap per week
Aphid species PLRV/PVY vectors
green peach aphid   1 (6.3)     1 (3.9)   3 (15.0)
potato aphid 1 (14.3) 1 (6.3)     5 (19.2)    
PVY vectors
bird cherry-oat aphid         2 (7.7)   6 (30.0)
corn leaf aphid         3 (11.5) 1 (12.5)  
English grain aphid   1 (6.3)          
greenbug   1 (6.3)          
sunflower aphid              
thistle aphid   5 (31.3)     3 (11.5)   2 (10.0)
turnip aphid 1 (14.3) 5 (31.3)     7 (26.9) 1 (12.5)  
other PVY vectors             1 (5.0)
Non-vectors and unidentified
non-vectors 5 (71.4) 1 (6.3)     4 (15.9) 4 (50.0) 7 (37.5)
unidentified   1 (6.3)     1 (3.9) 2 (25.0) 1 (5.0)
Totals 7 16 0 0 26 8 20

Wisconsin

Wisconsin Antigo
24 July
Rhinelander
24 July
  Aphids per trap per week
Aphid species PLRV/PVY vectors
green peach aphid    
potato aphid    
  PVY vectors
bird cherry-oat aphid    
corn leaf aphid    
English grain aphid    
greenbug    
sunflower aphid    
thistle aphid    
turnip aphid    
other PVY vectors    
  Non-vectors and unidentified
non-vectors    
unidentified    
Totals 0 0

South Dakota

South Dakota Watertown
25 July
  Aphids per trap per week
Aphid species PLRV/PVY vectors
green peach aphid  
potato aphid  
  PVY vectors
bird cherry-oat aphid 2 (33.3)
corn leaf aphid  
English grain aphid 1 (16.7)
greenbug  
sunflower aphid  
thistle aphid  
turnip aphid 2 (33.3)
other PVY vectors  
  Non-vectors and unidentified
non-vectors  
unidentified 1 (16.7)
Totals 6

Nebraska

Nebraska Cody
25 July
  Aphids per trap per week
Aphid species PLRV/PVY vectors
green peach aphid  
potato aphid 1 (2.1)
  PVY vectors
bird cherry-oat aphid  
corn leaf aphid 46 (97.9)
English grain aphid  
greenbug  
sunflower aphid  
thistle aphid  
turnip aphid  
other PVY vectors  
  Non-vectors and unidentified
non-vectors  
unidentified  
Totals 47