Aphid Alert 2001, No. 5, July 20
Aphid captures, week ending July 20
No green peach aphids have been captured in the first 6 weeks of trapping in 2001. Last year we began capturing green peach aphid in mid-June. We expect aphid captures to increase steadily throughout July and peak in early to mid-August. However, we are finding very few green peach aphids colonizing alternative and weed hosts, e.g., canola and wild mustard. First instar green peach aphid were found in leaf samples taken from a small potato field in Crookston, MN (0.7 GPA per 100 leaves, 1,400 samples). Aphid flight activity has increased over the previous week, but captures of potential vectors of PVY continued to be less than on the same date last year. Most common were aphids associated with small grains among which corn leaf aphid, Rhopalosiphum maidis, was most abundant.
As mentioned in the previous issue of the newsletter, the observed increase in small grain aphid flight increases the risk of PVY transmission. Unfortunately, management options for reducing in-season PVY transmission are limited because PVY is transmitted in a non-persistent manner by aphids. Non-persistent transmission is characterized by the contamination of aphid mouthparts with virus particles. However, it is a complex biological process and not simply a mechanical result of feeding. Acquisition and inoculation requires only seconds with no latent period in between. Aphids rapidly lose their virus charge after brief feeding on healthy plants. In terms of control, the key feature of non-persistent transmission is that virus transmission can occur within a very brief feeding probe. Thus, insecticides are not effective in controlling PVY because they are unable to kill probing aphids quickly enough to prevent virus transmission. In contrast to insecticides that target the vector, mineral oils target the transmission process itself. Last year we demonstrated that use of a mineral oil, 2% Aphoil (made by Agsco), can reduce PVY spread 40%. In our trial, phytotoxicity was not observed, but caution should be excised when using mineral oils because of the possibility of damaging foliage.
Locations for which data were not available when this page was created will be updated as results are received.
One suction trap and two pan traps are operated at each location, except at Crookston, Rosemount and Thief River Falls which have more. Data are reported as total aphid captures per three traps.
In 2001, no green peach aphids have been captured in the first 6 weeks of trapping. The usual pattern in previous years has been for aphid captures to increase steadily throughout July and peak in early to mid-August. However, we are finding very few green peach aphids colonizing alternative and weed hosts, e.g., canola and wild mustard.
Flight activity of other aphid species increased over the previous week, but numbers of potential vectors of PVY continued to be less than on the same date last year. Most common were aphids associated with small grains.
Guide to the Field Identification of Wingless Aphids on Potato
|Potato Aphid, Macrosiphum euphorbiae (Thomas)||Buckthorn Aphid, Aphis nasturii (Kaltenbach)||Foxglove Aphid, Aulacorthum solani (Kaltenbach)||Green Peach Aphid, Myzus persicae (Sulzer)||Cotton (=melon) aphid, Aphis gossypii (Kaltenbach)|
Entomology graduate student Min Zhu is working to associate weather patterns with the seasonal abundance of green peach aphid and virus spread. Min has compiled a list of wind events (greater than 12 h in duration) that could have transported aphids from southern sites to the Red River Valley of the North, for the years 1998-2001. She has computed what I have teasingly called the MinDex (Min Index) by multiplying the number of hours of such events by a 1-5 weighting, depending upon the date events occurred (5 for events May 15-31, 4 for events June 1-15, progressing to 1 for events July 15-31). Aphids arriving before May 15 typically would be exposed to killing frost; those arriving soon after would have the longest time to increase and contribute to the local population dispersing in July and August, those arriving after July 30 are assumed to contribute little to the local population.
Cumulative Min Index
Cumulative GPA per trap
|May 15-30||June 1-15||June 15-30||July 1-15||July 15-30|
What does this mean? Perhaps, nothing. But, the MinDex does suggest that early season wind events (prior to June 30) may determine seasonal abundance of green peach aphid (measured by cumulative captures of winged aphids per trap). If so, 2001 should be a year of low green peach aphid abundance.
Soybean aphid, Aphis glycines Matsumura, appears to be common on soybean in southeastern Minnesota.
This aphid tends to feed on plant terminals. Soybean aphid tends to have a highly aggregated distribution, i.e., most plants may have none, others will have large numbers. Soybean aphid appears to be resistant to some insecticides. University of Minnesota plots showed rapid increases in soybean aphid numbers after being sprayed with lambdacyhalothrin (Warrior) at St. Paul and with carbaryl (Sevin) at Rosemount. Presumably this resulted from the suppression of generalist predators.
Late Blight Alert!
Roger Jones, Department of Plant Pathology, University of Minnesota
Minnesota: In Minnesota, Cumulative Severity Values were exceeded and spray programs should have been initiated in: Crookston - Perham - July 20, Staples -July 20, Karlstad - July 18, Eldred - July 16, Felton - July 16, Stephen - July 16, Williams - July 16, Little Falls - June 15, Park Rapids - June 15, Brooten - July 9, Foxhome - July 9, Warren - July 3, Humboldt - July 2.
North Dakota: Cumulative severity values as of July 17 for irrigated sites ranged from 31 (Linton) to 66 (Hofflund), and for nonirrigated sites from 32 (Stephen, MN) to 51 (Warren, MN).
Wisconsin: Areas where late blight has been identified include the following: Amherst, Ellis, Hancock, and Plover. Environmental conditions and plant susceptibility favor the development of foliar disease. Apply fungicide at 7-10 days (see Wisconsin web site for recommendations for your area) at low rate.
Michigan: Late blight has been found in Michigan. Cumulative severity values are generally high and high rates and short spray intervals are recommended (see Michigan web site for recommendations for your area).