Aphid Alert:  Region-wide virus vector surveillance for the Minnesota and North Dakota potato industry

Edward B. Radcliffe,  radcl001@umn.edu

Aphid Alert 2001, No. 5, July 20

Published cooperatively by the
Department of Entomology, College of Agricultural, Food & Environmental Sciences, University of Minnesota, the University of Minnesota Extension Service, and the State Seed Potato Programs of 
Minnesota and North Dakota

Project funding provided by:
Minnesota State Legislature,
 Rapid Agricultural Response Initiative &
United States Department of Agriculture,
Cooperative State Research, Education and Extension Service,
Integrated Pest Management Program, North Central Region

WWW address:http://ipmworld.umn.edu/alert.htm

Contents:

Aphid captures

  Comparison of 2000 and 2001

  Aphid captures by location

MinDex (using weather data to forecast aphid abundance)

Pictures of potato aphids 

  Key to field identification

Update on soybean aphid

Late blight

Back issues of Aphid Alert 2001

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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.

Notes: 

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.
 

Be on the alert for cotton aphid.

In recent years years, we've received occasional reports of methamidophos (Monitor)-resistant aphids on potato.  Where we have investigated these, we found cotton aphid.  Cotton aphid is usually uncommon in the northern Great Plains, but in the subtropics they are considered important vectors of potato viruses.  Cotton aphid can transmit both PVY and potato leafroll.  This aphid reproduces best at high temperatures (above 85 F). Color of the nymphs varies from yellowish green to almost black.  The black cornicles (tube-like structures protruding from the abdomen) are distinctive.  On cotton, this aphid can be effectively controlled with thiamethoxam (Actara), imidacloprid (Provado) or pymethrozine (Fulfill).

The MinDex:

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.  

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.

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