Aphid Alert 1998, No. 5, August 28

Features of this issue

• Aphid biology revisited
• Aphid flight activity to date
• Aphid flight activity 10-19 August

Apologies are in order to readers of the printed version of this newsletter. The World Wide Web address (URL) given in the banner of of previous numbers of Aphid Alert was mistyped. The correct World Wide Web address (URL) for Aphid Alert is http://ipmworld.umn.edu/alert.htm. Alternatively, follow the link to Aphid Alert from the opening menu of Radcliffe's IPM World Textbook, http://ipmworld.umn.edu/.

A Primer About Internet Addresses

In an Internet address, the URL (Uniform Resource Locator) consists of 3 elements: the communications protocol used, the host server, and the location of the file on that server. The portion before the colon specifies the communications protocol used, the WWW uses http (Hypertext Transfer Protocol). The portion of the address beginning with double slashes, separated by "dots," and ending in a single slash is the name of the host server (computer), our host is //ipmworld.umn.edu/. The final portion of the address is a location on the host server, i.e., a path and file name, for the opening menu of Aphid Alert that is alert.htm.

Some Useful Internet Addresses

• Univeristy of Minnesota Home Page, http://twin-cities.umn.edu/
• University of Minnesota, College of Food, Agricultural and Natural Resource Sciences, https://www.cfans.umn.edu/
• University of Minnesota Extension Service, http://www.extension.umn.edu/
• Directory of University of Minnesota faculty, staff, and students: Search for an individual's address, phone, e-mail, http://search.umn.edu
• North Dakota State University, https://www.ndsu.edu/
• North Dakota State University, College of Agriculture, https://www.ag.ndsu.edu/academics
• Directory of North Dakota State University faculty, staff, and students: Search for an individual's address, phone, e-mail, https://www.ndsu.edu/directory/
• North Dakota State University, Late blight home page, https://www.ndsu.edu/potato_pathology/

More About the Trend to More Aphids

Photograph of an "aphid hole," Reprinted from Valley Potato Grower, April 1998.

Green peach aphid infestations of extraordinary severity have been experienced by many potato growers in Minnesota, North Dakota and Manitoba this year and last. Not surprisingly with such aphid pressure there has been a marked increase in the spread of potato leafroll virus and potato virus Y. Concomitantly, the Minnesota and North Dakota seed potato certification programs had seed lot rejections of unprecedented proportions in their 1997-98 winter tests. Net necrosis, a tuber condition associated with potato leafroll infection in the cultivar Russet Burbank resulted in substantial losses for a number of growers in North Dakota and Manitoba. "Aphid-holes" (see Figure 1) were reported by some growers. Aphid holes result from such heavy infestations that in local "hot spots" the vines prematurely senesce and die. In research at Staples in 1997, plots without aphid control sustained average yield losses of 154 cwt. per acre. Aphid pressure varies greatly from year to year and location to location making prediction of outbreaks impossible. Weather and pesticide use are probably the most important factors affecting populations.

Tubers showing "net necrosis." Picture from University of California, Statewide IPM Project, <http://www.ipm.ucdavis.edu/>

Net necrosis is problem primarily in Russet Burbank potatoes. It is associated with current-season PLRV infection, but infected tubers don't always exhibit this syndrome.

For more pictures of potato pests (insects, diseases, nematodes, and weeds) visit the University University of California, Statewide IPM Project, Pests of Potato.

Visit North Dakota State University's web site for information on potato late blight and early blight.

Importance of Natural Biological Control in Suppressing Green Peach Aphid

Green peach aphid is an insect with a remarkably capability for rapid population increase. In insecticide-treated plots we typically observe population growth rates in the range of r =1.40, i.e., the population increases 1.4 fold per day. Figure 1 shows the consequence of such explosive population growth if sustained for 10 days. At this rate of increase, an initial population of 1 aphid per leaf can be expected to increase to almost 30 per leaf by day 10 and to over 1,000 aphids per leaf in just 3 weeks. A modest reduction of rate of increase to r =1.12, i.e., a 20% reduction in rate of increase, e.g., as a consequence of natural enemies, would result increase of an initial population of 1 aphid per leaf to about 3 aphids per leaf on day 10, and 10 on day 21. We have often observed green peach aphid populations to increase at rates of r =1.4 per day (or even greater) for periods of a month or more.

On potatoes not treated with insecticides, natural enemies typically cause greater reductions in the rate of increase of green peach aphid than the 20% illustrated in Figure 1. Where natural enemies are not disrupted by the periodic application of pesticides, green peach aphid on potato seldom increases to densities that cause economic damage. Unfortunately, pesticides, both insecticides and fungicides, are essential in commercial potato production. Pesticides tend to be highly disruptive of naturally occurring biological control, suppressing or even eliminating beneficial insects (parasites and predators) and pathogens (entomophthoran fungi) of the aphids. Pesticide induced aphid outbreaks can then result (Figure 2). In Minnesota and North Dakota, green peach aphid is the species most likely to be "flared" by pesticide applications, but melon aphid, potato aphid, and buckthorn aphid can all respond in this way.

The take home message from this illustration is that growers should be aware that pesticides tend to disrupt biological control and are likely to "create" an aphid problem. When pesticides must be used on a tight schedule, as for late blight control, special attention should be paid to monitoring aphid populations. Timely application of an effective "aphidicide," e.g., Monitor® or Provado®, can hold a developing population in check. Once aphid numbers have reached densities much in excess of recommended treatment thresholds it is very difficult to re-establish control. Even effective aphidicides will not give satisfactory control in such situations because of the rapid rate of increase of the small proportion, but large number of individuals, that survive treatment.

Figure 1. Theoretical model of population growth of green peach aphid on potato.

Figure 2. Hypothetical model of green peach aphid population trends over the growing season from initial invasion and showing effects of naturally occurring biological control and disruptive effects of pesticides.

Photo Guide to the Most Common Potato-colonizing Aphids

Photographs reproduced below are from Insects infesting potatoes: a field guide, compiled by Duane A. Preston and published by the Red River Valley Potato Growers Association, North Dakota State Seed Department, Minnesota Seed Department, University of Minnesota Extension Service, and North Dakota State University Extension Service. The originals of these photographs were published in Aphids infesting potatoes in Canada: a field guide, by M. E. MacGillivray, Research Station, Fredericton, N.B. For a field key to the distinguishing characteristics for field identification of the aphids colonizing potato see Aphid Alert No. 2. For a picture of melon aphid see Aphid Alert No. 4.

Green peach aphid, non-winged (apterae)

Green peach aphid, winged

Potato aphid, non-winged

Potato aphid, winged

Buckthorn aphid, wingless

Buckthorn aphid, winged

Foxglove aphid, wingless

Foxglove aphid, winged

Aphid Trapping

During the week of August 10 through 19, aphid flight activity generally declined (Figure 5, Table 1). However, green peach aphid captures continued to increase and colonization of potato fields are now a common occurrence across the Valley. However, the process of colonization can be effectively reduced by following aphid control recommendations (please refer to previous issues of the Aphid Alert). For example, the trapping site near Climax continues to provide high aphid captures but leaf counts indicate no colonization. In potato fields where aphid management was somewhat relaxed, results of large scale aphid colonization is now especially striking. In these fields the severe aphid pressure did not only caused "aphid holes" but also whole fields were killed prematurely by aphids. Remember, aphid populations can double every two days which can rapidly lead to field sized "aphid holes"! These "aphid holes" in turn are producing countless numbers of winged green peach aphids invading nearby potato fields. Seed lots that are still green in late August and early September are at increased risk of late season virus infection! At present, early vine kill is the best management option for seed potato growers (quality versus quantity).

Aphid capture data of previous sampling dates have been added to Figure 5 from a trapping site near Hollandale, MN (only two green tile traps at Hollandale). However, due to logistical constrains current data from Hollandale and Little Falls are unavailable.

Figure 5. Aphid captures on six sampling dates, 1998

	Baker	Climax seed	Climax ** process	Karlstad	Williams	Hoople seed	Hoople process	Cando	Rolette
green peach aphid	1.5	2.4	9	3.7	2.4	2.7		8.9	6
PVY-vectors
birdcherry-oat aphid	2	1.2	1	0.5	11	1.4		1.3	0.5
corn leaf aphid	11.8	1.6	5	11.1	46.3	6.3		1.9	4
English grain aphid				0.5	2.4
green bug
pea aphid
potato aphid		0.4		0.5	1.2	3.2			0.5
sunflower aphid	45.8	62.4	62	41.2	20.7	44.3		48.1	53
thistle aphid	3.9	1.2	3	2.8	2.4	0.5		1.9
turnip aphid	2.5	2.4	4	8.8	1.2	9.5		7.6	8
Identified non-vector species	2	0	1	0	0.2	0.4		1.2	0
Unidentified	30.5	28.4	15	31	12.2	31.7		29.1	28
Total # captured	324	1976	250	451	88	881	no data	484	408