Aphid Alert 1998, No. 1, July 30

Welcome to Aphid Alert. The goal of this newsletter is to provide information concerning aphid flight activity in the Red River Valley. We hope that you will find the weekly articles and information concerning aphid flights useful in your decision making. Information that is included in this newsletter is collected from eight trapping sites established in Minnesota and North Dakota. These trapping sites have been established as part of the research initiative addressing questions of virus control in the Red River Valley. Funding for this project is provided by the 1998 Minnesota State Legislature. For a more detailed information about the project please see the first issue of Aphid Alert 1998 which is available on the Internet at the following address <http://ipmworld/umn.edu/alert.htm> and in the July 1998 issue of the Valley Potato Grower.

Trapping Sites

Aphids are being trapped at the following sites: Little Falls, Baker, Climax, Karlstad, Williams, Hoople, Cando and Rolette. Four green tile traps and a suction trap are used at each site, except in Rolette which has only tile traps. Also in Climax and Hoople two additional green tile traps have been placed in a nearby commercial potato fields (Pictures 1a, 1b). These green tile traps mimic potato foliage and give an unbiased measure of aphid landing rates. Suction traps are common monitoring devices for aphid flight activity in many parts of the world. Our suction traps are 6 feet tall PVC pipes of 8 inches in diameter and are equipped with a fan drawing 84 cubic feet of air per minute during daytime (Picture 2).

photo of a green tile trap for aphids, top down view

photo of a green tile trap for aphids, side view

Pictures 1a, 1b: Aphid monitoring with green tile traps.

photo of a suction trap for aphids, side view

Picture 2. Aphid monitoring with a suction trap.

Overview of PVY transmission

Colonizing aphid species

Potato colonizing aphids (those aphid species that reproduce on potato), such as green peach aphid (Myzus persicae), buckthorn aphid (Aphis nasturtii), and potato aphid (Macrosiphum euphorbiae), are the most efficient vectors of PVY.

Non-colonizing aphid species

However, a multi-species complex of non-colonizing aphids (those aphid species that reproduce on plants other than potato) associated with small grains, weeds and other crops are responsible for most of the PVY spread in the Red River Valley. These aphids are often inefficient vectors of PVY in contrast to green peach aphid with transmission efficiency typically less than 10%. In general, percent transmission refers to the percentage of aphids that, having fed on virus infected plant, will transmit the virus to a healthy plant. However, it has been estimated that the landing rate of non-colonizing aphids exceeds 2,000 aphids per plant per day during the peak flight activity which compensates for their lower transmission efficiency.

Among the common non-colonizing aphid species found in the Red River Valley, the following are the most important in PVY transmission:

  • birdcherry-oat aphid - Rhopalosiphum padi
  • corn leaf aphid - Rhopalosiphum maidis
  • English grain aphid - Sitobion avenae
  • green bug - Schizaphis graminum
  • pea aphid - Acyrthosiphon pisum
  • sunflower aphid - Aphis helianthi
  • thistle aphid - Capitophorus elaeagni
  • turnip aphid - Lipaphis erysimi

Why do non-colonizing aphids move to potato?

The host quality of small grains and weeds decline during maturation which triggers the production of winged individuals. These winged aphids leave the grain field or weed host in search for another suitable host, during which they align randomly on any green plant. Winged aphids are especially attracted to the green-dark interfaces of field edges and tend to land in greatest numbers on the margins of fields.

How is PVY acquired and transmitted?

Aphids must sample plant sap (taste) to determine its suitability as a host. This process of sap sampling takes only seconds and restricted to the outer cell layer of leaves (epidermis). It is during these brief feeding probes when PVY is acquired and transmitted. After acquiring PVY the aphid is only able to transmit the virus to the next one or two plants before losing its "virus charge". An aphid carrying PVY can be thought of as a flying dirty hypodermic needle. To transmit PVY again, an aphid must re-acquire the virus by feeding on another infected plant.

How to prevent PVY transmission?

Preventing PVY transmission is a very difficult task because insecticides are unable to kill aphids quick enough to prevent transmission. The only means of controlling PVY is to prevent aphids from directly landing on the crop and thus "cleaning" their virus charge prior to sampling potato.

Quick overview of PLRV transmission

PLRV vs. PVY

Transmission of PLRV is very different from that of PVY because PLRV is restricted to the vascular tissues (phloem tissue) of the plants. Since sap sampling (tasting) takes place only at the surface layer of the leaf, PLRV containing phloem is only reached by those aphids that colonize potato. Aphid species that colonize potatoes are the most important vectors of PLRV, such as green peach aphid, buckthorn aphid, and potato aphid. Of the aphids colonizing potato, green peach aphid is the most efficient vector of PLRV and is the aphid we monitor most closely.

How is PLRV acquired and transmitted?

It takes an aphid at least 10 minutes to reach the vascular tissues and acquire or transmit PLRV. After ingesting the plant sap the virus has to pass through the stomach wall into the blood then pass into the salivary gland before the aphid can transmit PLRV. This process takes between 24 to 48 hours. Once the virus is in the salivary gland the aphid can transmit PLRV for the rest of its life.

How to prevent PLRV transmission?

Control of PLRV lies in the effective control of green peach aphid. Because of the transmission characteristics of PLRV, insecticides can reduce the spread of PLRV. Admireâ incorporated at planting provides effective control of green peach aphid for about 100 days. The mode of action of Admireâ is slow requiring days to kill the aphids. For this reason Admireâ is not effective to prevent virus spread by PLRV infected winged aphids arriving into the field. These winged aphids are able to feed before they are killed by Admireâ . However, any nymphs deposited on treated plants will die before reaching maturity which will prevent within-field spread of PLRV. Research at the University of Minnesota has established that 3 wingless green peach aphid per 100 lower leaves is an effective threshold for foliar insecticide applications. To determine threshold, select 25 lower leaves in each of four areas in the field and count the aphids that are found on the underside of the leaf. Later issues of Aphid Alert will provide a quick key to identify aphids colonizing potato.

Aphid Sampling

Over the last two weeks winged aphid flights have been increasing at the trapping sites in MN and ND (Figure 1, Table 1). This increase in winged aphids corresponds to the buildup of aphids in maturing small grain fields, weeds and other crops. Sunflower and grain fields in close proximity to the Climax trapping site are most likely responsible for the large numbers of aphids caught in the Climax trap.

Figure 1. Aphid captures on the first and second sampling dates, 1998.

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

Important New Finding!

Large colonies of green peach aphids containing all life stages (winged and wingless aphids) have been found in a canola field near Crookston, MN.

Advisory

Seed potato fields in close proximity to canola fields may be at greater risk of invasion by green peach aphids. Monitor your field regularly by selecting lower leaves and if you find more than 3 green peach aphids per 100 leaves application of Monitorâ or Provadoâ is necessary to prevent PLRV spread.

Table 1. Aphid species (% of total capture, 07/13-07/21)

contributed by: Robert Suranyi, Dave Ragsdale, and Ted Radcliffe,
Department of Entomology, University of Minnesota

  Baker Climax
seed*
Climax
tablestock
Karlstad Williams Hoople
seed
Hoople
tablestock
Cando Rolette
green peach aphid     1 0.34          
PVY-vectors                  
birdcherry-oat aphid 2       5.6 2.2      
corn leaf aphid                   
English grain aphid     1   11.1 1.8 3.2    
green bug 2             16.6  
pea aphid       1          
potato aphid 3.8     1.7 5.6 1.8      
sunflower aphid 28.3 75.7 81 86   78.9 96.8 16.7  
thistle aphid 13.2 6.9 7.3 4.1 5.6 4.8   33.3  
turnip aphid 7.6   1.1     2.6      
Identified non-vector species 18.6 2.1 3.5 3.8 33.3 3.5   33.4  
Unidentified 24.5 15.3 5.2 3 38.8 4.4      
Total # captured 53 832 94 293 18 227 32 6 no data

* sub-sample of 378 aphids