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ISSUE 14  August 6, 1998

 

POTATO APHID UPDATE ON WEB

    The following information is available on the internet at:

        http://ipmworld.umn.edu/aphidalert/alert7.htm

    The updates and accompanying information are provided by: Robert Suranyi, Dave Ragsdale, and Ted Radcliffe, Department of Entomology, University of Minnesota.

    Some of the excerpts from their web page are provided here. The information pertains to aphids and the transmission of Potato Virus Y (PVY) and Potato Leaf Roll Virus (PLRV). On the web site there are graphics and includes a summary of aphid captures during one week intervals.

APHID ALERT NEWSLETTER

No. 2, 30 July 1998

    Information that is included in this newsletter is collected from eight trapping sites established in Minnesota and North Dakota. 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.

Aphid Sampling

    Over the last two weeks winged aphid flights have been increasing at the trapping sites in MN and ND. 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.

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 MonitorŽ or ProvadoŽ is necessary to prevent PLRV spread.

Treatment Guidelines

    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.

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.

Quick 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

    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. AdmireŽ incorporated at planting provides effective control of green peach aphid for about 100 days. The mode of action of AdmireŽ is slow, requiring days to kill the aphids. For this reason AdmireŽ 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 AdmireŽ . However, any nymphs deposited on treated plants will die before reaching maturity which will prevent within_field spread of PLRV.

Robert Suranyi
Dave Ragsdale
Ted Radcliffe
Department of Entomology
University of Minnesota

 

BUG BRIEFS

    Most insect concerns in the region continue to be with the same set of insects discussed last week.

    European corn borer moth captures continue to be at low levels. Reports from across the state indicate that egg laying and new larvae are at low levels. Infestations from the July peak appear to have been greatest in areas outside the southern Red River Valley. Perhaps storms, which occurred frequently during July, helped to reduce survival of larvae in many of the southern valley fields. Elsewhere, survival appears to have been very good.

    Red seed weevil in sunflower is being reported throughout the region. Levels reported from fields are quite variable. The most common report appears to be an average of 2 to 4 seed weevils per flower head. This level would be below the treatment threshold for oil seed sunflowers, but above threshold for confection flowers. Scouting fields will be very important this year to find those that are harboring threshold population levels.

    Sunflower midge damage is being assessed at this time. The majority of the larvae have dropped from the flower heads. Fields with significant scarring are blooming now and a more accurate measure of the impact they may have on a field will be possible.

    A few concerns have been raised about Potato leaf hopper in dry beans. Treatment thresholds for this insect are one leafhopper per trifoliate. The nymphs of the leafhopper can be found crawling on the undersides of the leaves. Nymphs are wingless and will not leave the field for sometime, feeding on the leaves near the site they hatched. When threshold levels are present, treatments should not be delayed until visible leaf injury has occurred. Injury is referred to as hopper-burn, appearing as brown areas at the tips of leaves.

Phillip Glogoza
Extension Entomologist


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