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Wednesday, January 16, 2013

What to watch for: Aphids in strawberries

There has been concern among some strawberry growers in the southeast regarding aphid vectored virus infections in plants from some sources. Dr. Barclay Poling, NC State University Horticulture Department, has reported on these observations as they developed in a series of alerts, found at the NCSU Strawberry Growers Portal.

Aphid vector viruses are rarely significant issues in annual strawberry because even if infections occur during the year, they happen later in the season and do not cause economic losses.  In addition, infection from multiple viruses (virus complexes) is often necessary for visual symptoms or loss to occur.  Where aphid vectored viruses can be significant is in perennial production and in nursery production.

It appears that plants suspected to be infected with viruses are restricted to a few plant sources, so for growers who do not have plants on their farm from suspected sources, no additional management is recommended.  However, for growers with suspect plants, management is recommended to prevent early season in field spread.  Systemically applied insecticides are likely the most effective method of managing aphids, and information on using these materials is in the Southern Regional Small Fruit Consortium Strawberry IPM Guide.  Importantly, these materials have application timing restrictions with respect to bloom to protect pollinators.  This restrictions should be carefully followed and are another good reason NOT to preventatively treat for aphids if you do not have suspect plants!  Most North Carolina growers with a need to treat should be able to meet these restrictions. If systemic insecticide applications cannot be made, the alternative foliar materials are more difficult to use because of pollinator concerns and should be applied only if aphids are present--which requires good scouting!

The recommended aphid scouting program is 40 randomly selected leaves per acre, which should be observed for live, non parasitized aphids. If only parasitized aphids are present, treatment may not be necessary.  This is particularly true if you have already treated, because parasitized aphid mummies will often hang on to plants when dead non parasitized aphids will wash away or decay.

Non parasitized and parasitized (brown, rounded) aphids on cabbage. Photo: IPM Images, Alton N. Sparks, Univ of Georgia.
The aphid species we are most likely to have present on strawberries in North Carolina are strawberry aphids, green peach aphids, and occasionally, potato aphids.  It can be important to distinguish between these species, particularly if faced with the need to treat populations with foliar insecticide applications, because aphid species may respond differently to insecticides.

All three aphids can be green but may also be orange to red. Potato aphids, with their larger size and long legs, are perhaps the most distinctive species on strawberries.

Potato aphids (Macrosiphum euphorbiae) nymphs, adults, and winged adults. Photo: IPM Images, Whitney Cranshaw, Colorado State University.
Strawberry aphids can be distinguished from green peach aphids by the presence of knobbed hairs covering their bodies, which can be observed under 20x or greater magnification. University of Florida Extension has a nice image illustrating these hairs.

Strawberry aphids (Chaetosiphon fragaefolii) posses knobbed hairs that can be used to distinguish them from green peach aphids. Photo: IPM Images, Jeffery Lotz, Florida Department of Agriculture and Consumer Services.
Green peach aphids are among the most common aphid pests in North Carolina crops and have a very wide host range.

Green peach aphid adult and nymph. Photo: Jim Baker, NC State University.

If aphids are observed in fields with suspected virus infected plants that do not appear similar to our common species, contact your extension agent and submit a sample to the NC State University Plant Disease and Insect Clinic (or your state resources) for identification.

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