Pennsylvania Small Grains: Beware Hessian Fly and Aphids in Fall-Planted Crops

    Hessian fly adult. Photo: Texas A&M AgriLife

    With silage harvest progressing, many farmers in Pennsylvania are turning, or have turned, their attention to establishing small grains. With fall-established small grains, it is wise to be aware of the risk from two insect pests of these crops: Hessian fly and aphids.

    While many Pennsylvania growers have never encountered Hessian flies, the past decade has seen an increasing number of outbreaks in eastern states, including Delaware, Virginia, and North and South Carolina.

    This pest is most problematic in wheat, but will also attack barley and, to a lesser extent, cereal rye, which tends to have good resistance against Hessian fly. This species has not been common because most farmers plant wheat after “fly-free dates”– dates after which egg-laying Hessian flies are not likely to be active.

    For Pennsylvania, fly-free dates have already passed (they range from 22 September – 1 October); recognize, however, that these fly-free dates are likely imperfect given the trend for warmer years due to climate change, so the proper dates are likely to be a few days later.

    With great adoption of small grains as cover crop species, some growers have been planting wheat and barley in late summer or early fall. These early-planted fields are available for egg-laying female flies and then can foster populations of Hessian fly larvae that can then emerge as adults in spring and further infest fields.

    Planting after flies are active avoids this trouble. Insecticides are generally ineffective for control of Hessian fly because larvae are protected by leaf sheaths and insecticides can not reach them. Therefore, the best tactic for farmers is to adhere to the fly-free dates and plant Hessian fly-resistant varieties of wheat or barley.

    For more information on Hessian fly in Pennsylvania, see our fact sheet.

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    Another risk to fall-planted small grains is a suite of “grain” aphids, including bird cherry-oat aphid, corn leaf aphid, greenbug, and English grain aphid, among others. In addition to small grains, grass hay and pasture grasses can be infested by these aphid species.

    Infestations of aphids are often diagnosed from the cab of a vehicle because some of these species can cause small grains to turn yellow, red, or even purple. These color changes can be caused by plant stress caused by aphid feeding; in the case of greenbug, they inject toxic saliva that alters plant physiology, influencing plant color.

    In some cases, this yellowing can be caused by barley yellow dwarf virus (BYDW), which can be difficult to diagnosis but is transmitted by many “grain” aphids.

    Some agricultural companies promote preventative insecticide applications to avoid aphids and possible virus infestation. We would like to stress, however, that preventative sprays provide inconsistent control of aphids and BYDV because insecticides provide only temporary protection.

    Integrated Pest Management provides more certain outcomes against aphids, and BYDV, because aphids are notoriously patchy and adjacent fields often having drastically different populations—scouting reveals these differences. We recommend scouting fields for aphids and only apply an insecticide when it makes economic sense.

    That is, when scouting reveals that the aphid population exceeds the economic threshold, which is 100 aphids per foot of row. If you suspect that your fields have an infestation of BYDV, consider sending plant samples to a commercial lab that can screen for the virus. Knowing that you have BYDV will not improve management of this year’s crop, but will provide insight on how to manage future plantings.

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