By John Tooker
With wheat planting beginning, it is wise to consider two insect pest concerns for this fall crop: Hessian fly and aphids.
While many Pennsylvania growers have never encountered Hessian flies, the past few years have 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 rye and barley. This species has not been common recently because most farmers plant wheat after “fly-free dates”— dates after egg-laying Hessian fly adults are not likely to be active.
For best estimates of fly-free dates in Pennsylvania, see this Department of Entomology website. Recognize, however, that these fly-free dates are likely imperfect given the trends for warmer years, so proper date might be a little later. Also, if your farm is at a higher elevation, the date is probably a little earlier than the rest of your county —this is an inexact science at this point, so just do your best.
With great adoption of small grains as cover crop species, some growers have been planting wheat, rye, and barley in late summer earlier. These early-planted fields are available for egg-laying female flies that can foster populations of Hessian fly larvae that can then emerge as adults in spring and further infest fields. Insecticides are generally not effective for control of Hessian fly so the best tactics for farmers is to adhere to the fly-free dates and plant Hessian fly-resistant varieties of wheat, rye, or barley.
Another risk to young wheat fields in autumn is a suite of “grain” aphids, including greenbug, bird cherry-oat aphid, corn leaf aphid, English grain aphid, and sugarcane aphid among others. Infestations of these aphids are often diagnosed from truck cab because some of these species can cause small grains to turn yellow, red or even purple. Greenbug and sugarcane aphid are well known for their tendency to cause color changes in their host plants. These color changes can be caused by plant stress caused by the aphids — in the case of greenbug, toxic saliva is injected by the aphids, or in some cases, barley yellow dwarf virus can be transmitted by many “grain” aphids.
We have heard that some agricultural companies are promoting preventive insecticide applications to avoid aphids and possible virus infestation. We would like to stress, however, that aphid populations are notoriously patchy with adjacent fields often having drastically different populations. Therefore, the best strategy for managing aphids is to use an IPM framework and only apply an insecticide when it makes economic sense — that is, when scouting reveals that the aphid population exceeds the economic threshold. The threshold published in the Penn State Agronomy Guide is 100 aphids per foot of row.