Fall Small Grain Insect Pest Management

David Owens, Extension Entomologist; owensd@udel.edu

Planting winter wheat and barley is coming up. There are two historically important insect pest groups to be mindful of: Hessian fly and the aphid complex. Hessian fly lays eggs on leaves and larvae move to the base of the leaf sheath and tiller to feed. Fall infestations are the most severe, resulting in dead or severely stunted tillers. This insect strongly prefers wheat but will go after barley. Hessian fly has not been an important pest in the mid-Atlantic for some time and my predecessor can count the number of affected fields she has seen over the course of several decades on one hand, although there is a hypothesis that this may change with increasing wheat cover crop and later and later first freeze events. The average date of the first freeze is called the ‘fly-free’ date. Planting after this date should minimize Hessian fly activity and impact. The fly-free date for Sussex County is October 10, Kent is October 8, and New Castle is October 3. At this time, insecticides are probably not necessary.

The other pest group that could affect fall small grain plantings is the aphid complex, and planting after the fly free date limits the amount of time that aphids will be active to colonize fall plantings. Our two most common aphids are bird cherry oat (BCOA) and English grain aphids (EGA). Winged BCOA is a black color while EGA is a green color with two dark thoracic lobes. Wingless EGA are green with long black antennae and long black tailpipes or cornicles. Wingless BCOA are a bulb shaped olive green color with red patches at the base of the cornicles.

BCOA is the most important vector for barley yellow dwarf virus. English grains are not considered to be an effective, economical vector. Numerous Extension recommendations indicate a fall aphid population exceeding 12 – 20 per foot between emergence and the beginning of tillering (Feekes 4) may benefit from an insecticide application if BCOA is present. If BCOA is not present, an insecticide application will most likely not provide a positive return on investment. If control is deemed necessary, the best options are insecticides containing pyrethroids (such as Warrior) or a pyrethroid-neonic premix like Endigo. What about seed treatment? Last year we had two locations in which wheat was planted after the fly-free date with and without a seed treatment. Subplots were treated with a foliar insecticide in November. The only aphid species present in the fall was English grain aphid, and in extremely low numbers, never exceeding 4 per foot in any individual plot. In the spring, large numbers of both BCOA and EGA aphids arrived and colonized these plots, meaning fall aphid management had no effect on spring population growth. BYDV was also practically non-existent, and it’s been a few years since BYDV epidemics have been observed. Some states much farther south have, in the past, reported a yield bump from an early spring insecticide application as opposed to a fall or a seed treatment application, but with our current low levels of BYDV, I think even this is unnecessary. Recent research from University of Maryland suggest that seed treatments may actually decrease parasitoid activity in the following spring. Other factors that reduce the BYDV risk is high seeding rate and a hot, dry preceding summer, which we certainly have had.

Bottom line: planting after the fly free date will minimize both Hessian fly and aphid activity. BYDV incidence has been low the last couple of years, and if BCOA is not present in fields, insecticides are not recommended. An insecticide option may help if planting before the fly free date, especially if growing a higher value small grain like malt barley and if BCOA is present.

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