Absence Of Aphids In Wheat Sown Early

DR. RAUL T. VILLANUEVA

 PRINCETON, KENTUCKY

In Kentucky, wheat is planted after October 4, depending on the latitudinal zone, to avoid egg oviposition by Hessian flies (fly-free dates to sow wheat) (Figure 1). However, for research purposes, wheat had been planted in research plots at the University of Kentucky Research and Education Center in Princeton on three dates: September 19, October 1, and October 19. The latter date would correspond to the fly-free date to sow wheat in Caldwell County in western Kentucky. Half of the plots of wheat were planted with seeds treated with a fungicide and the other half with seeds treated with an insecticide + fungicide. These early plantings were conducted mainly to evaluate the presence of Hessian flies, aphids, and any other pests in these fields.

Targeted Pests

Hessian fly

The Hessian fly, Mayetiola destructor(Diptera: Cecidomyiidae), was one of the key pests of wheat in the past. The adult oviposit eggs on the upper side of wheat leaves, and then after egg hatching, the maggots move between the leaf sheath and the stem and feed on the plant. Fall-infested wheat usually dies during the winter; spring-infested wheat produces grain but usually lodges before harvest. Economic infestations have been uncommon since the establishment of the fly-free dates to sow wheat and use of tolerant or resistant varieties in Kentucky.

Aphids

There are several aphid species affecting wheat, including the bird cherry oat aphid (Rhopalosiphum padi), English grain aphid (Sitobion avenae), and corn leaf aphid (Rhopalosiphum maidis). Aphids extract juices with their piercing-sucking mouthparts from wheat and, at the same time, may transmit barley yellow dwarf virus. Barley yellow dwarf virus is considered the most devastating problem in wheat if these plants are infected in the fall. Monitoring for aphids, conducting IPM-based insecticide sprays, and the use of tolerant cultivars are key factors to controlling and managing this disease.

Preliminary Observations

Hessian fly

So far, wheat plants infested with Hessian flies have not yet been found in any of the planting dates.

Aphid monitoring

In these fields (Figure 2), three well-trained entomologists (all using headbanded 4X magnification lenses) have been scouting for aphids and other pests every week since September 27. So far, no aphids have been found on most dates; 1 or 2 aphids were found in the six entire fields, corresponding to an average of 0.016 or 0.033 aphids per 1 ft-row/date. For this time and  crop age, the threshold is 3 aphids per food row. The aphids were found in either insecticide + fungicide or fungicide-only treated seed. The absence of aphids is surprising despite the extremely warm temperatures in September and October.  

Possible Reasons for the Absence of Aphids

The reason for the absence of aphids may be due to two causes:

(1) The environmental conditions in Kentucky during the spring and summer [severe drought in June and extremely high temperatures (higher than 100 °F) during the summer] may have negatively impacted the aphid populations; and

(2) Rains brought by Hurricane Helene [rains lasted for 3.5 days (September 26 to 29) with rainfall totals of 2 to 3 inches and up to 5 inches in some locations of western Kentucky] may have washed out aphids and other insects, causing them to drown, consequently reducing aphid populations.  ∆

DR. RAUL T. VILLANUEVA: University of Kentucky