• A few insect species can be present on wheat in the spring but the chances of them causing economic damage will depend on several factors
• The implementation of cultural and chemical practices, along with host plant resistance has made insect management of wheat a lot easier.
• Various aphid species can be found on wheat and treatment thresholds may vary with the aphid species present.
• Armyworms can cause economic damage when feeding occurs on the flag leaf or on the stem just below the head which can cause the head to fall off.

A few insect species may be present on wheat in the spring but the chances of them causing economic damage will depend on several factors. Temperature affects insect survival, development and the infestation level (number of insects) on the crop. The wheat’s growth stage at the time of feeding determines the risk of economic damage. In general, insect feeding after the head matures is not of serious concern though feeding during the head filling stages may greatly affect yields. Nonetheless, the implementation of cultural and chemical practices, along with host plant resistance has greatly improved the management of certain insect problems over the years.

HESSIAN FLY

This insect overwinters as a fully grown larva (maggot) inside cases that resemble flax seeds (Pic. 1). Once wheat comes out of dormancy in the spring, Hessian fly adults emerge and mate. The adults are mosquito-like in appearance less than 1/8” long. A female lives for about 4 days but during this time she can lay 250-300 eggs. Eggs hatch within 3-10 days into a legless maggot initially reddish in color before turning white (Pic. 1). Once they begin feeding, the maggots become stationary and may reach their maximum size in a couple of weeks. The maggots feed on the lower part of the stem but never enter the stem. However infested stems can break potentially causing yield losses. One of the most helpful tools in managing Hessian fly is planting wheat after the fly free period in the fall. This is a very widely adopted practice in the Midwest that has helped reduce damage to wheat in the fall. Resistant wheat varieties are also available and can result in poor survival and feeding cessation of the Hessian fly. Insecticide seed treatments are great tools to reduce insect damage in the fall but they will not be effective at preventing damage in the spring. Scouting when the heads begin to fill will help determine the presence and extent of damage. The presence of lodged stems in the spring can be an indication of Hessian fly damage.

Picture 1.  Young (reddish) and full grown (white) Hessian fly larvae with flaxseed-like puaria. (Source: GROWMARK, Inc.)

APHIDS

A number of aphid species can be found on wheat. For example in Illinois, greenbugs, bird cherry-oat (Pic.2), English grain and corn leaf aphids are common. Aphids suck plant juices but they are also vectors of Barley yellow dwarf virus (BYDV). Diagnosing BYDV can be complicated as the symptoms can resemble, nutrient deficiencies, other diseases or environmental stress. However, the combination of aphids present with yellow/ stunted plants and a maroon- purple coloration from the tip of the leaf down is usually associated with BYDV. The use of neonicotinoid seed treatments and foliar insecticides can reduce virus transmission in the fall. Aphids may re-infest wheat in the spring and yields may be impacted under high infestations. However, spring treatment thresholds for aphids on wheat have not been consistently defined across Universities. Some thresholds do appear to vary with the aphid species present from 25-50 aphids per plant (25 for greenbugs, 30 for corn leaf and bird cherry oat and 50 for English grain aphids) up to the boot stage. Other resources suggest that during the headfilling stages 50 -100 aphids or more per foot of row could indicate the need for control. Several pyrethroid and chlorpyrifos insecticides are labeled against aphids. Keep in mind that aphids have many natural enemies that can help keep their populations in check and that the unnecessary use of foliar insecticides can reduce the number of these natural controls and increase the risk for aphid outbreaks.

Picture 2.  Bird cherry oat aphid. (Source: GROWMARK, Inc.)

This insect feeds on leaves (Pic. 3) and stems of wheat and can cause economic damage when the feeding occurs on the flag leaf or on the stem just below the head which can cause the head to fall off. Some armyworms may overwinter in southern counties as a partly grown larva but a good number of moths migrate from the southern states in the spring. Armyworms feed mainly at night so scouting early in the morning or late in the evening will make it easier to find them. Scout areas with dense stands of wheat first prior to and following head emergence. If six healthy worms (¾  to 1 ¼ inch long) are found per foot of row or 2-3% of the heads have been clipped, an insecticide may be warranted. However, keep in mind that armyworms also have a few natural enemies that help keep them in check. When scouting, make notes about the presence of sick or parasitized armyworms but don’t take these numbers into account when determining your treatment thresholds. Only healthy, nonparasitized armyworms should be considered.

Picture 3.  Armworm damage to wheat (Source: GROWMARK, Inc.)