Arthropod Pests of Wheat in the Great Plains
Over 30 insect and mite species attack wheat in the United States of America. Most rarely cause damage to wheat or occur in localized areas, and therefore, are of minor economic significance. In the Great Plains, the greenbug (Schizaphis graminum) (Figure 1.3) and Russian wheat aphid (Diuraphis noxia) (Figure 1.4) are major pests that frequently cause damage to wheat over large parts of the region. When outbreaks occur, they must be managed to avoid significant yield losses.
Some insect or mite pests are important primarily because of the plant diseases they transmit. The bird cherry-oat aphid(Rhopalosiphum padi) (Figure 1.5) is a pest of wheat and barley in the region and can cause direct yield losses when populations are high during early crop growth stages. However, such infestations rarely occur. This aphid also transmits the virus that causesbarley yellow dwarf disease (BYDV), and even though economic losses from BYDV are usually low, they occasionally can be substantial. The Areawide Integrated Pest Management Program (AWIPM) demonstrated lower incidence of cereal aphids in more diversified cropping systems; however, due to the generally low levels of the disease during the study, reduced BYDV incidence could not be established.
The wheat curl mite (Aceria tosichella) is important in the High Plains. The wheat curl mite transmits three wheat viruses in the High Plains (wheat streak mosaic virus, High Plains virus, and Triticum mosaic virus). This disease complex is the most serious arthropod vectored cereal disease problem in the High Plains. Widespread outbreaks are rare, but isolated fields or groups of fields with severe disease occur in most years. Management of this disease complex involves cultural practices, control of volunteer wheat, and delayed planting dates. Diversifying the wheat-fallow system could have both positive and negative effects on virus epidemiology. Avoid growing wheat adjacent to wheat stubble (volunteer wheat) to reduce the potential forWSMV and HPV problems. In addition, the presence of sunflower or millet may allow for increased presence of alternate summer hosts for the mite, particularly summer annual grasses and volunteer wheat. The presence of nearby dryland corn, an alternate mite host, also can increase the potential for these diseases in some years.
The wheat stem sawfly (Cephus cinctus) is a pest in the northern part of the region. Host plant resistance and trap crops are used to control it. Reduced tillage has increased sawfly populations in Wyoming and Nebraska, but crop diversification can have the opposite effect. Cutworms and armyworms are occasionally important pests of wheat. The fall armyworm (Spodoptera frugiperda) attacks wheat seedlings in the autumn in the southern Great Plains and can destroy an entire crop. More commonly, damage is limited to skeletonizing of young wheat leaves. The armyworm (Pseudaletia unipuncta) is a problem at heading, and occasionally causes severe losses by clipping the stems just below the head. The army cutworm (Euxoa auxiliaries) feeds on wheat leaves in late autumn and again in spring, whereas the pale western cutworm (Agrotis orthogonia) feeds only in spring and cuts stems at the soil level. Both species can cause large losses. Climate plays an important role in determining outbreaks of armyworms and cutworms, so cropping practices have minimal effects on outbreaks. Insecticides are the primary management tool for these pests (reference Chapter 7-“Arthropod Pests of Wheat”).