Bird Cherry-Oat Aphid (Rhopalosiphum padi)
Wingless adult and nymphs. Photo Credit: G.W. Bishop and S.E. Halbert.
The bird cherry-oat aphid is the darkest aphid to be found on wheat in Kansas, although its color varies depending on the ambient temperature and its stage of growth. Nymphs are usually pale yellowish-green, darkening as they mature to a deep olive green in the adult stage. Normally, a dark brown patch is visible across the posterior portion of the abdomen spanning both cornicles, although it can be difficult to see on the darkest of specimens. Under very warm conditions, adults may be much paler in color, but the cornicles and terminal portions of the antennae are always black. When large colonies persist on wheat plants past the boot stage they can cause the flag leaf to twist into a corkscrew shape that can trap the awns, resulting in 'fish-hooked' heads. However, R. padi does not form dense colonies like other aphids, but tends to be widely distributed over the plant.
Rhopalosiphum padi is a host-alternating aphid – it normally exploits gramimaceous plants (secondary hosts) in summer and returns it its primary woody host (cherry) to complete a sexual generation in the fall. Sexual females (oviparae) lay overwintering eggs on the twigs of cherry which hatch into first-generation asexual females (fundrices) in the spring. By the time the daughters of these fundrices mature, the flush of new growth on the cherry trees hardens and is no longer a suitable food source, so virtually all develop into winged forms (alatae) that disperse to colonize suitable graminaceous plants such as wheat, oats or barley.
Although R. padi has been recorded from several hundred grasses, species that utilize the C4 metabolic pathway (e.g., corn and sorghum) are generally less suitable. However, it may form mixed colonies with other aphids that seem to increase the suitability of these plants for R. padi, as observed on sorghum during heavy infestations of the sorghum aphid. It is important to recognize that many R. padi populations can now skip the sexual generation (holocycle); when winters are sufficiently warm, asexual (anholocyclic) reproduction can continue year-round on graminaceous plants. With the advent of climate change, this is likely to occur at increasingly higher latitudes. Anholocyclic populations of R. padi occur in Oklahoma and possibly in southern Kansas. Rhopalosiphum padi can survive and reproduce at low temperatures; it occurs on wheat later in the fall, and earlier in the spring than other aphids, often while snow is still on the ground. The advent of warmer winters in Europe has resulted in year-round asexual reproduction by R. padi as far north as Poland where it has been recently linked to increased rates of disease transmission in spring-sown cereals.
The bird cherry-oat aphid can be considered a cryptic pest of wheat. Although its feeding causes no chlorosis or other visible damage to wheat plants, heavy infestations can reduce grain quality, protein content and test weight, and even impede protein assimilation by grazing cattle. However, its greatest economic impact results from its role as a key vector of plant viruses, especially Barley Yellow Dwarf Virus (BYDV). Virus infections are more damaging when they occur in early growth stages of the wheat plant and R. padi is well suited for this purpose. Although the hot summer weather in Kansas usually decimates aphid populations, R. padi can temporarily avoid extremes of temperature by descending to feed on the lowest parts of the stalk, at or below ground level. It is also able to feed actively in weather too cold for other aphids such as greenbug, enabling it to effectively colonize seedling wheat quite late into the fall.
The bird cherry-oat aphid is usually held below economic injury levels by the same natural enemies that provide effective biological control of other cereal aphids in Kansas wheat: lady beetles, lacewings, hover flies, and parasitic wasps. However, conditions that favor outbreaks of greenbug or Russian wheat aphid (for example, an abrupt shift back to cold temperatures after a warm spell in spring) also benefit R. padi and it will readily form mixed colonies with these aphids when they are abundant.
When these more directly harmful aphid species are present, decisions to apply pesticides should be driven by the total numbers of aphids present with the aim of controlling the more damaging species; materials applied for their control should be equally effective against R. padi. The economic threshold for R. padi alone is quite high, and treatment of wheat in vegetative stages is not recommended. From boot stage up until heading, count the number of aphids present on each of a series of 25 - 50 randomly selected tillers across a zig-zag transect of the field. Treatment with an insecticide broadly labeled for aphid control on wheat can be considered if an average of 50 or more aphids per tiller are present.
Note that contact insecticides will not reduce the incidence of virus transmission. Some studies suggest that seed treatment with neonicotinoids such as imidacloprid (Dyna-Shield Imidacloprid, Gaucho, or Gaucho XT) or thiamethoxam (Cruiser) reduce BYDV infection by suppressing aphid colony establishment during the seedling stage.
Additional Information can be found in the KSRE publication MF2823 Bird Cherry-Oat Aphids. Please refer to the most recent version of the Wheat Insect Management Guide for control options.
Updated 06/06/2024 by J.P. Michaud