Native to parts of Eurasia, Russian wheat aphid (RWA) has become a global pest for its rapid reproduction and considerable economic impact on crops, including wheat, barley, triticale, rye, oats, as well as pasture grasses.
RWA have a distinctive appearance, with several key characteristics that serve as key identifiers. Apterous (wingless) adults sport a pale green, elongated 'spindle' shaped body with notably short antennae, spanning one-third to half their length. Their siphunculi too are also very short, often not discernible to the naked eye. Also distinctive is its twin caudal processes at the end of the abdomen, giving the appearance of a 'double tail'. Winged adults, or alates, have proportional antennae and darker coloring, especially on the thorax.
Like other introduced aphids in Australia, RWA reproduces asexually, with females birthing live females. Thriving on actively growing plants, it infests newly emerged wheat seedlings in autumn, multiplying most during tillering, stem elongation, and booting stages.
The Russian wheat aphid can survive across a wide range of temperatures and may perform better at lower temperatures compared to other aphids affecting cereal crops.
RWA show a preference for feeding on the newest leaves of plants, often targeting the last two leaves that unfold. When their numbers are high, they can be found on various parts of the leaves. They gather in dense groups, typically on the lower parts or within leaves, which typically curl inwards due to feeding damage.
The damage they cause is characterized by several signs: leaves rolling lengthwise to create a sheltering tube for the aphids, the appearance of light to purplish streaks along the leaves, and changes in leaf coloration. Plants heavily infested with aphids might exhibit stunted growth and appear flattened, with tillers almost parallel to the ground. As the crop matures, wheat awns might get caught in the rolled leaves, resulting in a distinctive hooked appearance and bleaching of the wheat head, leading to reduced yield.
Currently, there is no documented resistance to insecticides in the Russian wheat aphid, and using chemicals for control is generally effective.
In cases where control methods fail in the field, it's often because of the aphid's hidden feeding habits. Insecticides might not adequately reach the tightly rolled leaves where the aphids take refuge.
Thus, effective control necessitates thorough application and the use of insecticides with either fumigant or systemic properties, such as chloropyrifos or pirimicarb.
While organophosphate insecticides can be efficient, their wide-ranging impact might inadvertently harm natural predators. The decision to employ foliar treatments hinges on the extent of infestation among seedlings or tillers.
Implementing seed treatments can provide initial protection during the early stages of growth.
Russian wheat aphid populations are significantly influenced by environmental factors. Survival outside their leaf shelter is impacted by elements like rainfall, drying winds, and the presence of predators and parasitoids. Rain events, for instance, dislodges aphids from upper leaves, and heavy rainfall can lead to as much as 50% mortality among them.
Notable natural enemies of Russian wheat aphids found in Australia include parasitoid wasps like Aphidius colemani, A. ervi, and Diaeretiella rapae, along with versatile predators like ladybird beetles (such as Coccinella spp. and Hippodamia spp.), lacewings (Chrysopa spp.), damsel bugs (Nabis sp.), hoverflies (Syrphus spp.), and even entomopathogenic fungi.
Cultural strategies for RWA include removing any leftover volunteer cereals and grasses in fields during the fallow period and other areas in the summer and autumn.
Further, adopting a strategy of planting winter cereals later can help delay and reduce initial aphid infestations. Crops that are sown early, such as cereals meant for pasture, might be more susceptible to Russian wheat aphid infestations. It's recommended to closely monitor these early sown crops.
Additionally, employing agronomic techniques to enhance crop strength and encourage the growth of a dense canopy can effectively restrain RWA populations and mitigate their impact on the crop.