Effects of Rainfall or Drought in the Abundance of Aphids in Winter Wheat
Raul T. Villanueva—Extension Entomologist
The bird cherry oat aphid (Rhopalosiphum padi) and the English aphid (Sitobion avenae) are the most abundant aphid species in small grains in Kentucky. They are key pest species because they are vectors of the Barley Yellow Dwarf Virus (BYDV). Virus transmission occurs during early stages of plant development when plants are more vulnerable to be affected by BYDV. These earlier infection of plants in the fall can produce stunted plants and reduce yields. Consequently, the abundances of aphids during the fall might be the critical period when aphid species that are carrying BYDV can potentially reduce incomes of growers.
Small grains are not planted during the summer in KY however, bird cherry oat and English aphids are present in alternative hosts. Bird cherry oat aphids in North America are hosted by the common choke-cherry (Prunus virginiana); whereas English aphids is hosted by several volunteer small-grains and weedy grass species such as rough barnyard grass, yellow foxtail, and green foxtail. Also, these two aphid species can be found in field corn (Figure 1) and millet planted in the summer.
Thus a question comes up here, would the rainfalls (or the absence of them) in summer months (July to August) influence the small grain aphid abundances during early stages of small grains development (from mid-October to December). Information on these type of events are based on anecdotal evidence. However, there have been several studies where models were developed using simulated droughts or rainfall events.
In simulated studies it has been shown that aphids reared on plants grown on drought events had heavy weights than aphids reared under normal conditions. Researchers explained that this happened because under drought, insects feed on plants that have higher concentrations of amino acids. However, in some cases the number of offspring produced was unaffected by the watering regime of the adult aphids’ host plants. Furthermore, it was shown a high correlation between insect body size, insect performance, and fecundity.
In a study conducted in Chile, 76% of bird cherry oat aphids and 69% of English grain aphids fell from the wheat seedlings when rainfall was simulated (30 mm/h for 30 min), in comparison with 11 to 18.4% from control, not subjected to simulated rainfall. Under natural rainfall (7.4 mm in 24 h), an average of 45 % of bird cherry oat aphids and English grain aphids were washed off the plants. These rainfalls also affected 58 % mummified aphids or empty mummies. However, it was found that several parasites species (Aphidius, Ephedrus and Praon) were able to emerge from the mummies washed to the ground, even after remaining under water for several weeks.
Rainfall events from the end of June to August in 2019 were not uniform in different regions of Kentucky (Figure 2). It appears that areas around Louisville have fewer rainfalls compared with Mayfield and Princeton. Thus, forecasting the aphid densities for Kentucky based on environmental conditions is a difficult task for the different regions. In general, the number of aphids colonizing the winter grains in autumn is largely dependent on the populations that have built up on grasses during the late summer. In Great Britain, warm, wet summers have been positively linked with large numbers of bird cherry oat aphid migrants in autumn.
Based on what is discussed above, rain events dislodge aphids from plants; whereas drought spells may produce aphids with heavier weights, but in general it is difficult to predict aphid populations for earlier stages of winter wheat development. In Kentucky, insecticide seed treatment is utilized by >80% of small grain farmers to protect plants from aphids. This is a preventive-prophylactic tool that protect fields for the 30 to 40 first days after seed emergence. Insecticide seed treatments is a re-cent practice in small grains, and as this practice is becoming generalized there is a potential risk that aphids may develop resistance in a near future. For this reason, growers and consultants need to continue scouting and monitoring for aphid populations to detect populations that may develop resistance to insecticide treated seeds. Scouting is an important IPM tool that will help with the identification of the pest and facilitate future control management practices if resistance start to develop within certain region.