Wheat: Earlier Aphid Occurrences May Be a Consequence of the 2017’s Warm Winter

Raul Villanueva, Extension Entomologist, University of Kentucky 

Figure 1. Pictures of the bird cherry oat aphid and
 a winged English grain aphid found in wheat fields in February 2017. (Photo credits Yaziri Gonzales).

In Kentucky there is a complex of aphid species that feeds on wheat. The bird cherry oat, the English
grain (Fig.1), the greenbug, and the corn aphids are the most important species. Their role as vectors of plant viruses, particularly Barley Yellow Dwarf Virus (BYDV), branded them as the key pest on
wheat grain production. These aphid species overwinter as nymphs, and can be active when temperatures are above 45⁰ F. It is known that BYDV infections are more damaging when they occur in early growth stages of the wheat plant. Thus, aphids have more opportunities to infect young plants under this climatological circumstances. 

Also, uncharacteristically warm temperatures (above 50⁰ F) were present during most days of November (Fig. 2). Based on historical records and comparing mean temperatures for the bimonthly periods November–December 1976, 1996, and 2016; and January-February 1977, 1997 and 2017 (periods were separate to facilitate the analysis). We can notice that mean temperatures (linear regression analysis) for 2016 and 2017 were higher than the previous year for the same periods,
respectively (Fig. 2). It may be possible that the continuous alterations on the climatological events are influencing these warmer temperatures (more frequent rains, storms out of the normal patterns, sudden ice storms, etc.) as shown on these 20-year intervals.

Figure 2. Temperatures (40⁰ F) for the months of November-December 1976, 1996, and 2016; and January-February 1977, 1997, and 2017. Regression lines shown that temperatures for 2016 and 2017 were  higher than for the previous years. (Figure was generated with temperatures of of Princeton, KY obtained from: http://weather.uky.edu/ky/data.php#KY_Monthly_Data).

Given the circumstances mentioned above, the warm winter in all likelihood will accelerate the development of both plants, and insects. To predict the biological events of organisms, accumulated degree days (AcDD) is used. The DD is based on temperatures above threshold temperatures that are specific for each organism. For wheat and aphids these are 45⁰ and 40⁰ F, respectively. These biological events re in turn used to schedule particular activities such as scouting and synchronizing insecticide sprays. For aphids and wheat, the AcDD starts on January 1 2017. Table 1 shows the AcDD for wheat and aphids from 2011 to 2017 for three counties of Kentucky. AcDD for 2017 are ahead of the AcDD of previous years.

Table 1. Accumulated degree days for wheat and aphid species for Caldwell, Monroe and Fayette Counties in Kentucky from January 1 to February 20, 2011 to 2017.  (Data source: http://weather.uky.edu/dd.php)

For wheat, the AcDD is far ahead compared to all the previous years, and that could set up wheat fields for freeze damage later in case an eventual freeze takes place. A later freeze on March or April can also affect insects. However, Dr. Michaud (Biocontrol Specialist) from the University of Kansas declared that earlier biological control organisms such as ladybug eggs and parasitoids can be disrupted by a cold front, and their recovery can be slower compared with insect pests.

Other pests such as Hessian fly maggots that are in root system of volunteer wheat or other plants also can be a problem. They can continue feeding as long as temperatures are above 40⁰ F. For this pest a chemical control would not work. However, for Hessian flies or BYDV there are resistant varieties that growers should have been using to reduce pest damages.

We have been sampling for aphids here at the UK’s Research and Education Center and other counties (Lyon, Trigg, Christian) and no aphids were found so far. However, we found aphids in early or late when? December. No aphids were found from mid-December to mid-February. However, aphids were found in low numbers after February 15 (when temperatures were >65⁰ F). Most of these aphids were nymphs although we had some adults and winged aphids.

 A question still remains about to spray or not to spray. There are many factors to consider but the most important is the economic value of the crop. The best alternative here is to continue monitoring for the presence of aphids and if the tallies are above the threshold levels indicated in Table 2, an insecticide spray need to be considered.

Table 2. The number of aphids per foot of wheat row required to support
an insecticide application for management of BYD

For more information check the following URL: • Predicting Insect Development Using Degree Days: http://entomology.ca.uky.edu/ef123 • Aphids and Barley Yellow Dwarf (Byd) in Kentucky Grown Wheat: http://www2.ca.uky.edu/entomology/entfacts/entfactpdf/ef121.pdf