In Kentucky, there is a complex of aphid species that feeds on wheat. Bird cherry oat (Figure1a), English grain (Figure1b), greenbug, and corn leaf aphid are the most important species. Their role as vectors of plant viruses, particularly barley yellow dwarf virus (BYDV), branded them as key pests of wheat grain production. These aphid species overwinter as nymphs and can be active when temperatures are above 45º 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 circumstance.
Also, uncharacteristically warm temperatures (above 50º F) were present during most days of November (Figure 2). Historical records and mean temperatures were compared for the bimonthly periods of November–December (1976, 1996, and 2016); and January-February (1977, 1997, and 2017) (periods were separate to facilitate the analysis). From this data, we notice that mean temperatures (linear regression analysis) for 2016 and 2017 were higher than the previous year for the same periods, respectively (Figure 2). It may be possible that the continuous alterations of climatological events are influencing these warmer temperatures (more frequent rains, storms out of the normal patterns, sudden ice storms, etc.) as shown during these 20-year intervals.
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. Degree day (DD) is based on temperatures above threshold temperatures that are specific for each organism. For wheat and aphids, these are 45º F and 40º F, respectively. These biological events are in turn used to schedule particular activities, such as scouting and synchronizing insecticide sprays. For wheat and aphids, the AcDD starts January 1 2017. Table 1 shows the AcDD for wheat and aphids from 2011 to 2017 for three counties in Kentucky. The AcDD for 2017 are ahead of the AcDD of previous years.
For wheat, the AcDD is far ahead compared to all the previous years, and that could set up wheat fields for freeze damage later should an eventual freeze take place. A late freeze in 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 systems of volunteer wheat or other plants can also be a problem. They can continue feeding as long as temperatures are above 40⁰ F. For Hessian fly maggots, 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 at the UK’s Research and Education Center and other counties (Lyon, Trigg, Christian), and we found aphids in early December. Then, aphids were not found from mid-December to mid-February. However, aphids were found in low numbers after February 15 (when temperatures were greater than 65º F). Most of these aphids were nymphs, although we observed some adults and winged aphids. In addition, we have been observing some parasitoids flying, and even collected some parasitized aphids (Figure 1c).
A question still remains about whether to spray or not to spray. There are many factors to consider, the most important is the economic value of the crop. Nonetheless, if a late freeze does not occur, natural enemies might be capable of holding aphids at populations below threshold levels. The best alternative 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 needs to be considered.
For more information check the following publications:
- 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
By Raul Villanueva, Extension Entomologist