Integrating Effects of Natural Enemies into Winter Wheat Greenbug Management

Integrating Effects of Natural Enemies into Winter Wheat Greenbug Management

Summary

Small grain aphids frequently attack wheat (Triticum aestivum L.) grown in the southern Great Plains of the United States. The most important aphids are frequently a mixture of species that include greenbug Schizaphis graminum (Rondani), Russian wheat aphid (Diuraphis noxia Mordviko), bird cherry-oat aphid (Rhopalosiphum padi L.), English grain aphid (Sitobion avenae Fabricius), and corn leaf aphid (Rhopalosiphum maidis Fitch). Aphid pests are of particular concern because they have been observed to damage wheat from plant emergence to heading and are often overlooked; aphids reproduce rapidly, and are often not detected by farmers until their populations reach deleterious levels
Recent research indicates that insecticide applications for aphid control are only occasionally needed. This is in part due to the fact that greenbugs are attacked by a number of predators and parasites: including lady beetles, parasitic wasps, spiders, damsel bugs, lacewing larvae and syrphid fly larvae. One of the most important natural enemies in the Southern Great Plains is the parasitic Hymenopteran Lysiphlebus testaceipes Cresson.
Decisions about greenbug management require taking samples to determine infestation levels. Historically this is accomplished by either selecting three tillers at each of 25 random locations throughout the field and calculating the mean number of greenbugs present per wheat tiller, or by examining a 0.3m long section of crop row at several random locations throughout the field to determine the mean greenbug population per foot of crop row. These populations were then compared to published guidelines of damage potential. Both of these sampling techniques are time consuming and difficult to accomplish. More recently, a simplified presence/absence (binomial) sequential sampling system for estimating greenbug densities has been developed. Coined “Glance ‘n go,” this method involves looking at randomly selected tillers and noting the presence or absence of greenbugs on each tiller. The number of tillers that were positive for greenbugs is then compared to a chart indicating the probable greenbug population density. Due to the simplicity and ease of using Glance ‘n go, producers are more likely to sample for greenbugs and make economic decisions about insecticide applications.
The Glance ‘n go method of greenbug sampling, while simple and easy to use, deals primarily with greenbug densities and does not include sampling for natural enemies such as parasites. Glance ‘n go only advises that if most of the greenbugs collected are mummies, then do not spray. Parasites are very important for biological control of aphid infestations in winter wheat. A review of several state extension publications, found few publications that actively recommended incorporation of natural enemies for greenbug control. To remedy this deficiency, a presence/absence sampling method for estimating parasite densities and their impacts in winter wheat fields has been developed (Glance ‘n go for parasites) and would make a good addition to the Glance ‘n go system. Being able to quickly predict parasite efficacy for greenbug control would allow producers to reduce the use of pesticides, thus increasing producer profits, reduce the incidence of greenbug resistance to insecticides and reduce negative impacts of pesticides on the environment.

Objectives/Performance Targets

Objective 1: Compare the greenbug management practices of Glance ‘n go (a binomial
sequential sampling system), Glance ‘n go for parasites (a combination of binomial sequential sampling for greenbugs and a binomial sequential sampling system for aphid parasitoids), no management, and prophylactic insecticide applications.
Objective 2: Determine yield of wheat fields managed by Glance ‘n go, Glance ‘n go for
parasites with wheat that is not treated with insecticide under any circumstances (no insecticide) and wheat grown without greenbugs (prophylactic insecticide).
Objective 3: Provide results of this research to the wheat growing community via field days and
publications such as extension publications.

Accomplishments/Milestones

As of 1 April 2004, four 30 by 30 m treatment plots at three sites were established in Caddo county, one site in Grady county, two sites in Canadian county, two sites in Payne county and two sites in Kay county. These five counties are located along a transect stretching from south central to north central Oklahoma. Sampling commenced in fall 2003 after seedling emergence. To date 58 data collection trips have been made for a mean of 6 trips to each site. Data collected at each site consisted of Glance ‘n go sampling as well as a reference sample consisting of absolute aphid and parasitoid counts taken from 60 randomly selected wheat tillers. In accordance with the research proposal, data will continue to be collected until wheat reaches the boot stage (this usually occurs in April) and a final data collection trip will be made just before the wheat is harvested to collect yield data.
There was a rather extensive greenbug outbreak throughout Oklahoma during the 2003-2004 growing season. This allowed for Glance ‘n go management decisions to be made at most of our research sites. Additionally, parasitism was detected in association with the greenbug outbreak that allowed for Glance ‘n go for parasitoids management decisions to be made as well. A better set of conditions couldn’t have hoped for in order to conduct this research.
Data collected from plots managed by Glance ‘n go for parasitoids has corresponded well with my previous parasitoid research. For the past five years, I have observed successful greenbug control in every instance when more than 5 percent of aphids present were parasitized. Additionally, I frequently observed unsuccessful biological control of greenbugs when less than five percent of aphids were parasitized. Every plot in this experiment where binomial sequential sampling indicated that the parasitism rate was greater than five percent, greenbug populations have decreased. These findings should further validate my previous research.
Preliminary results of this research were presented at the Southwestern Wheat Research and Education Consortium 2004 Annual Meeting at the Southwestern Research and Extension Center in Garden City Kansas on March 23, 2004 and further presentations are planned in the future. Results of this project will continue to be provided to the wheat growing community via field days and by publications such as extension publications along with publication in a scientific journal such as the Entomological Society of America’s Journal of Economic Entomology.

Impacts and Contributions/Outcomes

Though this research and demonstration project is not yet complete, it has already provided results that greatly improved the Glance ‘n go guidelines. We managed plots that were somewhat uneven with both seedling and more mature wheat at two sites. Glance ‘n go guidelines dictated that treatment for greenbugs was not warranted at each of these sites. However all the seedling wheat was quickly killed by greenbugs, while the more mature wheat (>4-leaf stage) easily survived. This oversight by the Glance ‘n go authors has been corrected to state that Glance ‘n go management cannot be applied to seedling wheat.
Additionally data collected at these research sites is being used to provide data for a remote sensing experiment being conducted in Oklahoma and Texas. Each of the fields was photographed by a infrared camera from an airplane. These photographs will be compared to aphid population measurements in an attempt to be able to estimate aphid infestations from aerial photographs.

Collaborators: