Biologically Intensive Pest Management of Greenbugs, Schizaphis Graminum (Rondani), on Grain Sorghum
Increased environmental awareness, the uncertain future of cheap insecticides, and wide spread occurrence of insecticide resistance indicates that substitution of cheap insecticides for comprehensive pest management expertise may not be an alternative in the future. Because greenbugs have numerous, naturally occurring parasitoids and predators that are highly effective under some conditions, sorghum pest management presents the unique opportunity of incorporating biological controls into existing pest management recommendations. The primary goal of this study is to develop field based management procedures that incorporate plant resistance and biological control into our sorghum management recommendations.
Specific objectives include: 1) Determine economic and biological benefits of greenbug resistant hybrids using on-farm comparisons; and 2) Evaluate and integrate predators and parasitoids, including inoculative releases of parasitoids, into management programs involving greenbug resistant versus susceptible sorghums. Procedures include small plot cage studies as well as field size plots to study the interaction of greenbugs and biological/natural control agents in resistant and susceptible sorghums. Economic and biological comparisons to insecticide-intensive management programs will be included in remaining studies.
In field size studies, fewer greenbugs and less damage occurred in 1998 compared to 1997. Although greenbug numbers were low, the results in 1998 showed similar trends as in 1997. Averaged over all fields, preliminary analyses indicates a 69 percent reduction in the number of greenbugs per plant (Sig., P=0.05) and a 71 percent reduction in leaf damage (Sig., P=0.05) in the greenbug resistant sorghum compared to susceptible sorghum. Unlike 1997, no significant difference in yield was detected among resistant and susceptible hybrids in 1998. Predator populations were higher in 1998 and contributed to keeping greenbugs under control compared to 1997. During the seedling to pre-boot sorghum development period in 1998, there were 34 percent more predators compared to 1997. During this same time period, corn leaf aphid populations were approximately five times higher in 1998 compared to 1997. The ratio of predators to greenbugs was higher in the resistant sorghum, 1:19 versus susceptible sorghum 1:49.
Over the entire season, parasitism rates were higher in 1998 (13.5 percent) than 1997 (2.3 percent). On August 10, 1998 (approximately four weeks post parasitoid infestation), parasitism was significantly higher (51.7 percent) in fields where parasitoids were released compared to non-release field (16.4 percent). These results indicate that development of procedures to encourage early corn leaf aphid populations and inoculative release of parasitoids hold promise as greenbug management tools.
For small plot and field cage studies, in the absence of parasitoids, resistant (antibiosis) and susceptible sorghums were killed by greenbugs, and heavy damage (approximately 84 percent leaf damage) occurred in tolerant sorghum in caged studies. When parasitoids were added, greenbug damage was reduced 68 percent in antibiosis, 29 percent (40 percent if based on total damage) in the tolerant, and 28 percent in the susceptible. Yield data is being analyzed. These data indicate the advantages of combining biological control and plant resistance to reduce pest damage.
In 1998, information from the first year studies were presented at four sorghum producer meetings, the University of Nebraska South Central Research and Extension Center field day, the University of Nebraska wheat field day, the joint Kansas/Nebraska Sorghum Board symposium, two presentations at the Entomological Society of American meeting, and was a part of the discussions at the 1998 aphid research works workshop. The information will be discussed at a 1999 University of Nebraska crop protection clinic.
North Central Region 1998 Annual Report.