Managing Greenbug Biotype and Insecticide Resistant Changes
By Hugo Zorrilla
Greenbugs are aphids that feed on grain sorghum causing severe economic losses each year. Management of greenbugs is done through the use of resistant grain sorghum hybrids and insecticides. Greenbug biotypes evolve, allowing populations to overcome both hybrid resistance and insecticide efficacy. This article will lay out facts on greenbugs, and describe briefly how biotypes with resistance to insecticides and hybrids have evolved. An overview of hybrid selection for greenbug resistance is also included.
Greenbug Distribution and Economic Impact
Greenbugs are capable of causing severe economic damage in small grains and sorghum crops and are found throughout Africa, North and South America, Asia, Australia and Europe. In terms of damage, greenbugs are economically important in Kansas, where they infest approximately 500,000 acres of sorghum each year. These pests can cause an average yield reduction of 10 bushels per acre, resulting in a crop reduction of 5 million bushels each year. At an average grain cost of $2.50 per bushel, Kansas experiences a $12.5 million economic loss each year due to greenbugs.
Assuming resistant hybrids will reduce losses by 70 percent: $12,500,000 x 0.7 = $8,750,000 could be saved if resistant hybrids were planted each year.
Greenbugs cause severe damage from seedling to grain filling stages in sorghum. When greenbugs feed, they inject a toxin into the plant, which causes reddish spots on the leaves. These reddish spots multiply as the greenbug population increases and the leaves begin to die. The aphids feed in colonies on the underside of the foliage and produce a heavy sticky substance called honeydew. Greenbugs can also transmit maize dwarf mosaic virus, which predisposes sorghum to charcoal stalk rot and subsequent lodging.
Description and Biology
The adult greenbug aphid is 1.6 mm in length, is light green, with a darker green dorsal abdominal stripe. Winged and wingless forms may be present in the same colony. Females produce living young (nymphs) without mating. Under optimal conditions a complete life cycle takes seven days and within 25 days each female can produce about 80 offspring.
Andrews, et al, list Rondina as first discovering greenbug in Italy in 1852. This pest was observed for the first time in the United States in 1884. A widespread outbreak in 1890 was disastrous to small grain crops over a large area of the country, extending from Texas to Indiana, including parts of Kentucky and Tennessee.
Greenbug, Schizaphis graminum (Rondani), is a major economical pest in sorghum. Biotype-C caused annual loses of $8-9 million in Kansas sorghum production from 1968 through the early 1980s. Beginning in 1981, biotype-E replaced biotype-C as the predominant pest in the High Plains sorghum production areas. Sorghum researchers developed products with resistance to biotypes-C and E, which greatly reduced commercial production losses to farmers. By 1988, at least eight (A - H) biotypes were documented as affecting small grains and/or sorghum. In 1989, Pioneer sorghum researchers observed greenbug damage in western Kansas on biotype-E resistant hybrids.
In 1990, many fields in southwestern Kansas with biotype-E resistant sorghum showed severe greenbug damage. Ken Kofoid and Tom Harvey, from Kansas State University, collected greenbug samples from biotype-E resistant hybrids growing in Stevens County. These were heavily infested with greenbugs and the plants showed considerable damage. When isolated in the laboratory, samples from Stevens County were designated as biotype-I.
In 1992, David Andrews, from the University of Nebraska-Lincoln; along with Paula Bramel-Cox and Gerald Wilde from Kansas State University, reported the identification of two new sources of resistance to biotype-I (subsequently named PI550607 and PI550610), in a collection of sorghum plant introductions from Russia. In the same year Harvey, Kofoid and Wilde reported the identification of a new greenbug biotype and designated this as biotype-K.
Greenbug resistance to organophosphate insecticides has been reported since the mid-1970s. Failures in greenbug control when using parathion and chlorpyrifos chemicals were first reported from southwestern Kansas in 1988. Due to favorable prevailing weather conditions in recent years, greenbug outbreaks have been limited. Very little insecticide has been applied, with predators and parasites significantly reducing greenbug populations.
Pioneer uses Cruiser¹ insecticide seed treatment to provide early season control of greenbugs, as well as chinch bugs, yellow sugarcane aphids and corn leaf aphids. Insecticide seed treatments can provide control equal to in-furrow products in Kansas, according to Wilde. A three-year, Hesston County study recently completed by Kansas State University, demonstrated an 8.5 bushel per acre advantage for insecticide treated grain sorghum seed, compared to seed that had no insecticide seed treatment. The difference in yield can be attributed to early season control or suppression of a number of insects, including greenbugs. Cruiser is not an organophosphate, but is a neonicotinoid insecticide. Resistant greenbug populations existing today are not cross-resistant with Cruiser, largely because of the difference in chemistry.
Wilde provided a summary of the 2001 greenbug biotype and insecticide resistance survey on sorghum in Kansas. Although insecticide resistance was found in several counties, the average percent is still low (see Table 1). The highest concentration of organophosphate insecticide resistance was shown in areas where a large number of fields were treated for greenbug in 2001.
Harvey made greenbug biotype determinations in several Kansas counties (see Table 2). Biotype-K constituted 23 percent of the collections and biotype-I was identified on the other 77 percent of insects collected.
Table 1. 2001 Kansas greenbug insecticide resistance survey
Source: G. Wilde, Kansas State University
Table 2. Number of biotype-E, I and K greenbugs collected in Kansas counties
Source: Tom Harvey, Hays Agric. Experiment Station, Kansas
Greenbug collections were tested on four replications each of Pioneer hybrids 8500 and 8505 to determine GBE and I and on Pioneer hybrid 550610 to determine the presence of GBK.
Pioneer Host Plant Resistance (HPR) Research Commitment
Host plant resistance (HPR) is an economical and effective integrated pest management tool for managing greenbug populations in grain sorghum. HPR involves breeding greenbug resistance into commercial hybrids. Greenbugs with insecticide resistance are still being reported in southwestern Kansas where greenbug biotypes-K and I are prevalent. Due to the presence of these insecticide resistant greenbugs, HPR will be even more important in the future.
In the summer of 1992, crosses were made between Pioneer elite sorghum lines and exotic sorghums carrying resistance to biotype-I greenbugs (PI550607 and PI550610). The process of backcrossing will continue in Argentina and Kansas plant breeding stations until an agronomically acceptable hybrid carrying biotype-I resistance is developed. By 1997, the first yield test using biotype-I resistant sources had been developed. Pioneer was able to provide a biotype-I resistant hybrid in 2000 (Pioneer brand hybrid 84G82). Pioneer sorghum research centers continue to move greenbug resistance into elite inbreds for future use.
Andrews, D.J., P.J. Bramel-Cox, and G.W. Wilde. 1992. New sources of resistance to greenbug Schizaphis graminum (Rondani), biotype 1 in sorghum Sorghum bicolor (L.) Moench, Kansas State Univ. Agron. Dept. Pub. 003.
Brooks, H.L. 1994. A historical look at the greenbug. Kansas State Univ. Ext. Specialist. Entom. Dept.
Harvey, T.L., K.D. Kofoid, T.J. Martin, and P.E. Sloderbeck. 1991. A new greenbug virulent to E-biotype resistant sorghum. Crop Sci. 31:1689-1691
Parker, R., 1995. Yield and economic return from at-planting insecticide treatments in sorghum, Lavaca County, Texas.
Russian Wheat Aphid Task Force. 1995. Economic Impact of the greenbug in the western United States 1992-1993. Great Plains Agricultural Council Pub.155.
Sloderbeck, P.E. 1996. High Plains Greenbug Newsletter. Kansas State Univ. vol. 5 no. 2
Teetes, G.L., K.V. Seshu Reddy, K. Leushner, and L.R. House. 1983. Sorghum insect identification handbook. ICRISAT bulletin no. 12.
Wilde, G., and R. Shufran, Kansas State Univ. Entom. Dept., and D. Kindler, USDA-ARS. 1996 Unpublished results of the 1996 greenbug biotype and insecticide resistance survey
Wilde, G., Summary of the 2001 greenbug biotype and insecticide resistance survey on sorghum in Kansas (letter to cooperators December 11, 2001).
Hugo Zorrilla earned a doctoral degree in plant breeding and genetics from Purdue University. He currently serves as Pioneer research scientist and manager of the Manhattan, Kansas research station. His key responsibilities are for development of sorghum hybrids, across three maturities, for Nebraska, Kansas, Missouri and Oklahoma. He has been with Pioneer Hi-Bred International since 1982.