Greenbug Populations Research Articles

Over-Summering and Biotypic Diversity ofSchizaphis graminum(Homoptera: Aphididae) Populations on Noncultivated Grass Hosts

Greenbug, Schizaphis graminum (Rondani), populations over-summering on noncultivated grass hosts may be implicated in early fall infestations in wheat. The purpose of this study was to examine the relationship between over-summering greenbugs on noncrop hosts and fall infestations on wheat. Since greenbug populations on noncultivated hosts may also act as reservoirs of virulence genes, the biotypes of collected aphids were also determined. The grass species present at three sites (two in Oklahoma and one in Kansas) were identified and a species richness curve was generated. Greenbugs were collected at these sites and their hosts and biotypes determined. At Hays, KS, a persistent over-summering greenbug population lead to an early fall infestation in wheat. At the sites in Oklahoma, where over-summering greenbugs were not detected, the fall infestation occurred 3 months later. Biotypes G, I, K, and a new biotype (i.e., previously undescribed) were found on noncultivated hosts at Hays, but only biotypes I and K were found on the cultivated wheat. Finding a new biotype supports the hypothesis that biotypic diversity (new combinations of virulence genes) is generated and maintained on noncultivated grasses, which may then act as reservoirs of virulence genes found in populations on crops.

Distribution and abundance of insecticide resistant greenbugs (Homoptera: Aphididae) and validation of a bioassay to assess resistance.

Laboratory bioassays were conducted to determine the toxicity of four insecticides (ethyl parathion, chlorpyrifos, malathion, and carbofuran) to insecticide-susceptible and resistant populations of greenbug, Schizaphis graminum (Rondani). These bioassays were used to develop and validate a discriminating concentration for assessing insecticide resistance in greenbug populations in the field. Samples from wheat and sorghum in two states, Oklahoma and Kansas, indicated that insecticide resistance persists in greenbug populations over a large area at a low level.

Identifying insecticide-resistant greenbugs (Homoptera: Aphididae) with diagnostic assay tests.

Laboratory bioassays were used to develop a diagnostic assay test for identifying greenburg, Schizaphis graminum (Rondani), populations that are insecticide-resistant. Petri dish assays with chlorpyrifos showed greenbug mortality should be monitored after 2 h of exposure. One-hour exposure did not kill a high percentage of susceptible greenbugs, and a 3-h exposure killed too many resistant greenbugs. Ethanol and methanol were both good solvents for mixing with chlorpyrifos in the petri dish assay. From the laboratory bioassays, four diagnostic concentrations of chlorpyrifos (3, 10, 30, and 100 ppm) were evaluated in the field by Texas A&M University agricultural research and extension entomologists across the Texas High Plains. Results from the diagnostic assay tests were compared with gel-electrophoresis resistance tests to validate resistance detection. The diagnostic assay tests gave the same greenbug resistance identification as the gel-electrophoresis analysis in 21 of 22 field bioassays in 1994 and 35 of 39 field bioassays in 1995. Diagnostic concentrations of 30 and 100 ppm chlorpyrifos killed > or = 85 and > or = 90%, respectively, of greenbugs identified by gel-electrophoresis as susceptible and < 40% and < 55%, respectively, of resistant greenbugs. The diagnostic assay technique is a quick, reliable, and inexpensive method for detecting insecticide resistance in greenbug populations.

Efficacy of pyramiding greenbug (Homoptera: Aphididae) resistance genes in wheat.

Durable resistance to greenbug, Schizaphis graminum (Rondani), in wheat is a goal of wheat improvement teams, and one that has been complicated by the regular occurrence of damaging biotypes. Simulation modeling studies suggest that pyramiding resistance genes, i.e., combining more than one resistance gene in a single cultivar or hybrid, may provide more durable resistance than sequential releases of single genes. We examined this theory by pyramiding resistance genes in wheat and testing a series of greenbug biotypes. Resistance genes Gb2, Gb3, and Gb6, and pyramided genes Gb2/Gb3, Gb2/Gb6, and Gb3/Gb6 were tested for effectiveness against biotypes E, F, G, H, and I. By comparing reactions of plants with pyramided genes to those with single resistance genes, we found that pyramiding provided no additional protection over that conferred by the single resistance genes. Based on the results of this test, we concluded that the sequential release of single resistance genes, combined with careful monitoring of greenbug population biotypes, is the most effective gene deployment strategy for greenbug resistance in wheat.

Arthropod Populations in Early Soybean Production Systems in the Mid-South

We compared the severity of insect problems in early and conventional soybean, Glycine max (L.) Merrill, production systems in the mid-South. The conventional soybean production system (cultivars in maturity groups V–VII planted in May) experienced significantly higher populations of late-season defoliators than the early soybean production system (cultivars in maturity group IV planted in April). However, the early soybean production system harbored significantly larger populations of southern green stink bug, Nezara viridula (L.) and threecornered alfalfa hopper, Spissistilus festinus (Say). Predators were significantly more abundant in the early soybean production system compared with the conventional soybean production system, early in the growing season. Late in the growing season, predator populations were lower in both productions systems and differences between the two systems were not significant. The results from the current study illustrated the benefits of early-planted early-maturing cultivars (early soybean production system) in avoiding lepidopterous and coleopterous defoliators that occur late in the growing season. However, our data also indicate that arthropod management will be essential in the early soybean production system because widespread use of this system will result in an abundance of suitable hosts for early-season pests.

Greenbug Resistance Management on Sorghum with Insecticide Mixtures: a Biological and Economic Analysis

Several tactics were evaluated for insecticide resistant greenbug, Schizaphis graminum (Rondani), control on sorghum. During the first 2 yr of this research, control by chlorpyrifos, carbofuran, and malathion and mixtures of the latter 2 chemicals plus piperonyl butoxide with chlorpyrifos were compared on sorghum. Chlorpyrifos provided <60% control. The mixtures of chlorpyrifos + carbofuran or malathion provided good control of insecticide resistant greenbug. The mixture of chlorpyrifos + piperonyl butoxide provided mediocre control of resistant greenbugs. In 1996, an experiment was designed to evaluate the effects of greenbug population insecticide resistance ratio and preexposure of greenbugs to chlorpyrifos on control by mixtures of chlorpyrifos with carbofuran or malathion. About half of the plots were presprayed with chlorpyrifos (0.28 kg [AI]/ha) 4 d before applying individual insecticides or mixtures. Plots were infested with 2 ratios of resistant: susceptible greenbugs (1:3 and 3:1, respectively). Application of mixtures of chlorpyrifos + malathion provided best control followed by chlorpyrifos + carbofuran and chlorpyrifos. Prespraying greenbugs with chlorpyrifos did not affect their control with subsequent insecticide resistance management mixtures. The difference between the 2 ratios was significant with the lowest greenbug densities in plots dominated by susceptible individuals. There was a significant yield response to greenbug densities. The highest yields were obtained in plots sprayed with chlorpyrifos + malathion mixtures. When chlorpyrifos was applied to greenbug populations dominated by resistant aphids, yields were lower than in the infested control plots where greenbugs were not sprayed. Economic analysis of the data further demonstrated the advantage of the chlorpyrifos + malathion mixture for controlling insecticide resistant greenbug.

Parasitism of Greenbugs (Homoptera: Aphididae) by Lysiphlebus testaceipes (Hymenoptera: Braconidae) in Grain Sorghum: Implications for Augmentative Biological Control

Field cage studies were conducted to describe the relationship between the per­ centage of Lysiphlebus testaceipes (Cresson) parasitism (as measured by aphid mummies) and densities of greenbug, Schizaphis graminumRondani, on grain sorghum, Sorghum bicolor L. In 1993 and 1994, a biotype E-susceptible grain sorghum hybrid was grown in field cages and L. testaceipes adults were released after each plant was infested with 20 biotype E greenbugs. The release rates were 0, 0.5, 1.0, and 2.0 wasps per plant in 1993, and 0, 0.16, 0.33, and 0.5 wasps per plant in 1994. Greenbugs and mummies were counted 1-2 times a week on all leaves of 2-4 randomly selected plants per cage. A release rate of0.33- 0.5 wasps per plant infested with 20 greenbugs maximized the number of mummies produced and prevented the greenbugs from reaching an economic threshold of 1,000 greenbugs per plant. Peak numbers of mummies occurred =400 -500 DD (10°C base) after the initial wasp release. Regression analyses showed that the greenbug population started decreasing when the percentage of parasitism (as measured by mummies) reached 20-30 %. Greenbugs in the absence of wasps significantly reduced yield in 1994, but not in 1993.

Relative Fitness of Greenbug (Homoptera: Aphididae) Biotypes E and I on Sorghum, Wheat, Rye, and Barley

Greenbugs, Schizaphis graminum (Rondani), present a well-documented example of the ability of insects to overcome designed host plant resistance. Four green bug biotypes have been identified on wheat, Triticum aestivum L., and 3 on sorghum, Sorghum bicolor (L.) Moench, since green bugs were identified as pests of small grains. Biotype E has been the predominant biotype since the mid-1980s. Greenbug biotype I is the most recent greenbug biotype identified in field plantings of small grains and may be replacing biotype E. Replacement of biotype E green bug by biotype I will depend on the fitness of biotype I on biotype E-resistant cultivars. We attempted to predict this event by assessing relative fitness of the 2 biotypes, defined by life history characteristics on sorghum, wheat, rye, Secale cereale L., and barley, Hordeum vulgare L. We used several sorghum cultivars or lines with different reported levels of resistance to greenbug biotypes E and I, and 1 cultivar or line each of wheat, barley, and rye with reported resistance to biotype E. Biotype E development time, intrinsic rate of increase ( rm ), and to a lesser extent number of offspring produced were adversely affected by E-resistant and I-resistant sorghum cultivars or lines, whereas biotype I was only adversely affected by I-resistant cultivars or lines. Even on I-resistant cultivars or lines, biotype I outperformed biotype E. Wheat line CI 1795 and barley cultivar 'Post', previously identified as resistant to greenbug biotype E, also showed resistance to green bug biotype I. No differences occurred in the reproductive characteristics of greenbugs between E-resistant and E-susceptible rye cultivars. Given these differences in fitness on both E- and I-resistant sorghum and the importance of sorghum to greenbug population dynamics in the central and northern plains (compared to wheat, barley, and rye), it is likely that as increasing acreage is planted to these sorghum cultivars biotype I will replace biotype E.

Greenbug systemic effect on barley phosphate influx

Greenbugs ( Schizaphis graminum Rond.) cause considerable yield loss in cereals. Local feeding damage of greenbug-infested leaves includes collapsed mesophyll cells, chlorosis, alterations in photosynthesis and respiration. However, this damage cannot explain rapid changes taking place in plant metabolism (inhibition of new leaf primordia and new root differentiation, within a few hours after attack), or the early death of such plants. This study was aimed at determining whether greenbug feeding induces systemic damage to barley. The phosphate influx by roots of susceptible and tolerant barley ( Hordeum vulgare L.) plants was evaluated as an estimate of aphid systemic damage. Phosphate (P)-influx was determined at two plant growth stages, with two levels of greenbug infestation, at two different greenbug life stages. Plants grown in hydroponics in a glasshouse were infested for 0 (control), 3, 6, 12, 24, 48, and 72 h with the Argentinean biotype C greenbug. The P-influx was not significantly affected in tolerant barley plants by greenbug infestation. In contrast P-influx was significantly reduced 6 h post-infestation in the susceptible cultivar. Plants with one expanded leaf suffered a significantly greater reduction in P-influx than plants with two expanded leaves. By 48 h after infestation, the P-influx of the two-expanded-leaf treatment was similar to that of the controls, whereas P-influx in plants with one expanded leaf remained significantly less than on the controls 72 h after infestation. A larger greenbug population resulted in greater reduction in P-influx. Adult greenbugs, but not third stage nymphs, affected P-influx. In summary, the intensity of greenbug-induced systemic damage was greater when young plant stages were infested by the aphid. Reductions of P-influx may become critical under increasing natural infestation levels.

Greenbug (Homoptera: Aphididae) Biotypes: Selected by Resistant Cultivars or Preadapted Opportunists?

Future advances in wheat, Triticum aestivum L., and sorghum, Sorghum bicolor (L.) Moench, resistance to greenbug, Schizaphis graminum (Rondani), will likely come from introduction of resistance transgenes into high-performance cultivars. First-generation resistance transgenes will be single genes that impart antibiosis traits (similar to Bacillus thuringiensis endotoxins in transgenic corn, Zea mays L.). This approach to pest management is incompatible with interpretations of simulation models that predict that deployment of antibiosis resistance controlled by single genes drives the development of new, virulent pest biotypes. This dichotomy must be addressed if full advantage is to be taken of the new, powerful tools of molecular biology for plant protection against insects. In this study, the specific insect-plant interactions of green bugs on wheat and sorghum were examined to understand the relationship between the deployment of plant resistance and the development of new green bug biotypes. From this analysis, there was no relationship between the use of resistant wheat and the development of new green bug biotypes. For sorghum, with only 3 of the 11 biotypes could there be any correlation between the use of resistant hybrids and the development of new biotypes. Even with these 3 biotypes, no clear cause-and-effect relationship was established. Based on analysis of these specific insect-plant interactions, we propose that future plant resistance efforts focus on the use of the most effective resistance genes, despite past predictions of what effect these genes may have on greenbug population genetics.

Occurrence of Insecticide Resistant Greenbugs (Homoptera: Aphididae) in Kansas, Texas, Oklahoma, and Colorado and Suggestions for Management

Polyacrylamide gel electrophoresis was used to resolve esterase isozymes that were related to insecticide resistance in greenbug, Schizaphis graminum (Rondani). This report documents the occurrence of esterase polymorphisms in the Midwest and assesses differences in regional occurrences. In total, 13,863 individual greenbugs from Kansas, Texas, Oklahoma, and Colorado were tested between 1991 and 1995. The occurrence of resistant greenbugs appeared to be greatest in the southwest crop reporting district of Kansas and the north plains crop reporting district of Texas. The overall incidence of pattern 1 resistant greenbugs declined from 7.1% in 1991 to <1% in 1994 and 1995. Pattern 2 resistant greenbug occurrence varied from year to year (5.7-29.8%). Several explanations for the spatial occurrence of resistant greenbugs and several techniques for managing resistant greenbug populations are offered.

Generation of clonal diversity by sexual reproduction in the greenbug, Schizaphis graminum.

In the United States, the greenbug, Schizaphis graminum (Rondani), reproduces primarily by apomictic parthenogenesis. Although a periodic sexual cycle exists, the extent to which it occurs naturally and its influence on the genetic variability of greenbug populations is unclear. Length variation in the intergenic spacer (IGS) of the rRNA cistron in the greenbug indicates that populations are made up of many genetically distinct clones. Previous laboratory studies have shown the stability of the IGS within parthenogenetic clones. By inducing the sexual reproductive cycle of the greenbug, we conducted both Intra- and inter-clone matings and studied the inheritance of the IGS in the offspring. In both mating schemes, rearrangements in the IGS were apparent. IGS diversity found among the offspring could be attributed to unequal cross-over and probably other molecular drive events during meiosis. Periodic sexual reproduction is a primary mechanism for the generation and maintenance of genetic variability in greenbug populations, and explains the level of clonal diversity found in previous studies.

Description of Three Isozynte Polyntorphisms Associated with Insecticide Resistance in Green bug (Homoptera: Aphididae) Populations

Esterases have been associated with insecticide resistance in several aphid species. This report documents existence of 3 est erase polymorphisms associated with insecticide resistance in the green bug, Schizaphis graminum (Rondani). Est erases were resolved using polyacrylamide gel electrophoresis and Rf values were calculated for the 6 est erase bands that resolved. The 3 est erase polymorphisms observed have been named no noticeable elevated est erase, pattern 1 and pattern 2. Est erase bands visible on the polyacrylamide gel appear to have no elevated activity in the no noticeable elevated est erase strain. A single est erase band appears to be elevated in pattern 1 individuals and 3 est erase bands appear to be elevated in pattern 2. After testing >;10,000 green bugs, no individual green bug has been detected with both pattern 1 and pattern 2 banding. It is unknown whether the 2 elevated est erase forms are allelozymes or isozymes. Surface-residue bioassay data are presented that support the hypothesis that these elevated est erases may be important in insecticide resistance.

Seed Treatment Evaluated for Greenbug Control in Sorghum, 1994

Abstract Seed of three sorghum hybrids treated with 0, 2 or 4 oz AI of Gaucho per cwt were furnished by Gustafson, Inc. The hybrids used in the study were NC+ 271 (greenbug susceptible), DK 56 (biotype- E resistant), and Cargill 607E, (biotype-I resistant). All three hybrids were pretreated with Captan and Concept and the DK 56 was also pretreated with Apron. The seed was planted on 24 May in a modified Latin square design with three replications. Each plot was two rows wide (5 ft) and 22 ft long. The seed was planted at the rate of 5 g per row using a cone planter. On 23 Jul corn leaf aphid and greenbug populations were monitored by cutting off two plants per plot and counting the number of aphids on the plant and removing the whorl and counting the number of aphids observed as the leaves were unrolled. On 4-5 Aug, two plants were cut off at the base and visually examined to estimate greenbug numbers. Yield data were collected by machine harvesting the plots on 27 Oct.

YIELD OF SPRING WHEAT IN RELATION TO LEVEL OF INFESTATION BY GREENBUGS (HOMOPTERA: APHIDIDAE)

AbstractThe effect of greenbug, Schizaphis graminum (Rondani), infestations on yield components of early- and late-planted spring wheat, Triticum aestivum L., were measured in 2 successive years in eastern South Dakota. Greenbug populations in plots generally increased rapidly and peaked when wheat plants were in boot stage. Populations then began a steady decrease until anthesis, at which time they were near zero. The number of seeds per spikelet and the number of spikelets per square metre were consistently significantly negatively correlated with aphid-days, whereas average seed weight was significantly negatively correlated with aphid-days for only one planting. A linear regression model that included different intercepts for each spring wheat planting and a common slope relating yield to greenbug feeding days (aphid-days) provided a good fit to the data. Based on the model, a loss of 41 kg of grain per hectare is expected for each 100 aphid-days that accumulate per tiller.

Variation in Fecundity of Greenbug (Homoptera: Aphididae) Biotypes on Resistant and Susceptible Barley

Variation in fecundity of apterous adults of three greenbug, Schizaphis graminum (Rondani), biotypes on seedlings of resistant and susceptible barley was studied in an environmental chamber programmed for 25:20°C and 16:8 (L:D) h. The objective of the test was to obtain baseline data on variability of biotypes B, C, and E on resistant and susceptible cultivars. Fecundity distributions for three biotypes (B, C, and E) on susceptible ‘Wintermalt’ were normal, but non-normality was evident for the three biotypes on resistant ‘Post’. For the non-normal distributions, significant skewness values were found in the direction of higher fecundity. Greenbug reproduction on some plants of ‘Post’ barley was greater than expected, but whether this was caused by variation in greenbug populations or variation in the host plants is unclear.

Biotypes and Clonal Variation in Greenbug (Homoptera: Aphididae) Populations from a Locality in California

Considerable genetic variation in virulence, fecundity, and adult weight was found between and within clones of greenbug, Schizaphis graminum (Rondani), populations near Davis, Calif. Eight clones were established from individual apterous aphids collected from four fields within 50 km of each other. Aphids from these clones differed in their ability to damage wheat variety ‘TAM 107’ and in their weight and fecundity on several sorghum varieties. Based on these differences, the clones could be separated into four distinct groups. Lineages of four clones, maintained for 14–16 successive generations by selecting either large or small aphids, differed in their fecundities. These differences could not be explained by maternal effects. If biotypes of the greenbug are determined by the damage they cause to particular plant varieties, as recently proposed, then clones that differ in fitness traits such as fecundity and weight cannot be considered different biotypes. However, to develop strategies for reducing the rate of aphid adaptation to resistant varieties, we must understand such variation in fitness on a local scale, information that is not obtained in general biotype surveys.

Induced Disulfoton Resistance in Greenbugs (Homoptera: Aphididea)

Field studies were done to determine whether continuous exposure to the systemic insecticides al bicarb, carbofuran, disulfoton, and terbufos would induce or select for insecticide-resistant green bugs, Schizaphis graminum (Rondani). The extent of resistance was determined with a surface residue bios say for each of the insecticides. In field experiments, green bug populations exhibited 21- to 104-fold increases in resistance to disulfoton. Green bugs exposed to the other insecticides did not exhibit significant increases in resistance. Greenhouse studies were done to determine if disulfoton resistance resulted from a change within a genotype (green bug clone) or was simply the result of selection of resistant genotypes from a genetically diverse population. Green bugs originating from a single aphid susceptible to insecticides developed 65- to 300-fold increases in resistance. Similar results were obtained when the study was repeated with progeny from a different green bug. These results indicate that resistance developed within green bug clones after exposure to disulfoton for 3 wk or longer. When resistant green bugs were tested for cross-resistanceto carbofuran, dimethoate, malathion, and terbufos, a significant increase in cross-resistance to dimethoate (nine-fold) and terbufos (two-fold) was observed, but cross-resistance to carbofuran and malathion was not significant.

Rearing History Affects Demography of Greenbugs (Homoptera: Aphididae) on Corn and Grain Sorghum

Biotype E greenbugs, Schizaphis graminum (Rondani), were reared on corn, Zea mays L., to assess their ability to develop, reproduce, and survive on this host. Four corn germ plasm accessions (Antigua, OH-45, AP-670, and 2570) were studied along with a susceptible grain sorghum, Sorghum bicolor (L.) Moench, (NC + 630x) and a resistant sorghum (PI 264453) as controls. Corn-reared and sorghum-reared greenbugs were tested to evaluate the effect of host history on performance. Daily mortality and fecundity schedules were measured and net reproductive rate, intrinsic rate of population increase, finite rate of increase, and population doubling time were calculated. Plant entry had a significant effect on development time for the first two instars but not the second two, which may be because of physiological adaptation to the novel hosts. All life-history measures except generation time indicated greater potential for greenbug population growth on sorghum than on corn. However, greenbugs still had sufficiently high population growth on corn to indicate that corn was a good host. Better performance on both sorghums and Antigua corn compared with the other corn entries suggested the possible involvement of the MBOA–DIMBOA complex in resistance to greenbugs. Longevity was greater and population doubling time was shorter for corn-reared than for sorghum-reared greenbugs. Host history also interacted with plant entry such that corn-reared greenbugs were more fecund on corn than sorghum-reared ones, whereas the opposite was true on sorghum. Assessment of these data indicates that greenbugs have a potential to adapt to corn as a true host and develop a biotype injurious to corn.

Experimental Evaluation of Predators and Parasitoids in Suppressing Greenbugs (Homoptera: Aphididae) in Sorghum and Wheat

The effectiveness of endemic predators and parasitoids as biological control agents of the greenbug, Schizaphis graminum (Rondani), was experimentally evaluated during 1985 and 1986 in Kansas winter wheat, Triticum aestivum , and grain sorghum, Sorghum bicolor. Mechanical exclusion was used as the experimental method of evaluation. The coccinellids Coleomegilla maculata lengi Timberlake, Hippodamia convergens Guerin, and Scymnus spp. were the predominant predators of greenbugs in sorghum and comprised 89% of the predator population. Lysiphlebus testaceipes (Cresson) was the most abundant primary parasitoid in sorghum. The natural enemy complex was able to suppress experimental greenbug densities at all stages of sorghum growth from 3-leaf through the boot stage. The impact of L., testaceipes on greenbugs was sporadic between locations. In winter wheat, H. convergens was the most abundant natural enemy and significantly suppressed greenbug densities. Natural enemies, primarily coccinellids, consistently reduced experimental greenbug populations in winter wheat and grain sorghum.