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a Dep. of Crop Sci
b USDA-ARS and Dep. of Crop Sci., Univ. of Illinois, 1101 West Peabody Dr., Urbana, IL 61801
* Corresponding author (ghartman{at}uiuc.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONREFERENCES |
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Abbreviations: IPM, integrated pest management • MG, maturity group • RH, relative humidity • VIPS, Variety Information Program for Soybeans
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONREFERENCES |
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A native of Asia, the soybean aphid was first found in the Midwest in 2000 (Hartman et al., 2001). It rapidly spread throughout the region and into other parts of North America (Patterson and Ragsdale, 2002). High aphid populations can reduce crop production directly when their feeding causes severe damage such as stunting, leaf distortion, and reduced pod set (Sun et al., 1990). Yield losses attributed to the aphid in some fields in Minnesota during 2001, where several thousand aphids occurred on individual soybean plants, were >50% (Ostlie, 2002) with an average loss of 101 to 202 kg ha–1 in those fields (Patterson and Ragsdale, 2002). In earlier reports from China, soybean yields were reduced up to 52% when there was an average of about 220 aphids per plant (Wang et al., 1994) and plant height was decreased by about 210 mm after severe aphid infestation (Wang et al., 1996). An additional threat posed by the aphid is its ability to transmit certain plant viruses to soybean such as Alfalfa mosaic virus, Soybean dwarf virus, and Soybean mosaic virus (Sama et al., 1974; Iwaki et al., 1980; Hartman et al., 2001; Hill et al., 2001; Clark and Perry, 2002).
Aphis glycines and close relative A. gossypii, the cotton or melon aphid, are the only aphid species found colonizing soybean in the USA. In other parts of the world, A. craccivora, Aulacorthum solani, and other species have been found colonizing soybeans (D. Voegtlin, personal communication, 2003).
Aphis glycines has a heteroecious, holocyclic life-cycle pattern (Guang-xue and Tie-sen, 1982). Rhamnus cathartica (buckthorn) is the primary host of A. glycines (Hartman et al., 2001) and soybean is a secondary host. In autumn, when the soybean crop matures, the aphid moves to R. cathartica, where mating and oviposition occurs. The egg stage overwinters on R. cathartica. During the following spring, the eggs hatch and a few generations are produced before alatae (winged females) fly to soybean.
Because A. glycines is a recent pest in the USA, a comprehensive integrated management approach to control the aphid has yet to be developed. Research to evaluate the efficacy of currently available insecticides and other control measures has just begun. Researchers in Minnesota have developed recommendations for the use of insecticides to control the aphid (Ostlie, 2002). Until other components of integrated pest management (IPM) are developed, soybean producers will need to rely on the use of insecticides to control the aphid.
An integral component of an IPM program to control aphids is plant resistance (Auclair, 1989; Harrewijn and Minks, 1989). Insect resistance can significantly reduce input costs for producers (Luginbill, 1969). Resistance was reported in G. soja (Sun et al., 1990), a close relative of G. max (Hymowitz, 1970), and other wild relatives (Zhuang et al., 1996). There are no reports of resistance in G. max. A report from Indonesia indicated that there was no resistance in a test of 201 soybean cultivars and breeding lines (Sama et al., 1974).
There are three basic kinds of resistance: tolerance, antixenosis, and antibiosis (Smith, 1989). Knowledge of the mechanism of resistance is necessary to develop effective screens to identify resistant plants. Choice tests, where aphids are allowed to choose their preferred hosts, help identify resistance, but do not distinguish between the types of resistance. Nonchoice tests, where aphid movement is restricted to a single host, help distinguish antibiosis, the effect of resistance on the biology of the aphid, from antixenosis, nonhost preference. Field tests to measure yield in the presence of aphid infestation help identify tolerance, the ability to produce similar yield in the presence or absence of aphids.
Resistance can be eroded by the rise of biotypes that can overcome resistance (Auclair, 1989; Smith, 1989). There is no information on the existence of biotypes in A. glycines.
The objectives in this study were to identify resistance to the soybean aphid in soybean, determine type of action of resistance (antibiosis, antixenosis, tolerance), examine variation among aphid clones toward plant resistance, and compare the agronomic performance of resistant and susceptible cultivars under severe aphid infestation and when protected by a systemic insecticide. The studies were conducted in the greenhouse and in the field.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONREFERENCES |
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Aphid Culture
Three clones of soybean aphids were established from single first-instar nymphs isolated from different aphid collections. One clone was from Urbana, IL, one from Brown County, Illinois, and the third clone came from Oxford County, Iowa. David Voegtlin (Illinois Natural History Survey, Urbana, IL 61801) confirmed the identification of the aphids as A. glycines. Aphids were reared on a continuous supply of seedlings of the soybean cultivar Williams 82 grown inside plant growth chambers at 22°C, the optimum temperature for population development (Hirano et al., 1996) and under continuous 300 µmol m–2 s–1 PAR irradiation, and 70% relative humidity (RH). The Urbana clone was used in all resistance-screening experiments described below.
Plant Culture
Plants in greenhouse experiments were grown in soilless potting medium (Sunshine Mix, LC1, Sun Gro Horticulture Inc., Bellevue, WA),1 in plastic multi-pot inserts (Hummert Intl., Earth City, MO) (pot sizes ranged from 30 x 40 x 60 mm to 60 x 60 x 60 mm), contained inside plastic trays without holes (Hummert Intl., #F1020) (Exp. 1–13, 16), or in 125- x 87.5-mm plastic azalea pots (Hummert Intl.) (Exp. 14, 15). Size and number of multi-pots per insert used depended on the number of test entries and the experimental design. Before planting, multi-pots were filled to capacity with the potting medium and then the medium was premoistened to field capacity. Two seeds of each test entry were placed in a shallow depression (
10 mm deep) made by lightly pressing a finger into the potting medium, and were covered with 5-mm layer of course-textured vermiculite (Strong-Lite, Sun Gro Horticulture Inc.). Slow release nutrient pellets (Nutricote 18-6-6, Hummert Intl.) were evenly spread over the surface of the vermiculite to a density of 3 to 5 pellets per pot. Seedlings were thinned to one plant per pot after emergence.
Choice Tests
Choice tests, where aphids were allowed to move to preferred host plants, were conducted in an air-conditioned, insecticide-free greenhouse, dedicated to aphid testing, and maintained at 22 to 25°C with supplemental continuous illumination provided by a mixture of 1000-W high-intesity discharge and high-pressure sodium lamps.
In most of the choice tests (Exp. 1–6, 9, 10, 13, 16), leaves from Williams 82 plants infested with the Urbana clone were placed on top of VE–stage seedlings (Fehr and Caviness, 1977) arranged in randomized complete blocks with three or four replications and with a row of two to four plants per experimental unit. There were 50 to 200 aphids of all stages on each infested leaf used to transfer aphids to the test plants. The aphids moved from the infested leaves to the test seedlings within the first day after transfer, and after about 7 to 14 d, they distributed themselves on preferred hosts and developed colonies. To avoid disturbing the aphids, plants were bottom watered by flooding the trays containing the plants as needed.
Resistance was evaluated at periodic intervals after infestation with estimates of aphid colonization and plant damage or by actual counts of aphid populations on each experimental unit that consisted of a row of two to four plants. An aphid index was calculated by multiplying the estimate of aphid population size, 0 to 3, where 0 = no aphids observed to 3 = high aphid density (>100 aphids per plant), by the rating of plant damage, 0 to 3, where 0 = no perceptible damage to 3 = severe leaf distortion and stunting, or plant death, giving a range of possible index values from 0 to 9 (Hill et al., 2002).
Indirect aphid infestation was done during the V1–stage (Fehr and Caviness, 1977) in Exp. 7, 8, and 11 and during the Vc–stage (Fehr and Caviness, 1977) plants in Exp. 12 by exposing test plants to viparous alatae from neighboring infested plants in the greenhouse test plants. Aphid populations were counted 5 (Exp. 8), 7 (Exp. 7), and 9 d (Exp. 11, 12) after infestation.
Nonchoice Tests
Characterization of the type of resistance in resistant germplasm was studied in nonchoice tests conducted in a plant growth chamber with environmental conditions as described above. Two methods to apply and confine aphids to test plants were used. Experiments were arranged in completely randomized design with four replications and one plant per replication.
In the first method (Exp. 14), a single viviparous alate was placed on the abaxial side of the lamina of the center leaflet of a new, fully expanded trifoliolate leaf of individual V1– to V2–stage plants (Fehr and Caviness, 1977) with the aid of a moist camel's hair paint brush. Aphids were isolated on the leaves by attaching leaf cages over the aphids to restrict their movement. The cages were made with 1-mm-thick plastic tubing with a 10-mm i.d., cut 12 mm long, and covered with plastic mesh with 100-µm openings (Sterling Net Co.) glued on one end. On the opposite end of the cage tubing, a 4-mm-wide x 4-mm-thick foam ring with a 8-mm i.d. and 12-mm o.d. was centered and glued on to provide a seal between the cage and leaf surface when attached to the leaf. Cages were placed over the aphids with the foam end down on the leaf surface and were fastened to the leaf with a metal clip held closed by spring tension. Alates were placed on four individual plants of each test entry. New aphid offspring from each alate placed on each plant were counted and removed daily. After 12 d, the cumulative number of aphid offspring on each plant was determined.
For the second method (Exp. 15), two first instar nymphs were placed together on the abaxial side of a unifoliate leaf of four different V1–stage test plants of each test entry with a moist camel's hair paint brush. Individual plants were isolated in 100- x 300-mm clear plastic cylindrical cages, with 4-mm-thick walls, and with 80- x 180-mm side windows plus the top covered with a plastic mesh with 100-µm openings (Sterling Net Co.). The open bottom of a cage was pressed into the soilless medium about 10 mm deep to prevent aphid escape. Total aphid populations on each plant were counted at 3- to 4-d intervals, up until 13 d after placing the nymphs on the test plants.
Test of Variability of Aphid Clones
Resistant germplasm was challenged with the three aphid clones that were established, as described above, in a choice test to evaluate the effect of resistance on different soybean aphid clones (Table 1, Exp. 16). Single plants of five resistant and three susceptible cultivars were arranged in a split-plot design, with clones organized as main plots and cultivars as subplots, with three replications. The clones were kept isolated by placing plants in 32-pot inserts inside 360- x 510- x 380-mm cages made with 4-mm-thick clear Plexiglas. A 250- x 380-mm window was cut in the top of each cage for aeration and was covered with a plastic mesh with 100-µm openings (Sterling Net Co.). Aphids were transferred onto VE–stage seedlings and aphid population and damage ratings were recorded weekly for 3 wk.
Field Cage Experiment
A field experiment was conducted at Urbana, IL, in 2002 to measure the agronomic performance of resistant and susceptible cultivars under severe aphid infestation and when protected by a systemic insecticide (Exp. 17). Plots of eight cultivars, three resistant and five susceptible, were arranged in a split block design with four replicates arranged in a RCB. Main plots were cultivars and subplots received either no insecticide treatment or a granular formulation of the systemic insecticide imidacloprid (1% G, Marathon, Olympic Horticultural Products, Mainland, PA) by applying a top-dress band at a rate of 4.2 mL m–1 of row when plants were transplanted into the cage. Plants of each cultivar were grown in a growth chamber maintained at 28°C, 70% RH, and 300 µmol m–2 s–1 irradiation controlled by a timer programmed to give a 12-h photoperiod for 4 wk. The short photoperiod was required to synchronize the floral development of the plants of cultivars from several MGs. Six R1–stage (first bloom) (Fehr and Caviness, 1977) plants were transplanted on 5 June 2002 into 0.3-m single-row main plots with 5-cm spacing between each plant. There were eight main plots in a range and eight ranges, with two ranges per replication. One of the ranges in each replication comprised the subplot that was treated with insecticide and the other range in a replication was the subplot that was not treated with the insecticide. Plots within a range were separated by 0.6 m and ranges were separated by 0.3 m. The experiment was contained inside a 6- x 6- x 2.1-m field cage with a 52 x 52 threads per linear inch mesh polypropylene covering supported by a galvanized steel frame (Redwood Empire Awning and Furniture Co., Santa Rosa, CA). The distance from the cage covering and the plots was from 0.6 to 0.9 m. Controlled-release nutrient pellets (Nutricote 14-14-14 N-P-K, Hummert Intl.) were top-dressed around the plants at a rate of 5 g per plant. The soil type at the Urbana, IL, location was a Parr silt loam (fine-loamy, mixed, active, mesic Oxyaquic Argiudolls). Plants were irrigated with an overhead sprinkler as needed. Immediately after transplanting, leaves infested with aphids from the Urbana clone were evenly distributed among the plots. One month later, aphid indices were recorded for each plot. At this point, plants reached the R2 to R3 stage (Fehr and Caviness, 1977) depending on the entry, MG, and the length of time needed to recover from transplanting. Reproductive development of Dowling (MG VIII) and Jackson (MG VII) was slower than the entries adapted to the location (MGs II–IV). At the R6 stage (Fehr and Caviness, 1977), early to mid-September for adapted entries, the average plant height for each plot was measured. Height of Dowling and Jackson plants was measured very late in the season, 8 November, just before the first killing frost event, because they had not reached the R6 stage. At the R8 stage (Fehr and Caviness, 1977) for the adapted entries, late September to late October, physiological maturity date for each plot was recorded and plots were hand harvested by cutting each plant at the soil line. Plants of Dowling and Jackson did not mature before they were killed by frost. All surviving plants of each plot were bulked and dried. After drying, the total dry plant mass, seed mass, number of pods, number of seeds, and 100-seed mass were measured. Dry mass of Dowling and Jackson plants included leaves because they were gathered after being killed by frost and before they matured. For each parameter, means per plant were calculated for each plot. An analysis of variance was calculated for each parameter. Comparisons (single degree of freedom contrasts) of the means in the imidacloprid treatment vs. no insecticide treatment for each parameter were calculated for each cultivar.
Statistical Analyses
All statistical data analyses were performed with the aid of JMP Version 5 (SAS Institute Inc., Cary, NC). Aphid population counts were first transformed to log10 +1 before performing analysis of variance and least squared means were detransformed. Mean separation was done by calculating the LSD at P = 0.05 when treatment means were significantly different (P < 0.05) in the ANOVA.
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RESULTS |
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There were no significant (P = 0.37) differences among the entries in the test of 79 cultivars adapted to the southern USA (Table 1, Exp. 5). All but three of the cultivars tested had aphid indices >5. The cultivar Stonewall had an index of 4.0, and Asgrow AG6202 and SS RT7499N both had indices of 4.5. The susceptible checks had indices >6. Stonewall had an aphid index of 8.5 when retested in Exp. 10.
Significant (P < 0.001) differences in aphid indices were found in the test of 106 Asian soybean cultivars 21 d after aphid infestation (Table 1, Exp. 6). However, all had indices >5, indicating a lack of resistance to the soybean aphid in this collection of cultivars. The susceptible checks had indices >8.
In tests of aphid population development on the Clark isolines with various types and densities of pubescence (Table 1; Exp. 7, 8), significant differences were found in both experiments (Table 2; P < 0.001 in the first test and P = 0.009 in the second test). Population development on all isolines was significantly higher or not significantly different from the susceptible check Williams 82 in both choice tests of those lines (P = 0.05). Two isolines with Pd genes conditioning dense pubescence had the highest populations in the first experiment, and all three dense pubescent isolines with Pd genes had the highest populations in the second experiment. A glabrous isoline, L62-1385, had significantly lower populations than the dense pubescent isolines L62-1686 and L76-1815 in the first experiment and significantly lower than L62-1686 and L77-1040 in the second experiment (P = 0.05).
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Aphid Control with Resistance Compared with Use of a Systemic Insecticide
There were significant (P = 0.006) differences (Table 7) in aphid indices among the eight genotypes in the treatment without imidacloprid in the field cage experiment (Table 1, Exp. 17). Indices for Dowling, Jackson, and PI 71506 were significantly lower than Pioneer 93B01, Ina, Loda, and Pana; however, they were not significantly lower than Williams 82. Very few if any aphids were observed on Dowling plants throughout the season, and no plant damage occurred. There were up to about 20 aphids observed on Jackson plants and up to about 50 aphids on PI 71506 plants observed at any given time during the season. Discernable foliar damage did not occur on Jackson plants, while mild leaf yellowing and minor leaf distortion did occur on PI 71506, presumably caused by aphid feeding as it was not observed on plants in the imidacloprid treatment. Dense aphid populations built up on all five of the susceptible cultivars until they progressively declined as a result of decreased availability of susceptible tissue because of severe damage caused by aphid feeding. Peak populations reached several thousand aphids on susceptible plants. Moderate leaf yellowing and leaf distortion occurred on Williams 82, while severe foliar damage occurred on the other susceptible cultivars, including strong leaf yellowing, gross leaf distortion, and severe desiccation of plant tissues. All plants of Loda, Pana, and Pioneer 93B01 in the treatment without imidacloprid died before maturation (R7–stage).
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As expected, development of Dowling (MG VIII) and Jackson (MG VII) plants was retarded compared with the entries adapted to the location (MGs II–IV) and they did not fully mature before the end of the experiment; however, differences between the insecticide and noninsecticide treatments within a cultivar for some agronomic parameters were evident. There was a significant (P = 0.01) treatment by cultivar interaction for 100-seed mass. In particular, single-degree-of-freedom contrasts or comparisons made between the treatments with and without imidacloprid indicated that Dowling resistance gave similar or equal protection against the soybean aphid as did imidacloprid (Table 7). No parameter means for Dowling in the treatment without imidacloprid were significantly different from the imidacloprid treatment means and the means were nearly identical for each parameter. Significant differences were found between the treatments in most parameters for Jackson and PI 71506, indicating that resistance in those cultivars did not protect the plants against the aphid as well as the resistance in Dowling did. In fact, Jackson and PI 71506 plants appeared to be stunted in the insecticide-free treatment, as indicated by significant differences in height between the imidacloprid and insecticide-free treatments. However, the agronomic performance of the susceptible cultivars, in particular Pioneer 93B01, Loda, Ina, and Pana, was much more severely affected by aphid feeding. Plants of those cultivars were not only severely stunted in the treatment without imidacloprid, but aphid feeding also killed many plants. Productivity of Ina and Loda was reduced to nearly zero by aphid feeding. Although aphid populations were also high on Williams 82, the effects of aphid feeding on agronomic performance were not as strong as on the other susceptible cultivars.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONREFERENCES |
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All nine resistant genotypes belonged to MGs IV through VIII. It seemed possible that resistance could be found in current cultivars belonging to those MGs; however, no resistance was found in commercial cultivars belonging to those MGs that were tested in this study. This suggested that resistance present in ancestral germplasm did not persist through the development of current commercial soybean cultivars in the central and southern USA, probably because there was no selection pressure to aid breeders in identifying, selecting, and maintaining resistance without the presence of the soybean aphid in North America. The lack of resistance in the 106 Asian cultivars tested (Table 1, Exp. 6) was unexpected because they were developed where selection pressure was assumed to have existed, although there are no known reports of the existence of resistance in G. max in the Asian literature.
Nonchoice tests (Table 6) demonstrated that the resistance in Dowling, Jackson, and Palmetto had a strong antibiotic effect on the soybean aphid, as indicated by the lack of population development on those cultivars, apparently due to a negative impact on fecundity, and high mortality. In contrast, resistance in PI 71506 and CNS appeared to be primarily antixenosis or nonpreference type because high aphid populations were clearly discouraged on those cultivars in choice tests, while in the nonchoice tests, population development was not significantly lower than the development on susceptible cultivars. Although alates of uniform age were not used in Exp. 14 (Table 1), the magnitude of differences between resistant accessions and susceptible genotypes (Table 6) was great enough to limit the importance of variability in population development due to potential bias of the age of adult used to initiate colonies. The resistance in all five resistant genotypes appeared to be expressed during all plant stages, suggesting that they would likely provide season-long protection.
Resistance in Jackson was probably derived from its parent Palmetto (USDA-ARS National Genetic Resources Program, 2003). The effect of both cultivars on aphid population development, survival, and fecundity were similar (Table 5). The origin of the resistance in Dowling was less clear because of the differences in the type of resistance expression in Dowling and its grandparent CNS (Table 5), as noted above. The genetic relationship between Dowling resistance and the resistance in Jackson is unknown; however, it seems likely that the resistance in PI 71506 is not genetically related to either the Jackson or Dowling resistance because of the differences in type of resistance expression. The inheritance and genetic relationships of these resistance sources is currently under study.
Dowling resistance, in particular, was demonstrated to provide protection to the soybean aphid equal to imidacloprid (Table 7). Imidacloprid was an effective aphicide in the field experiment, but it is not labeled for use in production of soybean. Other chemical substances that are labeled for use on soybean may be as effective in controlling the soybean aphid (Kim et al., 1987; Ostlie, 2002); however, genetic resistance is likely to be most economical (Luginbill, 1969), particularly if combining its use in an integrated control program with chemical and other control methods (Wang et al., 1998) maximizes its durability.
In contrast to reported effects of soybean pubescence on insect pests, such as reduced damage due to feeding by plant hoppers, Empoasca fabae (Harris) (Elden and Lambert, 1992), reduced feeding of the Mexican bean beetle, Epilachna varivestis Mulsant (Gannon and Bach, 1996), reduced leaf damage caused by the false melon beetle, Atrachya menetriesi Falldermann (Kanno, 1996), increased resistance to defoliation by lepidopterans (Lambert et al., 1992), and effects on the probing behavior of other aphid virus vectors (Gunasinghe et al., 1988), there was no protective benefit of dense pubescence against the soybean aphid (Table 2). In fact, the ability of the soybean aphid to colonize soybean irrespective of pubescence may have given it a selective advantage during its co-evolution with G. max. Soybean aphids feeding underneath trichomes may be protected from predation and parasitism.
Although a small sample of clones was tested (Table 1, Exp. 16), the lack of differences among the three clones indicated that the aphid population in the geographic region surveyed consisted of a single biotype or possibly the same clone. Nonsignificant clone by cultivar interaction indicated that variability in virulence toward the resistant cultivars, or host specialization, did not exist in the region. That suggested that the resistance would be effective throughout the region, initially at least, until genetic variability for virulence was introduced by mutation, migration, or another mechanism.
The screening methods used in this study proved to be efficient in screening large numbers of genotypes for resistance. Resistant phenotype expression, low aphid population densities with minimal plant damage, was clearly distinguishable from susceptible responses. Resistance identified in the greenhouse tests held true in the field test.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONREFERENCES |
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Received for publication March 10, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSDISCUSSIONREFERENCES |
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