Rating Soybean Germplasm for Whitefly (Homoptera: Aleyrodidae) Infestations'

R. M. McPherson

University ofGcorgia, Coastal Plain Experiment Station Department of Entomology 'J'ifton, Georgia 31793 USA

J. Agric. Entomol. 13(t): 65-71 (Jll.nunry 1996)

ABSTRACT Thirty-six soybean varieties and breeding lines in Maturity Groups VII and VIII were rated for infestations of whiteflies, primarily the silverlcaf whitefly, Benll:sia argenti(olii Bellows and Perring, but also for low populations of bandedwinged whitefly, Triaieurodes al>Utilonea (Haldeman), All entries were visually examined for the presence of whiteflies twice during the season and given an infestation rating score of 0-10. Each numerical score was derived from a t.ransformed scale of estimated percent. leaf arca infested, with each succeeding number increasing by an increment of 10% of the leaf area covered with whiten)' immat.ures and sooty mold. Mean infestation scores ranged from 1.0 to 7.3 in t.he Group VII entries and from 2.6 to 7.5 in the Group VIn entries. Leaves from each entry also were observed under a dissecting microscope to compare the immature whiteny densities to the infestation rating scores. The damage rating score was highly correlated to the whiteny immature density. The visual rating system provided an effective and feasible method of comparing whiteny infestation levels between numerous soybean entries. Over 90% of the adult whitenies captured on yellow sticky traps placed in each entry was B. argerlti{olii.

KEY WORDS Soybean, whiteflies, Bemisia argcTlti{olii, 13. tabaci, l'rialeurodes abutiionea, host plant resistnnce

The silverleaf whitefly (SLWF), Bemisia argenti{olii Bellows and Pening, previously reported as the sweetpotato wh.itefly, B. tabaci (Gennadius) (Perring et at. 1993), has become a major economic pest of many row crops and ornamentals throughout the United States (Faust 1992), causing plants to become wilted, stunted, and less vigorous (Smith et aJ. 1970). This pest is becoming more abundant in Georgia soybeans (McPherson & Lambert 1995). High populations of SLWF result in heavy honeydew secretions on which sooty mold develops (Vaishampayan & Kogan 1980). The bandedwinged whitefly (BWWF), Trialeurodes abutilonea (Haldeman), also is present on soybeans at low population levels (McPherson & Lambert 1995). Although whiteflies are considered a minor pest on soybean, they have become more abundant and persistent in recent years and have caused serious injury to the crop in isolated

I Accepted for publication" November 1995.

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66 J. Agric. Entomol. Vol. 13, No.1 (1996)

areas (Johnson & Nuessly 1995). Soybeans appear to be a highly preferred host for SLWF, and crop injury is more pronounced in hot, dry weather (Johnson & Nuessly 1995). Similar plant injury responses also are reported on cotton (Flint et a!' 1994).

Currently, there is little information available for the management of whitenies on soybeans (Johnson & Nuessly 1995). There appears to be some whitefly-resistant soybean germplasm, at least under greenhouse conditions (Lambert et a!. 1995), but these data have not been validated under field conditions. A southern regional effort (Southern Regional Information Exchange Group 32) is underway in six states to evaluate numerous soybean breeding lines and cultivars for resistance to insect defoliation (Burton 1995). This uniform field test was utilized in 1992 to develop an infestation rating system for evaluating whitefly infestations on different soybean germplasm.

Materials and Methods

Thirty-six soybean cultivars and breeding lines, primarily in Maturity Groups VII and VIIl, were planted in Tift County, Georgia, Coastal Plain Experiment Station, on 23 June 1992. The entries were planted in a randomized block design with three replications in plots measuring two rows wide (0.9 m row spacing) by 6.1-m long with 1.8 m alleys between blocks. The entries were separated by maturity group, with the Maturity Groups VII entries included in one test and the Maturity Group VIII entries in a second group to minimize any maturity effects between entries. 'Lamar' and 'Centennial' (Group VI), 'GATIR81-296' (Group VII), and 'Cobb' and 'PI229,358' (Group VIII) were included as standard soybean entries in both tests. The test site was conventionally plowed and a pre-plant tank mix of Prowl (2.4 Llha, American Cyanamid Company, Wayne, New Jersey 07470) and Vernam (2.7 Llha, Drexel Chemical Company, Memphis, Tennessee 38109) was incorporated into the soil for grass and broadleaf weed control. Insecticides were not applied on the test site throughout the growing season.

On 21 September all plots were rated for whiteny infestations, when the plants were in the late R5 (pods filling with seeds) or R6 (pods with full-sized seeds) growth stage. The fou.r uppermost trifoliates were visually examined on all plants on each two-row plot and assigned an average infestation score of 0 to 10, using percent area infested guidelines established by James (1971). Each numerical score was derived from a transformed scale of estimated percent leaf area infested, with each succeeding number representing an increase in the infested area by an increment of 10%. For example, a "0" score represented no whitefly immatures or sooty mold present, a liS" score represented an average of SO% leaf arca covered with whitefly immatures and sooty mold. a "5" score represented an average of 50% of the leaf area covered, and a "10" score represented total coverage of the leaf area. All plots were reevaluated for whiteny infestations on 1 October, just prior to plant senescence. Following the first ratings on 21 September, a 7.6-cm X 12.6-cm yellow sticky card (Olson Products, Medina, Ohio) was attached to a 91.4-cm cane stake and placed in the center of each

I\kPHERSON: Rating Soybean for \Vhitelly Infestations 67

plot just above the plant canopy, to sample whitefly species composition. All traps were collected 24 h later and the numbers of adult SLWF and BWWF were recorded. Just prior to placing the trap in the plot, three upper leaflets were randomly collected from each of three plants in each plot, labeled, bagged, and returned to the laboratory for counting whitefly immatures. The lower surface of the midvein region of each leaflet was observed under a dissecting microscope (\Vild-Heerbrugg) set at l2x. The total number of immatures was recorded for the 2.54-cm2 viewing area (about the size of a dime) under the microscope. 'Whitefly infestation ratings and immature numbers per 2.54 cm2 leaf area were analyzed for each maturity group using an ANOVA (P = 0.05) and means \\'ere separated using Duncan's multiple range test (SAS Institute 1985). The GLM procedure of SAS was used on the raw data to run a second-degree polynomial regression model (P = 0.05) to determine whitefly ratings as a linear or quadratic function of immature densities on the leaf (SAS Institute 1985).

Results and Discussion

There were significant differences in whitefly infestation ratings among the 18 entries in the Maturity Group VII test examined on both 21 September and 1 October ranging f"om 1.0 to 7.3 (Table 1). On both sampling dates, N88-91 had very low natural whiteny infestations while N89-1, G89­5066, G89-5172, and F90-1054 had moderate infestation levels. All other entries in the Group VII test had relatively high whiteny infestations, especially the cultivars Lamar and Cobb, and the breeding lines G88-51l1, G89-5037, D89-9121, GaTIR81-296, and PI229,358.

Significant differences also were observed among the 18 entries in the Maturity Group VIIl (Table 2), Infestation ratings were the lowest (2.6) on F90-724 and F90-988, and nine entries had ratings of 6.0 or higher on 1 October,

The population densities of whiteflies per 2.54 cm2 were significantly different between entries and ranged from six to 181.3 immatures in the Maturity Group VII test (Table 1). The linear correlation between whitefly rating and leaf density for the Group VII entries was significant (P = 0.01) and the correlation coefficient (r2 ) was 0.81. The population densities of whitenies per 2.54 cm2 leaf area also were significantly different between the entries in the Maturity Group VIII test and ranged from 15.3 to 73. The linear correlation between rating and density also was significant (P = 0.02) for the Group VIn entries with an r2 of 0.58.

Over 90% of the whiteflies captured on 21 September on yellow sticky cards was SLWF with only a few BWWF present. The traps were only used for a single 24-h period during the peak whiteny activity in soybeans. B\VWF may have been more numerous during the early season when overall population densities were low. However, when the peak whitefly population and resultant plant injury occurred, SL\VF was the predominant species on soybeans.

[t is interesting to note that the overall infestation ratings in the Maturity Group VII and VIII tests were similar on 21 September, 4.9 and

a InfcstoLion ruting scores from 0-10; 0 = no whitcOies or sooLy mold, 10 = entirc foliage covered. Column means followed by the same Icttcr are not significanLly different, Duncan's multiple range LeSt <P = 0.05).

69 McPHERSON: Rating Soybean for Whitefly Infestations

Table 2. Differential infestation ratings and population densities (immaturesl 2.54 cm2 upper lean of whiteflies on soybean germplasm in Maturity Group VIll field test, Georgia, 1992.

Infestation ratingsa

Whiteflies Soybean Maturity per 2.54 cm2 Entries Group 21 Sept. 1 Oct. 21 Sept.

G88-5234 VIII 6.0 abc 6.0 abc 60.7 ab

G89-5247 VIII 3.0 e 4.3 cder 15.7 d

G89-5180 VIII 5.3 abc 5.6 abed 33.0 bed

G89-146 VII! 4.0 cde 4.0 der 24.7 cd

SC89-1117 VIII 6.6 ab 7.0 ab 54.3 abc

SC89-2076 VIII 5.6 abc 5.6 abed 33.7 bed

F90-648 VIII 5.0 cde 5.3 bcde 39.3 abed

F90-700 VITI 3.3 de 3.6 er 19.0 cd

F90-716 VlU 4.0 cde 4.3 cder 20.0 cd

F90-724 VIII 2.6 e 3.0 r 24.7 cd

F90-988 VIIl 2.6 e 2.6 r 15.3 d

Braxton VII 5.6 abc 6.0 abc 42.7 abed

Crockett VIII 6.0 abc 7.0 ab 66.0 ab

Lamar VI 7.3 a 7.6 a 73.0a

Centennial VI 6.6 ab 6.6 ab 53.0 abc

Cobb VUI 6.0 abc 6.3 ab 53.0 abc

GATIR8I-296 VII 6.3 ab 6.6 ab 42.7 abed

P1229,358 VIII 6.6ab 7.0 ab 62.7 ab

II Infestation rat.ing scores from 0·10; 0 = no whit.eOies or sooty mold, 10 =entire folillge covered. Column menns li)lIowed by the 8UI110 letter arc nUl, signiliCll.ntly dilfcrcnt, Dunclln's multiple rallgc test. (P = 0.05).

70 J. Agric. Entomol. Vol. 13, No.1 (1996)

5.1, respectively. However, the actual whitefly densities were nearly twice as high in the Group VII test compared to the Group VIII test, 75.5 and 40.8 immatures/2.54 cm2 leaf area, respectively. The Group VIII entries were probably overrated on 21 September due to morc honeydew/sooty mold on the foliage of these entries because of an increase in adult SLWF. Lambert (1995) reported that later· maturing soybean varieties (Maturity Group VIII) have more adults and eggs on the foliage in mid September than earlier­maturing varieties (Maturity Group VII) that are beginning to senesee. Our finding that the overall infestation ratings for the "Maturity Group VIII entries increased from 5.1 to 5.5 from 21 September to 1 October, while the Group VII infestation ratings only increased from 4.9 to 5.1, would support that assumption. If this rating scale does overrate the leaf area infested when high populations of adults are present, then it is occurring uniformly across all entries within the Maturity Group, and it would balance out within a couple of weeks with the increased number of immatures resulting from the ovipositing adults.

In conclusion, the visual whitefly infestation rating scale of O~10 can be effectively used to measure differences in infestation levels among soybean entries. Counting whitefly densities on individual leaves is very time consuming. In this study, it took 3-5 min to count just one leaf sample when populations densities were high, whereas an entire plot could be rated for estimated whitefly density in less time. Sticky traps can be used to observe adult whitefly activity in the soybean field and for species identification. They also have been shown to be useful for monitoring whiteflies in the greenhouse (Gillespie & Quiring 1987). Several of the Maturity Group VII and VIII soybean entries examined in this study demonstrated a moderate to high level of host plant resistance to SLWF. These entries would be good candidates for breeding programs to develop resistant soybean varieties.

Acknowledgment

This study was supported in part through State and Hatch funds allocated to the Georgia Agricultural Experiment Station. I thank Dr. Judith K. Brown at the University of Arizona for identifying the whitefly species collected from Georgia soybeans, M. Padgett and D. Taylor for technical support, and J. Ruberson and J. Dutcher for reviewing the manuscript.

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71 l\'lcPHERSON: Rating Soybean for Whiten)' Infestations

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