FALL ARMYWORM' DAMAGE RATINGS ON CORN AT VARIOUS INFESTATION LEVELS AND PLANT DEVELOPMENT STAGES'

B. R. Wiseman and N. W. Widstrom Southern Grain Insects Research Laboratory, USDA, ARS

Tifton, GA 31793-0748

Abstract: Fall annyworm, Spodoplcra {rugiperda (J. E. Smit.h), larval infeslations at the six­leaf stage of plant development on both sweet and dent corn, Zea mays Linnacus, at the rates of 10, 20, and 40 fall armyworm larvae/plant caused a significantly higher leaf·feeding damage rating than infestations at the 12-leaf stage. Two applications of larvae resulted in significantly more damage than one application. Rllting 10 individual plants per plot is a slightly more precise method than the plot rating method of assessing damage differences.

Key words: Spoduptera (rugiperdll, maize, insect resistance, Zea mays, damage ratings

J. Agric. Entomol. 1(2): 115-119 (April 1984)

Large numbers of insects are essential to most successful host plant resistance (HPR) breeding programs. The use of a quick and easy system to artificially infest and evaluate plants is equally important. Artificial infestation devices that permit the manipulation of several lepidopterous insects have been developed and are in current use in HPR programs (Wiseman et al. 1974, 1980; \\'iseman and Widstrom 1980; Davis and Williams t979; and Guthrie 1975).

Wiseman and Widstrom (1980) compared several methods of artificial infestation with fall armyworm (FAW), Spodoptera {rugiperda (J. E. Smith), larvae on whorl­stage corn. They found that every plant of dent corn in the 10- to 12-leaf stages should be infested with two applications of at least 20 flrst-instar larvae per plant. The purpose of the study reported here was to compare whole plot ratings and individual plant damage ratings of 6· and 12·leaf stages of sweet and dent corn, Zea mays Linnaeus, infested once or twice with 10, 20, and 40 FAW larvae/ plant.

MATERIALS AND METHODS

Dent (pioneer 3369A) and sweet (Silver Queen) com hybrids were used in these experiments each year for 1979 and 1980, and only com in 1981. Experiments within a given year were planted on the same date. The experimental design was a split-split plot with eight replications. Whole plots were plant stages (6· or 12-leaf) of development. Subplots were the number of applications, and the final split consisted of plots infested with 0, 10, 20. and 40 FAW larvae/plant. The corn was planted in 6.1-m rows 76 em apart; plants were thinned to ca. 30 cm apart; and one row of border was planted between each treatment plot. Normal (accepted) cultural practices were carried out in the planting and maintenance of experimental plots. Using the mechanical device used by Wiseman and Widstrom

1 LBl'lJ)OI''TEI{A: Noctuidae

2 In cllQl~rlllion with the Univ. of Gil. ColI. of A",'Tic. ~;:lper. Sm., CoaRlll1 Plain Sl<ltion. Tifton. GA. Received for puhliC1llion 18 Junl' 198:1; accepted 27 September 198:1.

115

116· J. Agric. Entomol. Vol. I, No.2 (1984)

(1980), 10 plants/plot of each treatment combination (l2·leaf stage infestation was made ca. 14 dafter 6-leaf stage) were infested with neonate FAW larvae. The second application was made the day following the initial application. Damage was visually rated (0-9) (Wiseman and Davis 1979) on the 10 plants/plot, individually and collectively (whole plot). 15 d after the initial infestation.

Analyses of variance were calculated within years and corn types using the plot means obtained from 10 individual plant ratings or whole plot ratings objectively composited for each IO-plant group. Duncan's multiple range test was used to separate damage rating differences.

RESULTS AND DISCUSSION

Tables 1 and 2 show the data illustrating the visual leaf damage ratings for the 6- and 12-leaf stages of sweet and dent plant development. A significant stage-by­infestation level interaction occurred for each com type in each year, indicating that the check behaved differently than the infestation levels for the respective sweet and dent com types. The check ratings were ca. 4 units different from any infestation level for the 6-leaf stage and ca. 2 units less than any infestation level for the 12­leaf stage. At the 6-leaf stage of development, adding more larvae than la/plant did not increase substantially (17% for 20 larvae/plant and 12% for 40 larvae per plant) the resultant damage ratings. However, at the 12-leaf stage of plant deve­lopment, the addition of more larvae per plant increased the resultant damage ratings (42% for 20 larvae/plant and 27% for 40 larvae/plant) even though the damage ratings were almost one-half those at the 6·leaf stage of development. Successive increases in larval infestation rate significantly increased leaf damage ratings in every case for the sweet corn and in 28 of 36 cases for the dent corn. Exceptions occurred with individual dent corn plant ratings at the 6· and 12-leaf stages of plant development in 1980 and 1981, and for the plot ratings at the 6­leaf stage in 1981. In general, in these cases, 40 larvae/plant did not result in a significant increase in leaf damage ratings over 20 larvae/plant.

Table 1. Mean fall armyworm larval damage ratings on sweet corn after infesting with 10, 20, and 40 larvae/plant combined over one and two applications. 1979-1980·.

6-lear stage 12-1ear stage

Infestation 1979 1980 1979 1980

rate IP PR IP PR IP PR IP PR

o 0.0 a 0.0 a 0.4 a 0.5 a 0.0 a 0.0 a 0.2 a 0.3 a 10 4.6 b 4.4 b 5.1 b 5.3 b 2.3 b 1.9 b 3.7 b 3.8 b 20 5:4 c 5.4 c 5.9 c 6.1 c 3.3 c 3.4 c 4.6 c 5.3 c 40 6.0 d 6.2 d 6.5 d 6.9 d 4.6 d 4.9 d 5.3 d 6.3 d

• Mellns within .. column followed by the NOme leiter lire not aignil'icuntly difFerent U-0.05) lUI upllratcd by DUnClln'S multiple ranse teal. [I" .. mClll1 of individual plant ralin~s IIml PR .. mellll or plot ratin~8.

Table 3 shows visual damage ratings for the 6- and l2-leaf stages of plant development averaged over infestation levels and rates, excluding the check (0 infestation rate). In every paired comparison, visual ratings were significantly higher for the 6·leaf stage than for the 12-leaf stage of plant development for both sweet

Table 2. Mean fall armyworm larval damage ratings on dent corn after infesting with 10,20, and 40 larvae/plant combined over one and two applications, 1979-81.­

• Mlt'lI1U ...ilhin a column lo(kr...·ed by thlt' same Jettu are not signific:ant!y dilfe~nt (P - 0.05) as Kparated b). Dunun·~ multipl... Tllnge Ul<I.. IP - mean of individual planl ntint: and PR = mean

of plol ratings.

6-leof stage 12-leo[ stage

Infestation 1979 1980 1981 1979 1980 1981 :5 en

rate IP PR IP PR IP PR IP PR IP PR IP PR OJ .:

0 0.1 0.0 • 0.6 • 0.6 • 0.5 • 0.6 • 0.3 • 0.2 • 0.5 a 0.5 • 0.0 • 0.0 • z• >

10 3.4 b 3.3 b 5.2 b 5.1 b 4.8 b 5.0 b 2.1 b 2.1 b 2.1 b 1.6 b 3.7 b 2.9 b 20 4.3 c 4.4 c 5.7 c 6.1 c 5.2 c 5.2 b 3.1 c 3.2 c 2.8 c 2.5 c 4.8 c 5.1 c 40 4.9 d 5.5 d 6.5 d 7.0 d 5.2 c 5.2 b 3.9 d 4.3 d 3.1 c 3.5 d 5.1 c 6.0 d

() o 3

~. ". .., >::;;

.,

118 J. Agric. Entomol. Vol. 1, No.2 (1984)

Table 3. Mean fall armyworm larval damage ratings on sweet and dent corns infested at the 6- and 12-1eaf stages of plant development, 1979-81+.

Sweet corn Dent corn

stage IP PR IP PR IP PR IP PR IP PR Leaf 1979 1980 1979 1980 1981

6 5.3 a 5.3 a 5.8 a 6.0 a 4.2 a 4.4 a 5.8 a 6.1 a 5.1 a 5.1 a 12 3.4 b 3.4 b 4.6 b 5.1 b 3.0 b 3.2 b 2.7 b 2.5 b 4.5 b 4.6 b

Mean 4.4 ns 4.4 5.2 • 5.6 36 • 3.8 4.3 ns 4.3 4.8 ns 4.9 t IP or PH ~'OJumn means ..ithin felln nnd corn types followed hy the urnI' letter and overall IP and PH year means

sellsratcd by ns are nOI significantly different W .. 0.05) as separnted lJ)' Duncan'a multiple Tange lUI. II' - mean of individual plant ratings and PH - mean of plot ratings. Means do not include ratings on uninfeBted checkH.

and dent corn. Plot ratings were generally higher than individual plant ratings, but no significant differences (P < 0.05) occurred between the individual plant ratings and plot ratings for the sweet com infestations in 1979 and the dent com infestations in 1980-81. However, significant differences were found between the plot and individual plant damage ratings in 1979 for dent corn and in 1980 for sweet corn. In some cases, the analysis of plot ratings produced a coefficient of variation of ca. 2X higher than that for individual plant analysis. When leaf damage rating means for the 6- and 12-leaf stages are compared (Tables 1-3), consistently higher ratings were evident for the plot ratings in both 6- and 12- leaf stages of plant development.

Table 4 shows the damage ratings for the two larval applications when damage ratings were averaged over leaf stages and larval infestation rates, excluding the damage ratings for the check (0 infestation rate). [n every case except one, two applications of larvae significantly increased the resultant visual damage ratings for both sweet and dent corn.

Since the 6-leaf stage of plant development for both sweet and dent corn received significantly more damage than the 12-leaf stage, the lack of susceptibility of the 12-leaf stage may be due to environmental influences. We have experienced in recent years that later artificial infestations (after June 1) of FAW do not tend to produce an adequate establishment. Predators and parasites have been collected

Table 4. Mean fall armyworm larval damage ratings on sweet and dent corn after infestation with one or two applications of larvae, 1979-81-.

No. of 1979 1980 1981

Applications IP PR IP PR IP PR

Sweet corn 1 4.0 a 3.9 a 4.5 a 4.8 a 2 4.7 b 4.9 b 5.8 b 6.4 b

Dent corn 3.4 a 3.5 a 3.9 a 3.9 a 4.5 a 4.3 a

2 3.8 a 4.2 b 4.6 b 4.8 b 5.1 b 5.5 b - II' nr I'll ColUlIlllllr means "'ithin com lype" followed by the sOllie leller lire !lot significalltly diff"nmt 1/' - 0.05) us

"eparliled by tJuncan's ne"" multiple runj(t:' I"S!. IP .., meau of indiYiduul plnut ratincs and PH "" mean of plot ratings. M~on~ li" nOI include ratinl(s nf IIninfcHl"d cheeks.

119 \\'1SEMAN and WIDSTROM: ArLificial Inresilltion of Corn with FAW

in greater numbers from laler whorl-infested com (Martin and Wiseman, unpublished data). Thus, indications nre that parasites or predators could be reducing effective establishment of FAW on the 12-leaf stage of plant development, thereby reducing the number of FAW larvae and causing a reduction in the actual damage and resultant visual damage ratings. If this occurred, relative assessment of the damage was still detectable and easy to separate statistically. Therefore, for both stages of plant development and for sweet and dent corn, we were able to detect differences at the 6- and 12-leaf stages and between the 10,20, and 40 larvae/plant infestation rates.

In summary, significantly higher visual damage ratings resulted on the 6-leaf stage as compared to the 12-leaf stage of plant development for both sweet and dent corn. Two applications of FAW larvae resulted in significantly higher damage ratings than one application for both leaf stages and sweet and dent corns. The dent corn damage ratings are in agreement with our earlier findings (Wiseman and Widstrom 1980). All three lan'al infestation rates (10, 20, and 40 larvae/plant) produced significantly higher damage ratings than the check. and ratings signifi­cantly different from each other. It is suggested for initial screenings at the 6-leaf stage of infestation that plot ratings be used, but for breeding or more detailed studies, that individual plants be visually rated at both 6- and 12-leaf stages of plant development.

ACKNOWLEDGEMENTS

We thank J. L. Skinner and Charles Mullis for their technical assistance in this study.

REFERENCES CITED

Davis, F. M.. lind W. P. WilIinms. 1979. More efficient procedures for saeening corn gena· lypes for resistance to second-brood Southwestern COfll borer. J. Econ. EnlOmol. 72: 801­803.

Guthrie, W. D. 1975. Entomological problems involved in developing host plant resistance programs. Iowa State J. Res. 49: 519·525.

Wiseman. B. R., and F. M. Davis. 1979. Plant resistance to the fall armyworm. Fla. Entomol. 62, 123-130.

Wiseman, B. R., F. M. Davis, and J. E. Campbell. 1980. Mechanical infestation device used in fall armyworm plant resistance programs. Fla. Entomo!. 63: 425-432.

Wiseman, B. R., and N. W. Widstrom. 1980. Comparison of met.hods of infesting whorl-stage com with fall armyworm larvae. J. Ecan. EntomoL 73: 440-442.

Wiseman, B. R.. N. W. Widstrom. and W. W. McMillian. 1974. Methods of application and numbers of eggs of the corn earwonn required to infesl ears of com artificially. J. Econ. EnLOmol. 67: 74-76.