David J. Isenhour

CHEMICAL AND ULTRASTRUCTURAL ANALYSIS OF CORN CUTICULAR LIPIDS AND THEIR EFFECT ON FEEDING BY FALL ARMYWORM LARVAE

The cuticular lipid composition of lower and upper leaves of five genotypes of field-grown corn,Zea mays L., was determined by combined gas chromatography-mass spectrometry. Surface lipids of the upper leaves had a higher proportion ofn-alkanes (45–52%) than the lower leaves, while the lower leaves had higher percentages of fatty alcohols (12–18%) than the upper leaves. Scanning electron microscopy showed that the upper leaves of two corn genotypes, MpSWCB-4 and Cacahuacintle Xs, had a smooth amorphous appearance, while the lower leaves had a dense array of wax crystals.Spodoptera frugiperda (J.E. Smith) larvae weighed more and developed more rapidly when they were reared on diet containing corn foliage from which the cuticular lipids had been removed than when they were fed untreated foliage. However, growth was not inhibited when larvae were fed diet containing the cuticular lipid extracts or individual cuticular lipid components.

DISTRIBUTION OF FALL ARMYWORM (LEPIDOPTERA: NOCTUIDAE) PARASITOIDS ON FIVE CORN GENOTYPES IN SOUTH GEORGIA

Five genotypes of corn were planted on four different dates in south Georgia during the spring of 1991. Plants were infested with neonate fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), larvae and collected either seven or twelve days later. Larvae were reared in the laboratory on artificial diet to determine distribution of FAW parasitoids. Larvae recovered from the resistant genotype MpSWCB-4 had the highest percent parasitism (44.30%), while the susceptible genotype Pioneer 3192 had the lowest parasitism (31.70%). Aleiodes laphygmae (Gahan) was the most abundant parasitoid, attacking a total of 12.8% of the 16,120 FAW larvae recovered. Archytas marmoratus (Townsend) parasitized 10.0% of the FAW larvae while Ophion flavidus Brulle and Cotesia marginiventris (Cresson) parasitized 7.9% and 6.3%, respectively, of the recovered FAW. Ten additional parasitoid species emerged from the FAW, but none of these species attacked more than 1% of the recovered FAW larvae.

Fall Armyworm Symposium: Effect of Corn Foliar Cuticular Lipids on the Movement of Fall Armyworm (Lepidoptera: Noctuidae) Neonate Larvae

Genotypes of corn known to vary in their resistance to fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), were utilized in a study designed to examine the plant resistance role of surface ultrastructure and cuticular lipids. Scanning electron microscopy showed dramatic differences between the ultrastructural appearance of lower (4th) and upper (8th) leaves of the resistant MpSWCB-4 and the susceptible Cacahuacintle Xs. In a choice test, FAW larvae preferred the upper leaves from which the cuticular lipids had been removed over untreated leaves of these two genotypes. Larval behavior was monitored by video camera on the adaxial and abaxial surfaces of upper and lower leaves of MpSWCB-4 and Cacahuacintle Xs and on foliage samples with and without cuticular lipids. FAW showed more nonacceptance behavior on the untreated foliage than on the chloroform-extracted foliage. FAW larvae traveled greater distances and crawled faster when they were on upper leaves rather than lower leaves and when they were on the abaxial leaf surface rather than the adaxial surface. However, no difference in behavior was found when larval movement was monitored on the cuticular lipid extracts from the foliage of two resistant and two susceptible corn genotypes.

Resistance to fall armyworm in converted sorghums.

Field experiments were conducted in 1988 to evaluate newly converted sorghum, Sorghum bicolor (L.) Moench, germplasm for both whorl and panicle resistance to feeding by larvae of the fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith). In the whorl-stage resistance study, over 20 and 30 of the entries were significantly more resistant to FAW feeding than the resistant check at 7 and 14 days after infestation, respectively. Significant differences also were found among the entries for the number of FAW larvae that established per panicle 4 days after infestation.

Functional response of Cotesia marginiventris (Cresson) (Hym., Braconidae) to Spodoptera frugiperda (J. E. Smith) (Lep., Noctuidae) on meridic diet containing resistant or susceptible corn genotypes

The functional response of female Cotesia marginiventris (Cresson) was determined for 5 host densities of fall armyworm at two temperatures. The effect of meridic diets containing a susceptible or resistant corn genotype on functional response was also evaluated. C. marginiventris females exhibited a Type‐II functional response at both temperatures. There was no significant (P > 0.05) difference in rates of parasitism between fall armyworm reared on meridic diet containing a resistant corn genotype versus those reared on diet containing a susceptible genotype.

The impact of toxaphene applied as a post-emergence herbicide for control of sicklepod, Cassia obtusifolia L., on arthropods associated with soybean

Abstract The impact on arthropod populations of toxaphene (camphechlor) used as a herbicide was compared with cultivation and pre-emergence herbicide usage for control of sicklepod, Cassia obtusifolia L., in soybean. Soybean yields and weed control were also evaluated. Pest resurgence due to toxaphene was found to be limited, but significantly more soybean plants had main stems girdled due to the threecornered alfalfa hopper, Spissistilus festinus (Say), than those plants where toxaphene had not been applied. Both toxaphene usage and cultivation reduced early season predator abundance significantly compared with soybean where weeds were not controlled. However, the greater abundance of predators in soybeans with weeds appeared to be attributable to the increase in habitat diversity provided by the weeds and soybeans and not to a reduction in predator numbers caused by toxaphene in the treated plots. Neither soybean yields, nor the degree of sicklepod control, differed significantly between toxaphene usage and cultivation. Permethrin usage significantly reduced both pest and predator abundance in soybean.

Bioassay for Screening Plant Accessions for Resistance to Fall Armyworm (Lepidoptera: Noctuidae) Using Artificial Diets

Laboratory bioassays demonstrated that formulation of artificial insect diet influenced the expression of antibiosis to fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), in grain sorghum, Sorghum bicolor (L.) Moench. Larvae of FAW were reared on a diet formulated with pinto bean and a diet formulated with no pinto bean, both supplemented with dried milk stage florets of resistant and susceptible sorghum genotypes. The sorghum lines showed a significantly higher antibiosis to FAW when mixed in the diet formulated with no pinto bean than when mixed in the diet containing the bean. Larvae that were fed the no-bean diet supplemented with resistant sorghum genotypes weighed one-half to one-third less, required longer to pupate, and resulted in lighter pupae than larvae reared on mixtures containing the bean. The results obtained when the larvae were reared on the bean diet were significantly correlated (P = 0.0001, r > 0.80, n = 18) with those recorded when the no-bean diet was used for the FAW variabl...

Spodoptera frugiperda resistance in developing panicles of sorghum accessions

Experiments were conducted in 1989 to evaluate converted sorghum accessions for preflowering extended panicle and soft-dough stage panicle feeding resistance to fall armyworm, Spodoptera Frugiperda (J. E. Smith), by incorporating plant tissue into artificial insect diet. Spodoptera Frugiperda biological parameters measured were larval and pupal weights, mean duration of the larval stage, time to adult eclosion, survivorship, fecundity, net reproductive rate, intrinsic rate of increase, and relative fitness. The converted lines IS 2246C, IS 2403C, IS 2825C, IS 7007C, IS 8337C, IS 12592C, IS 12612C, IS 12657C, IS 12666C, and IS 12681C were more resistant to both preflowering and soft-dough stage panicle feeding by S. frugiperda than the resistant check NK Savanna 5. The genotypes IS 1340C, IS 2553C, IS 2569C, IS 6911C, IS 7498C, IS 12219C, IS 12617C, IS 12662C, and IS 12664C were more resistant than the resistant check to S. frugiperda feeding in the soft-dough stage, but not in the preflowering stage of panicle development. Plant antibiosis was the major mechanism of resistance in these lines at both stages of the panicle development.RésuméDes lignées naines de sorgho, Sorghum bicolor (L.) Moench ont été évaluées pour déterminer leur résistance à la chenille légionnaire, Spodoptera Frugiperda (J. E. Smith) durant la phase préfloraison et la phase patte douce de la panicule. La résistance a été étudiée en incorporant des échantillons de panicule des différentes lignées dans le diète de l’insecte. Les paramètres biologiquesde l’insecte qui ont été mesurés étaient le poids des larves, le poids des nymphes, la durée de la phase larvale, le temps d’apparition de l’adulte, le taux de survivance, la fécondité, le taux net de réproduction, le taux intrinsec d’accroissement et l’aptitude relative. Les lignées naines IS 2246C, IS 2403C, IS 282SC, IS 7007C, IS 8337C, IS 12592C, IS 12612C, IS 126S7C, IS 12666C et IS 12681C étaient plus résistantes à la chenille que le témoin résistant NK Savanna 5 durant la phase préfloraison aussi bien que la phase patte douce de la panicule. Les lignées IS 1340C, IS 2S53C, IS 2S69C, IS 6911C, IS 7498C, IS 12219C, IS 12617C, IS 12662C et IS 12664C étaient plus résistantes que le témoin résistant durant la phase patte douce de la panicule, mais pas durant la phase préfloraison. L’antibiose était le méchanisme majeur de la résistance chez ces lignées de sorgho aux deux phases de la panicule qui ont été testées.