Effects of fast and slow-wilting soybean genotypes on fall armyworm (Spodoptera frugiperda) growth and development

Abstract

Soybean (Glycine max) is the most important plant protein source, and Fall Armyworm (FAW, Spodoptera frugiperda) is considered a major pest. This study aimed to examine the impact of FAW feeding on soybean accessions that vary in their water use efficiency (WUE) traits, by examining FAW growth and life history parameters along with plant growth response to pest damage. Soybean accessions were grown in a greenhouse and exposed to FAW larval feeding for 48 h at three different soybean growth stages: V3, R3, and R6. The growth and development of the FAW and soybeans were monitored. Results showed that faster wilting soybean accessions grow taller and have more leaves than slower wilting accessions, but yield was higher in slower wilting soybean accessions. FAW experienced the highest mortality on mid-stage (R3) soybean plants, but they gained the least mass on early stage (V3) soybean plants. These results can assist in better understanding plant insect-interactions at different life stages in both soybean and FAW with implications for management.

Keywords: Canopy wilting; fall armyworm; mass gain; mortality; pupal mass; water use efficiency.

Figure 1.

Soybean leaf with a 2^nd instar FAW larvae enclosed in a mesh bag.

Figure 2.

Damage assessment scale on soybean leaves. 0 = 0% damage, 1 = 25% damage, 2 = 50% damage, 3 = 75% damage, 4 = 100% damage.

Figure 3.

Feeding initiation study set up. From right to left: sampled leaves in falcon tubes, digital microscope, laptop, stopwatch, and discard bin.

Figure 4.

(A) mean mass gained by FAW when exposed to various soybean growth stages for 48 hours. Different letters denote significant differences in mean mass as determined by post hoc analysis using Tukey’s test (p=.001). (B) mean mass gained by FAW after larvae were exposed to either fast or slow wilting genotypes for 48 hours. Different letters denote significant differences in mean mass as determined by post hoc analysis using student t-test (p=.0110).

Figure 5.

A) mean pupal mass after larvae were exposed to various soybean growth stages for 48 hours. Different letters denote significant differences in mean mass as determined by post hoc analysis using Tukey’s test (p=.001). B) mean pupal mass after larvae were exposed to either fast or slow wilting genotypes for 48 hours. Different letters denote significant differences in mean mass as determined by post hoc analysis using student t-test (p=.334).

Figure 6.

A) Mean adult mass after larvae were exposed to various soybean growth stages for 48 h. Let ‘NS’ denote that there are no significant differences in mean mass as determined by post hoc analysis using Tukey’s test (p=.216). B) Mean adult mass after larvae were exposed to either fast or slow wilting genotypes for 48 h. ‘NS’ denote that there are no significant differences in differences in mean mass as determined by post hoc analysis using student t-test (p=.563).

Figure 7.

A) Mean mortality of FAW larvae across early, mid, and late soybean growth stages. Different letters denote significant differences in mean mortality across soybean growth stages as determined by post hoc analysis using Tukey’s test (p= .0001). B) Mean mortality of FAW across fast wilting genotypes and slow wilting genotypes. Different letters denote significant differences in mean mortality between fast and slow wilting soybean genotypes as determined by post hoc analysis using student t-test (p=.0360).

Figure 8.

A) Mean damage assessment across early, mid, and late soybean growth stages. Different letters denote significant differences in mean damage inflicted across soybean growth stages as determined by post hoc analysis using Tukey’s test (p=.0781). B) Mean damage assessment of FAW across fast wilting genotypes and slow wilting genotypes. Let ‘NS’ denote that there are no significant differences in mean damage inflicted between fast and slow wilting soybean genotypes as determined by post hoc analysis using student t-test (p=.4755).

Figure 9.

Mean plant height across different soybean growth stages (early, mid, late). Different letters denote significant differences in mean plant height as determined by post hoc analysis using Tukey’s test (p<.05). B) Mean plant height across fast wilting and slow wilting soybean genotypes. Different letters denote significant differences in mean plant height as determined by post hoc analysis using student t-test (p=.0001).

Figure 10.

A) Mean number of leaves across different soybean growth stages (early, mid, late). Different letters denote significant differences in a mean number of leaves as determined by post hoc analysis using Tukey’s test (p=.0001). B) Mean number of leaves across fast and slow soybean genotypes. Different letters denote significant differences in a mean number of leaves as determined by post hoc analysis using a student t-test (p=.002).

Figure 11.

A) Mean number of trichomes on different soybean leaf sides (adaxial, abaxial). Different letters denote significant differences in a mean number of leaves as determined by post hoc analysis using a student t-test (p=.0001). B) Mean number of trichomes across fast and slow soybean genotypes. Different letters denote significant differences in a mean number of leaves as determined by post hoc analysis using a student t-test (p=.0025).

Figure 12.

A) Mean number of seconds it took FAW larvae of different instars to begin feeding on soybean leaves (early, late). Different letters denote significant differences in mean number of leaves as determined by post hoc analysis using a student t-test (p=.0001). B) Mean number of seconds it took FAW larvae to begin feeding on fast and slow soybean genotypes. Let ‘NS’ denote that there are no significant differences in the mean it took FAW larvae to begin feeding using the student t-test (p=.1780).

Figure 13.

Percentage of FAW larvae refused to feed on fast and slow soybean genotypes. Letter ‘NS’ denote that there are no significant differences. It took FAW larvae to begin feeding using a student t-test (p=.5330).

Figure 14.

Mean number of pods on fast and slow wilting soybean genotypes. Different letters denote significant differences in the mean number of pods found on soybean plants using a student t-test (p=.0001).