Exposure to Cry1 Toxins Increases Long Flight Tendency in Susceptible but Not in Cry1F-Resistant Female Spodoptera frugiperda (Lepidoptera: Noctuidae)

Abstract

The fall armyworm (JE Smith) (Spodoptera frugiperda) is a polyphagous pest targeted by selected Cry and Vip3A insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) that are produced in transgenic Bt corn and cotton. Available evidence suggests that sublethal larval exposure to Cry1Ac increases flight activity in adult Spodoptera spp. However, it is not known whether this effect is also observed in survivors from generally lethal exposure to Cry1Ac. Moreover, while multiple cases of field-evolved resistance to Bt proteins have been described in the native range of S. frugiperda, the effect of resistance on flight behavior has not been examined. Long-distance migratory flight capacity of S. frugiperda is of concern given its ongoing global spread and the possibility that migrants may be carrying resistance alleles against pesticides and Bt crops. In this study, we used rotational flight mills to test the effects of generally lethal exposure to Cry1Ac in susceptible and sublethal exposure in Cry1F-resistant S. frugiperda strains. The results detected altered pupal weight after larval feeding on diet containing Cry proteins, which only translated in significantly increased tendency for longer flights in female moths from the susceptible strain. This information has relevant implications when considering current models and assumptions for resistance management of Bt crops.

Keywords: Cry1Ac; Cry1F; Spodoptera frugiperda; dispersion; fall armyworm; flight mill; induced flight; migratory behavior; resistance.

Figure 1

Representative aggregate output from 20 rotation flight mills (RFMs) monitoring distance flown during 12 h by S. frugiperda moths. Flight activity for each individual moth is shown as a blue line, while the average flight is shown as a red line with an orange lower area. Moths in this example were from the Benzon strain and were fed control diet as larvae. Orange line circle includes moths that would be classified as “migratory” based on flying >2 km in the first 12 h of testing. Purple line circle includes moths that flew <2 km and would be classified as “resident”.

Figure 2

Percentage of moths in the resident (flying < 2 km in 12 h, yellow) and migratory (flying > 2 km in 12 h, black) categories depending on strain, sex and diet treatment during the larval stage. Samples are named on the left using one letter abbreviation for strain (B = Benzon, P = 456LS4D), sex (M = male, F = female), and diet treatment (D = control diet, C = Cry1Ac, F = Cry1F).