Abstract
Simple SummaryFall armyworms from Mexico are susceptible to four Bacillus thuringiensis (Bt) proteins, while those from Puerto Rico have field-evolved resistance to Cry1F and Cry1Ac and are susceptible to Cry2Ab2 and Cry1A.105. Implications for fall armyworm management are discussed.Fall armyworm is one of the main pests of conventional and Bacillus thuringiensis (Bt) corn in many countries in the Americas, Africa, Asia and in Australia. We conducted diet-overlay bioassays to determine the status of susceptibility to four Bt proteins (Cry1A.105, Cry2Ab2, Cry1F and Cry1Ac) in three different populations of fall armyworm from Mexico, and one population from Puerto Rico. Bioassays showed that fall armyworms from Puerto Rico were resistant to Cry1F with a resistance ratio 50 (RR50) higher than 10,000 ng/cm2 and to Cry1Ac with a RR50 = 12.2 ng/cm2, displaying the highest median lethal concentration (LC50) values to all Bt proteins tested. The effective concentration 50 (EC50) values further confirmed the loss of susceptibility to Cry1F and Cry1Ac in this population. However, LC50 and EC50 results with Cry1A.105 and Cry2Ab2 revealed that fall armyworm from Puerto Rico remained largely susceptible to these two proteins. The Mexican populations were highly susceptible to all the Bt proteins tested and displayed the lowest LC50 and EC50 values to all Bt proteins. Our results suggest that Cry1F and Cry1Ac resistance is stable in fall armyworm from Puerto Rico. However, this population remains susceptible to Cry1A.105 and Cry2Ab2. Results with Mexican fall armyworms suggest that possible deployment of Bt corn in Mexico will not be immediately challenged by Bt-resistant genes in those regions.
Highlights
The greatest differences were observed in the LC50 s and effective concentration 50 (EC50) s for Cry1F and Cry1Ac, with larvae from the Mexican populations needing the lowest concentrations, while larvae from the Puerto Rico (PR) population requiring approximately 12- to 100-fold more (Figure 1, Tables 2 and 3)
For PR, the LC50 and EC50 values for Cry1F were close to or greater than 10,000 ng/cm2, which was the highest concentration that could be attained with the stock solution
An extensive work characterizing Cry1F resistance in fall armyworms from Brazil [59] has determined that resistance to this Bacillus thuringiensis (Bt) protein can be due to the interaction of several resistant alleles, and can develop independently in different populations in the presence of selection pressure. This highlights the importance of studying resistance case by case, even with the same species, since resistance may evolve through different mechanisms. This is a unique research study that shows the stability of resistance of fall armyworm from Puerto Rico to Cry1F and resistance to Cry1Ac, but the lack of cross-resistance to Cry1A.105 and Cry2Ab2
Summary
Fall armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is native to the Americas and is a major pest in the southern United States and Latin America, affecting important crops such as corn, rice, sorghum, and cotton [1,2]. Management of fall armyworm is complicated by its outstanding migratory. Insects 2020, 11, 831 capacity, as it can move long distances between countries and even regions [5]. In 2016, it was first detected in West Africa, and it spread to 28 African countries within a year of detection [6]. By 2020, fall armyworm was officially confirmed in Yemen, several countries in Southeast Asia, and Australia, threatening the production of corn, but rice, sorghum, and other crops [7,8]
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