The fall armyworm strain associated with most rice, millet, and pasture infestations in the Western Hemisphere is rare or absent in Ghana and Togo

Abstract

The moth pest fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is now present throughout much of the Eastern Hemisphere where it poses a significant economic threat to a number of crops. Native to the Western Hemisphere, fall armyworm is one of the primary pests of corn in the Americas and periodically causes significant economic damage to sorghum, millet, cotton, rice, and forage grasses. This broad host range is in part the result of two populations historically designated as host strains (C-strain and R-strain) that differ in their host plant preferences. Reports of infestations in Africa have to date mostly been limited to the C-strain preferred crops of corn and sorghum, with little evidence of an R-strain presence. However, this could reflect a bias in monitoring intensity, with the R-strain perhaps being more prevalent in other crop systems that have not been as routinely examined for the pest. Because knowledge of whether and to what extent both strains are present is critical to assessments of crops at immediate risk, we analyzed specimens obtained from a systematic survey of pasture grass and rice fields, habitats typically preferred by the R-strain, done contemporaneously with collections from corn fields in Ghana and Togo. Substantial larval infestations were only observed in corn, while pheromone trap capture numbers were high only in corn and rice habitats. Little to no fall armyworm were found in the pasture setting. Comparisons with a meta-analysis of studies from South America identified differences in the pattern of strain-specific markers typically found in fall armyworm collected from rice habitats between the two hemispheres. Genetic tests of specimens from rice and corn area traps failed to show evidence of differential mating between strains. These results are consistent with the R-strain being rare or even absent in Africa and, at least for the Ghana-Togo area, this R-strain lack does not appear to be due to limitations in pest monitoring. The implications of these results to the crops at risk in Africa and the accuracy of existing molecular markers of strain identity are discussed.

Genetic studies of fall armyworm indicate a new introduction into Africa and identify limits to its migratory behavior

“…Consistent with previous studies the frequency of TpiR remains very low 14 , 27 . From a total of 1722 specimens tested only 18 were TpiR.…”

“…DNA from individual specimens were isolated as previously described 27 . In brief, specimens were homogenized in 1.5 ml of phosphate buffered saline (PBS, 20 mM sodium phosphate, 150 mM NaCl, pH 8.0) using a tissue homogenizer (PRO Scientific Inc., Oxford, CT) or hand-held Dounce homogenizer then pelleted by centrifugation at 6000 g for 5 min.…”

Genetic Characterization of Fall Armyworm in Africa Indicates a Limited Capacity for Transcontinental Population Movements and Evidence of a Second Introduction

“…This con guration is found at variable frequencies in the Western Hemisphere, with the highest frequencies associated with C-strain preferred host plants 18,24 . These observations suggest that the COI-RS TpiC population is behaving as the C-strain with respect to plant host use and this seems to be the case in Africa as reports of agricultural damage by FAW has consistently been limited to C-strain plants like corn and sorghum 7,27 .…”

“…The situation in Africa differs from the Americas in that while both COI-CS and COI-RS are detected at high frequency, the TpiR haplotype is very rare in Africa, present in less than 1% of all specimens even in collections from habitats dominated by R-strain preferred plant hosts 27 . The Africa population is dominated by two COI Tpi con gurations, COI-CS TpiC present in 41% of specimens and COI-RS TpiC at 47%, with the former preferentially found in the western Africa grouping of collection sites and the latter in eastern Africa (Fig.…”

“…Both larvae and adult males from pheromone traps were stored refrigerated or air dried at ambient temperature until transport by mail to CMAVE, Gainesville, FL USA for DNA preparation. DNA from individual specimens were isolated as previously described 27 . In brief, specimens were homogenized in 1.5 ml of phosphate buffered saline (PBS, 20 mM sodium phosphate, 150 mM NaCl, pH 8.0) using a tissue homogenizer (PRO Scienti c Inc., Oxford, CT) or hand-held Dounce homogenizer then pelleted by centrifugation at 6000g for 5 min.…”