Throughout the soybean growing areas of the southern U.S. and South America, lepidopteran caterpillars are the most important pest complex affecting the crop. Millions of pounds of insecticides are sprayed on the crop each year in an effort to minimize their impact—a practice that is not always successful and entails significant exposure risks to the environment and farm workers alike. A variety of lepidopteran species occur in soybeans, and proper identification is essential to ensure adequate control and avoiding unnecessary applications. While the most important and commonly encountered species are velvetbean caterpillar (Anticarsia gemmatalis) and soybean looper (Chrysodeixis includens), others include soybean podworms (Helicoverpa zea in the U.S.; H. gelotopoeon and—now—H. armigera in Brazil and Argentina), sunflower looper (Rachiplusia nu), bean shoot moth (Crocidosema aporema), and armyworms of the genus Spodoptera. The last group contains several species that can affect soybean, and while they have traditionally been considered minor pests of the crop a number of species have increased in importance during the past few years.
I have been conducting soybean field trials in both the U.S. and South America for many years now and have had an opportunity to photograph most of the species known to occur on soybean in these regions. Identification of armyworm larvae can be rather difficult due to their similarity of appearance, lack of distinctive morphological differences (e.g. number of prolegs), and intraspecific variability in coloration. Conclusive identification is not always possible, especially with younger larvae; however, the different species do exhibit subtle characters that can usually allow for fairly reliable identification of large larvae. Considering the dearth of direct comparative resources—either in print or online—I offer this quick guide to the six armyworm species that I’ve encountered in soybean.
Spodoptera frugiperda (fall armyworm). This is not the most important armyworm pest of soybean, in contrast to its great importance in other crops such as corn and cotton. It is, however, the most widely distributed of the species, occurring in both the southern U.S. and throughout soybean growing areas of Brazil and Argentina. When problems do occur on soybean they are usually a result of larvae moving from grassy weeds to small soybean plants in late-planted or double-crop fields. Larvae can damage all stages of soybean, from seedlings (cutting them off at ground level) to later stages by feeding primarily on foliage and even pods. Larvae are somewhat variable in coloration but are distinctive among armyworms by virtue of the pinaculae (sclerotized tubercles) visible over the dorsum, each bearing a single stout seta. Four pinaculae are present on each of the abdominal segments, with those on the eighth abdominal segment forming a square, and larvae also exhibit a pronounced inverted, white, Y-shaped mark on the head.
Spodoptera exigua (beet armyworm). This species is better known as a pest of vegetables but will occasionally damage soybean in the southern U.S. In soybean larvae prefer to feed on foliage of seedling plants but will, if present during reproductive stages, also feed on blossoms and small pods. Late-instar larvae can be rather variable in appearance, but most tend to be green above and pinkish or yellowish below with a white stripe along the side. Larvae can be confused with Spodoptera eridania (southern armyworm) because of a dark spot that might be present on the side, but in southern armyworm the spot is on the first abdominal segment while in beet armyworm (when present) it is on the mesothorax.
Spodoptera ornithogalli (yellow-striped armyworm). This species is widely distributed throughout North and South America, but its status as an occasional pest of soybean is limited practically to the southeastern U.S. It is often encountered in soybean in low numbers but can reach pest status in double-crop fields with small plants that have been planted after wheat (similar to fall armyworm). Compared to other species in the genus the larvae are rather uniform in appearance, exhibiting paired, black, triangular spots along the back of each abdominal segment with thin to prominent yellow stripes running lengthwise adjacent to and not interrupted by the spots. Larvae oftentimes have an almost black velvety appearance with distinctly contrasting bright yellow stripes.
Spodoptera eridania (southern armyworm). This species is, like fall armyworm, widely distributed from the southern U.S. through Brazil and Argentina. In the U.S. it occurs only sporadically on soybean, usually causing “hot spots” of damage by groups of many larvae hatching from a single egg mass and skeletonizing the nearby foliage before dispersing as they grow larger. In Brazil and Argentina this species has emerged during recent years as one of the most important armyworm pests of soybean, especially in regions where cotton is also grown. Larvae can be somewhat variable in appearance and, in South America, can be easily confused with those of the black armyworm (S. cosmioides), both of which often exhibit prominent black markings on first and eighth abdominal segments and a subspiracular light-colored line along the length of the thorax and abdomen. Southern armyworm, however, rarely exhibits an additional black marking on top of the mesothoracic segment. Additionally, when the subspiracular line is present it is interrupted by the black marking on the first abdominal segment and is less distinct in front of the spot than behind, and if the line is not present then the black spots on top of the first abdominal segment are larger than those on top of the eighth abdominal segment.
Spodoptera cosmioides (black armyworm). No accepted English common name exists for this strictly South American species that was previously considered a synonym of the North and Central American species Spodoptera latifascia. In Brazil it has been referred to by such names as “lagarta preta” (black caterpillar) and “lagarta da vagem” (pod caterpillar). The latter name has also been applied to other soybean pests, including southern armyworm, so to me “black armyworm” seems the most appropriate English name to adopt. Like southern armyworm, this species is a sometimes pest of cotton and in recent years has become increasingly important in soybean throughout Brazil and northern Argentina. Larvae often resemble and can be easily confused with those of southern armyworm; however, there is almost always a dark spot on top of the mesothoracic segment that is lacking in southern armyworm. Additionally, the light-colored subspiracular line, when present, is not interrupted by the black spot on the first abdominal segment and is equally distinct in front of and behind the spot. When the line is not present the black spots on top of the first abdominal segment are smaller than than those on top of the eighth abdominal segment.
Spodoptera albula (unbarred or gray-streaked armyworm). While known to occur in extreme southern U.S., this species has been cited as a pest of soybean only in Brazil, although its importance has not matched that of southern or black armyworm. Like most armyworms it is polyphagous, but this species seems to prefer amaranth (Amaranthus spp.). Larvae of this species can be distinguished from other South American armyworms that feed on soybean by the trapezoidal black marking on the mesothorax (usually semicircular to slightly trapezoidal in black armyworm), the black marking on the first abdominal segment not larger than that on the sixth abdominal segment, both of which are smaller than those on the seventh and eight abdominal segments, the white-only rather than white and orange dorsolateral stripe, and the triangular black markings on the abdominal segments each with a small white spot in the middle or at the apex of the marking.
Copyright © Ted C. MacRae 2013