Life at 8X—Guide to lepidopteran eggs on soybean
September 3, 2012 5 Comments
Most of you are aware of my passion for beetles, but in reality that is just my evenings-and-weekends gig. By day, I am an agricultural entomologist conducting research on insect pests of soybean. I’m not sure how many latent soybean entomologists there may be among readers of this blog, but for this installment of “Life at 8X” I thought it would be interesting to feature eggs of several of the more important lepidopteran species that infest soybean in the U.S. Soybean is primarily a New World crop, and of the many lepidopteran species that attack soybean on these two continents, most belong to the great family Noctuidae (owlet moths). The species shown here include the most important species in North America, and in some cases South America as well.
See this post for details on photographic technique; however, note that most of the photos in this post that were shot at 8X have been cropped slightly (~10–15%) for composition (should I call this post “Life at 9X”?).
Anticarsia gemmatalis. Velvetbean caterpillar (“oruga de las leguminosas” in Argentina; “lagarta-da-soja” in Brazil) has long been the most important lepidopteran soybean pest throughout the New World. In North America its attacks are confined to the lower Mississippi River delta and southeastern Coastal Plain, but in South America nearly 100% of the soybean growing area is subject to attack. Eggs of this species are laid almost exclusively on leaf undersides throughout the canopy and are intermediate in size compared to the other species shown below (~7,000 eggs per gram). They are distinctive in their slightly flattened spherical shape and turn pinkish as they age and the developing larva takes form inside the egg.
Chrysodeixis includens (=Pseudoplusia includens). Soybean looper (“oruga medidora falsa” in Argentina; “lagarta falsa-medideira” in Brazil) was until recently primarily a North American pest with the same southern occurrence as velvetbean caterpillar. In recent years, however, it has gained importance in Brazil and northern Argentina as well, with its impact magnified by the capacity to develop resistance against most of the insecticides that have been used to control it. The egg of this species is quite small (~10,000 eggs per gram) and are are irregularly spherical with a somewhat translucent, crystalline appearance. Like velvetbean caterpillar, eggs of this species are laid almost exclusively on the leaf undersides, but the moths exhibit a clear preference for the middle or upper canopy depending upon plant growth stage.
Helicoverpa zea. Soybean podworm is better known in other crops as corn earworm, cotton bollworm, or tomato fruitworm (a testament to its polyphagous nature), and in South America the common names are even more diverse depending on both crop and country (“gusano bellotero,” “gusano cogollero del algodón,” “gusano elotero,” “isoca de la espiga en maíz,” or simply “bolillero” in Argentina; “lagarta-da-espiga-do-milho” or ”broca-grande-do-fruto in Brazil). While it has long been considered a secondary pest of soybean in North America, recent years have seen a marked increase in its incidence across the mid-south growing areas. Unlike the above two insects, larvae of this species feed not only on foliage but also directly on pods, typically breaching the pod wall and consuming the developing seeds inside. This method of feeding not only causes direct yield impacts but also affords some protection to larvae from insecticide applications.
Also unlike the first two insects, eggs of this species can be laid anywhere on the plant—leaves (upper or lower surface), petioles, stems, pods, and even flowers. The eggs are rather large compared to the other species shown here (~3,500 eggs per gram) and assume a distinctive barrel shape when laid on the leaf. The creamy-white coloration, often with a light brown ring below the apex, is also distinctive compared to the previous two species. Eggs laid on pods tend to be attached to trichomes (hairs) rather than the pod surface, in which case they take on an almost perfectly spherical shape.
Heliothis virescens. Like the previous species, tobacco budworm has only recently gained attention as a pest of soybean. This importance, however, seems to be confined to Brazil (where it is known as “lagarta-das-maçãs”), while in North America it is usually found in combination with H. zea at minor levels. This is bad news for South American farmers; like soybean looper, tobacco budworm has developed resistance to all the insecticides that have been used against it in significant quantities. The oviposition and feeding behaviors of this species are very similar to those of H. zea, with eggs again laid on all parts of the plant and being very similar in appearance to those of H. zea except their slightly smaller (approx. 5,000 eggs per gram). In practical terms, eggs and young larvae of H. virescens and H. zea can be reliably distinguished only through species-specific immunoassay (Greenstone 1995) or feeding disruption bioassay using a diagnostic concentration of Bacillus thuringiensis ( Bailey et al. 2001).
Spodoptera frugiperda. Fall armyworm is a minor pest of soybean that rarely reaches economically damaging levels. However, its incidence in South America (where it is called ”oruga militar tarde in Argentina and “lagarta-militar” in Brazil) has increased somewhat with the adoption of no-till cultivation of soybean. The species prefers grass hosts, but when these are knocked down by applications of post-emergence herbicides the larvae then move onto the soybean plants and continue feeding. Unlike any of the above species, eggs are laid in distinctive masses that are covered by abdominal setae and wing scales for protection. These eggs are also small (~8,500 eggs per gram), exhibit much finer and more numerous ridges than the above species, and are often colored orange, pink, or light green.
Bailey, W. D., C. Brownie, J. S. Bacheler, F. Gould, G. G. Kennedy, C. E. Sorenson & R. M. Roe. 2001. Species diagnosis and Bacillus thuringiensis resistance monitoring of Heliothis virescens and Helicoverpa zea (Lepidoptera: Noctuidae) field strains from the southern United States using feeding disruption bioassays. Journal of Economic Entomology 94 (1):76–85.
Greenstone, M. H. 1995. Bollworm or budworm? Squashblot immunoassay distinguishes eggs of Helicoverpa zea and Heliothis virescens (Lepidoptera: Noctuidae). Journal of Economic Entomology 88(2):213–218.
Copyright © Ted C. MacRae 2012