Bollworms rising!

One of the most pernicious pests that U.S. farmers have battled is the larval stage of Helicoverpa zea (Lepidoptera: Noctuidae). This insect is destructive enough to have earned not just one official common name, but four (corn earworm, cotton bollworm, soybean podworm, and tomato fruitworm)—one for each of the crops in which it has attained major pest status. It isn’t only North American farmers, however, that must deal with this pest, but South American farmers as well. For many decades, corn and cotton have been its most important hosts in North America, but in recent years its importance has increased steadily in soybean as well, particularly across the mid-south. In South America, however, it seems satisfied—curiously—to confine its attacks to corn. Lest you think that South American farmers are getting off easy, there are other species of Helicoverpa in South America that are causing problems of their own. Perhaps the most troubling one is H. armigera, the Old World bollworm¹—a sister species to H. zea (Goldsmith & Marec 2010) native to Africa, Asia, and Australia and just as polyphagous as H. zea that was recently found infesting corn, cotton, soybean, and other crops in several areas of Brazil.

¹ Interestingly, in the Old World this species is called the “American bollworm,” despite the fact that it did not come from the Americas at all. I guess neither hemisphere wants to take the blame for this species.

Helicoverpa gelotopeon (South American bollworm) | Buenos Aires Province, Argentina

Helicoverpa gelotopeon (South American bollworm) | Buenos Aires Province, Argentina

While we wait to see what impact H. armigera ends up having in South America, another species of the genus is quietly rising from the ranks of secondary to primary pest further south on the continent. For many years, Helicoverpa gelotopeon (or South American bollworm) has been a sometimes pest of cotton and other crops in Argentina, Chile and Uruguay (Evangelina et al. 2012), but in a situation that mirrors the rise of H. zea on soybean in North America, the incidence of H. gelotopeon has grown during the past few years in the more southern soybean growing areas of South America as well. Like its North American counterpart, this insect causes not only indirect damage by feeding on the foliage of the plant during vegetative stages of growth (reducing photosynthetic capacity of the plant), but also direct damage by feeding on the developing pods during reproductive stages of growth. Predictably (and regrettably), farmers have responded by increasing applications of organophosphate insecticides, but the efficacy of these products—despite their relatively high toxicity—has often been inadequate to prevent yield losses. As a result, other management techniques and technologies will be required to keep this insect from having a major impact on soybean production in the temperate regions of Argentina.

Young larvae feed on foliage (note the very small caterpillar in the lower center area of the leaf).

Young larvae feed on foliage (note the very small caterpillar in lower left area of the leaf).

A mid-instar larvae feeding on soybean foliage.

A mid-instar larvae feeds on soybean foliage and exhibits the black pinacula characteristic of the subfamily.

Larger larvae feed on developing pods, breaching the wall of the pod to consume the seeds within.

Larger larvae feed on developing pods, breaching the wall of the pod to consume the seeds within.

Adults are slightly smaller than H. zea and a little darker with somewhat bolder markings.

The photographs in this post may well be the best—and perhaps even the only ones—available of this species. A Google image search turned up nothing, and have I been unable to find any literature with photographs of either the adults or the larvae and their damage. If you are aware of any please leave a comment with the citation.

REFERENCES:

Goldsmith, M. R. & Marec, F. 2010. Molecular Biology and Genetics of the Lepidoptera. CRC Press, Boca Raton, Florida, 368 pp.

Evangelina, P., F. Crepo & J. C. Gamundi. 2012. Evaluación del daño simulado de “oruga bolillera” Helicoverpa gelotopoeon (Dyar) en estados vegetativos del cultivo de soja. Unpublished report, Instituto Nacional de Tecnología Agropecuaria (INTA), 6 pp.

Copyright © Ted C. MacRae 2013

The “little soybean weevil”

Lepidopteran caterpillars are without question the most important pests affecting soybean in South America, while stink bugs run a close second in terms of economic impact and as the targets of insecticide applications. There are, however, a number of weevil species (order Coleoptera, family Curculionidae) whose incidence has increased during the past decade or so as the area planted to soybean continues its decade’s long expansion on the continent. The most important of these is Sternechus subsignatus, a  relatively large (and rather attractive black-and-yellow) species that was first detected in southern Brazil in the 1970s. It has since spread to northern Brazil and in recent years has also begun affecting soybean in Salta and Tucumán Provinces of northern Argentina (sometimes considered a distinct species, S. pinguis). Known locally as “picudo grande” (big weevil), adults clip the petiole of leaves and girdle the stems, leading to stand loss. One adult is capable of killing multiple plants, so that even light infestations can result in severe damage.

IMG_2439_enh_1080x720

Promecops carinicollis | Tucumán Province, Argentina

I’ve not yet seen “big weevils” for myself, but there are at least two other species that are showing up in soybean fields, particularly in Salta and Tucumán Provinces. During my recent visit to Argentina I happened upon a soybean field in northern Tucumán infested with one of them, Promecops carinicollis, a few photos of which I show here. This species is much smaller than S. subsignatus and is, thus, called “picudo chico” (little weevil)—certainly an appropriate name for the 3- to 4-mm long adults. While the integument is black, the body is densely covered with flat scales that form irregular white blotches on the elytra and otherwise give the beetle a mottled-brown appearance.

IMG_2436_enh_1080x720

Damage consists of adult feeding around the leaflet margins, giving them a scalloped appearance.

Like S. subsignatus, it is the adults that cause damage to the plants, although instead of the stems and petioles their feeding seems to be confined to the margins of the leaflets. This gives the leaflets a “scalloped edge” appearance that is quite distinctive and unlike the leaf damage caused by other leaf-feeding insects of soybean. The feeding causes a general reduction of the leaf surface area of the plant, which reduces the plant’s capacity to photosynthesize. However, as soybean has a rather high capacity to compensate for foliage loss by growing new foliage, especially during the earlier vegetative stages of growth, it would take rather high pressure by these weevils to cause enough damage to result in yield loss. It may be one of those soybean pests for which insecticide applications are made much more often than is warranted. The most important impact of this insect probably occurs just after seedling emergence, during which time feeding on the cotyledons and first leaves can weaken seedlings enough to cause stand loss.

Promecops carinicollis | Tucumán Province, Argentina

Beginning the process of making more Promecops carinicollis.

Copyright © Ted C. MacRae 2013

Life at 8X: MPMI Cover


The January 2013 issue of Molecular Plant-Microbe Interactions (volume 26, number 1) is now online. Why do I mention this? You may recall the cover photos of the soybean aphid, Aphis glycines, from my post —one of a series of posts I’ve done featuring insects photographed at 8X life-size.

MPMI is a publication of The American Phytopathological Society, and I have Dr. Gustavo MacIntosh at Iowa State University to thank for the appearance of these photos on the cover of this Special Focus Issue. Dr. MacIntosh is Associate Professor of Biochemistry, Biophysics and Molecular Biology and studies hormone-based defense mechanisms in soybeans. In a paper appearing in this special issue, Dr. MacIntosh and co-author Matthew Studham published the results of a study that suggests soybean aphids are able to “short-circuit” soybean defense mechanisms, making it easier for other pests (e.g., soybean cyst nematode) to colonize infested plants as well. Their study revealed large differences in transcription profiles of soybean varieties with and without an endogenous resistance gene (Rag1) in response to aphid infestation and suggested that the aphids are able to circumvent the defense response in susceptible plants by triggering activation of abscissic acid (normally associated with abiotic stress responses) as a “decoy” strategy (Studham & MacIntosh 2013). Plants infested with aphids have been shown to also become more susceptible to soybean cyst nematode—even varieties with genetic resistance to nematodes (McCarville et al. 2012). Dr. MacIntosh saw my photos when I posted them here and asked permission to submit them as candidates for the cover of the MPMI issue in which his paper was to appear.

Dr. Macintosh hopes that his research will enable the development of soybean varieties that will be more resistant to aphids and other pests.

REFERENCE:

McCarville, M. T., M. O’Neal, G. L. Tylka, C. Kanobe & G. C. MacIntosh. 2012. A nematode, fungus, and aphid interact via a shared host plant: implications for soybean management. Entomologia Experimentalis et Applicata 143(1):55–66 [DOI: 10.1111/j.1570-7458.2012.01227.x].

Studham, M. E. & G. C. MacIntosh. 2013. Multiple Phytohormone Signals Control the Transcriptional Response to Soybean Aphid Infestation in Susceptible and Resistant Soybean Plants. Molecular Plant-Microbe Interactions 26(1):116–129 [DOI: 10.1094/MPMI-05-12-0124-FI].

Copyright © Ted C. MacRae 2012

Life at 8X—Bandedwinged Whitefly

Trialeurodes abutiloneus (bandedwinged whitefly) | Obion Co., Tennessee

The world of minute insects can seem strange and even bizarre when compared to our relatively giant perspective. To the unaided human eye, this bandedwinged whitefly (Trialeurodes abutiloneus), measuring only 1 mm in length, looks like nothing more than a fleck of dandruff. Through a Canon MP-E 65mm 1–5X macro lens with 68 mm of extension tube (resulting in 8X magnification), however, we see an almost moth-like insect with a decidedly adorable “face” negotiating the “trichome forest” of a soybean leaf under-side.

A more conventional 2X view of a whitefly infestation on the underside of a leaf

Whiteflies (order Hemiptera, family Aleyrodidae) are tiny insects (more related to aphids than true flies) that colonize a variety of host plants, often building to extraordinary numbers and densities while sucking juices from the leaves. The bandwinged whiteflies in these photos were seen in a soybean field in northwestern Tennessee this summer and can be easily identified as this species due to the transverse, zig-zag bands on the forewings (Malumphy et al. 2010). In the photo above numerous eggs can also be seen distributed over the leaf surface—a sign that this population is about to explode given the numbers of eggs present.

Zooming in to 8X allows the zig-zag wing pattern to be seen easily.

Whiteflies are an occasional pest of soybean in the U.S., but yield reduction has been documented only in the southeastern U.S. by another species, Bemisia tabaci (sweet potato whitefly). Whiteflies are also occasionally seen on soybeans in the Midwest by B. tabaci or yet another species, Trialeurodes vaporarium (greenhouse whitefly); however, yield impacts in this area are rare. Trialeurodes abutiloneus is occasionally reported from soybean, but this species is actually more commonly encountered on sweet potato and malvaceous crops such as cotton and hibiscus (Clower et al. 1973). There was a lot of cotton growing in the area of this soybean field, so perhaps this infestation was a result of spillover from that crop.

Piercing/sucking mouthparts are inserted into the leaf for feeding.

“Adorable” and “cute” are not words that I’ve ever associated with whiteflies, but these ultra-closeup photographs give them a personality that I’ve not seen before. For an even more astounding view of the face of a greenhouse whitefly, see this incredible 16X photograph by Huub de Waard. Taken with the same lens as these—though I suspect with a 2X converter rather than extension tubes, it shows an amazing level of sharpness compared to the admittedly soft photos in this post. The larger aperture used (f/6.3) may also be a better choice than the small f/13 aperture I used in an attempt to preserve as much depth of field as possible but with which diffraction is likely significantly greater. Stay tuned as I do some more testing…

A cute couple!

REFERENCE:

Clower, D. F. & C. M. Watve. 1973. The bandedwinged whitefly as a pest of cotton, pp. 90–91. Proceedings of Beltwide Cotton Production and Research Conference, 11–12 January, Phoenix, Arizona. Cotton Council of America, Memphis, TN.

Malumphy, C., A. MacLeod & D. Eyre. 2010. Banded-winged whitefly Trialeurodes abutiloneus. Plant Pest Factsheet, The Food and Environment Research Agency (Fera), 4 pp.

Copyright © Ted C. MacRae 2012

Inchworm, Inchworm, Oh So Small

Soybean looper (Chrysodeixis includens) | 3rd-instar larva

Okay, I know this is not a real “inchworm” (generally restricted to caterpillars in the family Geometridae), but this young larva of a soybean looper (Chrysodeixis includens, family Noctuidae) is just too cute to not have an equally cute name. I believe it is an early 3rd instar, based on its small size (~7mm in length), lightly colored head capsule, and distinct bristles around the head and on the body—1st instars have a black head capsule, while 2nd instars have a light brown head capsule, and in both the bristles on the body are smaller and not as distinct.

This larva hatched from an egg laid on soybean by a laboratory-reared adult.

Copyright © Ted C. MacRae 2012

Life at 8X—Guide to lepidopteran eggs on soybean

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.

Anticarsia gemmatalis—velvetbean caterpillar


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.

Chrysodeixis includens (= Pseudoplusia includens)—soybean looper


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.

Helicoverpa zea—soybean podworm

Helicoverpa zea eggs on soybean pod


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).

Heliothis virescens—tobacco budworm

Heliothis virescens eggs on soybean pod.

As with H. zea, H. virescens eggs laid on pods tend to be stuck to hairs and assume a spherical shape.

This H. virescens egg has apparently died—note the shriveling and uniform black coloration.


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.

Spodoptera frugiperda—fall armyworm

Individual eggs inside the mass are covered by abdominal setae and wing scales.


REFERENCES:

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