Soybean Aphids in Iowa—2007 by Marlin E. Rice, Matt O’Neal, and Palle Pedersen Introduction Origin of Soybean Aphid The soybean aphid (Aphis glycines) is a The soybean aphid is native to eastern Asia, including China, Indonesia, and major pest for soybean growers in Iowa. Japan, where it is an infrequent pest Since the arrival of this Asian species to of soybeans. It was first detected in Iowa in 2000, significant outbreaks have North America in Wisconsin in July occurred, most noticeably in 2003 when 2000 and now occurs throughout aphid populations exceeded 3,000 aphids all Midwestern soybean production states. It is not known how this per plant in many fields. During this insect entered the United States, but year, Iowa soybean yields averaged historical records of other aphid in- 32 bushels per acre, a 16 bushel per acre terceptions by the U.S. Department of (or a 32%) reduction from 2002. This Agriculture suggest that the soybean yield reduction was partly due to the aphid most likely arrived from Asia, either carried by an international soybean aphid coupled with drought airline passenger or associated with conditions. Approximately 3 million horticultural cargo. acres in Iowa were sprayed with insecticides to reduce these damaging Distribution in Iowa populations, and a survey of 2,400 Iowa Since its discovery in northeastern Soybean aphids will colonize the upper stem and developing leaves on soybeans. farmers indicated that yield losses Iowa in August 2000, the soybean (Marlin E. Rice) aphid quickly spread across Iowa. reached 57.7 million bushels of soybeans. Twelve months later, the aphid However, in 2004, very low soybean was detected in western Iowa in aphid populations occurred throughout Woodbury County, and within two Iowa, with only 50,000 acres treated for years, the aphid had been found in soybean aphids. soybeans in every Iowa county. Description of Soybean Aphid Despite this year-to-year variation, The soybean aphid is the only the soybean aphid remains a threat to aphid in North America that will cause economic damage to soybeans. develop large colonies on soybeans. This publication reviews what is Therefore, large clusters of aphids currently known about the biology found on soybeans must be soybean aphids. There are both wingless and Winged and wingless soybean aphids on a of the soybean aphid and suggests soybean leaflet. (David Voegtlin) winged forms. Wingless soybean management strategies. aphid adults are about 1/16 inch in SP 247 Revised July 2007 length, pale yellow or green, and The soybean aphid have dark-tipped cornicles (tail may have 15 to 18 pipes) near the end of the abdomen. generations on soybean The winged form has a shiny black plants. Soybean aphid head and thorax with a dark green reproduction is affected abdomen and black cornicles. Aphids by temperature. The feed through piercing-sucking optimum temperatures mouthparts. for reproduction and longevity are 72 to 77 °F with the relative humidity below Soybean aphids clustering on a soybean stem. (Marlin E. Rice) 78 percent. Under optimum conditions maintained in a laboratory environment, soybean aphids have the potential to double their populations within 2 to 3 days. When temperatures exceed 81 °F, the Winged soybean aphid. (Marlin E. Rice) developmental time is lengthened. Biology and Seasonal Cycle However, in soybean A large colony of soybean aphids often consists of white, fields, soybean aphids shed skins and brownish carcasses killed by fungal The seasonal cycle of soybean pathogens. (Marlin E. Rice) experience less than aphids is complex. Eggs are laid on ideal conditions. buckthorn in the fall, overwinter Estimates of soybean aphid population growth in Iowa suggest that doubling there, and hatch in the spring, giving time is on the order of 9 to 13 days. A temperature-based forecast is available rise to wingless females. These on the World Wide Web (see Additional Information on page 16); note that wingless females on buckthorn this forecast is derived from a mathematical model based on data of soybean reproduce without mating aphid growth from laboratory-based conditions. (asexually), and the young develop into winged females that migrate in In late summer, the wingless females search of soybean or possibly other produce young aphids that develop host plants. There may be up to four into both winged females and males. generations on buckthorn in the These winged aphids migrate back spring. Females on soybean reproduce to buckthorn, where they reproduce without mating and produce wingless sexually. These mated females daughters that continue the cycle. subsequently lay eggs, beginning During the summer, winged aphids a new seasonal cycle that passes may develop during any generation through the winter. Soybean aphids will lay eggs near bud on soybean, which puts much of Iowa scales on limbs of buckthorn. soybean at risk because the aphids (David Voegtlin) are easily carried by winds to areas even where the aphid may not have overwintered locally. 2 Soybean Aphids in Iowa—2007 The first detection of soybean aphids Soybean is a secondary host because (T. repens), white sweet clover in Iowa has typically occurred near the aphids do not reproduce sexually (Melilotus alba), and yellow sweet Decorah in northeastern Iowa on on this plant. Additional secondary clover (Melilotus officinalis) can seedling soybean plants (V1–V2 hosts include crimson clover support low levels of reproduction, stage) during early to mid-June. But (Trifolium incarnatum) and red clover but they are extremely poor hosts in 2006, the first soybean aphids were (T. pretense). These are excellent hosts for the soybean aphid. Furthermore, found on May 31 near Ames. for soybean aphids and will support horse nettle has been observed as a high levels of aphid reproduction. summer host for soybean aphids. Host Plants To a lesser extent, berseem clover In summary, these observations (T. alexandrinum) and kura clover suggest that these plants may serve The primary host is a small woody (T. ambiguum) will support aphid as a “bridge” for soybean aphids to tree called buckthorn (Rhamnus reproduction, while white clover persist after leaving buckthorn but cathartica and Rhamnus alnifolia). before soybeans are available. Soybean aphids prefer seedling or sapling trees on which to lay Injury Symptoms in Soybean their eggs in the fall. Eliminating buckthorn might seem to be a logical In low numbers, soybean approach to reducing soybean aphid aphids can feed on soybeans populations, but this is impractical. and cause little or no Buckthorn grows widely across Iowa damage. Large populations in wooded areas and river bottoms can result in visual evidence and has been planted in shelterbelts of damage. Honeydew, a as windbreaks. sticky and shiny liquid excreted by the aphids as a by-product from ingesting large amounts of plant juices, accumulates on the top surface of leaves. Excessive honeydew Soybean aphids excrete large amounts of sticky honeydew that falls on leaves below them. Sooty mold permits the growth of sooty grows on the honeydew and turns the leaves black. mold, turning the leaves (Marlin E. Rice) dark and interfering with photosynthesis in the plant. Heavily plant. Feeding also competes with the infested plants may be stunted. Plants soybean plant for nutrients, which that become stunted during the early reduces the number of soybeans per Soybean aphids will lay eggs on buckthorn reproductive growth stages of soybean pod and, less frequently, the size of during the fall. The eggs will overwinter may have reduced pod set and seed soybeans. Therefore, protecting plants here. (Marlin E. Rice) counts, resulting in lower yields. during the flowering stages (R1–R2) and green-bean pod stages (R5–R6) Feeding by soybean aphids causes helps protect soybean yield. These flowers and small pods to abort, soybean stages typically occur from reducing the number of pods per early July into early August in Iowa. Soybean Aphids in Iowa—2007 3 Transmission of Viruses The soybean aphid can transmit several viruses, including alfalfa mosaic virus, bean yellow mosaic virus, and soybean mosaic virus. The soybean aphid is an efficient transmitter of soybean mosaic virus, requiring only 5 to 30 minutes of feeding time for efficient transmission. Soybean mosaic virus is of primary concern in Iowa because it can cause significant yield loss. No estimates are available of how much yield loss is due to soybean mosaic virus. This virus may be more important when it occurs in plants that also are infected with bean pod mottle virus that is transmitted by Soybean mosaic virus is transmitted by the soybean the bean leaf beetle. Plant-expressed symptoms of these two viruses aphid. Although a diagnosis cannot be made based are similar and are not easily separated visually from each other in on visual symptoms alone, the impact of the virus can the field. produce injury seen in the picture above. Yield loss due to the virus has not been fully studied in Iowa and management plans for preventing the transmission of the virus have not been developed. However, managing the direct feeding damage of the aphid to soybeans can prevent yield losses. Natural Enemies of Soybean Aphids The soybean aphid is attacked by The insidious flower bug Orius( as a mummy. In North America, a three types of natural enemies— insidiosus) is a small predatory insect limited number of wasps will attack predators, parasitoids, and pathogens. that can feed on 19 aphids per day. the soybean aphid in this way. In Asia, Below is a review of these as they cur- Although this is less than what lady there are several species of wasp that rently occur in Iowa soybean fields. beetles eat, Orius are present before parasitize soybean aphids and may soybean aphids arrive and thus are be responsible for the aphids’ limited In order of importance, predators are thought to help delay if not prevent pest status in that part of the world.