Agriculture and Environment Extension Agriculture and Natural Resources Publications 6-2004 Soybean Aphids in Iowa—2004 Marlin E. Rice Iowa State University, [email protected] Matthew E. O'Neal Iowa State University, [email protected] Palle Pedersen Iowa State University Follow this and additional works at: http://lib.dr.iastate.edu/extension_ag_pubs Part of the Agricultural Education Commons, and the Entomology Commons Recommended Citation Rice, Marlin E.; O'Neal, Matthew E.; and Pedersen, Palle, "Soybean Aphids in Iowa—2004" (2004). Agriculture and Environment Extension Publications. 86. http://lib.dr.iastate.edu/extension_ag_pubs/86 Iowa State University Extension and Outreach publications in the Iowa State University Digital Repository are made available for historical purposes only. Users are hereby notified that the content may be inaccurate, out of date, incomplete and/or may not meet the needs and requirements of the user. Users should make their own assessment of the information and whether it is suitable for their intended purpose. For current publications and information from Iowa State University Extension and Outreach, please visit http://www.extension.iastate.edu. Soybean Aphids in Iowa—2004 by Marlin E. Rice, Matt O’Neal, and Palle Pedersen THE SOYBEAN APHID (Aphis glycines) has become a major pest in Iowa. The largest infestation of soybean aphid occurred in 2003. Iowa soybean yields during 2003 averaged 32 bushels per acre. This was an average decrease of 16 bushels per acre (or a 32 percent reduc- tion) from the average yield of 48 bushels per acre in 2002. This was the lowest average yield since the floods of 1993. Delayed planting, flooding, and drought were partly responsible for these yield reductions, yet soybean aphids were one of the principal causes for these large Soybean aphids will colonize the upper yield losses. stem and newly developing leaves on soybeans. (Marlin E. Rice) he soybean aphid has been Origin of Soybean Aphid Distribution in Iowa found in Iowa soybean again The soybean aphid is native to Entomologists in Wisconsin reported Tin 2004. However, the extent eastern Asia, including China, finding widespread and large aphid of the potential problem is unknown. Indonesia, and Japan. It was first populations in southern Wisconsin This publication reviews what is detected in North America in soybean on July 20, 2000. By late known about the insect and makes Wisconsin in 2000. It is not known August the soybean aphid could be management suggestions for 2004. how this insect entered the Midwest, found in Iowa and by mid-September Included is a new economic thresh- but historical records of aphid it was detected or assumed to occur old based on a review of data from interceptions by the USDA suggest in all counties in the eastern half of field trials and university research. that the soybean aphid most likely the state (Fig. 1). Entomologists from 11 universities arrived in the United States from used this information to develop a Japan or China, either carried by an The following summer, soybean consensus recommendation for the international airline passenger or aphids were observed in western 2004 growing season. associated with horticultural cargo. Iowa in Woodbury County and within two years after entering the state, the soybean aphid had been detected in every Iowa county. SP 247 June 2004 Uninfested Presumed infested Infested Figure 1. Known distribution of soybean aphid in Iowa, September 8, 2000. Description of Soybean Aphid 1 Wingless soybean aphid adults are about ⁄16 inch in length, pale yellow or green, and have dark-tipped cornicles (tail pipes) near the end of the abdomen. Aphids feed through piercing-sucking mouthparts and have both wingless and winged forms. The winged form has a shiny black head and thorax with a dark green abdomen and black cornicles. The soybean aphid is the only aphid in North America that will reproduce on soybeans. There- fore, any small colony of aphids found on soybeans must be soybean aphids. Biology and Seasonal Cycle Winged and wingless soybean aphids on a soybean The seasonal cycle of soybean leaflet. (David Voegtlin) aphids is complex. Eggs are laid on buckthorn in the fall and overwinter there. The nymphs hatch in spring, giving rise to wingless females. There may be up to four generations Soybean aphids will lay eggs near on buckthorn in the spring. bud scales on limbs of buckthorn. These wingless females on (David Voegtlin) buckthorn reproduce without mating (asexually) and the young develop into winged females that migrate in search of soybean or possibly other host plants. Females on soybean reproduce without mating and produce wingless daughters that continue the cycle. During the summer, A large colony of soybean aphids often consists of white, winged aphids may develop during any generation on soybean, shed skins and brownish carcasses killed by fungal which places much of Iowa soybean at risk from pest invasion pathogens. (Marlin E. Rice) because the aphids are easily carried by winds to areas even where the aphid may not overwinter locally. – 2 – Soybean aphids reproduce faster in cooler environments. This may be a primary reason why aphids have caused more damage in northern Iowa than southern Iowa. The optimum temperatures for reproduction and longevity are 72–77°F with the relative humidity below 78 percent. During optimum 1 conditions, a soybean aphid population can double in size every 1 ⁄2 days. When temperatures exceed 81°F, the developmental time is lengthened. The soybean aphid may have as many as 15 to 18 generations annually. In late summer, the wingless females produce young that develop into both winged females and males. These winged aphids migrate back to buckthorn, Soybean aphids will lay eggs on buckthorn where they reproduce sexually. These mated females subsequently lay eggs, during the fall. The eggs will overwinter here. beginning a new (Marlin E. Rice) seasonal cycle that Host Plants passes through The primary host is buckthorn the winter. (Rhamnus cathartica and Rhamnus alnifolia). Soybean aphids seem to The first detection prefer seedling or sapling trees on of soybean aphids which to lay their eggs in the fall. in Iowa has typically Soybean is a secondary host because occurred near the aphids do not reproduce sexually Decorah in north- on this plant. Additional secondary east Iowa on hosts include crimson clover and seedling soybean Soybean aphids clustering on a soybean stem. (Marlin E. Rice) red clover. These are excellent hosts plants (V1–V2 stage). for soybean aphids and will support For the last four years, the first aphids on soybeans have been found high levels of aphid reproduction. on June 18 (2001), June 16 (2002), June 5 (2003), and June 8 (2004). To a lesser extent, berseem clover and kura clover will support aphid Injury Symptoms in Soybean reproduction, while white clover, Soybean aphids may distort soybean leaves and heavily infested plants may white sweet clover, and yellow sweet have yellow leaves. Honeydew, a sticky and shiny liquid excreted by the clover can support low levels of aphids as a by-product from ingesting large amounts of plant juices, reproduction, but they are extremely accumulates on the top surface of leaves. Excessive honeydew permits poor hosts of the soybean aphid. the growth of sooty mold, turning the leaves dark and interfering with photosynthesis in the plant. Heavily infested plants may be stunted. Plants that become stunted during the early reproductive growth stages of soybean may have reduced pod set and seed counts, resulting in lower yields. Feeding by large soybean aphid populations causes flowers and small pods to abort, reducing the number of pods per plant. Feeding also competes with the soybean plant for nutrients, which reduces the number and size of soybeans per pod. Therefore, protecting flowers (stages R1–R2) and developing pods (stages R3–R4) helps protect soybean yield. These soybean stages typically occur from mid-July into early August in Iowa. Soybean aphids excrete large amounts of sticky honey- dew that falls on leaves below them. Sooty mold grows on the honeydew and turns the leaves black. (Marlin E. Rice) – 3 – Transmission of Viruses Begin scouting for soybean aphids, The soybean aphid can transmit especially in northeastern Iowa, the several viruses including alfalfa first week of July. Check the upper mosaic virus, bean yellow mosaic two to three trifoliate leaves for virus, and soybean mosaic virus. aphids. Scout five locations per The soybean aphid is an efficient 20 acres. Field observations should transmitter of soybean mosaic virus, be made weekly. Aphids are most requiring from 5 to 30 minutes of likely to concentrate in the plant feeding time for efficient transmis- The multicolored Asian lady beetle is terminal. Look for ants or lady a major predator of soybean aphids. sion. Soybean mosaic virus is of (Marlin E. Rice) beetles on the soybean plant—they primary concern in Iowa because are good indicators of the presence it can cause significant yield loss, Fungal pathogens also reduce of aphids. Lady beetles feed on although this has not yet been soybean aphid populations. A fungal aphids, while ants tend the aphids documented in the state. This virus epidemic was observed in Wisconsin and “milk” them for honeydew. may be more important when it in 2000 and was believed partly occurs in plants that also are infected responsible for the decline in late- Estimate aphid population size with bean pod mottle virus that is season aphid densities. per plant. The best that can be transmitted by the bean leaf beetle. done is to count all the aphids on Plant-expressed symptoms of these Scouting Recommendations several leaves and plant terminals two viruses are similar and cannot Scouting methods for the soybean to establish what 100 or 250 aphids be separated visually from each other aphid are currently being investi- look like and then use this as in the field.