Aphis glycines (soybean)
Authors: Kelley Tilmon, Erin Hodgson, Matt O'Neal, David Ragsdale
The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a significant insect pest of soybean in the north-central region of the U.S., causing estimated yield losses of at least $2.4 billion annually if left untreated.
The soybean aphid is native to eastern Asia, where soybean was first domesticated. Soybean aphid was first detected in the U.S. in 2000. The first specimens were confirmed on soybean, Glycine max, in Wisconsin, and quickly spread within four years of its discovery across 22 states and three provinces of Canada. The soybean aphid is a pest predominantly in middle to high latitudes in the Midwester U.S. and the provinces of Ontario and Québec, Canada. It is seldom found south of Kansas, Missouri, Kentucky, or Virginia. In southern states it can sometimes be confused with cotton aphid (Aphis gossypii), which can occur on soybean in that region.
Wingless soybean aphids are pear-shaped, 1/16th inch (1.5 mm) long, and range from pale yellow to lime green in color (though on late-season soybeans, some aphids may be much smaller and paler in appearance). Adults have have dark-tiped cornicles (tailpipes) at the end of the abdomen. Winged soybean aphids have a dark thorax and cornicles, and transparent wings that extend well past the abdomen.
Life Cycle and Seasonal History
The life cycle of soybean aphid is heteroecious (i.e. alternates between summer and winter hosts) and holocyclic (i.e., alternates between sexual reproduction and asexual reproduction).
On soybean, which is the summer host plant, the aphids are all female and reproduce asexually by giving live birth to all-female offspring that are themselves only days away from being able to reproduce. This feature gives soybean aphids the ability to increase very rapidly when conditions are favorable. The optimal developmental temperature is 82°F (27.8°C). The upper developmental temperature is 94.8°F (34.9°C), and the lower developmental temperature is 47.5°F (8.6°C). Under ideal conditions (e.g., 77-86°F/25-30°C), soybean aphids are highly reproductive and can double populations in as little as 1.5-2 days. More typically populations can double every 6-7 days on average, have high per-capita reproduction and have about 15 asexual generations on soybean, although 18-20 generations have been recorded in China. Density on soybean can be quite high, sometimes reaching thousands per plant.
Following mid-summer population increases during soybean flowering (R1-R2),many of the wingless aphids will produce winged offspring in late summer which can colonize other areas within-field or can migrate potentially great distances. Factors that contribute to aphids producing migrants include plant nutrition, photoperiod and crowding, temperature, plant phenology, and the presence of aphids natural enemies.
At the end of the soybean growing season during the early autumn, the aphids migrate to their primary host plant, buckthorn (Rhamnus spp.), a shrub common in shelterbelts and woods in northern states. Buckthorn is a critical part of the soybean aphid life cycle – without this plant, they cannot spend the winter in a given area. Common buckthorn, Rhamnus cathartica, is most widely infested, though it may also overwinter on R. alnifolia. Common buckthorn is itself an invasive species and is widespread across North America, particularly north of 41°N latitude where plant densities of greater than 1,000/acre are known to occur. The fall migration of soybean aphids to buckthorn is regulated by photoperiod and temperature. Winged females (gynoparae) leave soybean in search of buckthorn, where they feed and deposit a cohort of wingless females (oviparae). Winged males from soybean seek the oviparae, where mating, oviposition and overwintering (egg stage) occurs.
The overwintering egg is cold hardy and can survive temperatures down to -29°F. Eggs hatch during spring and several generations develop on buckthorn. As soybeans germinate, colonies on buckthorn produce winged founder females, which colonize soybeans in the early vegetative growth stage (V1-V5) by late spring/early summer.
Plant Injury and Damage
Soybean aphids have piercing-sucking mouthparts that are used to feed on phloem sap. Heavily infested plants are stunted and may be covered with dark sooty mold growing on the sugary excretions (“honeydew”) that aphids produce. Heavy infestations can result in yellow and wrinkled leaves, stunted plants and aborted pods. At moderate infestation levels (i.e., <50 aphids per leaflet), soybean aphid can significantly reduce gas exchange and negatively affect photosynthetic rates. In general, populations tend to perform better in fields with potassium-deficient soils. In outbreak years soybean fields may be sprayed for aphids up to three times, usually with broad-spectrum insecticides.
Soybean aphid, like many aphids, can transmit plant viruses. It has been shown to transmit both Soybean mosaic virus and Alfalfa mosaic virus to soybean. It may also vector diseases to other temporally-visited crops, such as Cucumber mosaic virus in snap bean, Phaseolus vulgaris, and Potato virus Y in potato, Solanum tuberosum. Soybean aphid is able to transmit several other viruses, but the economical importance as a vector in North American soybean and other crops is still to be determined.
Scouting and Threshold
Economic injury levels and economic thresholds have been calculated based on 19 yield-loss experiments conducted over a three-year period in six states (IA, MI, MN, NE, ND, WI). These studies were conducted under field conditions which incorporated various naturally-occurring factors such as weather and the impact of natural enemies. During the early to mid reproductive stages (R1-R5) the threshold for treatment is when populations exceed 250 aphids per plant with 80% of the plants infested. This economic threshold is calculated to give lead-time to line up treatment before the economic injury level (c. 674 aphids/plant) is reached. Application of a foliar insecticide is recommended with three to seven days after populations reach the economic threshold. Once soybeans reached the R6 growth stage (full seed, when a pod on one of the four top nodes has green seeds that fill the pod to capacity), research has not shown a reliable yield gain from insecticide treatment. Awareness and use of these recommendations is common for most growers (70%) throughout the north central region of the US, and this approach has been shown to be more cost effective than a preventative approach of applying insecticide based on the growth stage of the plant irrespective of aphid population density.
Based on these and similar studies, scouts are advised to check fields weekly from mid June through late August, the period when aphids colonize soybean fields. When scouting for soybean aphids, walk a broad U or X pattern through the field and examine 20 to 30 plants total, spread out over the field, counting the number of aphids per plant and basing management decisions on the threshold guidelines described above. Aphids can occur in “hot spots” but treatment decisions should be based on a broad sample of randomly-selected plants. Producers who are near threshold should consider checking the field again before treatment (3-4 days after the initial treatment decision is made). If aphid numbers have decreased, or are still near the economic threshold and have not increased noticeably, or if many natural enemies such as ladybeetles are present, producers may wish to delay treatment, as populations can sometimes decline naturally before reaching damaging levels. Sampling protocols such as “speed scouting” have been developed to aid in the rapid assessment of soybean aphid populations relative to the established economic threshold, and can reduce sampling time appreciably. An animated video clips explaining the details of speed scouting for soybean aphid can be seen below.
Host Plant Resistance
Through intense screenings, host-plant resistance in the form of both antibiosis (reduced survival/fecundity) and antixenosis (non-attractive/repellency of establishment) to the soybean aphid has been found. Molecular mapping is ongoing, but at least four genes have been identified: Rag1, Rag2, Rag3 and Rag4 . Rag1 (an abbreviation for Resistance to Aphis glycines), is a single-gene source of antibiosis developed at the University of Illinois. In field trials it significantly reduces aphid populations compared to susceptible controls. Rag1 soybean lines first became commercially available in the US on a limited basis in 2009; varieties containing additional resistance genes are likely to follow. However, it should be noted that Rag1-containing soybeans are not aphid-free, and economically relevant populations can sometimes occur. Work to calculate economic injury levels and economic thresholds for Rag1 soybeans is currently underway.
Host-plant resistance as a management strategy is complicated by the appearance of insect populations that overcome host-plant resistance (often termed biotypes). Biotypes have already been identified which can overcome Rag1 and Rag2 resistance, and work in this area continues.
Field studies of soybean reveal a diverse community of natural enemies, which help suppress soybean aphid colonization and population growth. These natural enemies include ladybeetles, lacewings, minute pirate bugs, and entomophagous (insect-killing) fungi. The community of insect predators within soybean has apparently been altered by the arrival of the soybean aphid, and now includes more species that focus on aphids. Included in this community is Harmonia axyridis, an Asian ladybeetle which contributes to biological control of A. glycines in its native range, and is one of the dominant soybean aphid predators in the US.
Natural enemies contribute significantly to background control of the soybean aphid. Several field studies have demonstrated the contributions of existing communites of natural enemies to soybean aphid suppression. In the absence of predation, soybean aphid population growth is significantly faster (2–7 times). Prophylactic application of broad-spectrum insecticides or fungicides has the potential to ultimately exacerbate aphid pressure or cause secondary outbreak of other pests such as spider mites by removing the natural enemies that often keep pest populations in check.
Though surveys of natural enemies in North America reveal a rich community of natural enemies, parasitoids – a type of parasitic insects that can specialized on particular prey species – have been notably lacking from this community. Parasitoids that attack soybean aphids are an important part of the community of predators that suppress soybean aphid in China; however, it is not uncommon for a newly introducted pest insect to arrive without the specialist natural enemies of its native range. A classical biological control program, whose goal is to introduce parasitoid species from the native range that specialize on soybean aphid, has been underway. One particular species, Binodoxys communis (Hymenoptera: Braconidae) has undergone intense scrutiny for host specificity and ecological interactions with soybean and other aphids. Based on these studies, B. communis received federal approval for release in the US in 2007. Releases of B. communis in the Midwest are on-going, though their impact on A. glycines has not been measured, and is unlikely until populations of the parasitoid have established within their new range. Additional candidate parasitoids are in quarantine and maybe released in the near future.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com