Soybean Aphid Population Growth Research Articles

A field-based assessment of the parasitoid Aphelinus certus as a biological control agent of soybean aphid in North America

Damaging outbreaks of soybean aphid continue to occur in North America despite the valuable biological control services provided by resident natural enemies. The adventive parasitoid Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae) has recently established in North America and has been hypothesized to be a key component of the resident natural enemy community, but there have been few studies evaluating the efficacy of this parasitoid in suppressing soybean aphid populations. We used an exclusion cage study to quantify the effect of A. certus on soybean aphid population growth at four sites spanning western and east-central Minnesota from 2017 to 2019. There were minimal differences in soybean aphid population growth between experimental treatments that excluded natural enemies and control treatments, suggesting that parasitism of soybean aphid by A. certus did not have a strong impact on soybean aphid population growth during this study. Because, for example, A. certus larvae can reduce host reproduction prior to mortality (resulting in underestimates of effects in short-term studies), our results reflect the challenges of using exclusion cages to assess the effects of individual natural enemy species, especially those with complex life cycles.

The Effect of Temperature and Host Plant Resistance on Population Growth of the Soybean Aphid Biotype 1 (Hemiptera: Aphididae).

A laboratory experiment was conducted to evaluate direct and indirect effects of temperature on demographic traits and population growth of biotype 1 of the soybean aphid, Aphis glycines Matsumura. Our objectives were to better understand how temperature influences the expression of host plant resistance, quantify the individual and interactive effects of plant resistance and temperature on soybean aphid population growth, and generate thermal constants for predicting temperature-dependent development on both susceptible and resistant soybeans. To assess indirect (plant-mediated) effects, soybean aphids were reared under a range of temperatures (15-30 °C) on soybean seedlings from a line expressing a Rag1 gene for resistance, and life history traits were quantified and compared to those obtained for soybean aphids on a susceptible soybean line. Direct effects of temperature were obtained by comparing relative differences in the magnitude of life-history traits among temperatures on susceptible soybeans. We predicted that temperature and host plant resistance would have a combined, but asymmetrical, effect on soybean aphid fitness and population growth. Results showed that temperature and plant resistance influenced preimaginal development and survival, progeny produced, and adult longevity. There also appeared to be a complex interaction between temperature and plant resistance for survival and developmental rate. Evidence suggested that the level of plant resistance increased at higher, but not lower, temperature. Soybean aphids required about the same number of degree-days to develop on resistant and susceptible plants. Our results will be useful for making predictions of soybean aphid population growth on resistant plants under different seasonal temperatures.

Soybean Aphid Populations on Aphid Resistant and Susceptible Soybean Lines Growing under Potassium Deficiency

The soybean aphid is an important pest of soybean in North America. A positive link between soybean aphid population growth and potassium (K) deficiency, perhaps related to a higher free amino acid concentration in deficient plants, is generally accepted. At least one aphid resistance trait, Rag1, has a lower concentration of several free amino acids, and thus reduced nutritional quality. Since K appears to influence nutritional quality as well, the objective of this study was to determine if antibiosis resistance of two soybean aphid resistance traits was maintained under K deficiency. Over three seasons, aphid populations on resistant (LD05‐16060, Rag1; E06902, rag1b/rag3) and susceptible (SD01‐76R) lines were compared in field cages under K‐deficient and sufficient conditions. When lines were caged separately, the number of soybean aphids per plant on the susceptible line was significantly greater on deficient compared to fertilized plants. However, the Rag1 and rag1b/rag3 traits maintained resistance under the same conditions. When lines were caged together, and winged aphids from the susceptible line continuously deposited nymphs on the resistant lines, aphid populations on the resistant lines still remained far below the population on the susceptible line, and below the economic injury level of 674 soybean aphids per plant, under both K‐fertilized and K‐deficient conditions. Thus, resistance to soybean aphid was maintained in two soybean lines with different resistance genes, even when K was deficient.

Soybean Aphid (Aphididae: Hemiptera) Population Growth as Affected by Host Plant Resistance and an Insecticidal Seed Treatment

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) is a significant soybean pest in the north central United States. Insecticidal seed treatments and host plant resistance are two commercially available management tools. Here we investigate the efficacy of both management tools throughout the season. Soybean lines containing the soybean aphid resistance genes Rag1, Rag2, or both Rag1 + Rag2 were compared with a near-isogenic aphid-susceptible line. Each line was grown in field plots both with and without thiamethoxam applied to the seed. Individual plants from each plot were caged and infested with soybean aphids to measure the efficacy and potential interaction of aphid resistance and thiamethoxam. Aphid population growth rate was measured for each caged plant for 9-12 d after infestation. New cages were established each week from 34 d after planting (dap) to 92 dap to track seasonal variations in efficacy. Thiamethoxam reduced population growth only at the 42 dap time point and only for the susceptible, Rag1, and Rag2 lines. The lack of an effect of thiamethoxam on the Rag1 + Rag2 line was likely because of already high mortality from two resistance genes. Aphid resistance alone reduced population growth compared with the susceptible line at least till 55 dap for single-gene resistance and 63 dap for the two genes combined. Aphid resistance provided suppression of soybean aphid population growth throughout the season unlike the insecticidal seed treatment.

English

  Edamame are soybeans harvested at a physiologically immature (R6) stage as a specialty food item for fresh and processed (frozen) markets. The soybean aphid, Aphis glycines Matsumura, is a newly introduced insect pest of soybeans in North America. This field study was to provide baseline information on the impact of A. glycines on the edamame soybean. This study determined the population growth rate of A. glycines on two edamame soybean cultivars, ‘Butterbeans’ and ‘Envy,’ at two planting dates during 2004 and 2005 in Nebraska. Aphid population growth was significantly higher on 'Butterbeans' than on 'Envy' for the first planting date in both 2004 and 2005 seasons, whereas the second planting date only had significant higher soybean infestations on 'Butterbeans' during the 2005 season. The infestation difference was the greatest on plants at the late reproductive growth stages, R5 and R6, in 2005. Aphid’s infestation at 'Butterbeans' growth stages in 2005 was significantly different for the first and second planting dates. The aphids were higher on plants at the R6 and R5 growth stages than the other stages for first and second planting dates, respectively. However, ‘Envy’ growth stages in 2005 did not exhibit significantly different average aphid infestation during the first and second planting dates. This study revealed that soybean aphid population growth on edamame soybeans is affected by the planting date, season, and cultivar choice.   Key words: Growth stages, cultivars, infestation, planting date

Measuring the Benefit of Biological Control for Single Gene and Pyramided Host Plant Resistance for <I>Aphis glycines</I> (Hemiptera: Aphididae) Management

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an economically important pest in the north central United States. In the state of Iowa, economically damaging populations occurred in seven of 11 growing seasons from 2001 to 2011. The high frequency and economic impact of the soybean aphid makes it an ideal candidate for management by using host plant resistance. We compared an aphid-susceptible line to near-isolines that contain Rag1 and Rag2, both alone and pyramided together, to suppress aphid populations and protect yield. Each of four near-isolines, were artificially infested with aphids and grown in small plots in which the exposure to natural enemies was controlled by the use of cages, resulting in the following treatment groups: natural enemy free (only aphids), biocontrol (both aphids and natural enemies), and aphid free (no aphids or natural enemies). The seasonal accumulation of aphids and the population growth rates were measured for each line and an estimate of yield was measured at the end of the season. Soybean aphid population growth rate was reduced 20% by natural enemies alone, 44% by pyramided resistance, and 63% by the combination of natural enemies and pyramided resistance. This reduction in population growth rate resulted in a 99.3% reduction in the pyramid line's seasonal exposure to aphids. In the presence of natural enemies, all three resistant lines maintained aphid populations below the economic injury level and prevented yield loss. This study demonstrates the compatibility of biological control with soybean aphid host plant resistance and its utility, especially for single resistance gene lines.

Alate immigration disrupts soybean aphid suppression by predators

AbstractNatural enemies suppress many aphid populations, and yet, population outbreaks sometimes occur. The reasons predation fails to suppress such outbreaks are not clearly understood. While manipulating predators to examine their role in soybean aphid population growth, a natural immigration of soybean aphids occurred that enabled us to compare the roles immigration and predation played in population growth. Using predator exclusion cages, we found that predation on the top of the plant accounted for 42.3 ± 11.4% (mean ± SE) reduction in aphid population growth rates. When 90–100% of the canopy was exposed, predation failed to reduce aphid population growth because winged immigrants colonized plants, with an observed 6‐fold increase in alates compared to plants completely covered or exposing only the top nodes (approximately 10% of the total canopy). We conclude that reproduction by immigrants contributed to population growth rates sufficiently to compensate for predation. These results demonstrate that immigration can counteract high levels of predation and lead to aphid population growth rates that could result in outbreak population densities.

Biology of the Soybean Aphid, <I>Aphis glycines</I> (Hemiptera: Aphididae) in the United States

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a significant insect pest of soybean in the north-central region of the United States and southern Canada, and if left untreated can reduce yield value by $2.4 billion annually. The soybean aphid is native to eastern Asia, where soybean was first domesticated, and was first detected in the United States in 2000. It quickly spread within 4 years of its discovery across 22 states and three provinces of Canada. Heavy infestations can result in a covering of sooty mold, yellow and wrinkled leaves, stunted plants, and aborted pods leading to significant yield loss of 40% or more. It can also transmit plant viruses such as Soybean mosaic virus and Alfalfa mosaic virus. The soybean aphid has a complex life cycle that involves different physical forms, sexual stages, and two host plant species-soybean and buckthorn (the overwintering host). Plant nutrition, natural enemies, climate, and weather all affect population growth rate, but the typical population doubling time is ≈6-7 days. Though at present management is primarily through broad-spectrum insecticides, biological control has a significant impact on soybean aphid population growth, and aphid-resistant soybean varieties are becoming increasingly available.

Methyl Salicylate Attracts Natural Enemies and Reduces Populations of Soybean Aphids (Hemiptera: Aphididae) in Soybean Agroecosystems

Methyl salicylate, an herbivore-induced plant volatile, has been shown to attract natural enemies and affect herbivore behavior. In this study, methyl salicylate was examined for its attractiveness to natural enemies of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), and for its direct effects on soybean aphid population growth rates. Methyl salicylate lures were deployed in plots within organic soybean [Glycine max (L.) Merr.] fields. Sticky card traps adjacent to and 1.5 m from the lure measured the relative abundance of natural enemies, and soybean aphid populations were monitored within treated and untreated plots. In addition, exclusion cage studies were conducted to determine methyl salicylate's effect on soybean aphid population growth rates in the absence of natural enemies. Significantly greater numbers of syrphid flies (Diptera: Syrphidae) and green lacewings (Neuroptera: Chrysopidae) were caught on traps adjacent to the methyl salicylate lure, but no differences in abundance were found at traps 1.5 m from the lure. Furthermore, abundance of soybean aphids was significantly lower in methyl salicylate-treated plots. In exclusion cage studies, soybean aphid numbers were significantly reduced on treated soybean plants when all plants were open to natural enemies. When plants were caged, however, soybean aphid numbers and population growth rates did not differ between treated and untreated plants suggesting no effect of methyl salicylate on soybean aphid reproduction and implicating the role of natural enemies in depressing aphid populations. Although aphid populations were reduced locally around methyl salicylate lures, larger scale studies are needed to assess the technology at the whole-field scale.

Population Growth of Soybean Aphid,Aphis glycines, Under Varying Levels of Predator Exclusion

Although soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), has caused economic damage in several Midwestern states, growers in Missouri have experienced relatively minor damage. To evaluate whether existing predatory insect populations are capable of suppressing or preventing soybean aphid population growth or establishment in Missouri, a predator exclusion study was conducted to gauge the efficacy of predator populations. Three levels of predator exclusion were used; one that excluded all insects (small mesh), one that excluded insects larger than thrips (medium mesh), and one that excluded insects larger than Orius insidiosus (Say) (Hemiptera: Anthocoridae), a principal predator (large mesh). Along with manipulating predator exposure, timing of aphid arrival (infestation) was manipulated. Three infestation times were studied; vegetative (V5), beginning bloom (R1), and beginning pod set (R3). Timing of aphid and predator arrival in a soybean field may affect the soybean aphid's ability to establish and begin reproducing. Cages infested at V5 and with complete predator exclusion reached economic threshold within two weeks, while cages with predators reached economic threshold in four and a half weeks. Cages infested at R1 with complete predator exclusion reached economic threshold within five weeks; cages with predators reached economic threshold within six weeks. Cages infested at R3 never reached threshold (with or without predators). The predator population in Missouri seems robust, capable of depressing the growth of soybean aphid populations once established, and even preventing establishment when the aphid arrived late in the field.

Soybean Aphid Population Dynamics, Soybean Yield Loss, and Development of Stage‐Specific Economic Injury Levels

Stage‐specific economic injury levels (EILs) form the basis of integrated pest management for soybean aphid (Aphis glycines Matsumura) in soybean [Glycine max (L.) Merr.]. Experimental objectives were to develop a procedure for calculating EILs of the soybean aphid specific to the R2 (full bloom), R4 (full pod), and R5 (beginning seed) soybean development stages using the law of the diminishing increment regression model. Soybean aphid population growth over time appeared to follow the symmetrical bell‐shaped and logistic growth curve models. Peak soybean aphid population levels and rates of increase occurred at the R5 development stage and then declined sharply thereafter. Highest peak soybean aphid populations were 21,626 aphids plant−1 for infestations starting at V5, and 6446 aphids plant−1 for infestations starting at R2. Highest maximum aphid‐days plant−1 recorded were 537,217 for V5‐introduced soybean aphids and 148,609 aphid‐days plant−1 for R2‐introduced soybean aphids. On average, the calculated maximum possible yield loss was 75% for soybean aphid infestations starting at the V5 (five node) stage and 48% for soybean aphid infestations starting at the R2 stage. Interrelationships among the current or predicted market value of soybean, cost of soybean aphid control, and the yield potential of the soybean field were considered in the calculations of the stage‐specific EILs. Practical examples for calculating stage‐specific EILs are presented. Economic injury levels were calculated both as soybean aphids plant−1 and soybean aphid‐days plant−1. Use of these stage‐specific EILs may enable growers to manage soybean aphids more accurately.

Life history characteristics of Orius insidiosus (Say) fed Aphis glycines Matsumura

Abstract The soybean aphid, Aphis glycines Matsumura, is a new invasive pest of soybeans throughout most of the soybean production areas of North America. Field studies have demonstrated that the indigenous predator, Orius insidiosus (Say), is an important natural enemy of the soybean aphid early the soybean crop season. Because soybean aphid is newly introduced into North America, the life history characteristics of predators fed this aphid are not known. In laboratory assays, we measured the survival, development, longevity and reproduction of O. insidiosus fed 1, 3, 6 or 12 seconds to third instars of soybean aphid. O. insidiosus nymphal development decreased from 34.0 to 21.4 days as the number of soybean aphid nymphs provided increased from 1 to 6 aphid nymphs daily. Stage-specific mortality was highest at 68% for first instar O. insidiosus nymphs fed 1 soybean aphid nymph per day. Adult longevity (43.9 days) and fecundity (49.7 eggs per female) was highest for O. insidiosus fed 6 soybean aphid nymphs daily, but longevity (23.5 days) and fecundity (10.1 eggs per female) declined for adults fed 1 soybean aphid nymph daily. The intrinsic rate of increase of O. insidiosus ranged from 0.048 to 0.133. Compared to other prey species, soybean aphid is an adequate prey item for O. insidiosus . Our results suggest that O. insidiosus will be most effective in suppressing soybean aphid population growth in the initial phase of the aphid’s colonization of soybeans.

Suppression of Population Growth of the Soybean Aphid, <I>Aphis glycines</I> Matsumura, by Predators: The Identification of a Key Predator and the Effects of Prey Dispersion, Predator Abundance, and Temperature

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), has recently invaded North America from Asia and has become a major pest in soybean. Using field surveys and cage exclusion techniques, we identified the effect of natural enemies and abiotic factors on the growth of soybean aphid populations in 2004 and 2005. The soybean aphid population was significantly limited by natural enemies in the field. Generalist predators dominated the natural enemy community. One species, Orius insidiosus Say (Hemiptera: Anthocoridae) represented 85–90% of predators found. There was a significant negative relationship between aphid population growth and O. insidiosus abundance. For other predators, there were no relationships between abundance and aphid population growth. The spatial distribution of aphids among plants affected the impact of O. insidiosus on aphid population growth. When aphids were distributed in a clumped manner, increases in O. insidiosus numbers resulted in lower aphid growth rates. For randomly distributed aphids, there was no effect of O. insidiosus abundance on aphid population growth. Finally, we found no relationship between aphid population growth and degree-day accumulations. The potential of O. insidiosus to suppress soybean aphid population growth at low aphid numbers and the importance of the predator to soybean aphid integrated pest management are discussed.

Influence of Potassium Fertility on Soybean Aphid,Aphis glycinesMatsumura (Hemiptera: Aphididae), Population Dynamics at a Field and Regional Scale

Soybean aphid populations have been hypothesized to respond to nutrient availability in soybean plants. We examined how soil potassium (K) deficiency affects soybean aphid population at both regional and field level scales. First, we measured soil and leaf nutrient levels and monitored soybean aphid populations in 34 production soybean fields throughout Wisconsin. A principle component analysis (PCA) was used to examine the relationship between soil and leaf nutrients and soybean aphid population growth. Results showed that aphid population growth rate was negatively correlated with soil K and P and leaf K, N, P, and S, whereas peak aphid densities were positively correlated with the same suite of soil and leaf nutrients. In a manipulative field experiment, we established low, medium, and high K fertility soybean plots and determined life table parameters of soybean aphids. Aphids developing within clip-cages showed a significantly greater intrinsic rate of population increase and net reproductive rate in the low K treatments in comparison with the medium and high K treatments. In these same plots, naturally colonizing populations of soybean aphids also had significantly higher peak abundance and rate of population increase in the low K treatment compared with medium and high K treatments. In general, these findings indicate that soil K availability and leaf K levels affect plant quality and may play an important role in soybean aphid population dynamics.

Orius insidiosus (Say) as a predator of the soybean aphid, Aphis glycines Matsumura

Abstract The soybean aphid, Aphis glycines Matsumura, a pest of soybeans in Asia, invaded North America recently. The indigenous generalist predator Orius insidiosus (Say) is common in Midwestern US soybean fields throughout the soybean-growing season. From its phenology and abundance, we hypothesize that this predator could act to suppress soybean aphid population growth by attacking aphids during the critical period when aphids are first colonizing soybean fields. Therefore we conducted a series of laboratory assays of O. insidiosus to evaluate the conditions under which it may effectively suppress aphid population growth. We conducted a standard functional response experiment and found that O. insidiosus had a maximum attack rate of 19.25 (±0.06 SE) aphids in 24 h. We examined the ability of O. insidiosus to suppress aphid population growth over a four-day period in microcosms with different numbers of aphids, plant sizes, and plant numbers. We found that predators were able to effectively suppress aphid population growth, but that their efficacy depended upon the number of prey present and characteristics of the plant on which prey were found. Further, we showed that O. insidiosus was able to suppress aphid population growth below starting densities for an extended time period (12 days). These results suggest that under certain conditions, O. insidiosus can effectively suppress aphid population growth and that they may be key factors influencing aphid population dynamics in soybeans.