B. P. McCornack

Economic Threshold for Soybean Aphid (Hemiptera: Aphididae)

Abstract Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), reached damaging levels in 2003 and 2005 in soybean, Glycine max (L.) Merrill, in most northern U.S. states and Canadian provinces, and it has become one of the most important pests of soybean throughout the North Central region. A common experimental protocol was adopted by participants in six states who provided data from 19 yield-loss experiments conducted over a 3-yr period. Population doubling times for field populations of soybean aphid averaged 6.8 d ± 0.8 d (mean ± SEM). The average economic threshold (ET) over all control costs, market values, and yield was 273 ± 38 (mean ± 95% confidence interval [CI], range 111–567) aphids per plant. This ET provides a 7-d lead time before aphid populations are expected to exceed the economic injury level (EIL) of 674 ± 95 (mean ± 95% CI, range 275–1,399) aphids per plant. Peak aphid density in 18 of the 19 location-years occurred during soybean growth stages R3 (beginning pod formation) to R5 (full size pod) with a single data set having aphid populations peaking at R6 (full size green seed). The ET developed here is strongly supported through soybean growth stage R5. Setting an ET at lower aphid densities increases the risk to producers by treating an aphid population that is growing too slowly to exceed the EIL in 7 d, eliminates generalist predators, and exposes a larger portion of the soybean aphid population to selection by insecticides, which could lead to development of insecticide resistance.

Demography of Soybean Aphid (Homoptera: Aphididae) at Summer Temperatures

Abstract Soybean aphid, Aphis glycines Matsumura, is now widely established in soybean, Glycine max L., production areas of the northern United States and southern Canada and is becoming an important economic pest. Temperature effect on soybean aphid fecundity and survivorship is not well understood. We determined the optimal temperature for soybean aphid growth and reproduction on soybean under controlled conditions. We constructed life tables for soybean aphid at 20, 25, 30, and 35°C with a photoperiod of 16:8 (L:D) h. Population growth rates were greatest at 25°C. As temperature increased, net fecundity, gross fecundity, generation time, and life expectancy decreased. The prereproductive period did not differ between 20 and 30°C; however, at 30°C aphids required more degree-days (base 8.6°C) to develop. Nymphs exposed to 35°C did not complete development, and all individuals died within 11 d. Reproductive periods were significantly different at all temperatures, with aphids reproducing longer and producing more progeny at 20 and 25°C than at 30 or 35°C. Using a modification of the nonlinear Logan model, we estimated upper and optimal developmental thresholds to be 34.9 and 27.8°C, respectively. At 25°C, aphid populations doubled in 1.5 d; at 20 and 30°C, populations doubled in 1.9 d.

Management Recommendations for Soybean Aphid (Hemiptera: Aphididae) in the United States

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is the primary pest of soybean, Glycine max L., in the north central region. After more than a decade of research and extension efforts to manage this pest, several consensus management recommendations have been developed for sustainable and profitable soybean production. A summary of integrated pest management (IPM) tactics for soybean aphid are discussed, including cultural, genetic, economic, and chemical controls. To date, sampling and timely foliar insecticides are routinely recommended to protect yield and delay genetic resistance to insecticides. Host plant resistance is a new tool that can regulate populations and reduce the reliance of insecticides to control soybean aphid. A combination of these management tools also will reduce overall production costs and minimize negative environmental effects such as human exposure, and mortality of beneficial insects and other animals.

Within-Plant Distribution of Soybean Aphid (Hemiptera: Aphididae) and Development of Node-Based Sample Units for Estimating Whole-Plant Densities in Soybean

Abstract The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is capable of reducing soybean, Glycine max (L.) Merr., yield up to 40% during severe outbreaks. Frequent sampling, which can be costly and time-consuming, is essential to making informed management decisions. However, one way to decrease sampling effort is to use a reduced sample unit when possible. The objectives of this study were to describe the vertical distribution of soybean aphid within soybean over time and to define node-based sample units of varying sizes by testing the ability of selected units to accurately estimate whole-plant aphid density. Within-plant distribution of soybean aphid changed significantly with time. However, the average nodal position where soybean aphids were found on soybean remained within the top half of the plant at all three locations studied across all sample dates. Consequently, selecting the node with the highest aphid density multiplied by the total number of infested nodes (NMAX′) was the best predictor of aphids on remaining soybean components in both the original (r2 = 0.855) and validation (r2 = 0.824) data sets. For sample units that included more than a single node to estimate densities, a weighted formula, which incorporated changes observed in the within-plant aphid distribution, improved model performance (higher r2 values) and reduced variability around parameter estimates compared with a node-averaged formula. Our results suggest that smaller sample units provide reliable estimations of whole-plant aphid density throughout the growing season for differently maturing soybean, which is essential to their use in pest management decisions and development of future sampling plans.

One gene versus two: A regional study on the efficacy of single gene versus pyramided resistance for soybean aphid management

ABSTRACT The soybean aphid (Aphis glycines Matsumura) is a threat to soybean production in the Midwestern United States. Varieties containing the Rag1 soybean aphid resistance gene have been released with limited success in reducing aphid populations. Furthermore, virulent biotypes occur within North America and challenge the durability of single-gene resistance. Pyramiding resistance genes has the potential to improve aphid population suppression and increase resistance gene durability. Our goal was to determine if a pyramid could provide improved aphid population suppression across a wide range of environments.Weconducted a small-plot field experiment across seven states and three years. We compared soybean near-isolines for the Rag1 or Rag2 gene, and a pyramid line containing both genes for their ability to decrease aphid pressure and protect yield compared with a susceptible line. These lines were evaluated both with and without a neonicitinoid seed treatment. All aphid-resistant lines significantly decreased aphid pressure at all locations but one. The pyramid line experienced lower aphid pressure than both single-gene lines at eight of 23 location-years. Soybean aphids significantly reduced soybean yield for the susceptible line by 14% and for both single-gene lines by 5%; however, no significant yield decrease was observed for the pyramid line. The neonicitinoid seed treatment reduced plant exposure to aphids across all soybean lines, but did not provide significant yield protection for any of the lines. These results demonstrate that pyramiding resistance genes can provide sufficient and consistent yield protection from soybean aphid in North America.

A Simple Method for In-Field Sex Determination of the Multicolored Asian Lady Beetle Harmonia axyridis

Abstract The multicolored Asian lady beetle, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), has become a popular study organism due to its promise as a biological control agent and its potential adverse, non-target impacts. Behavioral and ecological research on H. axyridis, including examinations of its impacts, could benefit from non-destructive or non-disruptive sexing techniques for this coccinellid. External morphological characters were evaluated for H. axyridis (succinea color form) sex determination in laboratory and field studies. The shape of the distal margin of the fifth visible abdominal sternite accurately predicted H. axyridis sex for all beetles examined. Males consistently had a concave distal margin, while females had a convex distal margin. In addition, pigmentation of the labrum and prosternum were both significantly associated with H. axyridis sex; males had light pigmentation and females had dark pigmentation. Labrum and prosternum pigmentation increased from light to dark with decreasing rearing temperature and increasing time after adult eclosion for females. Male pigmentation was only affected by a decrease in rearing temperature. Validation through in-field collections indicated that these predictors were accurate. However, labrum pigmentation is a more desirable character to use to determine sex, because it is more accurate and easily accessible. Therefore, we recommend using labrum pigmentation for in-field sex determination of H. axyridis. Implications of this diagnostic technique for applied and basic research on this natural enemy are discussed.

Physiological Constraints on the Overwintering Potential of the Soybean Aphid (Homoptera: Aphididae)

Abstract The soybean aphid, Aphis glycines Matsumura, has a heteroecious, holocyclic life cycle. Soybean aphids overwinter as eggs, hatch in the spring, reproduce asexually, and undergo three or more generations on buckthorn, Rhamnus spp., before migrating to a secondary host such as soybean, Glycine max (L.) Merr. The ability of different soybean aphid life stages to survive low temperatures potentially experienced during fall or winter is not known. The objectives of this study were to determine the supercooling point (SCP) of various soybean aphid life stages and to determine the annual probability that winter temperatures within the North Central region of the United States would equal or fall below the mean SCP of soybean aphid eggs. Aphid eggs are considered the most cold-hardy stage; therefore, their SCP can be used as a conservative estimate for aphid overwintering mortality. In our study, eggs had the lowest mean SCP (approximately −34°C) among all life stages, whereas gynoparae and oviparae had the highest mean SCPs (approximately −15°C). During the winter, extreme low air temperatures are likely to reach or exceed the mean SCP of soybean aphid eggs in portions of northern Minnesota, northern Wisconsin, and the upper peninsula of Michigan. Thus, widespread successful overwintering in the northern United States and southern Canada is less likely than in Illinois, Indiana, Ohio, Iowa, southern Minnesota, southern Wisconsin, and the lower peninsula of Michigan.

Spatial Distribution of Aphis glycines (Hemiptera: Aphididae): A Summary of the Suction Trap Network

ABSTRACT The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an economically important pest of soybean, Glycine max (L.) Merrill, in the United States. Phenological information of A. glycines is limited; specifically, little is known about factors guiding migrating aphids and potential impacts of long distance flights on local population dynamics. Increasing our understanding of A. glycines population dynamics may improve predictions of A. glycines outbreaks and improve management efforts. In 2005 a suction trap network was established in seven Midwest states to monitor the occurrence of alates. By 2006, this network expanded to 10 states and consisted of 42 traps. The goal of the STN was to monitor movement of A. glycines from their overwintering host Rhamnus spp. to soybean in spring, movement among soybean fields during summer, and emigration from soybean to Rhamnus in fall. The objective of this study was to infer movement patterns of A. glycines on a regional scale based on trap captures, and determine the suitability of certain statistical methods for future analyses. Overall, alates were not commonly collected in suction traps until June. The most alates were collected during a 3-wk period in the summer (late July to mid-August), followed by the fall, with a peak capture period during the last 2 wk of September. Alate captures were positively correlated with latitude, a pattern consistent with the distribution of Rhamnus in the United States, suggesting that more southern regions are infested by immigrants from the north.

Within-plant bottom-up effects mediate non-consumptive impacts of top-down control of soybean aphids.

There is increasing evidence that top-down controls have strong non-consumptive effects on herbivore populations. However, little is known about how these non-consumptive effects relate to bottom-up influences. Using a series of field trials, we tested how changes in top-down and bottom-up controls at the within-plant scale interact to increase herbivore suppression. In the first experiment, we manipulated access of natural populations of predators (primarily lady beetles) to controlled numbers of A. glycines on upper (i.e. vigorous-growing) versus lower (i.e. slow-growing) soybean nodes and under contrasting plant ages. In a second experiment, we measured aphid dispersion in response to predation. Bottom-up and top-down controls had additive effects on A. glycines population growth. Plant age and within-plant quality had significant bottom-up effects on aphid size and population growth. However, top-down control was the dominant force suppressing aphid population growth, and completely counteracted bottom-up effects at the plant and within-plant scales. The intensity of predation was higher on upper than lower soybean nodes, and resulted in a non-consumptive reduction in aphid population growth because most of the surviving aphids were located on lower plant nodes, where rates of increase were reduced. No effects of predation on aphid dispersal among plants were detected, suggesting an absence of predator avoidance behavior by A. glycines. Our results revealed significant non-consumptive predator impacts on aphids due to the asymmetric intensity of predation at the within-plant scale, suggesting that low numbers of predators are highly effective at suppressing aphid populations.

Alate immigration disrupts soybean aphid suppression by predators

Natural 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.