Aphid Species and Feeding Location on Canola Influences the Impact of Glucosinolates on a Native Lady Beetle Predator

Aphids that attack canola (Brassica napus L.) exhibit feeding preferences for different parts of canola plants, which may be associated with brassica-specific glucosinolates. However, this idea remains untested. Furthermore, canola aphid species employ different strategies for tolerating glucosinolates. While the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), excretes glucosinolates, the cabbage aphid Brevicoryne brassicae (L.) (Hemiptera: Aphididae) sequesters them. Given the different detoxification mechanisms, we predicted that both aphid species and aphid feeding location would affect prey suitability for larvae of the predator, Hippodamia convergens (Guérin-Méneville) (Coleoptera: Coccinellidae). We hypothesized that aphids, specifically glucosinolate-sequestering cabbage aphid, reared on reproductive structures that harbor higher glucosinolates concentrations would have greater negative effects on predators than those reared on vegetative structures which have lower levels of glucosinolates, and that the impact of aphid feeding location would vary depending on the prey detoxification mechanism. To test these predictions, we conducted experiments to compare 1) glucosinolates profiles between B. brassicae and M. persicae reared on reproductive and vegetative canola structures, 2) aphid population growth on each structure, and 3) their subsequent impact on fitness traits of H. convergens. Results indicate that the population growth of both aphids was greater on reproductive structures, with B. brassicae having the highest population growth. B. brassicae reared on reproductive structures had the highest concentrations of glucosinolates, and the greatest adverse effects on H. convergens. These findings suggest that both aphid-prey species and feeding location on canola could influence populations of this predator and, thus, its potential for biological control of canola aphids.

Effect of Temperature on Plant Resistance to Arthropod Pests

Temperature has a strong influence on the development, survival, and fecundity of herbivorous arthropods, and it plays a key role in regulating the growth and development of their host plants. In addition, temperature affects the production of plant secondary chemicals as well as structural characteristics used for defense against herbivores. Thus, temperature has potentially important implications for host plant resistance. Because temperature directly impacts arthropod pests, both positively and negatively, distinguishing direct effects from indirect effects mediated through host plants poses a challenge for researchers and practitioners. A more comprehensive understanding of how temperature affects plant resistance specifically, and arthropod pests in general, would lead to better predictions of pest populations, and more effective use of plant resistance as a management tactic. Therefore, the goals of this paper are to 1) review and update knowledge about temperature effects on plant resistance, 2) evaluate alternative experimental approaches for separating direct from plant-mediated indirect effects of temperature on pests, including benefits and limitations of each approach, and 3) offer recommendations for future research.

Parasitism of Adult Pentatomidae by Tachinidae in Soybean in the North Central Region of the United States

Stink bugs (Hemiptera: Pentatomidae) are agricultural pests of increasing significance in the North Central Region of the United States, posing a threat to major crops such as soybean. Biological control can reduce the need for insecticides to manage these pests, but the parasitism of stink bugs by Tachinidae (Diptera) is poorly characterized in this region. The objective of this study was to evaluate the rate of parasitism of stink bugs by tachinids over 2 yr from nine states across the North Central Region. Parasitism was assessed by quantifying tachinid eggs on the integument of stink bug adults. Parasitism rates (i.e., percent of adult stink bugs with tachinid eggs) were compared across stink bug species, states, stink bug sex, and years. The mean percent parasitism of stink bugs by tachinids was about 6% across the region and did not differ among stink bug species. Mean percent parasitism was significantly higher in Missouri than in northern and western states. In addition, male stink bugs had significantly higher mean percent parasitism than females. Stink bug species commonly found in soybean in the region showed some parasitism and are therefore potentially vulnerable to oviposition by these parasitoids. This is the first study to characterize the level of parasitism of stink bugs by tachinids across the North Central Region.

Hessian Fly (Diptera: Cecidomyiidae) Attraction to Different Wavelengths and Intensities of Light-Emitting Diodes in the Laboratory

Monitoring of Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), populations is important for targeted management methods. Also, effectiveness of monitoring efforts is critical to surveillance efforts in regions of the world without this pest. Current Hessian fly monitoring traps rely purely on a single attractant, the female sex pheromone, which is ineffective for monitoring females in the population. Our objective was to examine another attractant targeting vision of both males and females. To do this, we evaluated emitted light of various wavelengths and intensities produced by light-emitting diodes (LEDs). Hessian flies were released in the center of a four-leaf clover-shaped arena, which contained an LED within each collection cup at the apex of each of the four cloverleaves. Initially, we examined a range of colors, including red, amber, green, and blue, with wavelengths of 624, 590, 527, and 472 nm, respectively. Both sexes of Hessian fly preferred green LEDs; therefore, we examined specific wavelengths within the green spectrum (502, 525, and 565 nm), and varied light intensities (4, 8, 12, and 16 W/m2). Specifically, females preferred wavelengths in the lower region of the green spectrum (502 and 525 nm), and both sexes preferred high intensity light (16 W/m2). This is the first report of Hessian fly attraction to select emitted wavelengths and intensities from LEDs under controlled conditions. Leveraging these results into new trap designs will add a second sensory modality to the existing trap; however, future studies are needed to assess attraction to LED traps under field conditions.

Incorporating biological control into IPM decision making

Of the many ways biological control can be incorporated into Integrated Pest Management (IPM) programs, natural enemy thresholds are arguably most easily adopted by stakeholders. Integration of natural enemy thresholds into IPM programs requires ecological and cost/benefit crop production data, threshold model validation, and an understanding of the socioeconomic factors that influence stakeholder decisions about biological control. These thresholds are more likely to be utilized by stakeholders when integrated into dynamic web-based IPM decision support systems that summarize pest management data and push site-specific biological control management recommendations to decision-makers. We highlight recent literature on topics related to natural enemy thresholds and how findings may allow pest suppression services to be incorporated into advanced IPM programs.

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.

Preference and Performance of Hippodamia convergens (Coleoptera: Coccinellidae) and Chrysoperla carnea (Neuroptera: Chrysopidae) on Brevicoryne brassicae, Lipaphis erysimi, and Myzus persicae (Hemiptera: Aphididae) from Winter-Adapted Canola

In the southern plains of the United States, winter-adapted canola (Brassica napus L.) is a recently introduced annual oilseed crop that has rapidly increased in hectares during the past 10 yr. Winter canola fields are infested annually with populations of Brevicoryne brassicae (L.) and Lipaphis erysimi (Kaltenbach), and these Brassica specialists are known to sequester plant volatiles from host plants, producing a chemical defense system against predators. Myzus persicae (Sulzer) is also common in winter canola fields, but as a generalist herbivore, does not sequester plant compounds. These three aphid species are expected to affect predator survival and development in very different ways. We conducted laboratory studies to 1) determine whether Hippodamia convergens (Guérin-Méneville) and Chrysoperla carnea (Stephens) larvae demonstrate feeding preferences among winter canola aphids and 2) describe the suitability of these prey species. Predators demonstrated no significant preference among prey, and each aphid species was suitable for predator survival to the adult stage. However, prey species significantly affected development times and adult weights of each predator species. Overall, predator development was delayed and surviving adults weighed less when provided with L. erysimi or B. brassicae, which sequestered high levels of indole glucosinolates from their host plants. Our results indicate that although common winter canola aphids were suitable prey for H. convergens and C. carnea, qualitative differences in nutritional suitability exist between Brassica-specialist aphids and the generalist M. persicae. These differences appear to be influenced by levels of sequestered plant compounds that are toxic to aphid predators.

Comparison of Relative Bias, Precision, and Efficiency of Sampling Methods for Natural Enemies of Soybean Aphid (Hemiptera: Aphididae)

Generalist natural enemies play an important role in controlling soybean aphid, Aphis glycines (Hemiptera: Aphididae), in North America. Several sampling methods are used to monitor natural enemy populations in soybean, but there has been little work investigating their relative bias, precision, and efficiency. We compare five sampling methods: quadrats, whole-plant counts, sweep-netting, walking transects, and yellow sticky cards to determine the most practical methods for sampling the three most prominent species, which included Harmonia axyridis (Pallas), Coccinella septempunctata L. (Coleoptera: Coccinellidae), and Orius insidiosus (Say) (Hemiptera: Anthocoridae). We show an important time by sampling method interaction indicated by diverging community similarities within and between sampling methods as the growing season progressed. Similarly, correlations between sampling methods for the three most abundant species over multiple time periods indicated differences in relative bias between sampling methods and suggests that bias is not consistent throughout the growing season, particularly for sticky cards and whole-plant samples. Furthermore, we show that sticky cards produce strongly biased capture rates relative to the other four sampling methods. Precision and efficiency differed between sampling methods and sticky cards produced the most precise (but highly biased) results for adult natural enemies, while walking transects and whole-plant counts were the most efficient methods for detecting coccinellids and O. insidiosus, respectively. Based on bias, precision, and efficiency considerations, the most practical sampling methods for monitoring in soybean include walking transects for coccinellid detection and whole-plant counts for detection of small predators like O. insidiosus. Sweep-netting and quadrat samples are also useful for some applications, when efficiency is not paramount.

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

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 awide range of environments. We conducted 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.

Sequential sampling for panicle caterpillars (Lepidoptera: Noctuidae) in sorghum

Panicle caterpillars comprise an economically important insect pest complex of sorghum throughout the Great Plains of the United States, particularly in Kansas, Oklahoma, and Texas. The sorghum panicle caterpillar complex consists of larvae of two polyphagous lepidopteran species: the corn earworm, Helicoverpa zea (Boddie), and fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae). Sampling for panicle caterpillars in sorghum fields is usually accomplished by the beat bucket sampling technique with a fixed sample size of 30 beat bucket samples of one sorghum panicle each per 16.2 ha of field. We used Wald's sequential probability ratio test for a negative binomial distribution to develop a sequential sampling plan for panicle caterpillars. In total, 115 sorghum fields were sampled in Kansas, Oklahoma, and Texas from June to August 2010. Panicle caterpillars had an aggregated distribution of counts confirmed by Pearson's chi-square statistic for lack of fit to the negative binomial distribution for each sampled field. A sequential sampling plan was developed using a high threshold (an economic threshold) of 0.5 caterpillars per sorghum panicle, a low threshold (a safe level) of 0.20 caterpillars per panicle, and fixed error rates (alpha = 0.10 and beta = 0.05). At caterpillar densities > 0.45 and < 0.12 per panicle, the average number of panicles inspected to make a decision was less than the current recommendation of 30. In a 2013 validation test of 25 fields, the expected number of samples taken from average sample number curve was in close agreement with the number of samples required using the sequential plan (r2 = 0.93), and all fields were correctly classified when compared with a fixed sample size result. The plan improved upon current sampling recommendations for panicle caterpillars in sorghum because at known acceptable fixed error rates fewer samples were required when caterpillars are scarce or abundant, whereas more samples were required to make decisions with the same acceptable error rates when densities were near the economic thresholds.

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.

Sorghum seed maturity affects the weight and feeding duration of immature corn earworm, Helicoverpa zea, and fall armyworm, Spodoptera frugiperda, in the laboratory

Corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae), and fall armyworm, Spodoptera frugiperda J.E. Smith, are occasional pests in sorghum, Sorghum bicolor L. Moench (Poales: Poaceae), and can be economically damaging when conditions are favorable. Despite the frequent occurrence of mixed-species infestations, the quantitative data necessary for developing yield loss relationships for S. frugiperda are not available. Although these species share similar biological characteristics, it is unknown whether their damage potentials in developing grain sorghum panicles are the same. Using no-choice feeding assays in the laboratory, this study examined larval growth and feeding duration for H. zea and S. frugiperda in the absence of competition. Each species responded positively when exposed to sorghum seed in the soft-dough stage, supporting evidence for the interactions between host-quality and larval growth and development. The results of this study also confirmed the suitability of using laboratory-reared H. zea to develop sorghum yield loss estimates in the field, and provided insights into the biological responses of S. frugiperda feeding on developing sorghum seed.

Crop residue and residue management effects on Armadillidium vulgare (Isopoda: Armadillidiidae) populations and soybean stand densities

In general, Armadillidium vulgare (Latreille) are considered nonpests of soybean [Glycine max (L.) Merrill], but changes in soil conservation practices have shifted the pest status of this organism from an opportunistic to a perennial, early-season pest in parts of central Kansas. As a result, soybean producers that rotate with corn (Zea mays L.) under conservation tillage practices have resorted to removing excess corn residue by using controlled burns. In a 2-yr field study (2009-2010), we demonstrated that residue removal in burned compared with unburned plots (measured as previous crop residue weights) had minimal impact on numbers of live and dead A. vulgare, soybean seedling emergence, and isopod feeding damage over time. Specifically, removal of residue by burning did not result in higher emergence rates for soybean stands or less feeding damage by A. vulgare. In a separate study, we found that number of live A. vulgare and residue weights had no consistent relationship with seedling emergence or feeding damage. Furthermore, seedling emergence was not impacted by higher numbers ofA. vulgare in unburned plots, indicating that emergence in this study may have been influenced by factors other than A. vulgare densities. These studies demonstrate that removing residue through controlled burning did not impact seedling emergence in presence of A. vulgare and that residue and feeding damage to seedlings did not consistently relate to A. vulgare densities. Other factors that may have influenced a relationship between residue and live isopod numbers, such as variable moisture levels, are discussed.

Development and validation of node-based sample units for estimating soybean aphid (Hemiptera: Aphididae) densities in field cage experiments

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is currently the most important insect threat to soybean, Clycine max (L.) Merr., production in the North Central United States. Field cage studies are a key tool in investigating the potential of natural enemies and host plant resistance to control this pest. However, a major constraint in the use of cage studies is the limited number of treatments and replicates that can be used as aphid densities frequently become so large as to limit the number of experimental units that can be quantified. One way to overcome this limitation is to develop methods that estimate whole-plant aphid densities based on a reduced sampling plan. Here, we extend an existing method, node-sampling, used for estimating aphid populations in open field conditions and apply it to caged populations. We show that parameters calculated under open field conditions are inappropriate to estimate caged populations. In contrast, using four independent data sets of caged populations and a cross-validation technique, we demonstrate that a three-node sampling unit and a weighted formula provide accurate and robust estimates of whole-plant aphid density. This method reduced the number of aphids counted per plant by and average of 60%, with greater reductions at higher aphid densities. We further demonstrate that nearly identical statistical results were obtained when whole-plant or node-sampling estimates were used in the analysis of two case studies. The reduced sample unit method developed here saves time without sacrificing efficiency so that more plants, replications, or studies can be conducted that will lead to improved soybean aphid management.

Within-plant distribution of soybean aphid (Hemiptera: Aphididae) and development of node-based sample units for estimating whole-plant densities in soybean

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 (N(MAX)') 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.

Economic threshold for soybean aphid (Hemiptera: Aphididae)

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.

Economic threshold for soybean aphid (Hemiptera: Aphididae)

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

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 degrees C with a photoperiod of 16:8 (L:D) h. Population growth rates were greatest at 25 degrees C. As temperature increased, net fecundity, gross fecundity, generation time, and life expectancy decreased. The prereproductive period did not differ between 20 and 30 degrees C; however, at 30 degrees C aphids required more degree-days (base 8.6 degrees C) to develop. Nymphs exposed to 35 degrees 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 degrees C than at 30 or 35 degrees C. Using a modification of the nonlinear Logan model, we estimated upper and optimal developmental thresholds to be 34.9 and 27.8 degrees C, respectively. At 25 degrees C, aphid populations doubled in 1.5 d; at 20 and 30 degrees C, populations doubled in 1.9 d.