Invasive Cereal Aphids of North America: Ecology and Pest Management

High-throughput molecular gut content analysis of aphids identifies plants relevant for potato virus Y epidemiology

Aphids are phloem-feeding insects that reduce crop productivity due to feeding and transmission of plant viruses. When aphids disperse across the landscape to colonize new host plants, they will often probe on a wide variety of nonhost plants before settling on a host suitable for feeding and reproduction. There is limited understanding of the diversity of plants that aphids probe on within a landscape, and characterizing this diversity can help us better understand host use patterns of aphids. Here, we used gut content analysis (GCA) to identify plant genera that were probed by aphid vectors of potato virus Y (PVY). Aphids were trapped weekly near potato fields during the growing seasons of 2020 and 2021 in San Luis Valley in Colorado. High-throughput sequencing of plant barcoding genes, trnF and ITS2, from 200 individual alate (i.e., winged) aphids representing nine vector species of PVY was performed using the PacBio sequencing platform, and sequences were identified to genus using NCBI BLASTn. We found that 34.7% of aphids probed upon presumed PVY host plants and that two of the most frequently detected plant genera, Solanum and Brassica, represent important crops and weeds within the study region. We found that 75% of aphids frequently probed upon PVY nonhosts including many species that are outside of their reported host ranges. Additionally, 19% of aphids probed upon more than one plant species. This study provides the first evidence from high-throughput molecular GCA of aphids and reveals host use patterns that are relevant for PVY epidemiology.

Unconventional Strategies for Aphid Management in Sorghum

Since the invasion of the sorghum aphid Melanaphis sorghi (Theobald), farmers in the sorghum (Sorghum bicolor L. Moench) production region in the Great Plains of the U.S. have faced significant crop damage and reduced yields. One widely used practice to aid in managing sorghum aphids is pest monitoring, which often results in field-level insecticide applications when an economic threshold is reached. However, relying on this traditional management practice includes the application of insecticides to non-infested plants. To reduce insecticide usage in sorghum, we proposed spraying individual plants when aphids are present or absent compared to traditional spraying based on a standard economic threshold using field replicate plots over two summer seasons. The experimental results of this study indicated fewer aphids in plots managed with an economic threshold, followed by randomly sprayed and plant-specific treatments compared with the untreated control treatment. Therefore, compared with traditional management, those treatments can be alternative strategies for managing aphids on sorghum within our field plot study.

Cereal Aphid Parasitoids in Europe (Hymenoptera: Braconidae: Aphidiinae): Taxonomy, Biodiversity, and Ecology

Cereals are very common and widespread crops in Europe. Aphids are a diverse group of herbivorous pests on cereals and one of the most important limiting factors of cereal production. Here, we present an overview of knowledge about the taxonomy, biodiversity, and ecology of cereal aphid parasitoids in Europe, an important group of natural enemies contributing to cereal aphid control. We review the knowledge obtained from the integrative taxonomy of 26 cereal aphid primary parasitoid species, including two allochthonous species (Lysiphlebus testaceipes and Trioxys sunnysidensis) and two recently described species (Lipolexis labialis and Paralipsis brachycaudi). We further review 28 hyperparasitoid species belonging to three hymenopteran superfamilies and four families (Ceraphronoidea: Megaspillidae; Chalcidoidea: Pteromalidae, Encyrtidae; Cynipoidea: Figitidae). We also compile knowledge on the presence of secondary endosymbionts in cereal aphids, as these are expected to influence the community composition and biocontrol efficiency of cereal aphid parasitoids. To study aphid-parasitoid-hyperparasitoid food webs more effectively, we present two kinds of DNA-based approach: (i) diagnostic PCR (mainly multiplex PCR), and (ii) DNA sequence-based methods. Finally, we also review the effects of landscape complexity on the different trophic levels in the food webs of cereal aphids and their associated parasitoids, as well as the impacts of agricultural practices and environmental variation.

Landscape Complexity has Mixed Effects on an Invasive Aphid and Its Natural Enemies in Sorghum Agroecosystems

Landscapes with more complex composition and configuration are generally expected to enhance natural enemy densities and pest suppression. To evaluate this hypothesis for an invasive aphid pest of sorghum, Melanaphis sorghi Theobald (Hemiptera: Aphididae), sampling in sorghum fields for aphids and natural enemies was conducted over two years in a southern U.S. coastal production region. Landscape composition and configuration of crop and noncrop elements were assessed using correlation and multivariate regression modeling to detect relationships with insects at different spatial scales. Significant models found more complex landscape configuration, particularly the amount of habitat edges, was associated with increased aphid and natural enemy abundance. Composition associated with noncrop habitats had the opposite effect. Numerical response of natural enemies was taxa dependent, with parasitism lower as landscape complexity increased, while predator numerical response was not affected by landscape complexity. These results indicate landscape complexity may increase both aphid and natural enemy abundance, but with decreasing parasitism and little association with predator numerical response. These relationships are likely contingent on overall environmental suitability to aphid population increase as results were less evident in the second year when average aphid abundance regularly exceeded the economic threshold. This study supports the importance of configuration, especially habitat borders, as a critical metric for determining pest-natural enemy dynamics within a large-scale cereal agroecosystem.

Effect of Abiotic Climatic Factors on the Gonadal Maturation of the Biocontrol Agent Sphaerophoria rueppellii (Wiedemann, 1830) (Diptera: Syrphidae)

Simple Summary

Knowledge about the morphology and functioning of the male and female reproductive system in insects is key to understanding their reproductive biology, and to assessing the effects that environmental factors, such as temperature or photoperiod, can have on oviposition, fecundity, and lifespan. This knowledge is particularly interesting in those species that are mass-reared, as in the case of the predatory syrphid Sphaerophoria rueppellii. Given the lack of published information regarding sexual maturation in syrphids, this type of study, applied to beneficial insects used as biological control agents, offers, firstly, the chance to improve their mass breeding under controlled conditions and, secondly, to know their capability for pest control response under field conditions. Our results show that photoperiod and temperature affect development and gonad maturation in S. rueppellii males and females.

Abstract

The hoverfly Sphaerophoria rueppellii is currently one of the most effective predators commercially available for aphid pest control. However, knowledge of the reproductive system of males and females of this syrphid is limited. The present article aims to report how changes in the temperature and photoperiod may affect development of the gonads (ovaries and testes), oviposition, and fecundity during the lifespan of S. rueppellii. Four environmental conditions (14L:10D, T: 20 ± 1 °C; 12L:12D, T: 20 ± 1 °C; 14L:10D, T: 25 ± 1 °C; and 12L:12D, T: 25 ± 1 °C) were used to determine oviposition, hatching percentage, and lifespan during a period of 30 days after the adult emergence. The maturation of the ovaries was done under three treatments (barley leaves with aphids always available; barley leaves two days per week with aphids available; no barley leaves available), and in the same environmental conditions noted above. Males at 14L:10D, 20 ± 1 °C; and 14L:10D, 25 ± 1 °C; were used to analyze and study the maturation of the testes. Females at 14L:10D; T: 25 ± 1 °C showed a significant difference in oviposition, percentage of hatching, and rate of eggs. A detailed description of the male and female gonads was undertaken, and it was determined that the conditions in which males sexually mature early are at 14L:10D, 25 ± 1 °C. These results will improve the application of S. rueppellii in crops, for the control of aphid pests.

Natural Enemies, Mediated by Landscape and Weather Conditions, Shape Response of the Sorghum Agroecosystem of North America to the Invasive Aphid Melanaphis sorghi

The sorghum (Sorghum bicolor [L.]) agroecosystem of North America provided an opportunity to evaluate agroecosystem response to an invading insect herbivore, Melanaphis sorghi (Theobald) (sorghum aphid) (previously published as Melanaphis sacchari Zehntner) (Hemiptera: Aphididae) onto a widely planted crop that experiences a range of agro-landscape and weather conditions. Initial sorghum risk assessments after M. sorghi's invasion in the mid-2010s provided forecasts of range expansion and annual migration, which were based on aphid life history, extent of sorghum cultivation and susceptibility to M. sorghi, and weather (aphid-plant-weather [APW] risk scenario). A more comprehensive risk assessment proposed here brings top-down forces of M. sorghi-natural enemy interactions to the forefront as mediated by agro-landscape and weather conditions (aphid-enemy/landscape-weather mediated [AE/LW] risk scenario). A hypothesis of regional differences in aphids and natural enemies and sensitivity to agro-landscape and weather was tested using empirical data of insect, landscape, and weather data across 5 years and four regions (two in the U.S. Great Plains [South GP and North GP], one farther south (South), and one in the southeast U.S. [South E]). Natural enemies were widespread with two parasitoids and four coccinellid species common across regions, but regional variation in M. sorghi and natural enemy abundance was detected. The AE/LW risk scenario accounted for natural enemy abundance and activity that was highest in the South region, functioned well across agro-landscape and weather conditions, and was accompanied by average low M. sorghi abundance (~23 M. sorghi per leaf). Positive correlations of natural enemy-M. sorghi abundance also occurred in the South GP region where M. sorghi abundance was low (~20 M. sorghi per leaf), and selected natural enemy activity appeared to be mediated by landscape composition. Melanaphis sorghi abundance was highest in the South E region (~136 aphids/leaf) where natural enemy activity was low and influenced by weather. The AE/LW risk scenario appeared suited, and essential in the South region, in assessing risk on a regional scale, and sets the stage for further modeling to generate estimates of the degree of influence of natural enemies under varying agro-landscape and weather conditions considered in the AE/LW risk scenario. Broadly, these findings are relevant in understanding agroecosystem resilience and recommending supportive management inputs in response to insect invasions in context of natural enemy activity and varied environmental conditions.

Oscillation, synchrony, and multi-factor patterns between cereal aphids and parasitoid populations in southern Brazil

In different parts of the world, aphid populations and their natural enemies are influenced by landscapes and climate. In the Neotropical region, few long-term studies have been conducted, maintaining a gap for comprehension of the effect of meteorological variables on aphid population patterns and their parasitoids in field conditions. This study describes the general patterns of oscillation in cereal winged aphids and their parasitoids, selecting meteorological variables and evaluating their effects on these insects. Aphids exhibit two annual peaks, one in summer-fall transition and the other in winter-spring transition. For parasitoids, the highest annual peak takes place during winter and a second peak occurs in winter-spring transition. Temperature was the principal meteorological regulator of population fluctuation in winged aphids and parasitoids during the year. The favorable temperature range is not the same for aphids and parasitoids. For aphids, temperature increase resulted in population growth, with maximum positive effect at 25°C. Temperature also positively influenced parasitoid populations, but the growth was asymptotic around 20°C. Although rainfall showed no regulatory function on aphid seasonality, it influenced the final number of insects over the year. The response of aphids and parasitoids to temperature has implications for trophic compatibility and regulation of their populations. Such functions should be taken into account in predictive models.

Field Assessment of Aphid Doubling Time and Yield of Sorghum Susceptible and Partially Resistant to Sugarcane Aphid (Hemiptera: Aphididae)

Since outbreaks were first detected in grain sorghum, Sorghum bicolor (L.) Moench (Cyperales: Poaceae), in 2013, sugarcane aphid, Melanaphis sacchari Zehntner has become a major annual pest in grain sorghum-producing regions of North America. Economic thresholds have been recommended for susceptible hybrids, but these recommendations may not be suitable for grain sorghum hybrids partially resistant to sugarcane aphid. The objectives were to evaluate the grain yield-aphid density relationship and field-based population growth rates of the aphid on sorghum hybrids susceptible and partially resistant to sugarcane aphid across multiple years, locations, and hybrids. These data verified previously established economic injury levels for susceptible hybrids. The observed maximum aphid density ranged from 6 to 451 aphids per leaf for resistant hybrids and from 67 to 1,025 for susceptible hybrids. Across 50 location-year combinations, the maximum aphid density observed on resistant hybrids decreased by 0-99%, compared to a susceptible hybrid at the same location (mean reduction = 80%). Doubling time for sugarcane aphid populations on partially resistant hybrids was up to 6.4-fold higher than on known susceptible hybrids. For 48 of the 50 location-years, yield loss attributable to sugarcane aphid was not detected on the partially resistant hybrids; therefore, an economic injury level was unable to be estimated. If an economic injury level exists for resistant hybrids, it is likely at an aphid population level that exceeds the levels experienced in this study. It remains prudent to monitor resistant hybrids for unusual leaf decay associated with aphid densities that exceed current economic injury levels used for susceptible hybrids.

Altering Planting Date to Manage Melanaphis sacchari (Hemiptera: Aphididae) Populations in Sweet Sorghum

Melanaphis sacchari Zehntner is a new pest of sweet sorghum in the United States, with the potential to cause complete crop failure. In Kentucky, sweet sorghum is normally planted in early May and harvested in late August or September. Planting sweet sorghum earlier in the season may avoid damaging levels of M. sacchari that develop in late summer. In a 2-yr field study, three different planting dates separated by a month (April, May, and June) were tested for their effect on M. sacchari densities and sweet sorghum yield. April (early) planted sweet sorghum was grown in greenhouses and transplanted to the field. May (mid) and June (late) planted sweet sorghum were direct seeded in the field. Melanaphis sacchari population densities were evaluated weekly starting in June. Sweet sorghum was harvested at the onset of the hard dough stage. Plots were split into two subplots, insecticide or noninsecticide, in the second year to control for planting date effect on yield. Early-planted sweet sorghum had lower aphid densities, but had lower yield relative to mid-planting date, which had the highest yield. Insecticide drenches in 2019 reduced cumulative aphid days in mid-plantings and late plantings, but did not significantly affect yield within planting dates. Seeding sweet sorghum earlier can reduce M. sacchari densities; however, this method alone may not provide the highest yields. We documented that the recommended planting date (May) for Kentucky produced the highest yield.

Effects of Landscape Composition on Wheat Stem Sawfly (Hymenoptera: Cephidae) and Its Associated Braconid Parasitoids

Several agroecological and integrated pest management strategies focus on landscape management to increase complexity and foster biodiversity. However, landscape complexity does not always enhance biological control and in some cases may lead to increased pest populations. We examined the prevalence of two Bracon parasitoids, Bracon cephi Gahan and Bracon lissogaster Muesebeck (Hymenoptera: Braconidae), and their host the wheat stem sawfly Cephus cinctus Norton, a major pest of wheat. We assessed the degree of noncrop and crop host plant use and responses to landscape composition. We found no instances of parasitism by either Bracon species in our three-year, statewide winter wheat survey but found small populations of Bracon in noncrop landscapes throughout eastern and western Colorado. We used model selection to examine how local (500 m scale) and landscape (5 km scale) cover of suitable noncrop and crop habitats potentially affects abundances of Bracon and wheat stem sawfly. Our best fit model for wheat stem sawfly suggests that a decrease in noncrop cover at the landscape scale leads to an increase in wheat stem sawfly infestation. Our best fit model for Bracon parasitism suggests an increase in wheat cover at the local level results in the greatest increase in the odds of parasitism by either species of Bracon. Herbaceous cover at local and landscape scales were also significant predictors of Bracon parasitism. The results of this study suggest that pest and natural enemies respond differently to landscape composition and these responses should be evaluated before management decisions are made.

Evaluation of Tactics for Management of Sugarcane Aphid (Hemiptera: Aphididae) in Grain Sorghum

The invasive sugarcane aphid, Melanaphis sacchari (Zehntner), is a devastating new pest of grain sorghum. Studies were conducted utilizing an integrated approach of four management tactics: planting date, insecticidal seed treatment, a foliar-applied insecticide, and plant resistance. Experiments were conducted in 2016 and 2017 at Griffin, Tifton, and Plains Georgia, and in 2016 in Texas, Alabama, and Oklahoma, United States. Early planting was effective in reducing damage and increasing yields when compared to the late planting. Use of a resistant variety reduced cumulative aphid-days, plant injury and usually prevented significant yield loss. Foliar application of flupyradifurone when aphids reached an economic threshold, was an effective management tactic preventing aphid injury and yield loss. Use of clothianidin seed treatment also reduced aphid injury and yield loss of the susceptible hybrid but generally did not prevent injury and yield loss of the resistant hybrid. We conclude that an earlier planting date coupled with a resistant variety and judicious use of an efficacious foliar-applied insecticide can effectively manage sugarcane aphid on grain sorghum. An insecticide seed treatment also may be useful to reduce the risk of sugarcane aphid damage to seedlings of susceptible hybrids.

Development of Economic Thresholds for Sugarcane Aphid (Hemiptera: Aphididae) in Susceptible Grain Sorghum Hybrids

Sugarcane aphid, Melanaphis sacchari Zehtner (Hemiptera: Aphididae), outbreaks on grain sorghum were first detected in the United States in 2013. The spread of sugarcane aphid across the sorghum-producing regions of North America necessitated increased insecticide use to mitigate economic loss. A field experiment to develop economic thresholds for sugarcane aphid was conducted 15 times across seven locations across the southern United States during 3 yr (2014-2016). Grain sorghum hybrids were evaluated by measuring yield in response to a range of aphid infestations. Yield-aphid population density relationships were described by linear function, which facilitated calculating economic injury levels and economic thresholds. The slopes of the yield-aphid density regressions were significant, negative, and relatively stable across locations, years, and agronomic conditions. The relationships aggregated into two groups, populations that exhibited relatively slow and fast population growth, and common economic injury levels were determined using control costs and market values of grain. Average economic injury levels of 37 and 102 aphids per leaf were most applicable to the two groupings of sorghum/aphid relationships and aphid population growth. Using field-based sugarcane aphid population doubling time estimated from weekly observations of aphid densities, economic thresholds were calculated, ranging from 19 to 132 aphids per leaf across the 15 locations-years. Without site-specific knowledge of a slow-growing aphid population and given cost and market price variability of the system, a 40 aphid per leaf threshold is most prudent to use across the range of hybrid, environmental, and market conditions experienced in this study.