Aphid-Induced Volatiles and Subsequent Attraction of Natural Enemies Varies among Sorghum Cultivars

The production of herbivore-induced plant volatiles (HIPVs) is a type of indirect defense used by plants to attract natural enemies and reduce herbivory by insect pests. In many crops little is known about genotypic variation in HIPV production or how this may affect natural enemy attraction. In this study, we identified and quantified HIPVs produced by 10 sorghum (Sorghum bicolor) cultivars infested with a prominent aphid pest, the sorghum aphid (Melanaphis sorghi Theobald). Volatiles were collected using dynamic headspace sampling techniques and identified and quantified using GC-MS. The total amounts of volatiles induced by the aphids did not differ among the 10 cultivars, but overall blends of volatiles differed significantly in composition. Most notably, aphid herbivory induced higher levels of methyl salicylate (MeSA) emission in two cultivars, whereas in four cultivars, the volatile emissions did not change in response to aphid infestation. Dual-choice olfactometer assays were used to determine preference of the aphid parasitoid, Aphelinus nigritus, and predator, Chrysoperla rufilabris, between plants of the same cultivar that were un-infested or infested with aphids. Two aphid-infested cultivars were preferred by natural enemies, while four other cultivars were more attractive to natural enemies when they were free of aphids. The remaining four cultivars elicited no response from parasitoids. Our work suggests that genetic variation in HIPV emissions greatly affects parasitoid and predator attraction to aphid-infested sorghum and that screening crop cultivars for specific predator and parasitoid attractants has the potential to improve the efficacy of biological control.

Plant size, latitude, and phylogeny explain within-population variability in herbivory

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.

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.

The discrepancy between fire ant recruitment to and performance on rodent carrion

Ants have not been considered important in the process of vertebrate carrion decomposition, but a recent literature review reported over 150 carrion-visiting ant species. Though many ant species have been observed to remove carrion tissue and consume carrion-exuded liquids, the significance of ant recruitment to vertebrate carrion is poorly understood. We conducted a combination of field and laboratory experiments to quantify red imported fire ant recruitment to rodent carrion and determine whether consuming rodent carrion is beneficial to ant colony performance. In the field, 100% of rat carcasses were rapidly colonized by fire ants at high abundances. In our laboratory experiment, the performance of mice-fed fire ant colonies was poor when compared to colonies that were fed mice and insects or insects only. Our results suggest that there is a discrepancy between high levels of fire ant recruitment to vertebrate carrion and the poor colony performance when fed carrion. We hypothesize that fire ants are attracted to vertebrate carrion not because it is a high-quality food, but rather because it hosts large numbers of other invertebrates that can serve as prey for fire ants, potentially showcasing an interesting case of tritrophic interaction in carrion ecology.

Distinct colony boundaries and larval discrimination in polygyne red imported fire ants (Solenopsis invicta)

Evaluating the factors that promote invasive ant abundance is critical to assess their ecological impact and inform their management. Many invasive ant species show reduced nestmate recognition and an absence of boundaries between unrelated nests, which allow populations to achieve greater densities due to reduced intraspecific competition. We examined nestmate discrimination and colony boundaries in introduced populations of the red imported fire ant (Solenopsis invicta; hereafter, fire ant). Fire ants occur in two social forms: monogyne (colonies with a single egg-laying queen) and polygyne (colonies with multiple egg-laying queens). In contrast with monogyne nests, polygyne nests are thought to be interconnected due to the reduced antagonism between non-nestmate polygyne workers, perhaps because polygyne workers habituate the colony to an odour unique to Gp-9^b -carrying adults. However, colony boundaries and nestmate discrimination are poorly documented, particularly for worker-brood interactions. To delimit boundaries between field colonies, we correlated the exchange of a ¹⁵ N-glycine tracer dissolved in a sucrose solution with social form. We also evaluated nestmate discrimination between polygyne workers and larvae in the laboratory. Counter to our expectations, polygyne colonies behaved identically to monogyne colonies, suggesting both social forms maintain strict colony boundaries. Polygyne workers also preferentially fed larval nestmates and may have selectively cannibalized non-nestmates. The levels of relatedness among workers in polygyne colonies was higher than those previously reported in North America (mean ± standard error: 0.269 ± 0.037). Our study highlights the importance of combining genetic analyses with direct quantification of resource exchange to better understand the factors influencing ant invasions.

Crop and Semi-Natural Habitat Configuration Affects Diversity and Abundance of Native Bees (Hymenoptera: Anthophila) in a Large-Field Cotton Agroecosystem

Simple Summary

Commercial cotton growing systems are one of the most intensely managed, economically, and culturally important fiber cropping systems worldwide. The composition and configuration of crop species and semi-natural habitat can have significant effects on ecosystem services such as pollination. Here, we investigated the local-scale effect on the arrangement of different crop fields and surrounding semi-natural habitat in a large-field commercial cotton system on the diversity and abundance of native bee pollinators. Using bee bowl traps at crop interfaces (cotton grown next to cotton, sorghum, or semi-natural habitat along with a natural habitat comparator), we found a total of 32 bee species in 13 genera across 3 families. The most abundant native bee was Melissodes tepaneca Cresson (>4000 individuals, ~75% of bees collected). A higher number of individuals were found in all cotton–crop interfaces compared to the cotton next to semi-natural habitat or natural habitat alone. Native bee communities were also found to be influenced by the crop interface. Communities of native bees in the cotton–crop interfaces tended to be more consistent in the number of bees and number of bee species. While cotton grown next to semi-natural habitat had a more diverse array of bees, the number of bees collected varied. These data suggest that native bee communities persist in large-field commercial cotton growing systems. Select species dominate (i.e., M. tepaneca) and thrive in this large-field cotton system where cotton–crop interfaces are key local landscape features. These data have implications for potential pollination benefits to cotton production. The findings contribute to a discussion regarding the role of large-field commercial cotton growing systems in conserving native bees.

Abstract

The cotton agroecosystem is one of the most intensely managed, economically and culturally important fiber crops worldwide, including in the United States of America (U.S.), China, India, Pakistan, and Brazil. The composition and configuration of crop species and semi-natural habitat can have significant effects on ecosystem services such as pollination. Here, we investigated the local-scale effect of crop and semi-natural habitat configuration in a large field (>200 ha in size) cotton agroecosystem on the diversity and abundance of native bees. The interfaces sampled included cotton grown next to cotton, sorghum or semi-natural habitat along with a natural habitat comparator. Collections of native bees across interface types revealed 32 species in 13 genera across 3 families. Average species richness metrics ranged between 20.5 and 30.5, with the highest (30.5) at the interface of cotton and semi-natural habitat. The most abundant species was Melissodes tepaneca Cresson (>4000 individuals, ~75% of bees collected) with a higher number of individuals found in all cotton–crop interfaces compared to the cotton interface with semi-natural habitat or natural habitat alone. It was also found that interface type had a significant effect on the native bee communities. Communities of native bees in the cotton–crop interfaces tended to be more consistent in species richness and abundance. While cotton grown next to semi-natural habitat had higher species richness, the number of bees collected varied. These data suggest that native bee communities persist in large-field cotton agroecosystems. Selected species dominate (i.e., M. tepaneca) and thrive in this large-field cotton system where cotton–crop interfaces are key local landscape features. These data have implications for potential pollination benefits to cotton production. The findings also contribute to a discussion regarding the role of large-field commercial cotton growing systems in conserving native bees.

Adding a Natural Enemy to Respond to Pest Immigration and Delayed Natural Enemy Releases in Augmentative Biological Control

Whether increased natural enemy density or adding a second natural enemy species will provide superior pest suppression in greenhouse augmentative biological control is unknown for many commercially available natural enemy species. In this study, we use sweetpotato whiteflies, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), on poinsettias, Euphorbia pulcherrima Willd. ex Klotzsch (Malpighiales: Euphorbiaceae), to determine whether adding Amblyseius swirskii (Athias-Henriot) (Acari: Phytoseiidae) to Eretmocerus eremicus Rose and Zolnerowich (Hymenoptera: Aphelinidae) is better for B. tabaci suppression compared with either natural enemy alone, both with and without challenges with whitefly immigration or delayed natural enemy releases. The number of whiteflies on caged poinsettias treated with different natural enemy release rates (single or double rate), natural enemy species (one or two species), natural enemy delayed release (weeks 4 and 8), and whitefly immigration treatments (introduced at week 4 or week 8) was censused biweekly for 16 wk. Both species used in combination provided similar or better suppression of whiteflies compared with either natural enemy alone. Both species combined also provided superior suppression of whiteflies when challenged with whitefly immigration or delays in natural enemy releases compared with E. eremicus alone. Whitefly immigration or delays in E. eremicus releases did not increase whitefly populations, suggesting that suppression of whiteflies by E. eremicus alone is relatively robust. This study found no evidence for negative interactions between E. eremicus and A. swirskii for suppressing B. tabaci.

Sugar Feeding Patterns for Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae) Mosquitoes in South Texas

Effective mosquito surveillance and management depend on a thorough understanding of the biology and feeding patterns unique to species and sex. Given that a propensity to sugar feed is necessary for some mosquito surveillance and newer control strategies, we sought to document the amount of total sugar in wild Aedes aegypti (L.) and Culex quinquefasciatus (Say) captured from five different locations in the Lower Rio Grande Valley (LRGV) of South Texas over 2 yr. We used Biogents Sentinel 2 (BGS2) traps in year 1 and aspirators, BGS2, and CDC resting traps in years 2 and 3 to collect adult mosquitoes. The hot anthrone test was used to quantify total sugar content in each mosquito. Additionally, the cold and hot anthrone tests were used to distinguish fructose content from total sugars for mosquitoes captured in 2019. Overall, Ae. aegypti females had significantly lower total sugar content than Ae. aegypti males as well as both sexes of Cx. quinquefasciatus. However, the percentage of Ae. aegypti positive for fructose consumption was four to eightfold higher than Ae. aegypti previously reported in other regions. The difference between locations was significant for males of both species, but not for females. Seasonality and trapping method also revealed significant differences in sugar content of captured mosquitoes. Our results reinforce that sugar feeding in female Ae. aegypti is less than Cx. quinquefasciatus, although not absent. This study provides necessary data to evaluate the potential effectiveness of sugar baits in surveillance and control of both Ae. aegypti and Cx. quinquefasciatus mosquitoes.

Red imported fire ant (Solenopsis invicta) aggression influences the behavior of three hard tick species

Few studies have documented the indirect effects of predators on tick behavior. We conducted behavioral assays in the laboratory to quantify the effects of a highly abundant predator, the red imported fire ant (Solenopsis invicta), on three species of ticks endemic to the southern USA: the lone star tick (Amblyomma americanum), the Gulf Coast tick (A. maculatum), and the Cayenne tick (A. mixtum). We documented ant aggression toward ticks (biting, carrying, and stinging) and determined the effects of ants on tick activity. Ticks were significantly less active in the presence of fire ants, and tick activity was negatively associated with ant aggression, but in many cases the effects of fire ants on ticks varied by tick species, stage, and engorgement status. For example, fire ants took half as long (~ 62 s) to become aggressive toward unfed A. americanum adults compared with unfed A. maculatum, and only ~ 8 s to become aggressive toward engorged A. maculatum nymphs. Correspondingly, the activity of unfed A. americanum adults and engorged A. maculatum nymphs was reduced by 67 and 93%, respectively, in the presence of fire ants. This reduction in tick activity translated to less questing by unfed ticks and less time spent walking by engorged nymphs. Our results suggest that fire ants may have important non-consumptive effects on ticks and demonstrate the importance of measuring the indirect effects of predators on tick behavior.

Neonicotinoid Insecticides Alter the Transcriptome of Soybean and Decrease Plant Resistance

Neonicotinoids are widely used systemic insecticides that have been associated with spider mite outbreaks on diverse plants. These insecticides have complex effects on plant physiology, which have been speculated to drive enhanced performance of spider mites. We used RNA-Seq to explore how neonicotinoids modify gene expression in soybean thereby lowering plant resistance. We exposed soybean (Glycine max L.) to two neonicotinoid insecticides, thiamethoxam applied to seeds and imidacloprid applied as a soil drench, and we exposed a subset of these plants to spider mites (Tetranychus cinnabarinus). Applications of both insecticides downregulated genes involved in plant-pathogen interactions, phytohormone pathways, phenylpropanoid pathway, and cell wall biosynthesis. These effects were especially pronounced in plants exposed to thiamethoxam. Introduction of spider mites restored induction of genes in these pathways in plants treated with imidacloprid, while expression of genes involved in phenylpropanoid synthesis, in particular, remained downregulated in thiamethoxam-treated plants. Our outcomes indicate that both insecticides suppress genes in pathways relevant to plant⁻arthropod interactions, and suppression of genes involved in cell wall synthesis may explain lower plant resistance to spider mites, cell-content feeders. These effects appear to be particularly significant when plants are exposed to neonicotinoids applied to soybean seeds.

Decreased small mammal and on-host tick abundance in association with invasive red imported fire ants (Solenopsis invicta)

Invasive species may impact pathogen transmission by altering the distributions and interactions among native vertebrate reservoir hosts and arthropod vectors. Here, we examined the direct and indirect effects of the red imported fire ant (Solenopsis invicta) on the native tick, small mammal and pathogen community in southeast Texas. Using a replicated large-scale field manipulation study, we show that small mammals were more abundant on treatment plots where S. invicta populations were experimentally reduced. Our analysis of ticks on small mammal hosts demonstrated a threefold increase in the ticks caught per unit effort on treatment relative to control plots, and elevated tick loads (a 27-fold increase) on one common rodent species. We detected only one known human pathogen (Rickettsia parkeri), present in 1.4% of larvae and 6.7% of nymph on-host Amblyomma maculatum samples but with no significant difference between treatment and control plots. Given that host and vector population dynamics are key drivers of pathogen transmission, the reduced small mammal and tick abundance associated with S. invicta may alter pathogen transmission dynamics over broader spatial scales.

Summer and fall ants have different physiological responses to food macronutrient content

Seasonally, long-lived animals exhibit changes in behavior and physiology in response to shifts in environmental conditions, including food abundance and nutritional quality. Ants are long-lived arthropods that, at the colony level, experience such seasonal shifts in their food resources. Previously we reported summer- and fall-collected ants practiced distinct food collection behavior and nutrient intake regulation strategies in response to variable food protein and carbohydrate content, despite being reared in the lab under identical environmental conditions and dietary regimes. Seasonally distinct responses were observed for both no-choice and choice dietary experiments. Using data from these same experiments, our objective here is to examine colony and individual-level physiological traits, colony mortality and growth, food processing, and worker lipid mass, and how these traits change in response to variable food protein-carbohydrate content. For both experiments we found that seasonality per se exerted strong effects on colony and individual level traits. Colonies collected in the summer maintained total worker mass despite high mortality. In contrast, colonies collected in the fall lived longer, and accumulated lipids, including when reared on protein-biased diets. Food macronutrient content had mainly transient effects on physiological responses. Extremes in food carbohydrate content however, elicited a compensatory response in summer worker ants, which processed more protein-biased foods and contained elevated lipid levels. Our study, combined with our previously published work, strongly suggests that underlying physiological phenotypes driving behaviors of summer and fall ants are likely fixed seasonally, and change circannually.

Intraspecific Variation among Social Insect Colonies: Persistent Regional and Colony-Level Differences in Fire Ant Foraging Behavior

Individuals vary within a species in many ecologically important ways, but the causes and consequences of such variation are often poorly understood. Foraging behavior is among the most profitable and risky activities in which organisms engage and is expected to be under strong selection. Among social insects there is evidence that within-colony variation in traits such as foraging behavior can increase colony fitness, but variation between colonies and the potential consequences of such variation are poorly documented. In this study, we tested natural populations of the red imported fire ant, Solenopsis invicta, for the existence of colony and regional variation in foraging behavior and tested the persistence of this variation over time and across foraging habitats. We also reared single-lineage colonies in standardized environments to explore the contribution of colony lineage. Fire ants from natural populations exhibited significant and persistent colony and regional-level variation in foraging behaviors such as extra-nest activity, exploration, and discovery of and recruitment to resources. Moreover, colony-level variation in extra-nest activity was significantly correlated with colony growth, suggesting that this variation has fitness consequences. Lineage of the colony had a significant effect on extra-nest activity and exploratory activity and explained approximately half of the variation observed in foraging behaviors, suggesting a heritable component to colony-level variation in behavior.

Effect of Carbohydrate Supplementation on Investment into Offspring Number, Size, and Condition in a Social Insect

Resource availability can determine an organism's investment strategies for growth and reproduction. When nutrients are limited, there are potential tradeoffs between investing into offspring number versus individual offspring size. In social insects, colony investment in offspring size and number may shift in response to colony needs and the availability of food resources. We experimentally manipulated the diet of a polymorphic ant species (Solenopsis invicta) to test how access to the carbohydrate and amino acid components of nectar resources affect colony investment in worker number, body size, size distributions, and individual percent fat mass. We reared field-collected colonies on one of four macronutrient treatment supplements: water, amino acids, carbohydrates, and amino acid and carbohydrates. Having access to carbohydrates nearly doubled colony biomass after 60 days. This increase in biomass resulted from an increase in worker number and mean worker size. Access to carbohydrates also altered worker body size distributions. Finally, we found a negative relationship between worker number and size, suggesting a tradeoff in colony investment strategies. This tradeoff was more pronounced for colonies without access to carbohydrate resources. The monopolization of plant-based resources has been implicated in the ecological success of ants. Our results shed light on a possible mechanism for this success, and also have implications for the success of introduced species. In addition to increases in colony size, our results suggest that having access to plant-based carbohydrates can also result in larger workers that may have better individual fighting ability, and that can withstand greater temperature fluctuations and periods of food deprivation.

Interaction webs in agroecosystems: beyond who eats whom

Studies of insect communities in agroecosystems have historically focused on a food web approach: who eats whom. Recent research has highlighted the importance of indirect effects in determining the abundance and distribution of insects and their effects on other insects and plants. These indirect interactions include apparent competition where an herbivore indirectly suppresses another herbivore by attracting shared predators, cases where predators or even other herbivores alter the behavior and/or physiology of herbivores in ways that result in decreased plant consumption, and mutualisms that can generate a network of indirect effects that alter the abundance of many species within a community. The consequences of these indirect interactions have been modeled and proof-of-concept studies have demonstrated their potential importance, but studies of the consequences of these interactions on crop yield are sorely needed. Documenting the prevalence and consequences of these indirect effects in multiple crops will allow researchers to compare and contrast responses across systems and identify key species or characteristics of agroecosystems that dictate when and where these effects are important. This research will ultimately allow growers to manipulate these interactions to increase ecosystem services provided by insects and increase crop yield.

Interactions between insect herbivores and plant mating systems

Self-pollination is common in plants, and limited seed and pollen dispersal can create localized inbreeding even within outcrossing plants. Consequently, insects regularly encounter inbred plants in nature. Because inbreeding results in elevated homozygosity, greater expression of recessive alleles, and subsequent phenotypic changes in inbred plants, inbreeding may alter plant-insect interactions. Recent research has found that plant inbreeding alters resistance and tolerance to herbivores, alters the attraction and susceptibility of plants to insects that vector plant pathogens, and alters visitation rates of insect pollinators. These results suggest that interactions with insects can increase or decrease inbreeding depression (the loss of fitness due to self-fertilization) and subsequently alter the evolution of selfing within plant populations. Future work needs to focus on the mechanisms underlying genetic variation in the effects of inbreeding on plant-insect interactions and the consequences of altered plant-insect interactions on the evolution of plant defense and plant mating systems.

The effect of diet and opponent size on aggressive interactions involving caribbean crazy ants (Nylanderia fulva)

Biotic interactions are often important in the establishment and spread of invasive species. In particular, competition between introduced and native species can strongly influence the distribution and spread of exotic species and in some cases competition among introduced species can be important. The Caribbean crazy ant, Nylanderia fulva, was recently introduced to the Gulf Coast of Texas, and appears to be spreading inland. It has been hypothesized that competition with the red imported fire ant, Solenopsis invicta, may be an important factor in the spread of crazy ants. We investigated the potential of interspecific competition among these two introduced ants by measuring interspecific aggression between Caribbean crazy ant workers and workers of Solenopsis invicta. Specifically, we examined the effect of body size and diet on individual-level aggressive interactions among crazy ant workers and fire ants. We found that differences in diet did not alter interactions between crazy ant workers from different nests, but carbohydrate level did play an important role in antagonistic interactions with fire ants: crazy ants on low sugar diets were more aggressive and less likely to be killed in aggressive encounters with fire ants. We found that large fire ants engaged in fewer fights with crazy ants than small fire ants, but fire ant size affected neither fire ant nor crazy ant mortality. Overall, crazy ants experienced higher mortality than fire ants after aggressive encounters. Our findings suggest that fire ant workers might outcompete crazy ant workers on an individual level, providing some biotic resistance to crazy ant range expansion. However, this resistance may be overcome by crazy ants that have a restricted sugar intake, which may occur when crazy ants are excluded from resources by fire ants.

Neonicotinoid insecticides alter induced defenses and increase susceptibility to spider mites in distantly related crop plants

BACKGROUND

Chemical suppression of arthropod herbivores is the most common approach to plant protection. Insecticides, however, can cause unintended, adverse consequences for non-target organisms. Previous studies focused on the effects of pesticides on target and non-target pests, predatory arthropods, and concomitant ecological disruptions. Little research, however, has focused on the direct effects of insecticides on plants. Here we demonstrate that applications of neonicotinoid insecticides, one of the most important insecticide classes worldwide, suppress expression of important plant defense genes, alter levels of phytohormones involved in plant defense, and decrease plant resistance to unsusceptible herbivores, spider mites Tetranychus urticae (Acari: Tetranychidae), in multiple, distantly related crop plants.

METHODOLOGY/PRINCIPAL FINDINGS

Using cotton (Gossypium hirsutum), corn (Zea mays) and tomato (Solanum lycopersicum) plants, we show that transcription of phenylalanine ammonia lyase, coenzyme A ligase, trypsin protease inhibitor and chitinase are suppressed and concentrations of the phytohormone OPDA and salicylic acid were altered by neonicotinoid insecticides. Consequently, the population growth of spider mites increased from 30% to over 100% on neonicotinoid-treated plants in the greenhouse and by nearly 200% in the field experiment.

CONCLUSIONS/SIGNIFICANCE

Our findings are important because applications of neonicotinoid insecticides have been associated with outbreaks of spider mites in several unrelated plant species. More importantly, this is the first study to document insecticide-mediated disruption of plant defenses and link it to increased population growth of a non-target herbivore. This study adds to growing evidence that bioactive agrochemicals can have unanticipated ecological effects and suggests that the direct effects of insecticides on plant defenses should be considered when the ecological costs of insecticides are evaluated.

Introduced fire ants can exclude native ants from critical mutualist-provided resources

Animals frequently experience resource imbalances in nature. For ants, one resource that may be particularly valuable for both introduced and native species is high-carbohydrate honeydew from hemipteran mutualists. We conducted field and laboratory experiments: (1) to test if red imported fire ants (Solenopsis invicta) competed with native ants for access to mutualisms with aphids, and (2) to quantify the effects of aphid honeydew presence or absence on colony growth of native ants. We focused on native dolichoderine ants (Formicidae, Dolichoderinae) because they are abundant ants that have omnivorous diets that frequently include mutualist-provided carbohydrates. At two sites in the southeastern US, native dolichoderine ants were far less frequent, and fire ants more frequent, at carbohydrate baits than would be expected based on their frequency in pitfall traps. A field experiment confirmed that a native ant species, Dorymyrmex bureni, was only found tending aphids when populations of S. invicta were suppressed. In the laboratory, colonies of native dolichoderine ants with access to both honeydew and insect prey had twice as many workers and over twice as much brood compared to colonies fed only ad libitum insect prey. Our results provide the first experimental evidence that introduced ants compete for access to mutualist-provided carbohydrates with native ants and that these carbohydrates represent critical resources for both introduced and native ants. These results challenge traditional paradigms of arthropod and ant nutrition and contribute to growing evidence of the importance of nutrition in mediating ecological interactions.

Winter severity predicts the timing of host shifts in the mosquito Culex erraticus

In temperate regions, seasonal epidemics of many mosquito-borne viruses are triggered when mosquito populations shift from feeding on avian to mammalian hosts. We investigated effects of temperature on the timing of bird-to-mammal shifts using an 8 year dataset of blood-meals from a mosquito (Culex erraticus) in Alabama, USA. As expected, Cx. erraticus shifted from avian to mammalian hosts each year. The timing of the shift, however, varied considerably among years. Harshness of the preceding winter (chill accumulation) explained 93 per cent of the variation in the timing of bird-to-mammal shifts, with shifts occurring later in years following harsher winters. We hypothesize that winter temperatures drive the timing of bird-to-mammal shifts through effects on host reproductive phenology. Because mosquitoes target birds during the nesting season, and bird nesting occurs later in years following colder winters, later nesting dates result in a concomitant delay in the timing of bird-to-mammal host shifts. Global increases in winter temperatures could cause significant changes in the timing of seasonal host shifts by mosquitoes, with prolonged periods of epidemic transmission of mosquito-borne diseases.

Seasonality directs contrasting food collection behavior and nutrient regulation strategies in ants

Long-lived animals, including social insects, often display seasonal shifts in foraging behavior. Foraging is ultimately a nutrient consumption exercise, but the effect of seasonality per se on changes in foraging behavior, particularly as it relates to nutrient regulation, is poorly understood. Here, we show that field-collected fire ant colonies, returned to the laboratory and maintained under identical photoperiod, temperature, and humidity regimes, and presented with experimental foods that had different protein (p) to carbohydrate (c) ratios, practice summer- and fall-specific foraging behaviors with respect to protein-carbohydrate regulation. Summer colonies increased the amount of food collected as the p:c ratio of their food became increasingly imbalanced, but fall colonies collected similar amounts of food regardless of the p:c ratio of their food. Choice experiments revealed that feeding was non-random, and that both fall and summer ants preferred carbohydrate-biased food. However, ants rarely ate all the food they collected, and their cached or discarded food always contained little carbohydrate relative to protein. From a nutrient regulation strategy, ants consumed most of the carbohydrate they collected, but regulated protein consumption to a similar level, regardless of season. We suggest that varied seasonal food collection behaviors and nutrient regulation strategies may be an adaptation that allows long-lived animals to meet current and future nutrient demands when nutrient-rich foods are abundant (e.g. spring and summer), and to conserve energy and be metabolically more efficient when nutritionally balanced foods are less abundant.

Winter biology of wetland mosquitoes at a focus of eastern equine encephalomyelitis virus transmission in Alabama, USA

At temperate latitudes, vectors and pathogens must possess biological mechanisms for coping with cold temperatures and surviving from one transmission season to the next. Mosquitoes that overwinter in the adult stage have been proposed as winter maintenance hosts for certain arboviruses. In the cases of West Nile virus (family Flaviviridae, genus Flavivirus) and St. Louis encephalitis virus (family Flaviviridae, genus Flavivirus), discovery of infected overwintering females lends support to this hypothesis, but for other arboviruses, in particular Eastern equine encephalomyelitis virus (family Togaviridae, genus Alphavirus, EEEV), overwintering of the virus in mosquito hosts as not been demonstrated. In the current study, we collected overwintering mosquitoes from a focus of EEEV transmission in the southeastern United States to determine whether mosquitoes serve as winter maintenance hosts for EEEV and to document overwintering biologies of suspected vectors. No virus was detected via reverse transcription-polymerase chain reaction of > 500 female mosquitoes collected during three winters. Investigation into the winter biologies indicated that Anopheles punctipennis (Say), Culex erraticus (Dyar & Knab), Culex peccator Dyar & Knab, and Uranotaenia sapphirina (Osten Sacken) overwinter as females. Females of these species were collected from hollow trees and emergence traps placed over ground holes. Southern magnolia, Magnolia grandiflora L., trees were preferred overwintering sites of culicine mosquitoes. Emergence from underground overwintering sites peaked in mid-March, when air temperatures reached 18-22 degrees C, and the first blood-engorged females of Cx. erraticus and Cx. peccator were collected during this same period. Blood-fed Culex territans Walker females were collected as early as mid-February. This work provides insight into the overwintering biologies of suspected virus vectors at a site of active EEEV transmission and provides limited evidence against the hypothesis that EEEV persists through intertransmission periods in overwintering mosquitoes.

Macronutrient content of plant-based food affects growth of a carnivorous arthropod

Many arthropods engage in mutualisms in which they consume plant-based foods including nectar, extrafloral nectar, and honeydew. However, relatively little is known about the manner in which the specific macronutrients in these plant-based resources affect growth, especially for carnivorous arthropods. Using a combination of laboratory and field experiments, we tested (1) how plant-based foods, together with ad libitum insect prey, affect the growth of a carnivorous ant, Solenopsis invicta, and (2) which macronutrients in these resources (i.e., carbohydrates, amino acids, or both) contribute to higher colony growth. Access to honeydew increased the production of workers and brood in experimental colonies. This growth effect appeared to be due to carbohydrates alone as colonies provided with the carbohydrate component of artificial extrafloral nectar had greater worker and brood production compared to colonies deprived of carbohydrates. Surprisingly, amino acids only had a slight interactive effect on the proportion of a colony composed of brood and negatively affected worker survival. Diet choice in the laboratory and field matched performance in the laboratory with high recruitment to carbohydrate baits and only slight recruitment to amino acids. The strong, positive effects of carbohydrates on colony growth and the low cost of producing this macronutrient for plants and hemipterans may have aided the evolution of food-for-protection mutualisms and help explain why these interactions are so common in ants. In addition, greater access to plant-based resources in the introduced range of S. invicta may help to explain the high densities achieved by this species throughout the southeastern United States.

Host reproductive phenology drives seasonal patterns of host use in mosquitoes

Seasonal shifts in host use by mosquitoes from birds to mammals drive the timing and intensity of annual epidemics of mosquito-borne viruses, such as West Nile virus, in North America. The biological mechanism underlying these shifts has been a matter of debate, with hypotheses falling into two camps: (1) the shift is driven by changes in host abundance, or (2) the shift is driven by seasonal changes in the foraging behavior of mosquitoes. Here we explored the idea that seasonal changes in host use by mosquitoes are driven by temporal patterns of host reproduction. We investigated the relationship between seasonal patterns of host use by mosquitoes and host reproductive phenology by examining a seven-year dataset of blood meal identifications from a site in Tuskegee National Forest, Alabama USA and data on reproduction from the most commonly utilized endothermic (white-tailed deer, great blue heron, yellow-crowned night heron) and ectothermic (frogs) hosts. Our analysis revealed that feeding on each host peaked during periods of reproductive activity. Specifically, mosquitoes utilized herons in the spring and early summer, during periods of peak nest occupancy, whereas deer were fed upon most during the late summer and fall, the period corresponding to the peak in births for deer. For frogs, however, feeding on early- and late-season breeders paralleled peaks in male vocalization. We demonstrate for the first time that seasonal patterns of host use by mosquitoes track the reproductive phenology of the hosts. Peaks in relative mosquito feeding on each host during reproductive phases are likely the result of increased tolerance and decreased vigilance to attacking mosquitoes by nestlings and brooding adults (avian hosts), quiescent young (avian and mammalian hosts), and mate-seeking males (frogs).

Crepuscular flight activity of Culex erraticus (Diptera: Culicidae)

Circadian patterns of flight activity in mosquitoes can influence pathogen transmission by regulating dispersal potential of vectors and contact rates between vectors and reservoir and/or dead-end hosts. We investigated circadian activity patterns of Culex erraticus (Dyar and Knab) at a wetland field site in central Alabama, by aspirating resting adults and questing females in the morning and evening hours, respectively. Mosquitoes were aspirated at regular time intervals to determine the time of day during which peak resting site-seeking and host-seeking activities occurred. Day-to-day variation in activity patterns due to wind, humidity, and temperature was examined using stepwise linear regression. We found a distinct peak in flight activity during the morning hours (2 h before and 2 h after sunrise) for females and males of Culex erraticus, the most commonly encountered species at the site. The exact time of the peak varied from day to day, and was largely a function of temperature. A less distinct peak in activity was observed for questing females in the evening, although flights generally commenced just after sunset and peaked 30-60 min after sunset. A significant amount of day-to-day variation in the number of questing females was attributable to relative humidity. Our study demonstrates predictable patterns of circadian activity for Cx. erraticus, a suspected bridge vector of eastern equine encephalitis virus. Moreover, these patterns are modulated by environmental conditions. This information may be used to develop vector control strategies and make predictions about factors that affect the spread of mosquito-vectored pathogens.

Robert F. Denno (1945-2008): insect ecologist extraordinaire

Robert F. Denno was widely recognized as one of the leading insect ecologists in the world. He made major contributions to the study of plant-insect interactions, dispersal, interspecific competition, predator-prey interactions, and food web dynamics. He was especially well known for his detailed and comprehensive study of the arthropods that inhabit salt marshes. Denno promoted a research approach that included detailed knowledge of the natural history of the study system, meticulous experiments that often pushed logistical possibilities, and a focus on important ecological questions of the day. He was an enthusiastic collaborator and excellent mentor who invested incredible amounts of time and energy in the training and placement of graduate students and postdoctoral associates. As a result, Denno's legacy will continue to shape the field of insect ecology for generations to come.

Vector-host interactions in avian nests: do mosquitoes prefer nestlings over adults?

The hypothesis that nestlings are a significant driver of arbovirus transmission and amplification is based upon findings that suggest nestlings are highly susceptible to being fed upon by vector mosquitoes and to viral infection and replication. Several previous studies have suggested that nestlings are preferentially fed upon relative to adults in the nest, and other studies have reported a preference for adults over nestlings. We directly tested the feeding preference of nestling and adult birds in a natural setting, introducing mosquitoes into nesting boxes containing eastern bluebirds (Sialia sialis), collecting blood-fed mosquitoes, and matching the source of mosquito blood meals to individual birds using microsatellite markers. Neither nestlings nor adults were fed upon to an extent significantly greater than would be predicted based upon their relative abundance in the nests, although feeding upon mothers decreased as the age of the nestlings increased.

Extrafloral nectar content alters foraging preferences of a predatory ant

We tested whether the carbohydrate and amino acid content of extrafloral nectar affected prey choice by a predatory ant. Fire ants, Solenopsis invicta, were provided with artificial nectar that varied in the presence of carbohydrates and amino acids and were then provided with two prey items that differed in nutritional content, female and male crickets. Colonies of fire ants provided with carbohydrate supplements consumed less of the female crickets and frequently did not consume the high-lipid ovaries of female crickets. Colonies of fire ants provided with amino acid supplements consumed less of the male crickets. While a number of studies have shown that the presence of extrafloral nectar or honeydew can affect ant foraging activity, these results suggest that the nutritional composition of extrafloral nectar is also important and can affect subsequent prey choice by predatory ants. Our results suggest that, by altering the composition of extrafloral nectar, plants could manipulate the prey preferences of ants foraging on them.

Distribution expansion of Culex coronator in Alabama

Culex coronator, a mosquito species common to the American tropics, has been recently documented from a number of temperate areas in the USA. Since 2002 specimens have been reported for the first time from Oklahoma, Louisiana, Mississippi, Alabama, and Florida. Here we provide new collection records for Cx. coronator in east-central Alabama. In October 2007, 2 larvae of Cx. coronator were collected from an artificial container in Tuskegee National Forest in Macon County, AL. The distribution of Cx. coronator in the USA seems to be expanding at a prodigious rate, for reasons that remain unclear.

Blood feeding patterns of potential arbovirus vectors of the genus culex targeting ectothermic hosts

Reptiles and amphibians constitute a significant portion of vertebrate biomass in terrestrial ecosystems and may be important arbovirus reservoirs. To investigate mosquito preference for ectothermic hosts, feeding indices were calculated from data collected in Tuskegee National Forest, Alabama, USA. Four mosquito species fed upon ectothermic hosts, with Culex peccator and Cx. territans feeding primarily upon ectotherms. These two species appeared to target distinct species with little overlap in host choice. Culex peccator was a generalist in its feeding patterns within ectotherms, and Cx. territans appeared to be a more specialized feeder. Six of eleven ectotherm species fed upon by Cx. territans were fed upon more often than predicted based upon abundance. Spring peepers were highly preferred over other host species by Cx. territans. Blood meals taken from each host species varied temporally, with some hosts being targeted fairly evenly throughout the season and others being fed upon in seasonal peaks.

Preference of female mosquitoes for natural and artificial resting sites

At a wetland study site in Tuskegee National Forest, AL, resting female mosquitoes were collected from natural and artificial resting sites to identify species-specific resting sites and to evaluate various artificial resting sites for their utility in collecting resting mosquitoes. Natural resting sites included small tree cavities, large tree cavities, and understory vegetation. Artificial resting sites included resting boxes, fiber pots, and plastic trash cans. We collected 12,888 female mosquitoes, representing 23 species in 8 genera, during the 6-month study. Each mosquito species demonstrated a preference for a particular type of resting site. Resting Aedes vexans females were collected almost exclusively from understory vegetation, while the great majority of Anopheles quadrimaculatus females were aspirated from large tree cavities. Culex erraticus and Cx. peccator females preferred trash cans over other available resting sites. Females of Cx. territans, although collected most commonly in large tree cavities, were also collected frequently from understory vegetation and trash cans. A multiple regression of resting-site parameters (excluding vegetation), including volume, surface area, and opening size, indicated that 50% and 20% of the variability associated with An. quadrimaculatus and Cx. territans collections, respectively, could be explained by opening size. Inner surface area and volume accounted for 33% and 12% of variation in Cx. erraticus and Cx. peccator collections, respectively. Thus, female mosquitoes generally preferred larger resting sites over smaller resting sites. Similarly shaped artificial resting sites (fiber pots and trash cans) yielded comparable numbers of females per unit of volume (for those species that preferred artificial resting sites), indicating that shape of the resting site is an important factor in resting-site preference. In addition, trash cans proved to be a valuable novel tool for collecting resting female mosquitoes.

Ecological consequences of interactions between ants and honeydew-producing insects

Interactions between ants and honeydew-producing hemipteran insects are abundant and widespread in arthropod food webs, yet their ecological consequences are very poorly known. Ant-hemipteran interactions have potentially broad ecological effects, because the presence of honeydew-producing hemipterans dramatically alters the abundance and predatory behaviour of ants on plants. We review several studies that investigate the consequences of ant-hemipteran interactions as 'keystone interactions' on arthropod communities and their host plants. Ant-hemipteran interactions have mostly negative effects on the local abundance and species richness of several guilds of herbivores and predators. In contrast, out of the 30 studies that document the effects of ant-hemipteran interactions on plants, the majority (73%) shows that plants actually benefit indirectly from these interactions. In these studies, increased predation or harassment of other, more damaging, herbivores by hemipteran-tending ants resulted in decreased plant damage and/or increased plant growth and reproduction. The ecological consequences of mutualistic interactions between honeydew-producing hemipterans and invasive ants relative to native ants have rarely been studied, but they may be of particular importance owing to the greater abundance, aggressiveness and extreme omnivory of invasive ants. We argue that ant-hemipteran interactions are largely overlooked and underappreciated interspecific interactions that have strong and pervasive effects on the communities in which they are embedded.

Efficacy of broadcast and perimeter applications of S-methoprene bait on the red imported fire ant in grazed pastures

The red imported fire ant, Solenopsis invicta (Buren), is a major pest in the United States because of its painful sting. Toxic bait has been an important management tool against fire ants, but site registrations prohibit applications of most baits on grazed pastures. Extinguish, containing the insect growth regulator methoprene, was selected for this study because it has a broad site registration that includes grazed pastures. The primary objective of this research was to evaluate the efficacy for control of red imported fire ants by using broadcast applications of methoprene bait at a label rate of 1,121 g/ha versus applications around the perimeter of a target area at the reduced rate of 280 g/ha. Grazed pastures in Lee County, Alabama, and Chambers County, Alabama, were selected for this study, with broadcast treatments, perimeter treatments, and controls replicated three times at each site. All mounds were counted and rated using the USDA population index before applications and then at 8 and 16 wk posttreatment. Perimeter applications did not significantly reduce S. invicta mound abundance, but bait treatments significantly reduced mound abundance at 16 wk posttreatment at site 1 where applications were conducted in early evening. However, broadcast applications were not effective at site 2 where treatments were conducted in early morning with warmer temperatures. Emergence of winged alates was observed at 12 wk posttreatment, followed by a high density of incipient mounds that may have masked the full treatment effect of methoprene applications at site 2. Methoprene bait was effective in reducing abundance of S. invicta only when full label rates were applied.

Genetic variation and covariation for resistance and tolerance to Cucumber mosaic virus in Mimulus guttatus (Phrymaceae): a test for costs and constraints

Genetic variation for resistance and tolerance to pathogens may be maintained by costs represented as genetic tradeoffs between these traits and fitness. The evolution of resistance and tolerance also may be constrained by negative genetic correlations between these defense systems. Using a complete diallel, we measured genetic variation and covariation for and among performance, resistance, and tolerance traits in Mimulus guttatus challenged with a generalist pathogen, Cucumber mosaic virus (CMV). Viral coat protein was detected by enzyme-linked immunosorbent assay (ELISA) in all inoculated plants, indicating that all plants were susceptible to infection, although the ELISA absorbance varied quantitatively across plants. Plants inoculated with CMV had significantly reduced aboveground biomass and flower production relative to controls, although date of first flower was unaffected by infection. All three of these performance traits showed moderate to high narrow-sense heritability (h2 = 0.32-0.62) in both inoculated and control plants. We found phenotypic variation for both tolerance of and resistance to our strain of CMV, but both displayed very low narrow-sense heritability (h2 < 0.03). We found no evidence of a trade-off between resistance and tolerance. We also found no evidence for a cost of resistance or tolerance. In fact, a significant genetic correlation suggested that plants that were large when healthy had the greatest tolerance when infected. Significant, positive genetic correlations found between performance of uninfected and infected plants suggested that selection would likely favor the same M. guttatus genotypes whether CMV is present or not.

Effectiveness of Metal–Metal and Metal–Organic Compound Combinations Against Plutella xylostella: Implications for Plant Elemental Defense

Plants that contain elevated foliar metal concentrations can be categorized as accumulators or, if the accumulation is extreme, hyperaccumulators. The defense hypothesis suggests that these plants may be defended against folivore attack, and recent research has indicated that metal concentrations at or below the accumulator range may be defensively effective. This experiment explored the toxicity of four metals hyper-accumulated by plants (Cd, Ni, Pb, and Zn) and asked if combinations of metals, or metals and organic chemicals, might broaden the defensive effectiveness of metals. Metals were used alone and in certain metal + metal (Zn plus Ni, Pb, or Cd) and metal + organic defensive chemical (Ni plus tannic acid, atropine, or nicotine) combinations. Artificial diet amended with these treatments was fed to larvae of the crucifer specialist herbivore Plutella xylostella. Combinations of metals and metals + organic chemicals significantly decreased survival and pupation rates, compared to single treatments, for at least some concentrations in every experiment. Effects of combinations were additive rather than synergistic or antagonistic. Because Zn enhanced the toxicity of other metals and Ni enhanced the toxicity of organic defensive chemicals, our findings suggest that the defensive effects of metals are more widespread among plants than previously believed. They also support the hypothesis that herbivore defense may have led to the evolution of metal hyper-accumulation by increasing the preexisting defensive effects of metals at accumulator levels in plants.