The goal of this protocol is to deliver animal-derived and artificial blood meals to Aedes aegypti mosquitoes through an artificial membrane feeder and precisely quantify the volume of meal ingested.
Females of certain mosquito species can spread diseases while biting vertebrate hosts to obtain protein-rich blood meals required for egg development. In the laboratory, researchers can deliver animal-derived and artificial blood meals to mosquitoes via membrane feeders, which allow for manipulation of meal composition. Here, we present methods for feeding blood and artificial blood meals to Aedes aegypti mosquitoes and quantifying the volume consumed by individual females.
Targeted feeding and quantification of artificial/blood meals have broad uses, including testing the effects of meal components on mosquito behavior and physiology, delivering pharmacological compounds without injection, and infecting mosquitoes with specific pathogens. Adding fluorescein dye to the meal prior to feeding allows for subsequent meal size quantification. The meal volume consumed by mosquitoes can be measured either by weight, if the females are to be used later for behavioral experiments, or by homogenizing individual females in 96-well plates and measuring fluorescence levels using a plate reader as an endpoint assay. Meal size quantification can be used to determine whether changing the meal components alters the meal volume ingested or if meal consumption differs between mosquito strains. Precise meal size quantification is also critical for downstream assays, such as those measuring effects on host attraction or fecundity. The methods presented here can be further adapted to track meal digestion over the course of days or to include multiple distinguishable markers added to different meals (like nectar and blood) to quantify the consumption of each meal by a single mosquito.
These methods allow researchers to singlehandedly perform high-throughput measurements to compare the meal volume consumed by hundreds of individual mosquitoes. These tools will therefore be broadly useful to the community of mosquito researchers for answering diverse biological questions.
We present a protocol for feeding modified blood meals to Aedes aegypti mosquitoes using an artificial membrane feeder and precisely measuring the meal volume consumed by each individual mosquito. This protocol can be flexibly adapted to alter the content of the meal or to compare the meal volume consumed by different experimental groups of mosquitoes.
The Ae. aegypti mosquito threatens global health by spreading pathogens that cause diseases including yellow fever, dengue fever, chikungunya, and Zika1,2,3,4,5. Ae. aegypti females are obligate blood-feeders; they must consume vertebrate blood to obtain the necessary protein for egg development, and each clutch of eggs requires a full blood meal from at least one host6,7,8. The female mosquito first bites her host by piercing the skin with her stylet and injecting saliva, which contains compounds that trigger the host’s immune response9. She then feeds by pumping blood through her stylet into her midgut. While consuming a blood meal from an infected host, she may ingest blood-borne pathogens6,8, which then migrate from the mosquito’s midgut to her salivary glands10. Female mosquitoes infected in this manner can spread disease by injecting pathogens along with saliva when biting subsequent hosts11,12. Understanding and quantifying the mechanisms of blood-feeding behavior are crucial steps in controlling the transmission of mosquito-borne diseases.
Many laboratory protocols for mosquito rearing and experimentation use live animals including mice, guinea pigs, or humans as a blood source13,14,15,16. The use of live animals imposes ethical concerns as well as complex requirements for personnel training, animal housing and care, and compliance with Institutional Animal Care and Use Committee (IACUC) policies. It also limits the types of compounds that can be delivered to mosquitoes, which constrains the studies that can be carried out17.
Artificial blood-feeding apparatuses, which typically use a membrane system to simulate host skin, are useful tools for studying blood-feeding behaviors that circumvent the need for the maintenance of live hosts. Whole blood can be purchased from a number of vendors and fed to mosquitoes using heated, water-jacketed artificial membrane feeders or similar devices18,19. In this protocol, we demonstrate the use of small, disposable membrane feeders termed “Glytubes”. This membrane feeder, previously published by Costa-da-Silva et al. (2013)20, can be easily assembled from standard laboratory equipment, making it ideal for delivering blood meals to moderate numbers of mosquitoes and straightforward to scale up for testing larger groups or multiple meal formulations. The Glytube is an inexpensive and efficient alternative to other commercial artificial feeders, which may require larger meal volumes and are more suitable for batch feeding large groups of mosquitoes on a single meal formulation21.
This protocol includes two sections: preparing/delivering artificial meals and quantifying consumption. In the first section, Glytubes are used as an efficient means to deliver manipulated diets. Whole blood may be substituted with an entirely artificial meal to compare the effects of blood substitutes in lieu of a blood meal. A recipe adapted from Kogan (1990)22 is presented here, although multiple artificial meal formulations have been developed23,24. Furthermore, feeding is a less invasive and less laborious method to introduce pharmacological compounds than injection. Due to the low total volume required for each meal (1–2 mL), this protocol provides an attractive delivery method to reduce the amounts of expensive reagents. Ae. aegypti females readily consume protein-free meals of saline solution with adenosine 5′-triphosphate (ATP)25,26, which provides a baseline for measuring the effects of single meal components. For example, Neuropeptide Y-like receptor 7 (NPYLR7) in Ae. aegypti is known to mediate host-seeking suppression after a protein-rich blood meal, and when NPYLR7 agonists are added to a protein-free saline meal, female mosquitoes exhibit host-seeking suppression similar to those that have consumed whole blood7.
In the second section, steps for quantifying the volume of each meal consumed by an individual female mosquito are presented. This assay is fluorescence-based and captures feeding status in higher resolution than methods in which females are classified as “fed” or “unfed” based on visual assessment of abdominal distension alone. By adding fluorescein to the meal prior to feeding, meal volumes ingested by individuals can be quantified by homogenizing each mosquito in a 96-well plate and measuring fluorescence intensity as a readout. This assay can measure differences in feeding vigor in response to variables such as meal composition or the mosquitoes’ genetic background. Precise quantification is critical for intermediate meal sizes, for example when females are offered suboptimal meals containing feeding deterrents or when they consume sucrose meals of variable sizes27. If fed mosquitoes are required for subsequent behavioral assays after meal size quantification, meal size can instead be calculated by weighing anesthetized females in groups and estimating the average increased mass per individual. Although less precise than fluorescein marking, weighing still provides an aggregated estimate of meal volume and allows examination of the meal’s effect on downstream processes, such as fecundity or subsequent host attraction. While blood meal size is variable and can be influenced by a myriad of factors11,28,29, ingested meal sizes measured with the methods described here are consistent with previous quantifications7,30,31.
Blood-feeding procedures were not performed using live animals or human hosts and complied with the guidelines set by The Rockefeller University Institutional Animal Care and Use Committee (IACUC) and Institutional Review Board (IRB).
1. Meal preparation
2. Meal delivery to mosquitoes
3. Quantification of consumed meals
Figure 1 presents a schematic for assembling the Glytube, whereas Figure 2 shows an overview of the experimental design to measure meal size using the fluorescence-based assay described here. Figure 3 provides representative fluorescein meal size measurements from a blood-feeding experiment. Figure 4, Figure 5, and Figure 6 illustrate a sampling of biological questions that can be addressed using this protocol. Applications of the protocol are wide-ranging and include altering blood meal composition, feeding pharmacological compounds, precisely quantifying sub-optimal blood meals or smaller nectar meals, and comparing feeding behavior across mosquito genotypes.
To generate a standard curve for meal volume calculations, fluorescence readings are plotted from the designated reference wells each containing an unfed mosquito and a known volume of the meal with 0.002% fluorescein (Figure 3A). Fluorescence readings from the remaining wells, which contain mosquitoes from either the negative control group of unfed mosquitoes or the experimental group of mosquitoes offered a meal, are compared to this standard curve to quantify the meal volume (µL) consumed by each mosquito (Figure 3B). To validate the baseline readings in this assay, it should be confirmed that mosquitoes from the unfed negative control group are not assigned a positive value of µL consumed (Figure 3B, left). Although all females in the experimental group were offered the blood meal, some mosquitoes fed (Figure 3B, middle) and some did not (Figure 3B, right). This result demonstrates that two types of data can be obtained from this protocol: 1) the percentage of total females that feed on a given meal, and 2) the volume ingested by the females that feed on a given meal.
This protocol can be used to deliver and quantify meals with various protein compositions. Figure 4A,B show data collected using meals with added fluorescein. The proportion of mosquitoes that fed and the meal volume they ingested, respectively, were calculated from the fluorescence readings. These readings are highly sensitive and allow for precise quantification of µL, but have the limitation that mosquitoes cannot be used for future live experiments. Figure 4C,D show data collected from an independent experiment with mosquitoes that were scored as fed or unfed by eye after they were offered meals without fluorescein. Meal size was calculated as average weight/female from groups of 5 mosquitoes. Although these weight measurements are less sensitive than fluorescence measurements, they allow the females to be recovered and used for further live experimentation. The proportion of mosquitoes that feed can vary across different experimental days, as reflected in Figure 4A and Figure 4C.
Figure 5 shows the volume consumed of meals containing drugs that regulate mosquito host-seeking behavior. In these experiments, females were offered blood, saline + ATP, or saline + ATP meals with 100 μM of the human NPY Y2 receptor agonist, TM30338. This drug alters host-seeking behavior through activation of Ae. aegypti NPY-like receptor 7. Measuring meal sizes is critical for the interpretation of experiments to assess the effect of this drug on post-blood-feeding behavior because it allows the researcher to calculate the dose consumed by each female.
In the previous examples, females were fed either blood or substitute blood meals, all of which resulted in 3–5 µL meals (Figure 3, Figure 4, Figure 5). This fluorescence-based assay can also be used to measure smaller and/or more variable meal sizes that cannot be accurately discerned from average group weight measurements. In Figure 6, the same fluorescence quantification protocol was used to measure nectar-feeding behavior by exchanging the Glytube for a cotton ball saturated with 10% sucrose containing 0.002% fluorescein. Nectar sugars cannot be presented in the Glytube assay because females cannot detect the presence of nectar sugars with the stylet and do not initiate feeding27. These data allow the researcher to determine that sugar meals are consistently smaller than blood meals, in agreement with previous work34 (Figure 6).
Figure 1: Setup of Glytube method used to feed meals to mosquitoes. (A) Schematic of a deconstructed Glytube used to feed blood and other meals to mosquitoes. (B) Schematic of a Glytube presented atop a container of mosquitoes with a mesh lid. Female mosquitoes can pierce through the mesh lid to feed. (C) Photographs of the Glytube (top), and female Aedes aegypti mosquitoes before, during, and after feeding (bottom, from left to right) on a Glytube-delivered meal. Mosquitoes are shown piercing through the mesh covering their container to access the membrane feeder. (D) Photographs showing the appearance of female Ae. aegypti mosquitoes that are unfed (left) and that have engorged on either an artificial blood meal (right, top) or a saline + ATP meal (right, bottom). The Glytube method was previously published in Costa-da-Silva et al. (2013)20. Photographs in (C) and (D) are courtesy of Alex Wild. Please click here to view a larger version of this figure.
Figure 2: Schematic of how to quantify meal size after Glytube blood-feeding protocol. (A) Mosquitoes are offered a meal with fluorescein (top, experimental group) or no meal (bottom, unfed negative control group). (B) Individual mosquitoes are added to a 96-well plate after terminating the feeding experiment. (C) Standard curve is generated using known amounts of meal containing 0.002% fluorescein. (D) Mosquitoes are homogenized to release any consumed fluorescein, and fluorescence levels in each well are quantified using a plate reader. This fluorescence quantification method is modified from Liesch et al. (2013)34. Please click here to view a larger version of this figure.
Figure 3: Glytube blood-feeding experiment with fluorescein-based quantification. (A) Standard curve measurements obtained from the wells where a mosquito from the unfed control group was added to a known quantity of meal containing 0.002% fluorescein (y-axis scale = arbitrary units). (B) Meal volume calculated using fluorescence readings for females in the unfed control group (left, black, n = 40), the experimental group that fed on blood (middle, red, n = 37), and the experimental group that did not feed on blood (right, red, n = 23). Each point represents a measurement from an individual female. Data are shown as median with range. Letters indicate statistically distinct groups, Kruskal-Wallis test with Dunn’s multiple comparison, p<0.01. These data were published in Jové et al. (2020)27. Please click here to view a larger version of this figure.
Figure 4: Quantification of meals with differing protein composition. Females were offered meals of either sheep blood (red), artificial blood with human blood proteins (Kogan (1990)22) (orange), or protein-free saline + ATP meal (aqua)7. (A) Percentage of females fed scored using fluorescence readings. Each point represents a group of 12–16 females. Data are shown as medians with ranges, n = 12. (B) Meal volume calculated using fluorescence readings. Each point represents a measurement from an individual female in a single trial from Figure 4A. Data are shown as medians with ranges, n = 12. (C) Percentage of females fully engorged after artificial membrane feeding, scored by eye. Each point represents the percent of females engorged from groups of 20–30 females. Data are shown as medians with ranges, n = 23. (D) Meal sizes scored as weight/female after feeding status was scored by eye. Weights were calculated as the average of groups of 5 mosquitoes. Data are shown as medians with ranges, n = 23. A–D: Letters indicate statistically distinct groups, Kruskal-Wallis test with Dunn’s multiple comparison, p<0.05. Please click here to view a larger version of this figure.
Figure 5: Quantification of meals with pharmacological compounds. Females consume meals of the same size of sheep blood (red), saline + ATP (aqua), and saline + ATP + 100 µM dose of human NPY Y2 receptor agonist TM30338 (dark blue). Meal volume calculated using fluorescence readings. Each point represents a measurement from an individual female. Data are shown as medians with ranges, n = 12. Letters indicate statistically distinct groups, Kruskal-Wallis test with Dunn’s multiple comparison, p<0.05. Please click here to view a larger version of this figure.
Figure 6: Quantification of smaller nectar meals. (A) Schematic of nectar-feeding assay. (B) Meal volume calculated using fluorescence readings for wild-type females offered meals of either water (blue, n = 36) or 10% sucrose (green, n = 53), each with 0.002% fluorescein, in the nectar-feeding assay. Each point represents a measurement from an individual female. Data are shown as medians with ranges. Letters indicate statistically distinct groups, Mann-Whitney test, p<0.05. These data were published in Jové et al. (2020)27. Please click here to view a larger version of this figure.
Artificial Blood Meal | |||
Concentration of Stock Solution (mg/mL) | Volume of Stock Solution in Meal (μL/mL) | Final Meal Concentration (mg/mL) | |
Protein Components* | |||
γ-Globulins | 50 | 300 | 15 |
Hemoglobin | 35 | 230 | 8 |
Albumin | 300 | 340 | 102 |
Total Protein | – | – | 125 |
Non-Protein Components | |||
Concentration of Stock Solution (mM) | Volume of Stock Solution in Meal (μL/mL) | Final Meal Concentration (mM) | |
NaCl | In γ-globulin stock | – | 5-10 |
NaHCO3 | In γ-globulin stock | – | 120 |
ATP | 200 | 5 | 1 |
Water | – | 125 | – |
*Protein components are prepared in stock solution of double-distilled water, except for γ-Globulins, which are dissolved in 400 mM NaHCO3 and include a variable amount of NaCl (2-4%) in the product. |
Table 1: Recipe for preparing artificial blood meals (adapted from Kogan (1990)22). Artificial blood consists of protein and non-protein components regularly found in human blood and provides the option to vary the ratios of these components. Mosquitoes can produce eggs after feeding on artificial blood7,22.
Saline Meal | |||
Component | Concentration of Stock Solution (mM) | Volume of Stock Solution in Meal (μL/mL) | Final Meal Concentration (mM) |
NaCl | – | – | – |
NaHCO3 | 400 | 300 | 120 |
ATP | 200 | 5 | 1 |
Water | – | 695 | – |
Table 2: Recipe for saline meal with ATP (adapted from Duvall et al. (2019)7). Protein-free saline meals can be used to deliver compounds of interest to mosquitoes while still mimicking the abdominal distension that occurs after blood-feeding, but without triggering the egg development that occurs when proteins are ingested.
For many laboratory applications, artificial membrane feeders offer distinct benefits compared to live hosts by allowing researchers the ability to directly manipulate the contents of the meal. Although multiple methods are available for artificial membrane feeding, the method described here offers advantages in flexibility, cost, and throughput. In comparison to other commercial membrane feeders, the Glytube assay requires a small meal volume, making it an efficient delivery mechanism for costly reagents, including drugs or pathogens, by minimizing the total volume required7,35. As both the protein-free saline and artificial blood meals promote engorgement, compounds or pathogens can be added to either meal as a high-throughput and non-invasive alternative to injections. Additionally, each component of the Glytube can easily be washed, replaced, or scaled up to deliver and quantify multiple meal types without cross-contamination of the feeding apparatus.
To quantify meal volumes consumed by mosquitoes, the fluorescence-based method enables more precise meal size quantification than weighing the mosquitoes before and after feeding. It should be noted that this method is an end-point assay. In contrast, weighing allows the mosquitoes to be kept alive for further experimentation. By using a plate reader, the fluorescence-based method can be easily scaled up for high-throughput quantification of meals consumed by hundreds of individual females.
To achieve high feeding rates, a combination of sufficient host cues must be present to activate female host-seeking behavior and attract females to the feeder. If mosquitoes are not crowding underneath the Glytube, the meal may not be properly warmed, or CO2 delivery may not be sufficient. Addition of human odor to the membrane surface reliably increases attractiveness of the artificial membrane. If mosquitoes are observed underneath the Glytube but fail to feed, the meal composition may be at fault. Females may not feed if the meal itself is not warm, the blood is too old, or if the additives to the meal are intrinsically aversive or cause an undesirable chemical reaction36. Additional ATP also reliably increases feeding rates and can be scaled up to a final concentration of 2 mM in each of the recipes provided. Females may not feed if the parafilm is not pulled taut across the Glytube cap; the parafilm should be uniformly transparent and should not buckle, as this prevents the female from being able to effectively pierce the parafilm with her stylet. If the meal leaks through the Glytube onto the mesh, the parafilm may have torn during the stretching process and should be replaced.
Changing the meal composition can also allow researchers to manipulate the length of time needed to clear the meal from the midgut as well as the subsequent host-seeking behavior. The meals presented here require 24–36 h for digestion7 similar to animal-derived blood. After feeding on any of these meals, females will suppress host-seeking during the digestion time window. Since the saline meal lacks protein, females return to host-seeking after the meal is cleared. If a faster return is desirable, researchers can choose alternate “quick clearing” saline meals that are excreted in approximately 6 h27. While the composition of the saline meal presented here is matched to directly compare results with the artificial blood meal, the “quick clearing” meal more closely matches physiological salt levels found in vertebrate blood.
The methods described here have limitations that should be considered before selecting the assay that is most suited to the researcher’s experimental goals. The fluorescein measurements described do not allow mosquitoes to be used again for additional experimentation. However, weight measurements can be taken prior to meal size quantification using the fluorescein assay. If weight and meal size are consistent across multiple trials for a given meal, weight can be used as a proxy in future experiments. Moreover, this protocol does not distinguish between deficits in host-seeking versus blood-feeding behavior; mosquitoes that show impairments in finding the membrane feeder will have a reduction in feeding rates and/or meal size. By adding a camera to record behavior throughout the assay, researchers can determine whether the females cannot find the Glytube, or whether they find the Glytube, but do not feed.
The assay described here can be adapted to explore many outstanding questions related to feeding behavior in mosquitoes. For example, the contribution of specific blood proteins can be explored by altering the ratio of constituent proteins or total protein concentration in the artificial blood meal. To evaluate meal sizes from multiple feeding events, dyes with distinct fluorescence spectra can be added to differentiate meals from unique sources37. This protocol can also be modified to separately stimulate the internal mouthparts that detect blood and that are used for ingestion (i.e., stylet), and the chemosensory appendages that contact skin (i.e., labium, legs) as the mosquito lands to begin blood feeding36. For example, if ligands are added directly to the meal, they do not contact the labium and legs, since the membrane is pierced only by the stylet. If ligands are added to the outer surface of the parafilm instead, they remain separated from the meal and may be contacted by the labium and legs36. Finally, the detailed kinetics of blood-feeding behavior are not well understood and the method presented here could be modified to combine high-resolution tracking with machine learning tools to extract behavioral readouts of locomotion, posture, and feeding dynamics38.
This protocol is aimed at being user-friendly and cost-effective, with the ability to serve researchers employing pharmacological and genetic manipulations to study mosquito blood-feeding and post-blood-feeding behavior.
The authors have nothing to disclose.
We thank Nipun Basrur, Adriana K. Rosas Villegas, Nadav Shai, and Trevor Sorrells for comments on the manuscripts, and Zhongyan Gong and Kyrollos Barsoum for technical assistance. We thank Alex Wild for photographs used in Figure 1. K.V. was supported by the Boehringer Ingelheim Fonds PhD fellowship. V.J. was supported in part by NIH T32-MH095246. This work was supported in part by a grant to The Rockefeller University from the Howard Hughes Medical Institute through the James H. Gilliam Fellowships for Advanced Study program to V.J. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. NSF DGE-1325261 to V.J. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
15 mL conical tubes | Fisher Scientific | 14-959-70C | |
3 mm diameter borosilicate solid-glass bead | MilliporeSigma | Z143928 | For use for bead mill homogenizer; not required if using pellet pestle grinder |
32 oz. high-density polyethylene (HDPE) plastic cup | VWR | 89009-668 | Example mosquito container used for feeding assays shown; alternate options can be used |
50 mL conical tubes | Fisher Scientific | 14-959-49A | |
96-well black polystyrene plate | ThermoFisher | 12-566-09 | |
96-well PCR plate sealing film | Bio-Rad | MSB1001 | Alternate options can be used |
96-well PCR plates | Bio-Rad | HSP9621 | Alternate options can be used |
Adenosine 5′-triphosphate (ATP) disodium salt hydrate | MilliporeSigma | A6419 | |
Albumin (human serum) | MilliporeSigma | A9511 | |
Aluminum foil | Fisher Scientific | 01-213 | Alternate options can be used to block light entering fluorescein container |
Balance | Fisher Scientific | 01-911 | Alternate options can be used |
Bead mill homogenizer | Qiagen | 85300 | Not required if using pellet pestle grinder |
Cotton ball | Fisher Scientific | 22456880 | For nectar-feeding; alternate options can be used |
Defibrinated sheep blood | Hemostat Laboratories | DSB100 | Alternate options can be used |
Drosophila CO2 fly pad | Tritech Research | MINJ-DROS-FP | Alternate options can be used |
Fluorescein | MilliporeSigma | F6377 | |
Fluorescence plate-reader | ThermoFisher | VL0000D0 | Alternate options can be used |
Gamma-globulin (human blood) | MilliporeSigma | H7379 | |
Glycerol | MilliporeSigma | G7893 | |
Hemoglobin (human) | MilliporeSigma | G4386 | |
Laboratory wrapping film – parafilm | Fisher Scientific | 13-374 | |
Magnetic stirrer | Fisher Scientific | 90-691 | Alternate magnetic stirrers can be used |
Microcentrifuge for 96-well plate | VWR | 80094-180 | Alternate options can be used |
Microcentrifuge Tubes | MilliporeSigma | 2236412 | Alternate options can be used |
Pellet pestle grinder | VWR | KT749521-1500 | Not required if using bead mill homogenizer |
Phosphate buffered solution (PBS) | Fisher Scientific | BW17-516F | Optional |
Razor blades | Fisher Scientific | 12-640 | Alternate options can be used, such as a lathe for better consistency of cutting |
Rubber bands | |||
Silicone tubing | McMaster Carr | Needed if using a fly pad for CO2 delivery | |
Sodium bicarbonate (NaHCO3) | Fisher Scientific | S233 | |
Sodium chloride (NaCl) | MilliporeSigma | S9888 | |
Stir bars | Fisher Scientific | 14-512 | Alternate magnetic stir bars can be used |
Translucent polypropylene storage box with removable lid | Example box used for feeding assays shown | ||
Vortex mixer | |||
Water bath | Alternate heating device may be used | ||
White 0.8 mm polyester mosquito netting | American Home & Habit Inc. | F03A-PONO-MOSQ-M008-WT | Alternate options can be used |